# Patent

## Title
Josephson junction readout for graphene-based single photon detector

## Abstract

A detector for detecting single photons of infrared radiation or longer wavelength electromagnetic radiation. In one embodiment a waveguide configured to transmit infrared radiation is arranged to be adjacent a graphene sheet and configured so that evanescent waves from the waveguide overlap the graphene sheet. In other embodiments a transmission line or antenna is coupled to the graphene sheet and guides longer-wavelength photons to the graphene sheet. A photon absorbed by the graphene sheet heats the graphene sheet. Part of the graphene sheet is part of the Josephson junction as the weak link, and a constant bias current is driven through the Josephson junction; an increase in the temperature of the graphene sheet results in a decrease in the critical current of the Josephson junction and a voltage pulse in the voltage across the Josephson junction. The voltage pulse is detected by the pulse detector.

## Background
CROSS-REFERENCE TO RELATED APPLICATION(S)
The present application is a continuation in part of U.S. application Ser. No. 14/846,013, filed Sep. 4, 2015, issued as U.S. Pat. No. 9,577,176 on Feb. 21, 2017, entitled “JOSEPHSON JUNCTION READOUT FOR GRAPHENE-BASED SINGLE PHOTON DETECTOR”, the entire content of which is incorporated herein by reference, which claims priority to and the benefit of U.S. Provisional Application No. 62/181,195, filed Jun. 18, 2015, entitled “JOSEPHSON JUNCTION READOUT FOR GRAPHENE-BASED SINGLE PHOTON DETECTOR”, the entire content of which is incorporated herein by reference.


BACKGROUND
1. Field
One or more aspects of embodiments according to the present invention relate to detection of photons, and more particularly to a detector for detecting individual photons of infrared light or longer wavelength electromagnetic radiation using a waveguide, transmission line, or antenna coupled to a graphene sheet, coupled, in turn, to a Josephson junction.
2. Description of Related Art
Detectors capable of detecting single photons have multiple applications, including applications in quantum communications. Such detectors, for high-energy photons, may be constructed according to a variety of designs. For low energy photons, such as photons with wavelengths of 1 micron or more, however, there is a gap in detector technology. In particular, existing communications systems may use a wavelength of 1550 nm, and other components, such as lasers, designed to operate at this wavelength may be readily available, resulting in applications for detectors operating at the same wavelength. Thus, there is a need for a single-photon detector for low-energy photons.
SUMMARY
Aspects of embodiments of the present disclosure are directed toward a detector for detecting single photons of infrared radiation or longer wavelength electromagnetic radiation. In one embodiment a waveguide configured to transmit infrared radiation is arranged to be adjacent a graphene sheet and configured so that evanescent waves from the waveguide overlap the graphene sheet. In other embodiments, a transmission line or antenna is coupled to the graphene sheet and guides longer-wavelength photons to the graphene sheet. A photon absorbed by the graphene sheet heats the graphene sheet. Part of the graphene sheet is part of the Josephson junction as the weak link, and a constant bias current is driven through the Josephson junction; an increase in the temperature of the graphene sheet results in a decrease in the critical current of the Josephson junction and a voltage pulse in the voltage across the Josephson junction. The voltage pulse is detected by the pulse detector.
According to an embodiment of the present invention there is provided a photon detector including: a graphene sheet; a first conductor connected to the graphene sheet, the first conductor being configured to guide electromagnetic waves to the graphene sheet, to produce an increase in a temperature of the graphene sheet when a photon of the electromagnetic waves is absorbed by the graphene sheet; a Josephson junction adjacent or including the graphene sheet, the Josephson junction having a gap coupled to electrons of the graphene sheet; and a circuit connected to two contacts of the Josephson junction, the circuit configured to detect a decrease in a critical current of the Josephson junction, the decrease corresponding to the increase in the temperature of the graphene sheet.
In one embodiment, the first conductor is a conductor of a log periodic antenna.
In one embodiment, the first conductor is one of two superconducting conductors of the Josephson junction.
In one embodiment, the photon detector includes a second conductor, wherein the first conductor, the graphene sheet, and the second conductor form a microstrip transmission line.
In one embodiment, the second conductor has a first end connected to the graphene sheet, and a second end connected to ground.
In one embodiment, the second conductor has: a first end connected to the graphene sheet, and an open second end, a length of the second conductor, from the first end to the second end, being one quarter of a wavelength, in the microstrip transmission line, of electromagnetic waves to be detected.
In one embodiment, the graphene sheet has an electron mobility of more than 100,000 cm2/V/s.
In one embodiment, the graphene sheet has an electron mobility of about 1,000 cm2/V/s.
In one embodiment, the graphene sheet substantially has the shape of a rectangle, the rectangle having a length and a width, the length being greater than or equal to the width.
In one embodiment, the photon detector includes a second conductor, wherein: the first conductor is connected to a first short edge of the graphene sheet, the second conductor is connected to a second short edge of the graphene sheet, and the first conductor, the graphene sheet, and the second conductor form a microstrip transmission line.
In one embodiment, the Josephson junction includes: a first superconducting conductor adjacent a first long edge of the graphene sheet; and a second superconducting conductor adjacent a second long edge of the graphene sheet.
In one embodiment, the length of the rectangle is less than 20 microns.
In one embodiment, the product of the length of the rectangle and the width of the rectangle is less than 1000 square microns.
In one embodiment, the first conductor is on a substantially flat substrate.
In one embodiment, the photon detector includes a first layer of hexagonal boron nitride immediately adjacent a first surface of the graphene sheet, and a second layer of hexagonal boron nitride immediately adjacent a second surface of the graphene sheet.
In one embodiment, the circuit includes a current source connected to the two contacts of the Josephson junction and configured to drive a constant bias current through the Josephson junction.
In one embodiment, the photon detector includes an amplifier connected to the two contacts of the Josephson junction.
In one embodiment, the photon detector includes a pulse detector connected to the amplifier, the pulse detector including a voltage reference and a comparator configured to compare a signal at an output of the amplifier to a voltage at an output of the voltage reference.
In one embodiment, the graphene sheet consists of a single atomic layer of graphene.
In one embodiment, the photon detector includes a refrigerator configured to cool the graphene sheet to a temperature below 4 K.



BRIEF DESCRIPTION OF THE DRAWINGS
Features, aspects, and embodiments are described in conjunction with the attached drawings, in which:
FIG. 1 is a block diagram of a single photon detector, according to an embodiment of the present invention;
FIG. 2A is a perspective view of a sensor assembly, according to an embodiment of the present invention;
FIG. 2B is a schematic side view of a graphene sheet sandwiched between two layers of hexagonal boron nitride, according to an embodiment of the present invention;
FIG. 3A is a graph of voltage across a Josephson junction and current flowing through the junction at a first temperature, according to an embodiment of the present invention;
FIG. 3B is a graph of voltage across a Josephson junction and current flowing through the junction at a second temperature, higher than the first temperature, according to an embodiment of the present invention;
FIG. 4 is a block diagram of a pulse detector according to an embodiment of the present invention;
FIG. 5 is a schematic plan view of a sensor assembly according to an embodiment of the present invention;
FIG. 6 is a perspective view of a communications system including mobile transceivers according to an embodiment of the present invention;
FIG. 7 is a graph of critical current probability distributions according to an embodiment of the present invention;
FIG. 8A is a hybrid schematic diagram and plan view of a portion of a single photon detector, according to an embodiment of the present invention;
FIG. 8B is a hybrid schematic diagram and plan view of a portion of a single photon detector, according to an embodiment of the present invention;
FIG. 9A is a plan view of a portion of a single photon detector, according to an embodiment of the present invention;
FIG. 9B is a plan view of a portion of a single photon detector, according to an embodiment of the present invention;
FIG. 9C is a plan view of a portion of a single photon detector, according to an embodiment of the present invention; and
FIG. 10 is a schematic diagram of a portion of a single photon detector, according to an embodiment of the present invention.



DETAILED DESCRIPTION
The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of a Josephson junction readout for a graphene-based single photon detector provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features.
Referring to FIG. 1, in one embodiment, a single-photon detector for infrared light includes an infrared waveguide 110, a graphene sheet 120, and a Josephson junction 130. The infrared waveguide 110, graphene sheet 120, and Josephson junction 130 form a sensor assembly, configured to receive photons, and to produce electrical signals when the photons are detected. In the sensor assembly, a photon may be absorbed by the graphene sheet 120 from the waveguide 110; the photon heats the graphene sheet 120, which is coupled to the Josephson junction 130, and the heating of the graphene sheet 120 causes a decrease in the critical current 310 of the Josephson junction 130. When the critical current 310 falls below a bias current 305 driven through the Josephson junction 130 from a bias circuit 138, the voltage across the Josephson junction 130 increases, i.e., a voltage pulse is produced. An amplifier 135 amplifies the voltage pulse, and a pulse detector 140 detects the voltage pulse. The combination of the Josephson junction 130, the bias circuit 138, the amplifier 135, and the pulse detector 140 forms a Josephson junction readout for detecting changes in the temperature of the graphene sheet 120.
In other embodiments, illustrated for example in FIGS. 8A-10 and described in further detail below, a single photon detector may detect radio frequency, microwave, or millimeter wave electromagnetic waves. Photons in these electromagnetic waves may be absorbed by the graphene sheet 120, producing a signal in a manner similar to that described above.
Referring to FIG. 2A, the waveguide 110 and the graphene sheet 120 of the sensor assembly may be fabricated as an integrated component on a substrate 210. When infrared electromagnetic waves propagate in the waveguide 110, evanescent waves outside the waveguide 110 overlap with, and couple to, the graphene sheet 120. This coupling allows photons of the electromagnetic waves to be absorbed by the graphene sheet 120, raising the temperature of the graphene sheet 120. When an infrared photon is absorbed by the graphene sheet 120, the absorption occurs primarily through interaction of the photon with the electronic degrees of freedom of the graphene sheet 120, the interactions between the photon and the nuclei of the graphene sheet 120 being significantly weaker. Electrons in the graphene sheet 120 are weakly coupled to phonons in the graphene sheet 120, and the total heat capacity of the electrons in the graphene sheet 120 is relatively small, so that the absorption of a photon results in a relatively large and rapid increase in the electron temperature. Moreover, the electrons in the graphene sheet 120 are strongly coupled to each other, and, as a result, the electron temperature within the graphene sheet 120 quickly becomes substantially uniform after the absorption of a photon. As used herein, the temperature of the graphene sheet 120 refers to the temperature of the electrons in the graphene sheet 120; immediately after the absorption of a photon, the electron temperature may differ from the phonon temperature.
The waveguide 110 may be fabricated by any of various methods known in the art for fabricating a waveguide 110 on the surface of a substrate 210. In one embodiment, a layer of silicon dioxide is formed on a silicon substrate 210, and patterned using photolithography to form the core of the waveguide 110 as a raised structure. A layer 220 of silicon nitride may then be formed over the waveguide 110 and the surrounding area, so that the waveguide 110 core has silicon nitride on both sides of it and above it. This structure may then be polished, so that the upper surface of the structure is flat and smooth. In other embodiments, the waveguide 110 may be formed by depositing a layer of silicon dioxide on a silicon substrate 210, depositing a layer of silicon on the silicon dioxide, and patterning the silicon layer using photolithography to form the waveguide 110 as a raised silicon structure. This structure may then be planarized, i.e., made flat and smooth, by depositing an additional layer of silicon dioxide and polishing it down to the top surface of the raised silicon structure. In other embodiments the waveguide 110 may be composed of another first material, surrounded by one or more other materials having a lower index of refraction than the first material. The resulting waveguide structure may have a thickness of between 50 nm and 2000 nm; in one embodiment it has a thickness of between 100 nm and 1000 nm. The transverse dimensions of the waveguide structure may be somewhat smaller or considerably smaller than the wavelength of the infrared light to be detected. The waveguide 110 may be single-mode or multi-mode when guiding light at the wavelength of the infrared light to be detected.
The substrate 210 may be substantially flat, e.g., within 1 micron of being flat over the area including the waveguide, and the waveguide may be formed, e.g., using one of the processes described above, in a layer having a thickness greater than 50 nanometers and less than 2 microns. The front end of the waveguide 110 may extend to the edge of the substrate 210 as shown, and off-chip coupling optics 240 may be used to launch infrared light into the waveguide 110. In other embodiments portions of the coupling optics 240 may be fabricated on the substrate 210. In some embodiments the waveguide 110 is omitted and infrared light propagating in free space illuminates the graphene sheet 120 directly.
The Josephson junction 130 may be composed of two adjacent superconducting patches 250, deposited on the substrate 210 at the edge of the graphene sheet 120, so as to overlap the edge of the graphene sheet 120. Each superconducting patch 250 may have a thickness in the range of 50 nm to 100 nm. The superconducting patches 250 may be separated by a small gap 260, having a width between 100 nm and 1 micron, e.g., 500 nm; this gap may act as a weak link. A diagnostic contact 230 may also be deposited on the substrate 210, at an opposite edge of the graphene sheet 120. Electrons in the two superconducting patches 250 are coupled by tunneling across the gap 260. The Josephson junction 130 overlaps the edge of the graphene sheet 120, and electrons in the graphene sheet 120 couple to the gap 260 of the Josephson junction 130. The wave functions of electrons in the two superconducting patches 250 extend into the gap 260 and as a result are coupled to the electrons of the graphene sheet 120, the wave functions of which also extend into the gap 260. Because of this coupling, the temperature of the electrons of the graphene sheet 120 affects the critical current 310 (FIGS. 3A and 3B) of the Josephson junction 130; the higher the temperature of the electrons of the graphene sheet 120, the lower the critical current 310 of the Josephson junction 130. The superconducting patches 250 may be located on the graphene sheet 120 at a position that is separated (e.g., separated by at least 1 micron) from the waveguide 110 to avoid direction interaction of photons in the waveguide with the superconducting patches 250; such interaction may otherwise destroy the superconductivity of the superconducting patches 250. The superconducting patches 250 may be formed of any of a number of materials known in the art that become superconductive at low temperatures, including niobium nitride, niobium titanium nitride, niobium diselenide, aluminum, niobium, niobium titanium, or lead. The contacts formed between the superconducting patches 250 and the graphene sheet 120 may be highly transparent (i.e., they may have low contact resistance); each contact may be a one-dimensional (1D) contact along a respective edge of the graphene sheet 120, formed in a manner described in further detail below. The superconducting materials employed in the Josephson junction 130, and other design parameters of the Josephson junction 130 may be selected to form a Josephson junction 130 with a critical temperature of the order of (i.e., comparable to) the temperature change produced in the graphene sheet 120 by the absorption of a photon. The lateral dimensions of the superconducting patches 250 may be selected to be greater than the lateral extent of the Cooper pairs formed in the superconducting patches 250 when they are in a superconducting state.
The two superconducting patches 250 of the Josephson junction 130 also serve as contacts. Conductors, e.g., gold wire or gold ribbon, may be bonded to the two superconducting patches 250 to connect the Josephson junction 130 to bias and readout circuitry. The bias and readout circuitry includes a bias circuit 138, configured as a current source, that drives a constant bias current 305 through the Josephson junction 130. The bias circuit 138 may include an operational amplifier 135 configured as a current source. Referring to FIG. 3A, the Josephson junction 130 has a current-voltage (I-V) curve with a superconducting portion corresponding to currents with a magnitude less than a critical current 310 within which the voltage across the Josephson junction 130 is zero. For currents exceeding the critical current 310, the voltage across the Josephson junction 130 is proportional to the current. The bias current 305 is selected to be less than the critical current 310 at the steady-state temperature of the graphene sheet 120 when no photons have been recently absorbed. Referring to FIG. 3B, when a photon is absorbed, the temperature increases, the critical current 310 decreases to a decreased critical current 315, and, if the decreased critical current 315 is less than the bias current 305, the voltage increases to a value proportional to the bias current 305.
After absorption of a photon, the temperature of the graphene sheet 120 initially increases as described above, and then decreases as the electrons of the graphene sheet 120 lose heat energy through several mechanisms, including coupling through the contacts (e.g., the diagnostic contact 230, and the two superconducting patches 250) coupling to the lattice, and coupling to the electromagnetic environment. As such, after a photon is absorbed, the temperature may increase and then decrease again, and as a result, the critical current 310 may decrease and the increase, and the voltage across the Josephson junction 130 may increase and then decrease, i.e., the Josephson junction 130 may produce a voltage pulse. The voltage pulse may be amplified by an amplifier 135 (FIG. 1), which may have sufficient bandwidth, e.g., a bandwidth of at least 100 MHz, or a bandwidth of 1 GHz or more, to amplify the voltage pulse, which may be of short duration because of the low heat capacity of the electrons of the graphene sheet 120. The amplifier 135 may include a quantum noise limited amplifier followed by a high electron mobility transistor (HEMT) amplifier. The Josephson junction readout may also include a matching network, e.g., an inductor-capacitor (LC) matching network, for transforming the impedance of the Josephson junction 130 to the input impedance of the amplifier 135 (which may be about 50 ohms). In some embodiments the quantum noise limited amplifier may be a radio frequency superconducting quantum interference device (RF SQUID) amplifier, or it may be a travelling wave parametric amplifier, or a tuned system parametric amplifier (TPA), or any other kind of amplifier with a suitable frequency response that is quantum noise limited or nearly quantum noise limited. In some embodiments the amplifier 135 does not include quantum noise limited amplifier, and has a HEMT amplifier as the first stage instead. The behavior of the Josephson junction 130 may be understood as the behavior of a phase particle in a tilted “washboard” potential in the RCSJ model. In some embodiments the Josephson junction 130 may exhibit a behavior referred to as latching on, that may occur when the phase particle remains in a local minimum of the potential, and that may result in the voltage across the junction not returning to zero after the absorption of a photon. In such a case, the readout circuit may be configured to detect latching on (for example, when the duration of a pulse exceeds a threshold), and the readout circuit may then reset the Josephson junction 130 by briefly setting the bias current to zero, causing the Josephson junction to return to a zero-voltage state, in a state change referred to as retrapping. The likelihood that the Josephson junction 130 will exhibit latching on behavior may be affected by the magnitude of the bias current; e.g., a Josephson junction 130 biased with a bias current near the critical current 310 may be more likely to exhibiting latching on behavior than a Josephson junction 130 biased with a bias current well below the critical current 310.
A pulse detector 140 connected to the Josephson junction 130 may be used to detect the absorbed photons. Referring to FIG. 4, the input to the pulse detector 140 may be a voltage from the output of the Josephson junction 130, which may be zero when no photons have been recently absorbed, and which may exhibit a voltage pulse when a photon is absorbed. The pulse detector 140 may have a set threshold voltage, and it may generate a digital output pulse, or increment a photon count register, each time the input voltage increases past the threshold voltage, i.e., crosses the threshold voltage with a positive time-derivative. For example, the pulse detector 140 may include a comparator 410, one input of which is (or is connected to) the input of the pulse detector 140, a second input of which is connected to a voltage reference 420 that defines the threshold voltage, and the output of which is connected to a digital processing circuit 430. For example the output of the comparator 410 may be connected to the clock input of a flip-flop or other edge-triggered circuit, so that the detection of a photon causes an essentially immediate change of state in the processing circuit 430.
The waveguide 110 may be straight, and, to increase the amplitude of the evanescent waves overlapping the graphene sheet 120, it may be part of an optical resonator, constructed, for example, by forming a reflector (e.g., a Bragg reflector) at each end of a section of the waveguide 110. Bragg reflectors may be formed by creating periodic defects in or adjacent the waveguide 110, e.g., by forming holes in or adjacent the waveguide structure with a focused ion beam. The reflector at the front end of the waveguide 110 (i.e., the end first encountered by the arriving infrared light) may be partially reflective, to allow the infrared light to enter the resonator, and the reflector at the other end (the “back” end) of the waveguide 110 may be highly reflective, to avoid allowing light to escape from the back end of the waveguide 110. In some embodiments only one reflector is used, at the back end of the waveguide 110.
In other embodiments the waveguide 110 may not be straight, but may have one or more curves, increasing the length of the section of waveguide 110 that is adjacent the graphene sheet 120, and from which evanescent waves may interact with the graphene sheet 120. A curved section in the waveguide may have a radius of curvature less than the length of the graphene sheet 120, and in the curved section the direction of the waveguide may change by 45 degrees or more. The curvature of the waveguide may at all points be sufficiently gradual, e.g., having a radius of curvature exceeding the wavelength in the waveguide, of the infrared light, to avoid excessive loss. The increased length of the section of waveguide 110 adjacent the graphene sheet 120 may increase the probability, for any photon launched into the waveguide 110, of being absorbed by the graphene sheet 120. The waveguide 110 may have a double spiral shape as illustrated in FIG. 2A. In other embodiments the waveguide 110 may have the shape of a single spiral, with the back end of the waveguide in the center of the spiral. The back end of the waveguide 110 may be at an edge of the substrate 210 as illustrated in FIG. 2A, or it may be elsewhere on the substrate 210, e.g., near the middle in the case of a waveguide 110 in the shape of a single spiral. Referring to FIG. 5, in one embodiment the waveguide 110 may have a meandering shape, covering a region under a portion of the graphene sheet 120 as illustrated. In yet other embodiments, the waveguide 110 may have one or more curves and also form part of a resonator, to further increase the absorption probability for photons launched into the waveguide 110.
Infrared light may be launched into the waveguide 110 by any of several systems known to those of skill in the art. For example, if the infrared light to be detected is initially travelling in free space, it may be coupled into the waveguide 110 using one or more suitable powered optics, such as lenses or curved mirrors. Referring to FIG. 6, transceivers using coupling to free-space propagating waves may be used for mobile communications e.g., between ships 680 and a tower 685. If the infrared light to be detected is initially propagating in an optical fiber, it may be launched into the waveguide 110 of the single photon detector using any suitable one of a variety of fiber-to-chip couplers known to those of skill in the art.
The graphene sheet 120 may be a single-layer sheet, i.e., it may be one atomic layer thick, or it may be a multi-layer graphene sheet 120, having, e.g., 2, 3, 4, or more layers. Referring to FIG. 2B, in one embodiment, the graphene sheet 120 is encapsulated in hexagonal boron nitride (hBN). As is known to those of skill in the art, a sandwich is formed, with the graphene sheet 120 sandwiched between two layers 290 of hexagonal boron nitride. Each layer 290 of hexagonal boron nitride may be between 4 nm and 40 nm thick; these layers 290 of hexagonal boron nitride may keep the surface of the graphene sheet 120 clean, i.e., they may prevent surface contamination from compromising the properties of the graphene sheet 120. The sandwich, composed of the two outer layers 290 of hexagonal boron nitride encapsulating the graphene sheet 120, may then be disposed on the portion of the substrate 210 that includes the waveguide 110.
Each hexagonal boron nitride layer 290 may be a single crystal, with an atomically flat surface facing the graphene sheet 120. Each hexagonal boron nitride layer 290 may be annealed, e.g., at 250° C. for 10-15 minutes, before the sandwich is assembled. The sandwich may be formed by first bringing a first layer 290 of hexagonal boron nitride into contact with the graphene sheet 120, resulting in adhesion of the graphene sheet 120 to the hexagonal boron nitride by van der Waals forces, and then bringing the graphene sheet 120, on the first layer 290 of hexagonal boron nitride, into contact with the second layer 290 of hexagonal boron nitride, resulting in adhesion, again by van der Waals forces, at the interface between the graphene sheet 120 and the second layer 290 of hexagonal boron nitride. The edges of the sandwich may then be etched, e.g. using plasma etching, so that the edges of the two layers 290 of hexagonal boron nitride and the edges of the graphene sheet 120 in the structure remaining after the etch process coincide (i.e., are aligned).
The graphene sheet 120 may be rectangular as illustrated in FIG. 2A, with a length and a width, the length being greater than or equal to the width. The total area of the graphene sheet 120 may be less than 1000 square microns. In one embodiment the graphene sheet 120 is about 10 microns by 10 microns. In one embodiment the graphene sheet 120 has a length in the range 1.0-100.0 microns and a width in the range 1.0-100.0 microns. An electrical contact 230 may be provided at one end of the graphene sheet 120. In one embodiment, contact is made with the edge of the graphene sheet 120, using a layer of metal deposited onto the edge of the sandwich.
For good performance, the graphene sheet 120 may be made as small as possible, kept as clean as possible, and operated at as low a temperature as possible. In one embodiment, the graphene sheet 120 is cooled to 4 K, using, for example, a pulse tube refrigerator or a Gifford-McMahon (GM) cooler. In other embodiments direct cooling with liquid helium, or with liquid helium in a partial vacuum (e.g., using a 1 K pot, to reach temperatures below 4 K) may be used to cool the graphene sheet 120.
Diagnostic circuitry may be connected to two contacts at opposite ends of the graphene sheet 120, e.g., to the diagnostic contact 230 and to one of the superconducting patches 250. The diagnostic circuitry may include components and connections that may be used for diagnostics, e.g., during manufacturing, operation, or service. For example, a bias tee may be used to drive a low-frequency current through the graphene sheet 120, modulating its temperature, and the presence of a corresponding modulation at the output of a power detector measuring the thermal noise (or “Johnson noise”) power at radio frequencies or at microwave frequencies may then be used to measure the differential thermal conductance or to verify the functioning of the device generally. The superconducting patches 250 are connected to the graphene sheet 120 electrically but not thermally. The Cooper pairs in superconductors do not conduct heat.
The thermal activation and the macroscopic quantum tunneling of the phase particle of the Josephson junction, both of which may in principle be sources of error, are exponentially suppressed and negligibly small when the bias current and temperature are small and low enough, respectively.
FIG. 7 shows a first probability distribution 710 of the peak voltage at the input of the pulse detector 140 (e.g., the voltage at the output of the amplifier 135) after a photon is absorbed, and a second probability distribution 720 of the voltage at the input of the pulse detector 140 in the absence of absorbed photons. The threshold voltage (“Vthres”) of the pulse detector 140 may be adjusted to adjust both the dark count rate and the quantum efficiency: if the threshold voltage is increased, the dark count rate and the quantum efficiency are both decreased, because the likelihood that amplifier noise will trigger a count is reduced, and the likelihood that a pulse resulting from the absorption of a photon fail to trigger a count is increased. Conversely, if the threshold voltage is decreased, the dark count rate and the quantum efficiency are both increased, because the likelihood that amplifier noise will trigger a count is increased, and the likelihood that a pulse resulting from the absorption of a photon will trigger a count is also increased.
The greater the separation between the first probability distribution 710, and the second probability distribution 720 (relative to the widths of these distributions), the higher the achievable quantum efficiency for a certain dark count rate will be, or, equivalently, the lower the achievable dark count rate for a certain quantum efficiency will be. The separation between the probability distributions may be affected by the size of the graphene sheet 120, the cleanliness of the graphene sheet 120, and the temperature of the graphene sheet 120 in the absence of absorbed photons. The smaller and cleaner the graphene sheet 120 is, and the lower its temperature, the greater will be the separation between the first probability distribution 710 and the second probability distribution 720. The cleanliness of the graphene sheet 120 may be quantified as an impurity density level, with a cleaner graphene sheet 120 having a lower impurity density level, or as an electron mobility, with a cleaner graphene sheet 120 having a higher electron mobility. In one embodiment a 10 micron by 10 micron square graphene sheet 120 is used, with an impurity density level of 109/cm2, corresponding to an electron mobility of about 500,000 cm2/V/s. In other embodiments the detector may work adequately with a graphene sheet having significantly lower electron mobility, e.g., electron mobility as low as 1,000 cm2/V/s.
In some embodiments, a graphene sheet and Josephson junction 130 may be used to detect radio frequency, microwave, or millimeter wave electromagnetic radiation. Referring to FIG. 8A, in one embodiment the electromagnetic radiation may be coupled (e.g., by a suitable receiving antenna, such as a horn antenna or a patch antenna) to an input transmission line 805, and the resulting guided waves may propagate toward the Josephson junction 130 on the input transmission line 805 (e.g., a microstrip transmission line), through an impedance matching network 810, and through a detector transmission line 815 (that includes the graphene sheet 120), to ground 820. The detector transmission line 815 is illustrated, in FIG. 8A, in part schematically (as a line) and in part in a plan view, as a conductor having substantially constant width. The Josephson junction 130 includes two superconducting patches 250 with a gap overlapping the graphene sheet 120. When a photon (of the received electromagnetic waves) generates current in the graphene sheet 120, the electron temperature of the graphene sheet may increase, resulting in a voltage pulse as described above. The detector transmission line 815 may be a superconducting transmission line (composed of superconducting material), and, as such, photon energy couples and dissipates into the graphene sheet 120 via the impedance matching network 810 whereas the superconducting contacts 815 are non-dissipative. A pulse detector (FIG. 1) may be connected to the two superconducting patches 250 to detect such a pulse. External bias circuitry (FIG. 1) may also be connected to the two superconducting patches 250 to provide a bias current to the Josephson junction 130. Chokes 825 may be used to isolate the detector transmission line 815 from the pulse detector and bias circuitry over the range of frequencies of the electromagnetic radiation to be detected.
Referring to FIG. 8B, in an embodiment similar to that of FIG. 8A, the received electromagnetic waves propagating on the input transmission line 805 may be capacitively coupled to the detector transmission line 815 (which may be a superconducting transmission line) by a coupling capacitor 830 (which may be a gap, as shown, in a microstrip transmission line). A half-wave (or “lambda-over-two”) microwave resonator 835 with the graphene sheet 120 at the center may result in the formation of standing waves having a current antinode 840 (and a voltage node) at or near the graphene sheet 120. The overlap of the current node 840 and the graphene sheet 120 can result in critical coupling of the single photon to the single photon detector for enhanced detection efficiency. If the graphene sheet is part of a sandwich (e.g., between layers of boron nitride), contact between the detector transmission line 815 and the graphene sheet 120 may be made at the edges of the sandwich, as described above with respect to the conductor 230 of the embodiment of FIGS. 2A and 2B.
Referring to FIG. 9A, in one embodiment a log periodic antenna includes two conductive patches 910, and may be used to couple microwave or millimeter wave electromagnetic radiation propagating in free space into a graphene sheet 120, at the center of the log periodic antenna, which may be coupled to a Josephson junction. Referring to FIG. 9B, the two conductive patches 910 may extend, at the center of the log periodic antenna, along the long sides of a rectangular graphene sheet 120, which may have on each of its other two (short) sides a superconducting patch (not shown) forming a Josephson junction, the gap of which is coupled to the graphene sheet 120. Referring to FIG. 9C, in another embodiment the two conductive patches 910 may extend, at the center of the log periodic antenna, along the short sides of a rectangular graphene sheet 120, which may have on each of its other two (long) sides a superconducting patch (not shown) forming a Josephson junction, the gap of which is coupled to, or contains a portion of, the graphene sheet 120. As in the embodiments of FIGS. 8A and 8B, the embodiments of FIGS. 9A, 9B, and 9C may include additional circuitry (not shown in FIGS. 9A, 9B, and 9C) including a circuit to provide bias to the Josephson junction, a pulse detector circuit, and chokes to isolate the received electromagnetic waves from the bias circuit, and from the pulse detector circuit. The embodiments of FIGS. 9A, 9B, and 9C may be suitable for detecting electromagnetic radiation in a frequency range extending from about 100 GHz to about 1 THz.
Referring to FIG. 10, in some embodiments a diplexer 1010 may be used to separate the received radio frequency, microwave, or millimeter wave electromagnetic waves and the pulse signals sent to the pulse detector circuit when photons are detected. The diplexer 1010 separates signals according to frequency, transmitting received electromagnetic waves from the input transmission line 805 at the high frequency (HF) port of the diplexer to the detector transmission line 815, and transmitting pulses from the Josephson junction 130 to the low frequency (LF) port of the diplexer, which may be connected to a pulse detector circuit. In the embodiment of FIG. 10, the Josephson junction 130 may include a graphene sheet 120, with a first superconducting patch 250, abutting one edge of the graphene sheet 120, connected, through a bias circuit 138 (e.g., bias tee) to the common port of the diplexer 1010, and a second superconducting patch 250 abutting an opposite edge of the graphene sheet 120, and connected to ground. The superconducting patches 250 may then act as part of the Josephson junction 130, and also as the conductors that guide the received electromagnetic waves (i.e., photons) to the graphene sheet 120, heating it and causing the Josephson junction 130 to produce pulses corresponding to the absorbed photons. The embodiment of FIG. 10 may be suitable for the detection of radio frequency or microwave electromagnetic waves in a frequency range extending from about 100 MHz to about 100 GHz.
In some embodiments the Josephson junction 130 is a ballistic Josephson junction 130, i.e., the mean free path of the electrons in the graphene sheet is greater than the gap of the Josephson junction 130. This may be accomplished, for example, by using a graphene sheet of suitably high quality, resulting in a long electron mean free path, and/or by narrowing the gap of the Josephson junction 130 to be less than the mean free path. In some embodiments the Josephson junction 130 is formed as a layered structure, with the graphene sheet 120 being formed as a second layer on top of a first superconducting patch 250 (forming the first layer of the structure), and with a second superconducting patch 250 being formed on top of the graphene sheet 120, forming a third layer of the structure.
In some embodiments a detector of radio frequency, microwave, or millimeter wave electromagnetic waves may be used to measure a qubit, e.g., in a quantum mechanical computer. For example, a microwave cavity may contain an electromagnetic mode storing one or more qubits, and a radio frequency, microwave, or millimeter wave single-photon detector constructed according to an embodiment of the present invention may be used to count photons exiting the cavity.
Although limited embodiments of a Josephson junction readout for a graphene-based single photon detector have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that a Josephson junction readout for a graphene-based single photon detector employed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof.

## Claims

1. A photon detector comprising:
a graphene sheet;
a first conductor connected to the graphene sheet, the first conductor being configured to guide electromagnetic waves to the graphene sheet, to produce an increase in a temperature of the graphene sheet when a photon of the electromagnetic waves is absorbed by the graphene sheet;
a Josephson junction adjacent or including the graphene sheet, the Josephson junction having a gap coupled to electrons of the graphene sheet; and
a circuit connected to two contacts of the Josephson junction, the circuit configured to detect a decrease in a critical current of the Josephson junction, the decrease corresponding to the increase in the temperature of the graphene sheet.

2. The photon detector of claim 1, wherein the first conductor is a conductor of a log periodic antenna.

3. The photon detector of claim 1, wherein the first conductor is one of two superconducting conductors of the Josephson junction.

4. The photon detector of claim 1, further comprising a second conductor,
wherein the first conductor, the graphene sheet, and the second conductor form a microstrip transmission line.

5. The photon detector of claim 4, wherein the second conductor has a first end connected to the graphene sheet, and a second end connected to ground.

6. The photon detector of claim 4, wherein the second conductor has:
a first end connected to the graphene sheet, and
an open second end,
a length of the second conductor, from the first end to the second end, being one quarter of a wavelength, in the microstrip transmission line, of electromagnetic waves to be detected.

7. The photon detector of claim 1, wherein the graphene sheet has an electron mobility of more than 100,000 cm2/V/s.

8. The photon detector of claim 1, wherein the graphene sheet has an electron mobility of about 1,000 cm2/V/s.

9. The photon detector of claim 1, wherein the graphene sheet substantially has the shape of a rectangle, the rectangle having a length and a width, the length being greater than or equal to the width.

10. The photon detector of claim 9, further comprising a second conductor, wherein:
the first conductor is connected to a first short edge of the graphene sheet,
the second conductor is connected to a second short edge of the graphene sheet, and
the first conductor, the graphene sheet, and the second conductor form a microstrip transmission line.

11. The photon detector of claim 10, wherein the Josephson junction comprises:
a first superconducting conductor adjacent a first long edge of the graphene sheet; and
a second superconducting conductor adjacent a second long edge of the graphene sheet.

12. The photon detector of claim 9, wherein the length of the rectangle is less than 20 microns.

13. The photon detector of claim 9, wherein the product of the length of the rectangle and the width of the rectangle is less than 1000 square microns.

14. The photon detector of claim 1, wherein the first conductor is on a substantially flat substrate.

15. The photon detector of claim 1, comprising a first layer of hexagonal boron nitride immediately adjacent a first surface of the graphene sheet, and a second layer of hexagonal boron nitride immediately adjacent a second surface of the graphene sheet.

16. The photon detector of claim 1, wherein the circuit comprises a current source connected to the two contacts of the Josephson junction and configured to drive a constant bias current through the Josephson junction.

17. The photon detector of claim 16, further comprising an amplifier connected to the two contacts of the Josephson junction.

18. The photon detector of claim 17, further comprising a pulse detector connected to the amplifier, the pulse detector comprising a voltage reference and a comparator configured to compare a signal at an output of the amplifier to a voltage at an output of the voltage reference.

19. The detector of claim 1, wherein the graphene sheet consists of a single atomic layer of graphene.

20. The photon detector of claim 1, further comprising a refrigerator configured to cool the graphene sheet to a temperature below 4 K.