Patent Publication Number: US-2023137266-A1

Title: Quantum electromagnetic field sensor and imager

Description:
This application claims the benefit of U.S. Provisional Patent Application 62/991,999, filed Mar. 19, 2020, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure generally relates to imaging devices. 
     BACKGROUND 
     An image sensor may be a semiconductor device for converting electromagnetic radiation into electric signals. Examples include charge coupled devices (CCDs), complementary metal-on-semiconductor (CMOS) devices, photodiode arrays, charge injection devices, hybrid focal plane arrays, etc. For electromagnetic radiation in the terahertz (THz) frequency range, e.g., millimeter-waves, conventional imagers may include micro-bolometers, photo-conductive devices, folded-dipole antennas, Schottky-barrier diodes, pyrometric devices, and Golay cells, e.g., opto-acoustic detectors. 
     SUMMARY 
     In general, the disclosure describes a sensor array for imaging electromagnetic (EM) radiation having frequencies in the megahertz (MHz), gigahertz (GHz), and terahertz (THz) ranges (MHz/GHz/THz EM radiation). In some examples, a vapor cell array may operate as a transducer to convert electromagnetic radiation having frequencies in a first range to electromagnetic radiation having frequencies in a second range. In some examples, compared to direct detection of electromagnetic radiation in the first range, electromagnetic radiation in the second frequency range may be more discernable, have a higher signal-to-noise (SNR) ratio, may be less expensive to detect, detectable with a smaller or light apparatus, and have a higher sensitivity. 
     In some examples, a vapor cell array may include a plurality of vapor cells including alkali atoms. The alkali atoms may be prepared in a Rydberg state in which the alkali atoms are excited such that one or more electrons have a very high principal quantum number, n. The alkali atoms in a Rydberg state may have loosely bound valence electrons that may be perturbed or ionized by collisions or external fields, e.g., MHz/GHz/THz radiation. In some examples, the alkali atoms of each vapor cell of a vapor cell array may be prepared by excitation via coupling light having a first frequency and probe light having a second frequency. In some examples, the frequency of one or both of the coupling light and the probe light may be ultraviolet, visible, or near infrared (UV/VIS/NIR) frequencies. The probe light may excite the alkali atoms from a first quantum energy level to a second quantum energy level, and the coupling light may excite the alkali atoms from the second quantum energy level to a third quantum energy level such that the alkali atoms are in a Rydberg state. 
     In some examples, the alkali atoms in the Rydberg state may exhibit electromagnetically induced transparency (EIT) for frequencies near the frequency of the probe light. The probe light may be configured to be detected by a detector array after transmission through a vapor cell of the vapor cell array. Perturbations of the alkali atoms in the Rydberg state via EM radiation in the MHz/GHz/THz frequency ranges may be detected via changes of an EIT spectral window of the probe light transmitting through the vapor cell array to the detector array, resulting in a signal of the probe light in the UV/VIS/NIR frequency ranges that may be proportional to the magnitude and frequency of the EM radiation in the MHz/GHz/THz frequency ranges. 
     In some examples, an imaging system includes an array of vapor cells and an array of detectors. Probe light and coupling light in the UV/VIS/NIR frequency range may be used to image incident electromagnetic radiation in the MHz/GHz/THz frequency range. 
     Accordingly, the techniques may provide one or more technical advantages that realize at least one practical application. For example, the techniques may improve the sensitivity and signal to noise ratio (SNR) of a MHz/GHz/THz electromagnetic radiation imaging system. The techniques may reduce the size, weight, and required power (SWaP), and cost of a MHz/GHz/THz electromagnetic radiation imaging system. 
     In some examples, this disclosure describes a sensor comprising a vapor cell including a vapor of alkali atoms, a first photonic integrated circuit (PIC) configured to direct light of a first wavelength into the vapor cell and incident on the vapor of alkali atoms, wherein the light of the first wavelength is configured to excite the alkali atoms to a first excited state from a ground state, and a detector configured to detect a response of the alkali atoms, after the alkali atoms are excited from the first excited state to a Rydberg state, to incident electromagnetic radiation. 
     In some examples, this disclosure describes a method comprising exciting alkali atoms in a vapor cell, via light of a first wavelength from a first photonic integrated circuit (PIC), to a second quantum state from a first quantum state, exciting the alkali atoms in the plurality vapor cells, via light of a second wavelength, to a Rydberg state, detecting a response of the alkali atoms in the Rydberg state to incident electromagnetic radiation, and outputting a signal proportional to the detected response. 
     In some examples, this disclosure describes an electromagnetic radiation detection array comprising a plurality of vapor cells arranged in a two-dimensional (2D) array, each vapor cell including a vapor of alkali atoms, a first photonic integrated circuit (PIC) configured to direct light of a first wavelength into each vapor cell of the plurality of vapor cells and incident on the vapor of alkali atoms, wherein the light of the first wavelength is configured to excite the alkali atoms to a first excited state from a ground state, a second PIC configured to direct light of a second wavelength into each vapor cell of the plurality of vapor cells and incident on the vapor of alkali atoms, wherein the light of the second wavelength is configured to excite the alkali atoms from the first excited state to a Rydberg state, and a plurality of detectors, each detector corresponding to one of the plurality of vapor cells and configured to detect a response of the alkali atoms in the Rydberg state to incident electromagnetic radiation. 
     The details of one or more examples of the techniques of this disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional block diagram illustrating an example imaging system, in accordance with the techniques of the disclosure. 
         FIG.  2 A  is a perspective view of an example vapor cell array, in accordance with the techniques of the disclosure. 
         FIG.  2 B  is a perspective view of a partially transparent block diagram illustrating an example imaging system  160 , in accordance with the techniques of the disclosure. 
         FIG.  3    is a cross-sectional illustration of example sensor, in accordance with the techniques of the disclosure. 
         FIG.  4    is a cross-sectional illustration of example sensor, in accordance with the techniques of the disclosure. 
         FIG.  5    is a flowchart of an example method of imaging electromagnetic radiation, in accordance with the techniques of the disclosure. 
         FIG.  6    is an illustration of an illustration of an example energy diagram of an alkali atom including at least one Rydberg state, in accordance with the techniques of the disclosure. 
         FIG.  7    is an example plot illustrating absorption signal responses of probe light as a function of probe light frequency detuning, in accordance with techniques of the present disclosure. 
         FIG.  8    is an illustration of an illustration of an example energy diagram of an alkali atom including at least one Rydberg state, in accordance with the techniques of the disclosure. 
         FIG.  9    is a block diagram of an example system  900  for sensing EM radiation, in accordance with techniques of the present disclosure. 
         FIG.  10    is a cross-sectional illustration of example sensor, in accordance with the techniques of the disclosure. 
         FIG.  11    is a cross-sectional illustration of example sensor, in accordance with the techniques of the disclosure. 
         FIG.  12    is a flowchart of an example method of imaging electromagnetic radiation, in accordance with the techniques of the disclosure. 
     
    
    
     Like reference characters refer to like elements throughout the figures and description. 
     DETAILED DESCRIPTION 
     Terahertz (THz) radiation may be used in a range of different applications such as telecommunications and wireless networks (use of millimeter-wave mobile broadband systems), antennas and advanced radar applications, environmental monitoring, counterterrorism, astronomic observation such as in small satellites and cubesats, characterizing materials, characterizing electromagnetic interference, medical testing, microwave background detection, and so forth. Some advantages of terahertz radiation are its low energy profile and non-ionizing profile, making it relatively harmless. 
     There are many possible applications using THz detectors and imagers. For example, THz detectors can be integrated into self-calibrated electric field and power sensors in the radio frequency (RF), microwave and millimeter-wavelength regimes, e.g., high energy applications with greater than 1 kilovolt/meter (kV/m) electric fields. In the area of security, THz imaging can be used to detect concealed cargo or weapons. Millimeter-wave imagers can be used for aeronautical applications; for example, to monitor ground movement of aircrafts in adverse weather conditions. In addition, millimeter-wave sensors can be applied in biological sensing to monitor vital signs at a large stand-off distance. 
       FIG.  1    is a cross-sectional block diagram illustrating an example imaging system  100 , in accordance with the techniques of the disclosure. In the example shown, imaging system  100  includes vapor cell array  102 , detector array  104 , first waveguide  106 , second waveguide  108 , imaging optics  110 , and computing device  120 . Imaging system  100  may also include coupling light source  130  and probe light source  140 .  FIG.  1    also illustrates processing circuitry  116  communicatively coupled to imaging system  100  and memory  124  communicatively coupled to processing circuitry  116 . While the cross-sectional view of imaging system  100  in  FIG.  1    illustrates imaging system  100  in one-dimension (1D), imaging system  100  may be a two-dimensional (2D) system, e.g., vapor cell array  102 , detector array  104 , first waveguide  106 , and second waveguide  108  may be two-dimensional arrays including a plurality of individual elements, such as detectors  114  and vapor cells  112 , in both the x-direction and y-direction. 
     Vapor cell array  102  may include a plurality of vapor cells  112 , each vapor cell  112  including a vapor of atoms, for example, alkali atoms. As noted above, although the cross-sectional view of  FIG.  1    illustrates vapor cell array  102  as a 1D array including a plurality of vapor cells  112  in the y-direction, vapor cell array  102  may be a 2D array including a plurality of vapor cells  112  in both the x-direction and the y-direction. In some examples, each vapor cell  112  is configured to be a transducer to convert electromagnetic radiation having frequencies in a first range to electromagnetic radiation having frequencies in a second range. For example, each vapor cell  112  may transduce, or convert, electromagnetic radiation  152  having frequencies in the megahertz (MHz), gigahertz (GHz), and terahertz (THz) ranges (MHz/GHz/THz radiation  152 ) to optical frequencies in petahertz (PHz) ranges. Stated in terms of wavelengths, each vapor cell  112  may convert electromagnetic radiation in the meter- to millimeter-wavelength ranges to electromagnetic radiation in the UV/VIS/NIR wavelength ranges. 
     In some examples, each vapor cell  112  may include a vapor of atoms configured to be exhibit electromagnetic induced transparency (EIT). For example, the alkali atoms of each vapor cell  112  may be prepared in a Rydberg state by driving the alkali atoms from a first quantum energy level to a higher second quantum energy level via probe light  142 , and further driving the alkali atoms from the second quantum energy level to a third higher quantum energy level via coupling light  132 , as further illustrated and described below with respect to  FIG.  6   . In the presence of a strong on-resonant coupling light  132 , e.g., coupling light  132  having a frequency closely matched to the energy gap between the second and third quantum states, the index of refraction of the vapor of alkali atoms of vapor cell  112  may be modified for frequencies near the frequency of probe light  142  resulting in an EIT window in the absorption spectrum of the alkali atoms near that frequency, e.g., the frequency of probe light  142 . As such, the absorption spectrum of the alkali atoms as a function of probe light  142  frequency detuning may be observed, as further illustrated and described below with respect to  FIG.  7   . 
     The EIT transparency window may have a spectral width less than 1 MHz, less than 10 MHz, or less than 100 MHz, and may allow for sub-Doppler precision in measurement of the response of the alkali atoms to MHz/GHz/THz electromagnetic radiation at room temperature. For example, perturbations of the energy level of the alkali atoms in the third quantum energy level, e.g., a Rydberg state, may be measured via changes in the EIT window detected via intensity of probe light  142  at detector array  104 . In other words, each alkali atom may act as an independent transducer converting incident MHz/GHz/THz electromagnetic radiation to an optical signal, and the ensemble of alkali atoms of vapor cell  112  may incoherently amplify the optical signal. 
     In some examples, probe light  142  may be UV/VIS/NIR light. For example, probe light  142  may be 780 nm laser light, and probe light source  140  may be a 780 nm laser. In some examples, coupling light  132  may be UV/VIS/NIR light. For example, coupling light  132  may be 480 nm laser light, and coupling light source  130  may be a 480 nm laser. 
     First waveguide  106  may be configured to transport and extract coupling light  132  to one or more vapor cells  112 . In some examples, first waveguide  106  may be a photonic integrated circuit (PIC) including a waveguide and one or more extraction features. As noted above, although the cross-sectional view of  FIG.  1    illustrates first waveguide  106  as a 1D structure having a height in the y-direction and a thickness in the z-direction, first waveguide  106  may be a 2D structure having a height in the y-direction, a width in the x-direction, and a thickness in the z-direction, and may include a plurality of structural features disposed in both the x-direction and the y-direction, e.g., corresponding to the vapor cells  112  of a 2D vapor cell array  102 . In some examples, first waveguide  106  may be configured to transport coupling light  132  via total internal reflection (TIR) to distribute coupling light  132  over a 2D area and extract coupling light  132  via extraction features on either a front surface or a back surface, e.g., surfaces within the x-y plane, or within first waveguide  106 . In some examples, the extraction features may be configured to frustrate TIR and direct coupling light  132  in a predetermined direction from a predetermined position of first waveguide  106 , e.g., towards one or more vapor cells  112 . In some examples, coupling light source  130  may inject coupling light  132  into one or more edges of first waveguide  106 , as illustrated in  FIG.  1   . In other examples, first waveguide  106  may be an optical element having optical power such as a lens or lenslet array, a diffraction grating, or any optical element configured to direct coupling light  132  to vapor cells  112  of vapor cell array  102 . In some examples, first waveguide  106  may not be waveguide transporting coupling light  132  via TIR but may be any other structure configured to direct coupling light  132  in a predetermined direction from a predetermined position of first waveguide  106 , e.g., an array of beam splitters. In some examples, system  100  may include a plurality of coupling light sources  130 , for example, distributed along one or more edges of first waveguide  106  and configured to inject coupling light into one or more edges of first waveguide  106 . In some examples, coupling light source  130  may inject coupling light into first waveguide  106  via any surface and at any position of first waveguide  106 , e.g., via emitting light to a coupling structure located on any surface of first waveguide  106  and configured to direct coupling light into first waveguide  106  for transport. 
     Second waveguide  108  may be configured to transport and extract probe light  142  to detector array  104  through vapor cell array  102 . In some examples, second waveguide  108  may be a PIC including a waveguide having one or more extraction features. As noted above, although the cross-sectional view of  FIG.  1    illustrates second waveguide  108  as a 1D structure having a height in the y-direction and a thickness in the z-direction, second waveguide  108  may be a 2D structure having a height in the y-direction, a width in the x-direction, and a thickness in the z-direction, and may include a plurality of structural features disposed in both the x-direction and the y-direction, e.g., corresponding to the vapor cells  112  of a 2D vapor cell array  102  and the detectors  114  of a 2D detector array  104 . In some examples, second waveguide  108  may be configured to transport probe light  142  via TIR to distribute coupling light  132  over a 2D area and extract probe light  142  via extraction features on either a front surface or a back surface, e.g., surfaces within the x-y plane, or within second waveguide  108 . In some examples, the extraction features may be configured to frustrate TIR and direct probe light  142  in a predetermined direction from a predetermined position of second waveguide  108 , e.g., towards detector  114  through vapor cell  112 . In some examples, probe light source  140  may introduce probe light  142  into one or more edges of second waveguide  108 , as illustrated in  FIG.  1   . In other examples, second waveguide  108  may be an optical element having optical power such as a lens or lenslet array, a diffraction grating, or any optical element configured to direct probe light  142  to detectors  114  of detector array  104  through vapor cells  112  of vapor cell array  102 . In some examples, second waveguide  108  may not be waveguide transporting probe light  142  via TIR but may instead be another structure configured to direct probe light  142  in a predetermined direction from a predetermined position of second waveguide  108 , e.g., an array of beam splitters. In some examples, system  100  may include a plurality of probe light sources  140 , for example, distributed along one or more edges of second waveguide  108  and configured to introduce probe light into one or more edges of second waveguide  108 . In some examples, probe light source  140  may introduce coupling light into second waveguide  108  via any surface and at any position of second waveguide  108 , e.g., via emitting light to a coupling structure located on any surface of second waveguide  108  and configured to direct coupling light into second waveguide  108  for transport. 
     In some examples, imaging system  100  may include one of first or second waveguides  106  or  108  configured to transport and extract both coupling light  132  and probe light  142 . In other examples, imaging system  100  may not include waveguides  106  and  108  and may direct coupling light  132  to vapor cell array  102  and probe light  142  to detectors  114  of detector array  104  through vapor cells  112  of vapor cell array  102  via any other means, e.g., an array of light sources  130  and  140 , via optical fibers, and the like. 
     Detector array  104  may include a plurality of detectors  114 . As noted above, although the cross-sectional view of  FIG.  1    illustrates detector array  104  as a 1D array including a plurality of detectors  114  in the y-direction, detector array  104  may be a 2D array including a plurality of detectors  114  in both the x-direction and the y-direction. Detectors  114  of detector array  104  may be configured to detect electromagnetic radiation, for example, infrared and/or visible light. Detectors  114  may be large-bandgap solid-state visible wavelength detectors configured to operate at without cooling, e.g., at room temperature. For example, detector array  104  may be a charge-coupled device (CCD) array, metal-oxide-semiconductor based array such as a complementary metal-oxide-semiconductor (CMOS) array or N-type metal-oxide-semiconductor (NMOS) array. Detectors  114  of detector array  104  may be configured to detect probe light from probe light source  140 . In some examples, detector array  104  may be configured to output one or more signals proportional to the detected electromagnetic radiation, e.g., the detected probe light. For example, detector array  104  may be configured to output a 2D image of detected probe light, the detected probe light corresponding to EM radiation to be detected  152  that is transduced and/or converted to probe light  142  via vapor cells  112 . In some examples, detector array  104  may be configured to output a pixelated 2D image corresponding to EM radiation to be detected  152  in two dimensions, e.g., an image comprising a plurality of pixels. 
     Imaging optics  110  may include a lens, a 1D or 2D lens array, diffraction gratings, stackable THz-focusing optics, and the like. In some examples, imaging optics are configured to direct and/or focus EM radiation to be detected  152  on vapor cells  112 . In some examples, system  100  may not include imaging optics  110  and may operate as a phased array. 
     In some examples, system  100  may include PICs for in-plane beam routing and detection, e.g., for coupling light  132  and probe light  142 . For example, one or more of light source  130 , light source  140 , detector array  104 , first waveguide  106 , and second waveguide  108  may be integrated/combined as a PIC. 
     Computing device  120  may be configured to receive signals from detector array  104  indicative of detected probe light proportional to EM radiation to be detected  152 . Computing device  120  includes computation engine  122 , memory  124 , communication unit  118 , processing circuitry  116 , one or more hardware user interfaces  128  (hereinafter “hardware user interface  128 ”), and one or more output devices  126 . In the example of  FIG.  1   , a user of computing device  120  may provide input to computing device  120  via one or more input devices (not shown) such as a keyboard, a mouse, a microphone, a touch screen, a touch pad, or another input device that is coupled to computing device  120  via one or more hardware user interfaces  128 . 
     Output devices  126  may include a display, sound card, video graphics adapter card, speaker, presence-sensitive screen, one or more USB interfaces, video and/or audio output interfaces, or any other type of device capable of generating tactile, audio, video, or other output. Output devices  126  may include a display device, which may function as an output device using technologies including liquid crystal displays (LCD), quantum dot display, dot matrix displays, light emitting diode (LED) displays, organic light-emitting diode (OLED) displays, cathode ray tube (CRT) displays, e-ink, or monochrome, color, or any other type of display capable of generating tactile, audio, and/or visual output. 
     Computing device  120 , in some examples, includes communication unit  118 . Communication unit  118  is configured to receive electrical signal input from one or more sensors, such as detectors  114 . Communication unit  118  may transmit to and/or receive electrical signal input/output from coupling light source  130 , probe light source  140 , vapor cells  112 , and imaging optics  110 , via a wired or a wireless connection. For example, computing device  120  may communicate via communication unit  118  to configure coupling light source  130 , probe light source  140 , vapor cells  112 , and imaging optics  110 . Communication unit  118  may be configured to convert the received electrical signals into a form usable by computing device  120 . For example, communication unit  118  may include software or hardware configured to convert a received signal input from an analog signal to a digital signal. In another example, communication unit  118  may include software or hardware configured to compress, decompress, transcode, encrypt, or decrypt a received signal input into a form usable by computing device  120 . In another example, communication unit  118  may include a network interface device to receive packetized data representative of image data and/or input/output data. In such examples, an intermediate device may packetize signals to produce the packetized data and send the packetized data to computing device  120 . In this manner, communication unit  118  may be configured to interface with, or communicate with any of detectors  114 , coupling light source  130 , probe light source  140 , vapor cells  112 , and imaging optics  110 . 
     Computation engine  122  may be implemented in circuitry. For instance, computation engine  122  may include processing circuitry  116 , which may be any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. The functions attributed to processors described herein, including computation engine  122  and processing circuitry  116 , may be provided by processing circuitry of a hardware device, e.g., as supported by software and/or firmware. Computation engine  122  may be configured to generate a digital image based on signals received from detector array  104 . Computation engine  122  may also be configured to control the output of light sources  130  and  140  and receive information indicative of the output of light sources  130  and  140 , e.g., feedback regarding brightness and spectral content of light sources  130  and  140 . In some examples, computing engine  122  may be configured to control imaging optics  110 , e.g., to change focus and zoom. 
     Processing circuitry  116  may be communicatively coupled to imaging system  100 , for example via communication unit  118 . For example, processing circuitry  116  may process signals received via communication unit  118  from detector array  104  and indicative of detected probe light proportional to EM radiation to be detected  152 . In some examples, processing circuitry  116  may control the output of light sources  130  and  140  and receive information indicative of the output of light sources  130  and  140 , e.g., feedback regarding brightness and spectral content of light sources  130  and  140 . In some examples, processing circuitry may communicate with imaging optics  110 , e.g., to change focus and zoom. 
     In some examples, computation engine  122  may include memory  124 . Memory  124  may include any volatile or non-volatile media, such as a random-access memory (RAM), read only memory (ROM), non-volatile RAM (NVRAM), electrically erasable programmable ROM (EEPROM), flash memory, and the like. Memory  124  may be a storage device or other non-transitory medium. Memory  124  may be used by processing circuitry  116  to, for example, store information related to imaging system  100 , such as images and image information, detector array  104  settings, light source  130  and  140  settings, and imaging optics  110  settings. In some examples, processing circuitry  116  may store image information or previously received data from electrical signals in memory  124  for later retrieval. In some examples, processing circuitry  116  may store determined values or any other calculated values, in memory  124  for later retrieval. 
     In some examples, computing device  120  may be integrated with imaging system  100 . In other examples, computing device  120  may be an external device, e.g., a computing device separate from imaging system  100  and configured to communicate with imaging system  100 . 
       FIG.  2 A  is a perspective view of an example vapor cell array  166 , in accordance with the techniques of the disclosure.  FIG.  2 A  illustrates a separated view of vapor cell array  162  as a layer of imaging system  160 . In the example shown, vapor cell array  162  includes a plurality of vapor cells  172  arranged in a 2D array. In some examples, vapor cells  172  may be quantum antennas, and vapor cell array  162  may be a quantum antenna array, a phased array, an imaging array, and/or a multi-spectral imaging array. 
       FIG.  2 B  is a perspective view of a partially transparent block diagram illustrating an example imaging system  160 , in accordance with the techniques of the disclosure. In the example shown, imaging system  160  includes coupling waveguide  166 , vapor cell array  162 , and a probe waveguide  168  arranged as a layered stack with coupling waveguide  162  adjacent to a detector array  164  and vapor cell array  162  between coupling waveguide  166  and probe waveguide  168 . In some examples, vapor cell array  162 , coupling waveguide  166 , and probe waveguide  168  may form a PIC stack. In the example shown, detector array  164  is illustrated as separated from the other layers but may be a part of the PIC stack in other examples. Each of vapor cell array  162 , detector array  164 , coupling waveguide  166 , and probe waveguide  168  may be substantially similar to vapor cell array  102 , detector array  104 , first waveguide  106 , and second waveguide  108  as described above with respect to  FIG.  1   .  FIG.  2 B  illustrates coupling light  182  (shown as dotted lines) propagating within coupling waveguide  166  along the y-direction as a plurality of “rows” corresponding to locations of vapor cells  172  and probe light  192  (shown as solid thicker lines) propagating within probe waveguide  168  along the x-direction as a plurality of “columns” corresponding to locations of vapor cells  172 . In some examples, coupling light  182  and probe light  192  propagating in orthogonal directions within coupling waveguide  166  and probe waveguide  168 , respectively, may allow room for coupling and probe light sources to be arranged along orthogonal edges of the PIC. In some examples, coupling light  182  and probe light  192  may propagate in any direction within coupling waveguide  166  and probe waveguide  168 , respectively, and do not need to propagate in orthogonal directions. 
     Coupling light  182  and probe light  192  may be substantially similar to coupling light  132  and probe light  142 , respectively. In some examples, probe waveguide  168  is configured to extract at least a portion of probe light  192  to propagate through vapor cells  172  to detector array  164 . In some examples, probe waveguide  168  may be a PIC including a waveguide and one or more extraction features. Coupling waveguide  166  may be configured to extract at least a portion of probe light  182  to propagate through vapor cells  172 . In some examples, coupling waveguide  166  may be a PIC including a waveguide and one or more extraction features. 
       FIGS.  3 - 7    illustrate example sensors for detecting electromagnetic radiation in the MHz/GHz/THz frequency range and example operating principles and will be described together below. 
       FIG.  3    is a cross-sectional illustration of an example sensor  200 , in accordance with the techniques of the disclosure. Sensor  200  may be an example of a single “pixel” of system  100  and/or imaging system  160  described above. In the example shown, sensor  200  includes a vapor cell  212  disposed between a probe waveguide  246  and a coupling waveguide  248 , and photodetector  214 . 
     In the example shown, vapor cell  212  includes vapor cell sidewalls  260 , vapor cell detector end wall  262 , and vapor cell front end wall  264  defining a volume and configured to hold alkali atoms within the volume. Vapor cell  212  may include a vapor of alkali atoms  266  in a vapor and an integrated vacuum pump (not shown). Vapor cell detector end wall  262  may be any material configured to hold alkali atoms within the volume and may be substantially transparent to coupling light  232  and probe light  242 . Vapor cell front end wall  264  may be any material configured to hold alkali atoms within the volume and may be substantially transparent to probe light  242  and EM radiation  252 , e.g., MHz/GHz/THz EM radiation. Vapor cell  212  may transduce, or convert, EM radiation  252  having frequencies in the megahertz (MHz), gigahertz (GHz), and terahertz (THz) ranges to optical frequencies in petahertz (PHz) ranges. Stated in terms of wavelengths, vapor cell  212  may convert electromagnetic radiation in the meter- to millimeter-wavelength ranges to electromagnetic radiation in the UV/VIS/NIR wavelength ranges. 
     Coupling waveguide  236  may be disposed on substrate  234 . Substrate  234  may be any material suitable for providing mechanical support for coupling waveguide  236  to support coupling waveguide  236  in a generally planar shape and may be substantially transparent to coupling light  232 . In some examples, substrate  234  may have a lower refractive index at coupling light  232  frequencies than coupling waveguide  236 , e.g., enabling TIR within coupling waveguide  236 . In some examples, sensor  200  may not include substrate  236  and coupling waveguide  236  may be made of suitable material and a suitable thickness to keep a generally planar shape. 
     Coupling waveguide  236  may include coupling light director  238 . In some examples, coupling waveguide  236  may be a PIC including a waveguide and one or more extraction features. Coupling light director  238  may be configured to extract coupling light  232  from coupling waveguide  236  and direct coupling light  232  to vapor of alkali atoms  266  within the volume of vapor cell  212 . For example, coupling light director  238  may comprise one or more extraction features such as a surface relief pattern on either of the surfaces of coupling waveguide  236  at which TIR occurs, and the surface relief pattern may be a diffraction grating. In other examples, coupling light director  238  may include a distribution of painted dots on either of the surfaces of coupling waveguide  236  at which TIR occurs, a variation in shape of either of the surfaces of coupling waveguide  236  at which TIR occurs (e.g., a taper, curve, discontinuity), a scattering material and/or structure within the bulk material of coupling waveguide  236  at the location of coupling light director  238 , and the like. 
     Coupling waveguide  236  may be arranged along any of vapor cell sidewalls  260 , vapor cell detector end wall  262 , or vapor cell front end wall  264 , and configured to extract coupling light  232  into vapor cell  212  to vapor of alkali atoms  266 . In other words, coupling light  232  may enter vapor cell  212  from any direction. In the example shown, coupling waveguide  236  is disposed along vapor cell detector end wall  262  between photodetector  214  and vapor cell  212  and is generally planar having its smallest dimension, e.g., thickness, perpendicular to photodetector  214  and confining coupling light  232  to propagation within its thickness and in the x-y directions via TIR. In other examples, coupling waveguide  236  may be disposed along vapor cell front end wall  264 , e.g., on the other side of vapor cell  212  and the same side of vapor cell  212  as probe waveguide  246 . In still other examples, sensor  200  may include a plurality of coupling waveguides  236  disposed between one or more vapor cells  212 , e.g., generally planar having its smallest dimension in either the x or y direction (e.g., perpendicular to waveguide  236  illustrated in  FIG.  3   ) and confining coupling light  232  to propagation within its thickness and in the y-z or x-z directions. 
     Coupling waveguide  236  may be made of any suitable material substantially transparent to coupling light  232 , e.g., UV/VIS/NIR light. For example, coupling waveguide  236  may be glass, polymer material, polycarbonate, polymethylmethacrylate (PMMA), and the like. 
     In the example shown, coupling waveguide  236  may be an “edge lit” waveguide, e.g., coupling light  232  may be introduced into coupling waveguide  236  from any of the edges of coupling waveguide  236 , e.g., any surface of coupling waveguide  236  including the smallest dimension of coupling waveguide  236 . In the example shown, coupling light  232  may enter coupling waveguide  236  via edge  270  and may propagate along waveguide  236  in the y-direction via TIR and may be extracted and directed towards vapor of alkali atoms  266  by coupling light director  238 . 
     Probe waveguide  246  may be disposed on substrate  244 . Substrate  244  may be any material suitable for providing mechanical support for probe waveguide  246  to support probe waveguide  246  in a generally planar shape and may be substantially transparent to probe light  242 . In some examples, substrate  244  may have a lower refractive index at probe light  242  frequencies than probe waveguide  246 , e.g., enabling TIR within probe waveguide  246 . In some examples, sensor  200  may not include substrate  244  and probe waveguide  246  may be made of suitable material and a suitable thickness to keep a generally planar shape. In some examples, substrate  244  may be substantially similar to substrate  234 . 
     Probe waveguide  246  may include probe light director  248 . In some examples, probe waveguide  246  may be a PIC including a waveguide and one or more extraction features. Probe light director  248  may be configured to extract probe light  242  from probe waveguide  246  and direct probe light  242  to photodetector  214  through vapor of alkali atoms  266  within the volume of vapor cell  212 . For example, probe light director  248  may comprise one or more extraction features such as a surface relief pattern on either of the surfaces of probe waveguide  246  at which TIR occurs, and the surface relief pattern may be a diffraction grating. In other examples, probe light director  248  may include a distribution of painted dots on either of the surfaces of probe waveguide  246  at which TIR occurs, a variation in shape of either of the surfaces of probe waveguide  246  at which TIR occurs (e.g., a taper, curve, discontinuity), a scattering material and/or structure within the bulk material of probe waveguide  246  at the location of probe light director  248 , and the like. 
     Probe waveguide  246  may be arranged so as to extract probe light  242  to photodetector  214  through vapor cell  212 . In some examples, vapor cell  212  may be analogous to an optical shutter for probe light  242  that may vary the amount of probe light  242  transmitted through vapor cell  212  due to variations of the EIT of the vapor of alkali atoms  266  within vapor cell  212  corresponding to an amount and/or frequency of incident EM radiation  252 . For example, changes to the EIT of vapor of alkali atoms  266  for probe light  242  may be caused by EM radiation  252  incident on vapor of alkali atoms  266 , and the amount of probe light  242  detected by photodetector  214  may be directly proportional to the amount and/or frequency content of EM radiation  252 . In this way, vapor cell  212  may be a transducer for the amount and/or frequency content of EM radiation  252  by converting a response to the amount and/or frequency content of EM radiation  252  incident on vapor cell  212 , namely, the EIT of vapor of alkali atoms  266  for probe light  242 , to a detected amount of probe light  242 . 
     In general, probe light  242  may propagate through vapor of alkali atoms  266  to photodetector  214 . In the example shown, probe waveguide  246  is disposed along vapor cell detector end wall  262  on the opposite side vapor cell  212  from photodetector  214  and is generally planar having its smallest dimension, e.g., thickness, perpendicular to photodetector  214  and confining coupling light  242  to propagation within its thickness and in the x-y directions via TIR. In some examples, probe waveguide  246  may be disposed in any direction along any of vapor cell sidewalls  260 , vapor cell detector end wall  262 , or vapor cell front end wall  264 , and configured to extract probe light  242  to photodetector  214  through vapor cell  212 . For example, probe waveguide  246  may be disposed along vapor cell front end wall  264 , e.g., on the other side of vapor cell  212  and the same side of vapor cell  212  as coupling waveguide  236 , extract probe light  246  into vapor cell  212  and through vapor of alkali atoms  266 , and a reflector (not shown) may reflect probe light  242  back through vapor of alkali atoms  266  and to photodetector  214 . In other examples, a reflector may be arranged so as to reflect probe light  242  to photodetector  214  after having propagated through at least a portion of vapor of alkali atoms  266 . For example, sensor  200  may include a plurality of probe waveguides  246  disposed between one or more vapor cells  212 , e.g., generally planar having its smallest dimension in either the x or y direction (e.g., perpendicular to waveguide  236  illustrated in  FIG.  3   ) and confining probe light  242  to propagation within its thickness and in the y-z or x-z directions. One or more probe light directors  248  may extract and direct probe light  242  to vapor of alkali atoms  266  through one or both of vapor cell sidewalls  260 , which may be substantially transparent to probe light  242 . A reflector (not shown) may be arrange so as to reflect probe light  242  entering vapor cell  212  via a sidewall  260  towards photodetector  214  after having propagated through at least a portion of the volume containing vapor of alkali atoms  266 . In general, coupling waveguide  236  and probe waveguide  246  may be disposed along, about, adjacent to, in contact with or separated from any of vapor cell sidewalls  260 , vapor cell detector end wall  262 , or vapor cell front end wall while being configured to extract coupling light  232  and probe light  242 , respectively, into vapor cell  232 . In some examples, probe waveguide may be disposed so as to direct probe light  242  into vapor cell  232  and such that the probe light  242  directed into vapor cell  232  is subsequently directed towards detector  214 , e.g., with or without subsequent light directors such as one or more mirrors, lenses, or gratings. 
     Probe waveguide  246  may be made of any suitable material substantially transparent to probe light  242 , e.g., UV/VIS/NIR light. For example, probe waveguide  246  may be glass, polymer material, polycarbonate, polymethylmethacrylate (PMMA), and the like. In some examples, probe waveguide  246  may be substantially similar to coupling waveguide  236  and may include one or more probe light director  248  substantially similar to coupling light director  238 . 
     In the example shown, probe waveguide  246  may be an “edge lit” waveguide, e.g., probe light  242  may be injected into probe waveguide  246  from any of the edges of probe waveguide  246 , e.g., any surface of probe waveguide  246  including the smallest dimension of probe waveguide  246 . In the example shown, probe light  242  may enter probe waveguide  246  via edge  280  and may propagate along probe waveguide  246  in the y-direction via TIR and may be extracted and directed towards vapor of alkali atoms  266  by probe light director  248 . 
     Photodetector  214  may be configured to detect electromagnetic radiation, for example, infrared and/or visible light. Photodetector  214  may be substantially similar to detectors  114  described above and may be one of an array of photodetectors  214 , e.g., a pixel detector in a 2D focal plane array of pixels. Photodetector  214  may be a large-bandgap solid-state visible wavelength detector configured to operate at without cooling, e.g., at room temperature. For example, detector array  214  may be a pixel of a charge-coupled device (CCD) array, metal-oxide-semiconductor based array such as a complementary metal-oxide-semiconductor (CMOS) array or N-type metal-oxide-semiconductor (NMOS) array. Photodetector  214  may be configured to detect probe light  242  and may be configured to output one or more signals proportional to the detected probe light  242 . 
       FIG.  4    is a cross-sectional illustration of an example sensor  300 , in accordance with the techniques of the disclosure. Sensor  300  may be an example of a single “pixel” of system  100  and/or imaging system  160  described above. Sensor  300  may be substantially the same as sensor  200  illustrated and described above with respect to  FIG.  3   , with the difference being that the coupling waveguide and probe waveguides may form the vapor cell end wall and vapor cell front wall, respectively, e.g., the vapor cell may be integrated with the coupling and probe waveguides. Sensor  300  may also include differences supporting integration of the vapor cell and the coupling and probe waveguides, e.g., coupling and probe waveguide materials suitable to be vapor cell walls. In the example shown, sensor  300  includes a vapor cell  312  disposed between a probe waveguide  346  and a coupling waveguide  348 , and photodetector  214 . 
     In the example shown, vapor cell  312  includes vapor cell sidewalls  360 , coupling waveguide  336  and probe waveguide  346  defining a volume and configured to hold alkali atoms within the volume. Vapor cell  312  may be substantially the same as vapor cell  212 , with the difference that coupling waveguide  336  and probe waveguide  346  may be the vapor cell detector and front end walls, respectively, and vapor cell sidewalls  360  may fully extend between coupling waveguide  336  and probe waveguide  346  to define the volume of vapor cell  312 . Vapor cell  312  may include a vapor of alkali atoms  266  within the volume and an integrated vacuum pump (not shown). 
     Coupling waveguide  336  may be configured to hold vapor of alkali atoms  266 . For example, coupling waveguide  336  may be made of a material suitable for holding alkali atoms  266 , still be transparent to coupling light  232  and probe light  244 , and have an index of refraction relative to vapor of alkali atoms  266  to enable TIR at the interface between coupling waveguide  236  a vapor of alkali atoms  266 , e.g., glass or any other suitable material. Coupling waveguide  336  and its arrangement with respect to the other components of sensor  300  may otherwise be substantially similar to coupling waveguide  236  illustrated and described above with respect to  FIG.  3   . 
     Coupling waveguide  336  may include one or more coupling light directors  338  configured to extract and direct coupling light  232  into vapor of alkali atoms  266 . In some examples, coupling waveguide  336  may be a PIC including a waveguide and one or more extraction features. Coupling light extractor  338  may be substantially similar to coupling light director  238  illustrated and described above, with a difference the coupling light director  338  may be configured for extraction of coupling light  232  into vapor of alkali atoms  266 , which may have a different index of refraction than air and/or vacuum. 
     Probe waveguide  346  may be configured to hold vapor of alkali atoms  266 . For example, probe waveguide  346  may be made of a material suitable for holding alkali atoms  266 , still be transparent to probe light  244  and EM radiation  252 , and have an index of refraction relative to vapor of alkali atoms  266  to enable TIR at the interface between probe waveguide  246  a vapor of alkali atoms  266 , e.g., glass or any other suitable material. Probe waveguide  346  and its arrangement with respect to the other components of sensor  300  may otherwise be substantially similar to probe waveguide  246  illustrated and described above with respect to  FIG.  3   . 
     Probe waveguide  346  may include one or more probe light directors  348  configured to extract and direct probe light  242  into vapor of alkali atoms  266 . In some examples, probe waveguide  346  may be a PIC including a waveguide and one or more extraction features. Probe light extractor  348  may be substantially similar to probe light director  248  illustrated and described above, with a difference the probe light director  348  may be configured for extraction of probe light  242  into vapor of alkali atoms  266 , which may have a different index of refraction than air and/or vacuum. 
       FIG.  5    is a flowchart of an example method  500  of imaging EM radiation, in accordance with the techniques of the disclosure. The method  500  is described with reference to sensors  200  and  300  illustrated and described with reference to  FIGS.  3 - 4   , energy level diagrams  600  and  800  illustrated and described below with reference to  FIGS.  6  and  8   , and plot  700  illustrated and described below with reference to  FIG.  7   . 
     Coupling light and probe light may be injected into a coupling waveguide and a probe waveguide, respectively, of a sensor ( 502 ). For example, coupling light  232  may be injected into coupling waveguide  236  and/or  336  via edge  270  and probe light  242  maybe injected into probe waveguide  246  and/or  346  via edge  280 . Coupling light  232  may propagate within coupling waveguide  236  and/or  336  and may be extracted from coupling waveguide  236  and/or  336  via one or more extraction features, such as coupling light director  238  and/or  348 . In some examples, coupling light  232  may be extracted at a plurality of locations via a plurality of coupling light directors  238  and/or  338 . Similarly, probe light  242  may propagate within probe waveguide  246  and/or  346  and may be extracted from probe waveguide  246  and/or  346  via one or more extraction features, such as probe light director  238  and/or  348 . Probe light  242  may be extracted at a plurality of locations via a plurality of probe light directors  248  and/or  348 . For example, sensor  200  and/or  300  may correspond to each “pixel” of 2D array imaging system  100  or  160  with each coupling light director  238  and/or  338  corresponding to a vapor cell  112  and/or  172  and detector  114  of imaging system  100  and/or  160 . Coupling light  232  may be extracted from coupling waveguide  236  and/or  336 , and probe light  242  may be extracted from probe waveguide  246  and/or  346 , for each pixel of the array. 
     Probe light  242  may excite alkali atoms in one or more vapor cells  112  from a first quantum state to a second quantum state, e.g., a ground state to a first excited state ( 504 ). For example, probe light  242  may be extracted from probe waveguide  246  and/or  346  and directed towards one or more photodetectors  214  through one or more vapor cells  112 ,  212  and/or  312  may excite alkali atoms in one or more vapor cells  112 ,  212  and/or  312  from a first quantum state to a second quantum state. In some examples, a photon of probe light  242  may be absorbed by an alkali atom, e.g., a rubidium atom, a cesium atom, and the like, in one or more vapor cells  112 ,  212 , and/or  312 . The energy absorbed by the alkali atom may drive an electron of the alkali atom into an excited state, e.g., an intermediate quantum state. The quantum state transition for a rubidium atom is illustrated in  FIG.  6   .  FIG.  6    is an illustration of an example energy diagram  600  of an alkali atom including at least one Rydberg state, in accordance with the techniques of the disclosure. In the example shown in  FIG.  6   , probe light  242  may be 780 nanometer (nm) wavelength laser light, which may drive a transition of a rubidium atom from the |5S 1/2 &gt; quantum state (e.g., a ground state), to the |5P 3/2 &gt; quantum state (e.g., an intermediate state). 
     Coupling light  232  may excite alkali atoms in one or more vapor cells  112  from the second quantum state to a third quantum state, e.g., from the first excited state to a Rydberg state ( 506 ). For example, coupling light  232  extracted from coupling waveguide  246  and/or  346  and directed towards one or more vapor cells  112 ,  212  and/or  312  may excite alkali atoms in one or more vapor cell  112 ,  212  and/or  312  from the second quantum state to a Rydberg state with a higher principal quantum number n, e.g., a third quantum state. In some examples, a photon of coupling light  232  may be absorbed by an alkali atom in the second quantum state, e.g., a rubidium atom, a cesium atom, and the like, excited by probe light  242 . The coupling light  242  energy absorbed by the alkali atom may drive an electron of the alkali atom into another excited state, e.g., a Rydberg quantum state. In the example shown in  FIG.  6   , coupling light  232  may be 480 nm wavelength laser light, which may drive a transition of a rubidium atom from the 5P 1/2  quantum state to the Rydberg state |a&gt;. As described above, alkali atoms in a Rydberg state may have loosely bound valence electrons that may be perturbed or ionized by collisions or external fields, e.g., MHz/GHz/THz radiation. The alkali atoms in the Rydberg state |a&gt; may exhibit electromagnetically induced transparency (EIT) for frequencies near the frequency of the probe light  242 . In the example shown, EM radiation  252  incident on vapor of alkali atoms  266  in Rydberg state |a&gt; may be on-resonant with the energy splitting between Rydberg state |a&gt; and Rydberg state |b&gt;, and may cause EIT in vapor of alkali atoms  266 , e.g., Autler-Townes splitting of an absorption line of probe light  242  propagating through vapor of alkali atoms  266 . 
     In some examples, each alkali atom of vapor of alkali atoms  266  of any of one or more vapor cells  112 ,  212 , and/or  312  may respond like an independent transducer, e.g., converting incident on-resonant EM radiation  252  to an optical signal response via probe light  242 . The ensemble of alkali atoms of vapor of alkali atoms  266  of any of one or more vapor cells  112 ,  212 , and/or  312  may amplify the signal incoherently. 
     In some examples, the alkali atoms may be laser cooled, e.g., to reduce motion of the alkali atoms in the vapor. In some examples, laser cooling of the alkali atoms may increase the stability of the atoms in the Rydberg state and/or any of the intermediate quantum states above the ground state. 
     One or more photodetectors  214  may detect an amount of probe light after the probe light propagates through one or more vapor cell, e.g., probe light  242  after propagating through one or more vapor cell  112 ,  212  and/or  312  ( 508 ). In some examples, the absorption spectrum of the alkali atoms as a function of probe light  242  detuning across the |5S 1/2 &gt;→|5P 3/2 &gt; resonance may be detected. In the absence of coupling light  232 , the probe light  242  absorption spectrum looks like a wide (near-GHz), room-temperature Doppler-broadened dip in the intensity level of the probe light  242  after it passes through one or more vapor cells  112 ,  212  and/or  312 . In the presence of strong on-resonant coupling light  232 , the index of refraction of vapor of alkali atoms  266  is modified around the probe light  242  resonance frequency such that a spectrally narrow transparency “window” is “opened.” In some examples, the EIT spectral “window” is a few MHz wide and appears as a narrow peak in probe light  242  intensity at the bottom of the Doppler profile. The peak may indicate a resonance condition of both probe light  242  and coupling light  232 , and the presence of the peak may indicate that the vapor of alkali atoms  266  may be well-coupled to a Rydberg state. In some examples, perturbations to the energy level of that state, e.g., Ω RF  as illustrated in  FIG.  6   , due to incident EM radiation  252  may be determined. For example, the signal response of probe light  242  detected by photodetector  214  may vary in proportion to perturbations to the energy level of a Rydberg state due to EM radiation  252  incident on vapor cell  212  and/or  312 , as described below with reference to  FIG.  7   . The spectral narrowness of the EIT peak allows for sub-Doppler precision in measurements of the room-temperature vapor-cell atomic response to EM radiation  252 . 
       FIG.  7    is an example plot  700  illustrating absorption signal responses of probe light  242  as a function of probe light frequency detuning, in accordance with techniques of the present disclosure. In the example shown, the absorption signal response examples correspond to the amount of probe light  242  detected by one or more photodetector  214  as a function of detuning, e.g., frequency sweeping and/or scanning, probe light  242 . In the example shown, a frequency of 0 Hz on the x-axis of plot  700  corresponds to the |5S 1/2 &gt;→|5P 3/2 &gt; resonance frequency, e.g., substantially near 780 nm (e.g., 780.2460209 nm or 384.228022 THz). By way of reference, the detuning range of plot  700  from ±80 MHz corresponds to a wavelength range of about ±0.0001625 nm. Plot  700  includes four probe light  242  signal response plots corresponding to four different amounts of 17.04 GHz radio frequency (RF) EM radiation  252  incident on vapor of alkali atoms  266 . Plot  702  is the absorption signal response of probe light  242  with 0 milliwatts (mW) of 17.04 GHz RF light (e.g., EM radiation  252 ) incident on vapor cell  212  and/or  312  and is a Doppler-broadened dip in the intensity level of the probe light  242  after it passes through vapor cell  212  and/or  312 . Plot  708  is the absorption signal response of probe light  242  with 1.0 mW of 17.04 GHz RF light incident on vapor cell  212  and/or  312  and includes a narrow peak in probe light  242  intensity at the bottom of the Doppler profile, e.g., an EIT window. Plots  704  and  706  are the absorption signal responses of probe light  242  with 0.2 mW and 0.5 mW of 17.04 GHz RF light, respectively, and illustrate relative changes in the magnitude and shape of the absorption signal response with differing amounts of 17.04 GHz RF light incident on vapor cell  212  and/or  312 . 
     Processing circuitry, e.g., processing circuitry  116 , may determine an amount and/or one or more frequencies of EM radiation, e.g., EM radiation  252 , based on the detected amount of probe light, e.g., probe light  252  by one or more photodetectors  214  ( 510 ). For example, processing circuitry may determine an amount and/or one or more frequencies of EM radiation  252  incident on vapor cell  212  and/or  312  based on an absorption signal response of probe light  242 . In some examples, processing circuitry  116  may determine an amount and/or one or more frequencies of EM radiation  252  incident on vapor cell  212  and/or  312  based on any of the amplitude, shape, and spectral content of the absorption signal response of probe light  242  after propagation of probe light  242  through vapor cell  212  and/or  312  as a function of detuning, wavelength scanning and/or sweeping, and/or frequency scanning and/or sweeping. In other words, e.g., an absorption signal response plots similar to absorption signal response plots  702 - 708  described above. 
     Processing circuitry, e.g., processing circuitry  116 , may form an image based on the determined amount of EM radiation, e.g., EM radiation  252 , received at a plurality of detectors ( 512 ). For example, processing circuitry may determine a grayscale and/or color representation of each “pixel” of imaging system  100  and/or  160  and/or a plurality of sensors  200  and/or  300 . 
     In some examples, imaging system  100  and/or  160 , and sensor  200  and/or  300  may be configured to excite alkali atoms in a vapor cell, e.g., vapor cell  112 ,  212 , and/or  312 , using a plurality of frequencies of coupling lights  232  and/or probe light  242 . For example, the example shown in  FIG.  6    illustrates a “three-level” system in which alkali atoms may be excited from a first energy level to a second, e.g., intermediate energy level via probe light  242 , and from the second energy level to a third energy level in which the alkali atoms are in a Rydberg state. In some examples, any of imaging system  100  and/or  160  and sensor  200  and/or  300  may be configured to excite alkali atoms in a vapor cell using a system including more or fewer levels. For example, any of imaging system  100  and/or  160  and sensor  200  and/or  300  may be configured to excite alkali atoms in a vapor cell using a two-level system, or any other number system. In some examples, a probe light sufficient to excite atoms from a first state, e.g., a ground state, directly to a Rydberg state may require probe light  242  having frequencies corresponding to far-UV light, which may make a two-level system difficult and/or costly to implement using current light sources and detectors. By way of contrast, a system having three or more levels may have a reduced difficulty and implementation cost due to greater availability of higher performing and lower cost light sources and detectors. 
       FIG.  8    is an illustration of an illustration of an example energy diagram  800  of an alkali atom including at least one Rydberg state, in accordance with the techniques of the disclosure. In the example shown in  FIG.  8   , probe light  242  may be 780 nanometer (nm) wavelength laser light, which may drive a transition of a rubidium atom from the |5S 1/2 &gt; quantum state (e.g., a ground state), to the |5P 3/2 &gt; quantum state (e.g., an intermediate state). Light  842  may be 776 nanometer (nm) wavelength laser light, which may drive a transition of a rubidium atom from the intermediate |5P 3/2 &gt; quantum state to another intermediate κ&gt; quantum state. Coupling light  832  may be 1260 nm wavelength laser light, which may drive a transition of a rubidium atom from the |c&gt; quantum state to the Rydberg state |a&gt;. In some examples, 1260 nm light sources may be more common, less costly, and more powerful compared with blue 480 nm laser light sources. 
     Coupling light  832  and light  842  may be delivered to vapor cell  112 ,  212 , and/or  212  via either of coupling waveguide  236  and/or  336  and/or probe waveguide  246  and/or  346 . For example, coupling waveguide  236  and/or  336  and probe waveguide  236  and/or  336  may be configured to receive and distribute a plurality of frequencies of light, e.g., any of coupling light  232  and  832 , light  842 , and probe light  242 , concurrently, and may be configured with a plurality of light directors configured to extract and direct one or more of the plurality of frequencies of light to a vapor cell, e.g., vapor cell  112 ,  212 , and/or  312 . 
     In some examples, any of sensors  200 ,  300 ,  1100 , and/or  1200  may be configured to excite alkali atoms, e.g., in any of vapor cells  112 ,  212 ,  312 ,  1112 , or  1212 , via multiple quantum states and/or energy levels. For example, either of waveguide  236  and  246  (e.g., PICs  236  and  246 ) may be configured to direct light of two or more wavelengths into any of vapor cells  112 ,  212 ,  312 ,  1112 , or  1212  and incident on the vapor of alkali atoms, and the light of each wavelength of the two or more wavelengths may each be configured to excite the alkali atoms from a lower excited state to a higher excited state sequentially from the first excited state to the Rydberg state. In some examples, exciting alkali atoms from a lower state to a Rydberg state via one or more intermediate states via the use of lower frequency (higher wavelength) excitation light as described above, e.g., a “three-or-more photon” excitation scheme, may reduce the photoelectric effect of the incident light on the alkali atoms relative to a “two-photon” excitation scheme such as described with reference to  FIG.  6   . For example, exciting alkali atoms via multiple quantum states and/or energy levels may reduce background charges in the vapor relative to a two-photon scheme due to a reduced photoelectric effect relative to the two-photon scheme due to use of lower energy excitation light, e.g., lower frequency light, to excite the alkali atoms to the higher states. In other words, the net effect of several lower energy transitions to excite the alkali atoms to a Rydberg state versus two higher energy transitions is a reduction in background charges that may accumulate in the vapor cell due to the photoelectric effect. In some examples, the reduction of the background charges may reduce noise in measuring the ionization of the excited alkali atoms in the Rydberg state due to incident EM radiation  252 , such as illustrated and described below with reference to  FIGS.  10 - 11   , and thereby increase the readout sensitivity of sensors  1100  and  1200  described below. For example, the amount of current flowing in circuits  1120  and or  1220  may correspond to both the ionization of the excited alkali atoms (signal) and any background charges (noise) in the vapor of alkali atoms. 
       FIG.  9    is a block diagram of an example system  900  for sensing EM radiation, in accordance with techniques of the present disclosure.  FIG.  9    includes a probe laser  942 , coupling laser  932 , vapor cell  912 , EM radiation  952 , detector  914 , lock-in amplifier  902 , function generator  904 , and acousto-optic modulator  906 . In some examples, lock-in amplifier  902 , function generator  904 , and acousto-optic modulator  906  may be configured in a feedback loop to tune coupling laser  932  to a resonant frequency of an alkali atom in vapor cell  912 . System  900  may be used to excite a vapor of alkali atoms in vapor cell  912  to a Rydberg state and determine an amount and/or frequency of EM radiation  952  in a MHz/GHz/THz frequency range based on EIT of the alkali atom vapor for probe light from probe laser  942  and/or perturbations of EIT for the probe light from probe laser  942 . 
       FIG.  10    is a cross-sectional illustration of example sensor  1100 , in accordance with the techniques of the disclosure. Sensor  1100  may be an example of a single “pixel” of system  100  and/or imaging system  160  described above. Sensor  1100  may be substantially the same as sensor  300  illustrated and described above with respect to  FIG.  3   , with the difference being that photodetector  214  is removed and electrical circuit  1120  is included. In the example shown, sensor  1100  includes a vapor cell  1112  disposed between a probe waveguide  346  and a coupling waveguide  348 , and photodetector  214 . 
     In the example shown, electrical circuit  1120  includes first electrode  1102 , second electrode  1104 , electrical power source  1106 , and ammeter  1108 . First and second electrodes  1102  and  1104  extend within vapor cell  1112  and may form a portion of one of the walls of vapor cell  1112 . In the example shown, first and second electrodes  1102  and  1104  are disposed on coupling waveguide  336  and probe waveguide  346 , respectively. First electrode may be connected to a first terminal, e.g., a negative or ground terminal, of electrical power source  1106 . Ammeter  1108  may be connected between a second terminal, e.g., a positive terminal of power source  1106  and second electrode  1104  and may be configured to measure a current flowing in circuit  1120 . Power source  1106  may be configured to apply a voltage across first and second electrodes  1102 ,  1104 . In some examples, power source  1106  and first and second electrodes  1102 ,  1104  may be configured to apply 100 volts with a 5 millimeter separation between first and second electrodes  1102 ,  1104 . 
     In operation, the amount of current flowing in circuit  1120  corresponds to the ionization of the alkali atoms within vapor cell  1112 . In some examples, the alkali atoms in a Rydberg state within vapor cell  1112  may be sensitive to EM radiation  252 , that is, the ionization of the alkali atoms changes corresponding to the frequency and/or amount of EM radiation  252  and may change in response to a change in EM radiation  252 , which in turn may cause a change in the amount of current flowing in circuit  1120 . Ammeter  1108  may then determine the amount of current flowing, and ammeter  1108  and/or circuit  1112  may be configured to output a signal correlated to the change in the current in circuit  1112 . 
       FIG.  11    is a cross-sectional illustration of example sensor  1200 , in accordance with the techniques of the disclosure. Sensor  1200  may be an example of a single “pixel” of system  100  and/or imaging system  160  described above. Sensor  1200  may be substantially the same as sensor  1100  illustrated and described above with respect to  FIG.  10   , with the difference being that coupling waveguide  336  and probe waveguide  346  are removed. In the example shown, coupling light  232  and probe light  242  may be directed to be incident on the alkali atoms of vapor cell  1212 , e.g., via a light direction means (not shown). 
     In the example shown, electrical circuit  1220  may be substantially similar to electrical circuit  1120  of  FIG.  10   , only first electrode  1102  and second electrode  1104  may be disposed at least partially on the inner surfaces of substrates  234  and  244 , respectively, rather than waveguides  336  and  346 , which are not included with sensor  1200 . In the example shown, first and second electrodes  1102  and  1104  extend within vapor cell  1212  and may form a portion of one of the walls of vapor cell  1212 . Similar to electrical circuit  1120  of  FIG.  10   , the amount of current flowing in circuit  1220  corresponds to the ionization of the alkali atoms within vapor cell  1212 . In some examples, the alkali atoms in a Rydberg state within vapor cell  1212  may be sensitive to EM radiation  252 , that is, the ionization of the alkali atoms changes corresponding to the frequency and/or amount of EM radiation  252  and may change in response to a change in EM radiation  252 , which in turn may cause a change in the amount of current flowing in circuit  1220 . Ammeter  1108  may then determine the amount of current flowing, and ammeter  1108  and/or circuit  1212  may be configured to output a signal correlated to the change in the current in circuit  1212 . 
       FIG.  12    is a flowchart of an example method  1300  of imaging electromagnetic radiation, in accordance with the techniques of the disclosure. The method  1300  is described with reference to sensors  1100  and  1200  illustrated and described with reference to  FIGS.  10 - 11   , energy level diagrams  600  and  800  illustrated and described below with reference to  FIGS.  6  and  8   , and plot  700  illustrated and described below with reference to  FIG.  7   . 
     As described above with reference to  FIG.  5    and steps  502 ,  504 , and  506 , coupling light and probe light may be injected into a coupling waveguide and a probe waveguide, respectively, of a sensor ( 1302 ), e.g., such as sensor  1100 , probe light  242  may be directed towards one or more vapor cells  1112  and/or  1212  may excite alkali atoms in one or more vapor cells  1112  and/or  1212  from a first quantum state to a second quantum state ( 1304 ), and coupling light  232  may be directed towards one or more vapor cells  1112  and/or  1212  and may excite alkali atoms in one or more vapor cells  1112  and/or  1212  from the second quantum state to a Rydberg state with a high principal quantum number n, e.g., a third quantum state ( 1306 ). 
     An electrical circuit may apply a voltage across two electrodes within one or more vapor cells  1112  and/or  1212  ( 1308 ). For example, a first electrode and a second electrode may be spaced apart within a vapor cell  1112  and/or  1212  and may be connected to a power source configured to apply a voltage across the electrodes. 
     An ammeter may determine and/or detect a current flowing through a circuit including the first and second electrodes and correlated to an ionization of the alkali atoms in one or more vapor cells ( 1310 ). For example, EM radiation  252  incident on the alkali atoms within vapor cell  1112  and/or  1212  may ionize the alkali atoms within vapor cell  1112  and/or  1212 . The ionization of the alkali atoms within vapor cell  1112  and/or  1212  may correlate to the frequency and/or amount of incident EM radiation  252 . Ammeter  1108  may detect the corresponding amount of current, and a change in EM radiation  252  in frequency and/or amount may cause a corresponding change in the amount of current detected by ammeter  1108 . 
     Various examples have been described. These and other examples are within the scope of the following claims. For purposes of this disclosure, the operations shown in the figures do not need to be executed in the manner suggested by the illustrations and, unless specifically stated so, may be executed in any order. Further, the term substantially is to be given its standard definition of to a great or significant extent or for the most part; essentially. 
     The techniques described in this disclosure may be implemented, at least in part, in hardware, software, firmware, or any combination thereof. For example, various aspects of the techniques may be implemented within one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic QRS circuitry, as well as any combinations of such components, embodied in external devices, such as physician or patient programmers, stimulators, or other devices. The terms “processor” and “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry, and alone or in combination with other digital or analog circuitry. 
     For aspects implemented in software, at least some of the functionality ascribed to the systems and devices described in this disclosure may be embodied as instructions on a computer-readable storage medium such as RAM, DRAM, SRAM, magnetic discs, optical discs, flash memories, or forms of EPROM or EEPROM. The instructions may be executed to support one or more aspects of the functionality described in this disclosure. 
     In addition, in some respects, the functionality described herein may be provided within dedicated hardware and/or software modules. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components or integrated within common or separate hardware or software components. Also, the techniques may be fully implemented in one or more circuits or logic elements.