Patent Description:
Various military applications, and commercial applications, such as spectroscopy or microwave imaging applications, may make use of a microwave detector, e.g., an array detector for microwaves. Related art microwave detectors may be large or may have poor performance or high power consumption, or may otherwise be poorly suited for use in an array detector. Thus, there is a need for an improved microwave detector.

<CIT> discloses a photon detector including a graphene-insulating-superconducting junction configured as a temperature sensor. Photons are absorbed by the graphene sheet of the graphene-insulating-superconducting junction, each absorbed photon causing a temporary increase in the temperature of the graphene sheet, and a corresponding change in the differential impedance of the graphene-insulating-superconducting junction. The graphene-insulating-superconducting junction is part of a resonant circuit connected as a shunt load between a radio frequency input transmission line and a radio frequency output transmission line. The transmission S-parameterfrom input to output is affected by the impedance of the resonant circuit which in turn is affected by the differential impedance of the graphene-insulating-superconducting junction, and therefore by the temperature of the graphene sheet. The absorption of photons is detected by detecting changes in the transmission S-parameter indicating temperature changes caused by the absorption of a photon.

<NPL>, discloses the bolometric radio frequency response of graphene based superconducting tunnel junctions.

<CIT> discloses a portable microwave radiation monitor utilizing an antenna formed in a dual Archimedean spiral. The ellipse ratio of the spiral is minimized by selective placement of resistive means over a portion of the antenna and the power density of the electric field is indicated on a meter connected via diode means to the inner terminals of the antenna.

According to an embodiment of the present invention, there is provided a system for detecting microwave power, the system comprising: an array of resonators comprising a first resonator and a second resonator, wherein: the first resonator comprises a graphene-insulating-superconducting junction; the first resonator has a first resonant frequency; the second resonator comprises a graphene-insulating-superconducting junction; and the second resonator has a second resonant frequency different from the first resonant frequency; a probe signal source, coupled to the first resonator and to the second resonator; and a probe signal analyzer, the probe signal analyzer being configured: to measure a change in amplitude or phase of a probe signal received by the probe signal analyzer from the probe signal source, and to infer, from the change in amplitude or phase, a change in microwave power received by the graphene-insulating-superconducting junction; wherein the probe signal source is a multi-frequency source, configured to operate at a frequency within <NUM>% of the first resonant frequency and at a frequency within <NUM>% of the second resonant frequency.

In some embodiments, the first resonator includes a tank circuit.

In some embodiments, the probe signal analyzer is configured to measure a reflected probe signal, the reflected probe signal being reflected from the first resonator.

In some embodiments, the system includes a directional coupler connected to the probe signal source, the first resonator, and the probe signal analyzer, the directional coupler being configured: to transmit an incident probe signal from the probe signal source to the first resonator, and to transmit the reflected probe signal from the first resonator to the probe signal analyzer.

In some embodiments, the system further includes an amplifier between the directional coupler and the probe signal analyzer.

In some embodiments, the system further includes a circulator, between the first resonator and the amplifier.

In some embodiments, the system further includes an attenuator between the probe signal source and the directional coupler.

In some embodiments, the system further includes a bias circuit connected to the graphene-insulating-superconducting junction and configured to drive a bias current through the graphene-insulating-superconducting junction.

In some embodiments, the system further includes a bias tee, connected between the bias circuit and the graphene-insulating-superconducting junction.

In some embodiments, the system further includes a power splitter and a mixer, the power splitter being connected between the probe signal source and the first resonator, and configured to divert a portion of the probe signal from the probe signal source to a local oscillator input of the mixer, the mixer having a radio frequency input connected to the first resonator and an intermediate frequency output connected to the probe signal analyzer.

In some embodiments, the probe signal source is configured to provide an unmodulated signal at a first frequency, the first frequency being between <NUM> and <NUM>.

In some embodiments, the first frequency is within <NUM>% of a resonant frequency of the first resonator when no microwave power is received by the graphene-insulating-superconducting junction.

In some embodiments, the probe signal analyzer is configured to measure a change in amplitude of the probe signal received by the probe signal analyzer from the probe signal source.

In some embodiments, the probe signal analyzer is configured: to measure a change in phase of the probe signal received by the probe signal analyzer from the probe signal source and to measure a change in amplitude of the probe signal received by the probe signal analyzer from the probe signal source.

In some embodiments: the graphene-insulating-superconducting junction includes a graphene sheet, and the graphene-insulating-superconducting junction is configured to receive the microwave power as current flowing from a first edge contact of the graphene-insulating-superconducting junction, through the graphene sheet, to a second edge contact of the graphene-insulating-superconducting junction.

In some embodiments, the system includes a twin slot input resonator connected to the first edge contact and to the second edge contact.

In some embodiments, the graphene-insulating-superconducting junction 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 some embodiments, each of the first layer of hexagonal boron nitride and the second layer of hexagonal boron nitride has a thickness greater than <NUM> and less than <NUM> micron.

In some embodiments, the graphene sheet consists of a single atomic layer of graphene.

Features, aspects, and embodiments are described in conjunction with the attached drawings, in which:.

The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of a microwave 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 functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features.

Referring to <FIG>, a graphene-insulating-superconducting junction <NUM> may include a graphene sheet <NUM> sandwiched between two layers <NUM>, <NUM> of hexagonal boron nitride, and a superconducting metal layer <NUM>, separated from the sandwich by an insulating layer <NUM>. The superconducting metal may be superconducting aluminum, or other two-dimensional superconductors such as niobium diselenide, for example, and the insulating layer may be composed of aluminum oxide, or other two-dimensional dielectrics such as hexagonal boron nitride, for example. The graphene-insulating-superconducting junction may be constructed on a substrate <NUM>. The thickness of each of the hexagonal boron nitride layers <NUM>, <NUM> may be in the range of <NUM> to <NUM>, e.g., the thickness may be <NUM>. The thickness of graphene sheet <NUM> may be <NUM> atomic layer, or a small number of atomic layers (e.g., <NUM>, <NUM>, or <NUM> layers, or as many as <NUM> layers). The junction contact of the graphene edge <NUM> may be a one-dimensional contact or a (two-dimensional) contact made via the overlapping surface. The insulating part of the junction may be the hexagonal boron nitride or the other insulating oxide material as mentioned above.

The graphene-insulating-superconducting junction may be a two-terminal device, with one terminal being the superconducting metal layer <NUM> and the other terminal being the graphene sheet <NUM> (to which a connection may be made using, for example, an additional edge contact). In other embodiments the graphene sheet may have two or more additional edge contacts. For example, one or two edge contacts (e.g., two edge contacts <NUM>, <NUM> as shown in <FIG>), may be used to feed the oscillating voltage and current of a microwave (or terahertz) signal (the signal to be detected) to the graphene sheet <NUM> for detection (one contact if the current return path is through the graphene-insulating-superconducting junction (e.g., if the signal is fed in on an unbalanced transmission line), or two contacts if the current return path is separate, e.g., if the signal is in on a balanced transmission line). The superconducting contact (or another edge contact) may be used to feed an inbound radio frequency or microwave signal to the graphene-insulating-superconducting junction to measure its differential resistance, as described in further detail below.

<FIG> shows an embodiment in which the insulating portion of the graphene-insulating-superconducting junction <NUM> is not an additional element but is instead part of the upper hexagonal boron nitride layer <NUM>, which is locally thinned to provide a suitable thickness for quantum tunneling. <FIG> is a perspective view in one embodiment, in which the combination of an insulating layer and superconducting metal layer (<NUM>/<NUM>), or a superconducting metal layer <NUM> (with a separate insulating element, as shown for example in <FIG>), forms a graphene-insulator-superconductor junction. The circuit can be closed by superconductor or metal contact <NUM>, whereas the superconductor or metal contact <NUM> can be used for the coupling of the signal to be detected into the detector.

In operation, the junction may be cooled to a temperature at which the metal layer <NUM> is superconducting; the graphene sheet may remain normal (i.e., non-superconducting). Like other normal-insulating-superconducting junctions, the graphene-insulating-superconducting junction may exhibit a current-voltage characteristic that is diode-like; in particular the current through the junction may be proportional to the product of (i) the square root of the absolute temperature of the normal element (i.e., the graphene element) of the graphene-insulating-superconducting junction and (ii) an exponential function of (a) the voltage across the junction, divided by (b) the absolute temperature of the graphene element of the graphene-insulating-superconducting junction.

The electron temperature of the graphene sheet may be out of equilibrium with the temperature of the lattice of the graphene sheet. It is primarily the electron temperature that is affected by the absorption of microwave power, and it is primarily the electron temperature (not the temperature of the lattice) that influences the differential resistance of the graphene-insulating-superconducting junction (discussed in further detail below). Accordingly, the term "absolute temperature" as used herein refers to the absolute temperature of the electrons of the graphene element of the graphene-insulating-superconducting junction, i.e., the absolute temperature of the electrons of the graphene sheet.

In some embodiments, the temperature of the superconducting element of the graphene-insulating-superconducting junction does not significantly affect the current-voltage characteristic of the graphene-insulating-superconducting junction. As, such, when the graphene-insulating-superconducting junction is operated in a biased condition (e.g., with a bias voltage applied to the graphene-insulating-superconducting junction, resulting in a bias current flowing through the graphene-insulating-superconducting junction), the small signal resistance (or "differential resistance") of the graphene-insulating-superconducting junction is a function both of the bias voltage, and of the absolute electron temperature of the graphene element of the graphene-insulating-superconducting junction. In some embodiments, under suitable bias conditions, the differential resistance decreases with an increasing electron temperature in the graphene.

In some embodiments, microwave power is received (and absorbed) by the graphene sheet <NUM>, resulting in an increase in the electron temperature of the graphene sheet <NUM>, and a corresponding change in the differential resistance of the graphene-insulating-superconducting junction <NUM>, which may be measured to infer the amount of microwave power received by the graphene-insulating-superconducting junction <NUM>. The differential resistance may be measured, e.g., by a circuit such as that shown in <FIG> and <FIG>.

<FIG> shows a sensor circuit <NUM>, which may be contained in, and kept at a low temperature (e.g., <NUM>) by, a cryogenic system <NUM>. A probe signal is received at a probe input <NUM>, and partially transmitted, by a directional coupler <NUM> to the detection resonator, or "readout resonator" <NUM>. The probe signal is partially reflected by the readout resonator <NUM> and the reflected probe signal is coupled, by the directional coupler <NUM>, to the sensor circuit output <NUM>. The amplitude and phase of the reflected probe signal may depend on the impedance of the readout resonator <NUM>, and, therefore, on the differential resistance of the graphene-insulating-superconducting junction <NUM>, which in turn may depend on the amount of microwave power (in the signal to be detected) received by the graphene-insulating-superconducting junction <NUM>.

As such, the sensor circuit <NUM> may be employed to measure microwave power. For example, if the frequency of the probe signal is approximately equal to the resonant frequency of the readout resonator <NUM>, then a change in the received microwave power may cause a change in the amplitude of the reflected probe signal; if the frequency of the probe signal is offset from the resonant frequency of the readout resonator <NUM>, a change in the received microwave power may cause a change in the amplitude and a change in the phase of the reflected probe signal. The readout resonator <NUM> may have a resonant frequency between <NUM> and <NUM>, e.g., <NUM>, and it may have a bandwidth that is approximately the reciprocal of the thermal response time of the (electron temperature of the) graphene sheet <NUM> (e.g., a bandwidth of approximately <NUM>), so that the readout resonator <NUM> does not degrade the response time of the system.

A bias current, supplied through the bias (or "DC") input <NUM>, flows through the graphene-insulating-superconducting junction <NUM>. The readout resonator <NUM> includes the graphene-insulating-superconducting junction, connected in parallel with a tank circuit <NUM>, which may consist of a resonator capacitor <NUM>, a resonator inductor <NUM>, and a blocking capacitor <NUM> (which may be sufficiently large to have a negligible effect on the frequency response of the tank circuit <NUM>), as shown. As used herein, a "tank circuit" is the parallel combination of an inductor and a capacitor, or such a circuit including other elements (such as the large blocking capacitor <NUM>) that do not significantly affect its resonant characteristics. In some embodiments the readout resonator is configured differently; for example, it may consist of the graphene-insulating-superconducting junction <NUM> coupled to a transmission line resonator (e.g., a quarter-wave resonator or a half-wave resonator). An input cascade <NUM> (which may include one or more attenuators) may be present (within the cryogenic system <NUM>) on the input signal path, to attenuate noise (e.g., thermal noise) that may be received at the probe input <NUM>, while adding thermal noise only at a level corresponding to the low temperature in the cryogenic system <NUM>. This may reduce the extent to which such thermal noise may heat the graphene sheet <NUM>.

A circulator <NUM> and an output cascade <NUM> may be present on the output path to reduce the extent to which noise generated by circuit elements connected to the sensor circuit output <NUM> may heat the graphene sheet <NUM>. The output cascade <NUM> may include, connected in a cascade, zero or more attenuators, zero or more circulators, and zero or more amplifiers, to preserve the outbound radio frequency signal and to reduce both reflections back to the graphene-insulating-superconducting junction <NUM> and backward-propagating noise, either of which may, if not suppressed, heat the graphene sheet <NUM>. The amplifiers may include one or more low-noise amplifiers, e.g., high electron mobility transistor (HEMT) amplifiers. In some embodiments a portion of the output cascade <NUM> is within the cryogenic system <NUM> and is kept at low temperature during operation, as shown in <FIG>. The output cascade <NUM> may also include amplifiers, such as a low noise amplifier <NUM> (<FIG>), which are outside the cryogenic system <NUM> (and which are therefore at room temperature, in operation), to further amplify the output signal and overcome the receiver electronics noise.

The blocking capacitor <NUM> may block the DC bias current which in the absence of this capacitor would flow directly to ground. The capacitance of the blocking capacitor <NUM> may be sufficiently large that at the resonant frequency of the readout resonator <NUM> the impedance of the blocking capacitor <NUM> is negligible compared to the impedance of the resonator capacitor <NUM>, and negligible compared to the impedance of the resonator inductor <NUM>. A coupling capacitor Ccouple may be connected in series with the readout resonator <NUM> as shown, to prevent bias current from flowing, e.g., through the directional coupler.

<FIG> shows a circuit that may be used to measure the impedance of the readout resonator <NUM>, and, thereby, the differential resistance of the graphene-insulating-superconducting junction <NUM> and the amount of microwave power received by the graphene-insulating-superconducting junction <NUM>. A probe signal source <NUM> supplies the probe signal to the probe input <NUM>, and the reflected signal is fed from the sensor circuit output <NUM> to a receiver <NUM>. As such, the receiver <NUM> receives the probe signal, after reflection from the readout resonator <NUM> (and after transmission through various elements, including, for example, the directional coupler <NUM> and the output cascade <NUM>).

A portion of the probe signal may be split off by a power splitter <NUM> as shown. The phase of this portion of the probe signal may be shifted by a phase shifter <NUM>, and used as a local oscillator signal for the receiver <NUM>. In the receiver <NUM>, the local oscillator signal may be fed to the local oscillator ("LO") input of a mixer <NUM>, the radio frequency ("RF") input of which is connected to the sensor circuit output <NUM> and the intermediate frequency ("IF") output of which is connected to an analyzer <NUM>. As such, the analyzer receives the probe signal, after reflection from the readout resonator <NUM> and after frequency conversion by the mixer <NUM> (and after transmission through various elements, including, for example, the directional coupler <NUM> and the output cascade <NUM>).

The analyzer <NUM> may include, e.g., an analog to digital converter (ADC) or threshold detector <NUM> and a processing circuit (discussed in further detail below) that may calculate the amplitude and phase of the reflected probe signal (e.g., from the DC signal at the IF output of the mixer for two different phase shifts applied by the phase shifter), and, from it, infer the impedance of the readout resonator <NUM>, and the amount of microwave power received by the graphene-insulating-superconducting junction <NUM>. The phase shifter may be controlled by the analyzer <NUM> (e.g., to alternate between values separated by <NUM> degrees). In other embodiments two mixers are used, each supplied with a respective local oscillator signal, the two local oscillator signals differing in phase by <NUM> degrees (so that together they may measure the in-phase and quadrature components of the reflected probe signal). In other embodiments, the power splitter <NUM> and the mixer <NUM> may be absent, and, for example, undersampling may be used to effect frequency downconversion, and a common frequency reference may be used by the probe signal source <NUM> and the processing circuit, to preserve phase coherence between the probe signal source <NUM> and the analyzer <NUM>.

The term "processing circuit" is used herein to mean any combination of hardware, firmware, and software, employed to process data or digital signals. Processing circuit hardware may include, for example, application specific integrated circuits (ASICs), general purpose or special purpose central processing units (CPUs), digital signal processors (DSPs), graphics processing units (GPUs), and programmable logic devices such as field programmable gate arrays (FPGAs). In a processing circuit, as used herein, each function is performed either by hardware configured, i.e., hard-wired, to perform that function, or by more general-purpose hardware, such as a CPU, configured to execute instructions stored in a non-transitory storage medium. A processing circuit may be fabricated on a single printed circuit board (PCB) or distributed over several interconnected PCBs. A processing circuit may contain other processing circuits; for example, a processing circuit may include two processing circuits, an FPGA and a CPU, interconnected on a PCB.

In some embodiments the processing circuit may infer the temperature of the graphene sheet <NUM> based on only the magnitude of the impedance of the readout resonator <NUM>; in other embodiments the processing circuit may use both the magnitude and the phase of the impedance of the readout resonator <NUM> to estimate the temperature. The relationship between the temperature and the amplitude and phase of the reflected probe signal may be derived analytically, or it may be measured, by adjusting the temperature (in the absence of microwave power received by the graphene-insulating-superconducting junction <NUM>) and observing the amplitude and phase of the reflected probe signal.

In some embodiments changes in the differential resistance of the graphene-insulating-superconducting junction may be measured otherwise, for example, by applying a constant bias (voltage or current) to the graphene-insulating-superconducting junction and detecting (with an amplifier having sufficient bandwidth to resolve the thermal time constant of the graphene sheet) the changes in current or voltage that result when microwave power is absorbed by the graphene sheet.

The microwave power to be measured may be coupled to the graphene-insulating-superconducting junction <NUM> in various ways, e.g., by a transmission line or waveguide connected to a microwave source, e.g., a receiving antenna. Referring to <FIG>, in some embodiments, a twin slot antenna (including a first slot <NUM> and a second slot <NUM>), operates as an input resonator <NUM> to couple microwave radiation propagating in free space to (the edge contacts <NUM>, <NUM> of) the graphene-insulating-superconducting junction <NUM>.

Referring to <FIG>, in some embodiments, a plurality of graphene-insulating-superconducting junctions <NUM>, each coupled to a respective input resonator and each coupled to a respective tank circuit <NUM>, may be arranged in an array (e.g., a two-dimensional array) as shown, having a common terminal <NUM>. The array may be incorporated into a circuit like that of <FIG> and <FIG>, being substituted, in that circuit, for the combination of the readout resonator <NUM> and the coupling capacitor Ccouple. In such an embodiment, a frequency-multiplexed readout may be used. For example, the probe signal source may be a multi-frequency source, i.e., a signal source capable of operating at a plurality of frequencies (e.g., capable of producing a comb spectrum, or capable of switching between different frequencies), and the tank circuits <NUM> of the array may have different resonant frequencies, so that the respective differential resistance of each of the readout resonators <NUM> may be measured by measuring the amplitude or measuring the phase of the probe signal reflected at the corresponding frequency.

Claim 1:
A system for detecting microwave power, the system comprising:
an array of resonators comprising a first resonator (<NUM>) and a second resonator (<NUM>), wherein:
the first resonator comprises a graphene-insulating-superconducting junction (<NUM>);
the first resonator has a first resonant frequency;
the second resonator comprises a graphene-insulating-superconducting junction (<NUM>); and
the second resonator has a second resonant frequency different from the first resonant frequency;
a probe signal source (<NUM>), coupled to the first resonator and to the second resonator; and
a probe signal analyzer (<NUM>),
the probe signal analyzer being configured:
to measure a change in amplitude or phase of a probe signal received by the probe signal analyzer from the probe signal source, and
to infer, from the change in amplitude or phase, a change in microwave power received by the graphene-insulating-superconducting junction;
wherein the probe signal source is a multi-frequency source, configured to operate at a frequency within <NUM>% of the first resonant frequency and at a frequency within <NUM>% of the second resonant frequency.