Device for detecting single microwave photons

Abstract

The invention relates to a device (100; 100') for detecting a single microwave photon, comprising: a multi-state quantum system comprising N qudits, N being greater than or equal to 1, the system being capable of making, in response to reception of an incident microwave photon, a number X greater than or equal to 2 of energy transitions between energy levels distributed between the N qudits, these energy transitions corresponding to respective frequencies f1,..., fx, a set of resonators (R1, R2,...) defining X+l electromagnetic modes of respective frequencies fM,1 to fM,x+1, a parametric pumping system for applying X pump tones of respective frequencies fp, i, respecting fM,i + fP,i = fi + fM,i+1 with each integer i ranging from 1 to X, so as to convert, in frequency, by parametric pumping, the excitation contained in an electromagnetic mode i into an excitation of the electromagnetic mode i+1 and a transition i of the qudit in question, a system for reading the energy levels of the one or more qudits of the quantum system delivering a datum providing information on whether an incident microwave photon has been received or not.

Description

[0001] Description

[0002] Title: Single microwave photon detection device

[0003] Technical field

[0004] The invention lies in the field of quantum sensors and relates more particularly to the field of power measurement instrumentation in the microwave range of the electromagnetic spectrum.

[0005] The invention relates to a device for detecting single photons in the microwave range. The microwave range corresponds to photon frequencies of the order of 1 to 20 GHz.

[0006] The invention also relates to a method for detecting single microwave photons implemented by such a device.

[0007] Prior art

[0008] High-performance single-photon detection devices exist in the optical and infrared domains. These devices have very high detection efficiencies (close to unity), low operating noise, and nearly continuous operating modes. They are notably used in quantum optics experiments, for example to prepare non-classical states, to generate entanglement, or to produce cryptographic keys shared remotely.

[0009] However, in the microwave range, photons are difficult to detect due to their very low energy. However, the detection of electromagnetic power in the microwave range is a major issue in various technological fields: telecommunications (notably 3G, 4G, Wi-Fi technologies), radar technologies, medical imaging or even chemical analysis (notably magnetic resonance).

[0010] The use of quantum sensors makes it possible to count single photons in the microwave domain and thus resolve the corpuscular aspect of electromagnetic radiation. Power detection is achieved by measuring the photon flux per unit time. This leads to power detection with optimal sensitivity. The article “Cyclically Operated Microwave Single-Photon Counter with Sensitivity of le-22 W / sqrt(Hz)”, Balembois et al., PRApplied (2023) describes a single-quantum microwave single-photon detection device.

[0011] Patent FR 3090891 B1 describes a device 1 for detecting a single microwave photon such as that shown in Figure 1.

[0012] The detection device 1 is a superconducting circuit containing a superconducting quantum bit 10, or qubit, which can take two quantum states denoted |g> (for ground, or the fundamental state) and |e> (for excited, or an excited state). The quantum bit 10 has a transition frequency f q . The energy difference between the states |g> and |e> is f q . h, where h is Planck's constant.

[0013] Quantum bit 10 is represented by a capacitor arranged in parallel to a Josephson junction in Figure 1.

[0014] An input resonator 12 is coupled to a photon transmission line 16 and to the quantum bit 10. The input resonator 12 oscillates at a frequency f a and allows an incident photon 2 to be collected.

[0015] An output resonator 14 is coupled to the quantum bit 10 and to a photon transmission line 18 by a capacitive coupling 15. The output resonator 14 oscillates at a frequency fi, and allows an output photon 3 to be released after the detection of the incident photon 2.

[0016] The input and output resonators 12, 14 are each represented by a capacitor arranged in parallel with an inductor in Figure 1.

[0017] A microwave pump tone 4, i.e. a coherent signal, oscillating at a frequency f p is applied to qubit 10 to activate the detection process. The pump tone 4 is transmitted to qubit 10 by a transmission line 20.

[0018] A microwave photon detection is achieved when the quantum bit 10 is excited, which is caused by the photon passing through the detection device.

[0019] Indeed, initially, the incident photon 2 enters the input resonator 12.

[0020] The incident photon 2 then combines with the pump tone 4. This activates a four-wave mixing in which the incident photon 2 is converted into an excitation of the quantum bit 10, which changes from the ground state |g> to the excited state |e>, as well as into an output photon 3 emitted in the output resonator 14. This four-wave mixing process is possible when the frequency of the parametric pump satisfies the energy conservation f P + f a = fq + fb. In other words, the energy of the incident photon 2 and a pump tone photon 4 must be equal to the energy of the excitation of the quantum bit 10 and an output photon 3.

[0021] The output photon 3 created in the output resonator is re-emitted as an outward propagating photon. In the absence of photon 3 in the output resonator, the quantum bit 10 cannot return to the ground state by following the reverse four-wave mixing process: it therefore remains in the excited state.

[0022] Measuring the state of quantum bit 10 determines whether it is in the ground state or the excited state, which corresponds to the detection of an incident photon 2. If quantum bit 10 is measured in the excited state, a counting event is recorded and the quantum bit is reset to its ground state. The detection process can then resume.

[0023] However, it is possible for quantum bit 10 to be excited independently of the detection of a microwave photon. This can be caused, for example, by thermal noise, a quasiparticle, or energetic radiation. In this case, a counting event will be recorded in the absence of an incident photon. These events therefore generate false positives, i.e., dark noise.

[0024] The main sources of dark noise are detector imperfections and thermal photons from the environment.

[0025] When the photon flux per unit time is very low, dark noise limits the sensitivity of photon counting.

[0026] Dark noise can be reduced by minimizing spontaneous excitations of the quantum bit, for example by improving device screening, thermalization, or improving device nanofabrication techniques. However, these precautions are not always sufficient.

[0027] Furthermore, to be sensitive, photon counting at microwave frequencies must be efficient. The probability of counting a detection event, when an incident photon impacts the input resonator, must be as large as possible, if possible arbitrarily close to unity.

[0028] The sensitivity of a microwave photon counting device can be quantified by a quantity denoted NEP (acronym for "noise equivalent power") which corresponds to the minimum power detectable with a signal-to-noise ratio of 1 for an integration time of one second. This quantity is expressed in Ws 1 / 2 and is defined for a photon counting device by the equation:

[0029] NEP = h..f a . Tcc / r] where h is Planck's constant, f a the operating frequency of the detector, has the dark count in Hz or s' 1 and r] the detector efficiency, i.e. the probability of obtaining a detection signal when a photon arrives at the detector.

[0030] Patent US9692423B2 relates to a quantum bit reading system that allows a number of incident microwave photons to be counted in a non-destructive manner.

[0031] There is a need for a single microwave photon detection device that minimizes dark noise while providing high efficiency.

[0032] The aim of the invention is to meet at least part of this need.

[0033] Statement of the invention

[0034] To this end, the invention relates, according to one of its aspects, to a single microwave photon detection device, comprising: a multi-state quantum system comprising N qudit(s), N being greater than or equal to 1, the system being capable of operating, in response to the reception of an incident microwave photon, a number X greater than or equal to 2 of energy transitions between energy levels distributed between the N qudit(s), these energy transitions corresponding to respective frequencies fi, ..., fx, a set of resonators defining X+l electromagnetic modes of respective frequencies fwa to fM,x+i, a parametric pumping system for applying X pump tones of frequencies f p ,i respective, checking fxi,i + f P,i = fi + fM,i+i with each integer i ranging from 1 to X, so as to convert into frequency, by parametric pumping, the excitation contained in an electromagnetic mode i into an excitation of the electromagnetic mode i+1 and a transition i of the qudit concerned, a system for reading the energy levels of the qudit(s) of the quantum system providing information on the reception or not of an incident microwave photon. The term "qudit" designates a quantum subsystem with at least two energy levels, that is to say capable of operating at least one energy transition. A qubit is a two-level qudit. A "qubit" thus has a ground state and an excited state.

[0035] The detection device is thus configured to convert a photon of the electromagnetic mode Mi and a pump tone of frequency f p ,i in an excitation of the qubit Qi and a photon of the electromagnetic mode Mi+i, for i between 1 and X inclusive.

[0036] The conversion process is known per se, being described in particular in patent FR3090891. It is activated by the combination of the photon of the electromagnetic mode Mi and the pump tone of frequency f p ,i and consists of a four-wave modulatable interaction.

[0037] The frequency pump tone f p ,i is preferably applied to qubit Qi.

[0038] Preferably, the detection device is configured so that the conversion of the photon from the electromagnetic mode Mx into an excitation of the qubit Qx has a transfer rate Tx less than or equal to the dissipation rate K W of the electromagnetic output mode Mx+i.

[0039] The incident photon can be transmitted to the input resonator by a transmission line coupled to this resonator. The output photon from the output resonator can be discharged by a transmission line coupled to this resonator.

[0040] The frequency of the incident photon can be between 1 and 25 GHz, advantageously between 1 and 20 GHz. In particular, it can be between 4 and 8 GHz.

[0041] The detection device is preferably operated at a cryogenic temperature below 1 K, preferably below 20 mK, in particular of the order of 10 mK. The quantum bits are preferably superconducting.

[0042] Preferably, the set of resonators comprises X+l resonators defining the X+l electromagnetic modes. Alternatively, the set of resonators comprises a number less than X+l resonators which define the X+l electromagnetic modes. In other words, according to this variant some resonators define more than two electromagnetic modes.

[0043] The quantum system can include at least one qudit on which more than one transition takes place, notably two transitions, or even the X energy transitions.

[0044] The quantum system may comprise at least two spatially distinct qudits, between which the X energy transitions are distributed. For example, at least one of the two qudits is a qubit. Both qudits may be qubits. At least one of the two qudits may have at least three energy levels, namely a ground state and two excited states.

[0045] The quantum system may include a qubit on which one of the energy transitions takes place and a qubit with at least three levels on which at least one other of the energy transitions takes place.

[0046] The qudit(s) are preferably transmon-type superconducting qudits.

[0047] The readout system can measure the state of each qudit. Alternatively, the readout system returns logical information about the state of the quantum system as a whole, without providing information specific to the individual state of each qudit.

[0048] For example, the readout system comprises at least one readout resonator, the readout resonator(s) being coupled to one or more corresponding qudits. The photon detector may comprise at least as many readout resonators as qudits.

[0049] The system may include an input resonator coupled to a microwave photon source and an output resonator coupled to a dissipative environment adapted to dissipate a microwave photon.

[0050] The input resonator may include a SQUID device configured to tune the frequency of the input resonator. This provides increased detection efficiency.

[0051] The output resonator can form one of the playback resonators of the playback system.

[0052] Another aspect of the invention relates to a method for detecting a single incident microwave photon using a microwave photon detection device, in particular as defined above, the detection device comprising a multi-state quantum system comprising N qudit(s), N being greater than or equal to 1, the system being capable of operating, in response to the reception of an incident microwave photon, a number X greater than or equal to 2 of energy transitions between energy levels, distributed between the N qudit(s), these energy transitions corresponding to respective frequencies fi, ..., fx, the method comprising the following steps: a / applying X pump tones of frequencies f p ,i respective, verifying fwa + f P,i = fi + fM,i+i with each integer i ranging from 1 to X, so as to convert into frequency, by parametric pumping, the excitation contained in an electromagnetic mode i into an excitation of the electromagnetic mode i+1 and a transition i of the qudit concerned, b / read energy levels of the qudit(s) of the quantum system and deduce information on the reception or not of an incident microwave photon by the detector.

[0053] At least one of the energy transitions can be a transition of a qubit from a ground state to an excited state.

[0054] At least one of the energy transitions can be a transition of a qudit from a ground state to an excited state or between two successive excited states.

[0055] The step of reading the energy levels of the qudit(s) may consist of detecting whether a majority of energy transitions have been made in the quantum system. Optionally, the quantum system includes an odd number X of energy transitions.

[0056] The step of reading the states of the qudit(s) can consist of detecting whether the X energy transitions have actually taken place.

[0057] The method may comprise, prior to step a / , a step of adjusting the bandwidth of the quantum system in which the amplitude Ai of the frequencies of the pump tones is determined so as to adjust said bandwidth to a desired value.

[0058] Another aspect of the invention relates to a device for detecting a single microwave photon comprising:

[0059] - a number N greater than or equal to 2 of quantum bits Qi, ..., QN with two energy levels whose ground state and an excited state are controllable and detectable, the transition between the ground and excited states of each quantum bit Qi having a frequency f q ,i for i between 1 and N inclusive,

[0060] - an input resonator Ri configured to receive a single incident microwave photon having a frequency f r .i, the input resonator being coupled to a first quantum bit Qi of the N quantum bits,

[0061] - an output resonator RN+I configured to evacuate an output photon and having a frequency f r .N+i, the output resonator being coupled to an N-th quantum bit QN of the N quantum bits,

[0062] - a number (N- 1) of bus resonators Rj configured to transmit an intermediate photon from the quantum bit Qj-i to the quantum bit Qj and each having a frequency f rj, for j between 2 and N inclusive,

[0063] - a number N of reading systems each coupled to one of the N quantum bits so as to detect the state of said quantum bit,

[0064] - a number N of parametric pump transmission lines each configured to transmit a pump tone to one of the N quantum bits and / or to one of the input, output or bus resonators, such that a photon from an input or bus resonator Ri and a pump tone of frequency f p ,i = f q ,i + f r ,i+i - fr,i can be converted into an excitation of the quantum bit Qi and a photon of the bus or output resonator Ri+i, for i between 1 and N inclusive.

[0065] Thanks to the detection device according to the invention, dark noise is greatly reduced. Indeed, the redundancy due to the use of a plurality of qudits makes it possible to limit false positives caused by unwanted excitation of the qudits. The impact of an incident photon on the detection device triggers a cascade of excitation of the qudits. Consequently, the detection of a photon can only be counted when the reading system detects that at least a majority of the possible energy transitions have actually taken place. An event that would trigger only a single energy transition or only a minority of energy transitions can therefore be identified as dark noise and discarded.

[0066] In a special case where the quantum system comprises a chain of qubits, the detection of a photon is only counted when at least several of the quantum bits are measured simultaneously in their excited state. An event that would trigger the excitation of only one quantum bit or a minority of quantum bits can therefore be identified as dark noise and discarded.

[0067] Furthermore, when the environment of the detection device is not sufficiently cooled, it can be populated with thermal photons which can cause erroneous counting and therefore generate dark noise. As will be seen later, the detection device according to the invention allows dynamic adjustment of its bandwidth to the frequency of the signal to be measured. This considerably reduces thermal dark noise.

[0068] According to an advantageous characteristic, the N quantum bits are superconducting quantum bits of the transmon type. According to another advantageous characteristic, each reading system comprises a resonator.

[0069] Preferably, the reading system coupled to the N-th quantum bit QN comprises the output resonator RN+I. It is then not necessary to couple a specific reading resonator to the last quantum bit QN since the output resonator RN+I makes it possible to read the state of the qubit.

[0070] Alternatively, the readout system coupled to the N-th quantum bit QN is distinct from the output resonator RN+I.

[0071] Advantageously, the input resonator, the output resonator and the bus resonators are superconducting microwave resonators.

[0072] According to an advantageous characteristic, the input resonator is coupled to a microwave photon source and the output resonator is coupled to a dissipative environment adapted to dissipate a microwave photon.

[0073] Advantageously, the input resonator comprises a SQUID device configured to tune the frequency of the input resonator.

[0074] The invention also relates to a method for detecting a single microwave photon implemented by means of a detection device as described above, comprising the steps of: a / applying to each quantum bit Qi a pump tone of frequency f p ,i = f q ,i + f r ,i+i - f r ,i, for i between 1 and N inclusive; b / measure the state of each quantum bit Qi.

[0075] According to an advantageous embodiment, the detection device comprises an odd number of quantum bits and the method comprises, after step b / , a detection step c / in which it is determined whether at least a majority (N+1) / 2 of the quantum bits is measured in the excited state, in which case a detection event is counted.

[0076] Alternatively, the method comprises, after step b / , a detection step cl / in which it is determined whether all the quantum bits are measured in the excited state, in which case a detection event is counted.

[0077] According to an advantageous characteristic, the method comprises, prior to step a / , a step aO / of adjusting the bandwidth of the detection device in which the amplitude Ai of the pump tones of frequency f p,i is determined so as to adjust said bandwidth to a desired value. By adjusting the amplitude of the pump tones, it is possible to adjust the bandwidth of the detection device. The well-chosen adaptation of the bandwidth of the device to the frequency of the photons to be detected makes it possible to greatly reduce the thermal dark noise.

[0078] Preferably, the amplitudes Ai of the pump tones are chosen so that the transfer rates Ti of photons in the resonators Ri are all equal to the same value T. The bandwidth of the device is then determined by the value of E.

[0079] Thus, to detect an incident microwave photon, the latter is first received in the input resonator of the detection device. In combination with the application of parametric pumping, the incident photon is converted into an excitation of the first quantum bit and a photon in the bus resonator coupled to the first quantum bit. This intermediate photon, in combination with the application of parametric pumping, is itself converted into an excitation of the next quantum bit and a photon. In the same way, all the quantum bits of the detection device are excited. A step of reading the state of the quantum bits then makes it possible to confirm the detection of an incident photon.

[0080] Brief description of the drawings

[0081] [Eig 1] Figure 1 shows a device for detecting single microwave photons according to the prior art;

[0082] [Eig 2] Figure 2 schematically represents a microwave photon detection device according to the invention;

[0083] [Eig 3] Figure 3 schematically represents the operation of the detection device according to one embodiment of the invention;

[0084] [Eig 4] [Eig 5] Figures 4 and 5 illustrate, schematically, the operation of the detection device according to a second embodiment of the invention;

[0085] [Fig 6] Figure 6 illustrates, schematically, the detection device according to a third embodiment of the invention;

[0086] [Fig 7a] Figure 7a illustrates, schematically, the excitation of a qudit in a first excited level;

[0087] [Fig 7b] [Fig 7c] [Fig 7d] Figures 7b to 7d are graphs illustrating the measurements made on the qudit in the first excitation level; [Fig 7e] Figure 7e illustrates, schematically, the qudit of Figure 7a in the ground state;

[0088] [Fig 7f] [Fig 7g] [Fig 7h] Figures 7f to 7h are graphs illustrating the measurements made on the qudit in the ground state;

[0089] [Fig 8a] Figure 8a illustrates, schematically, the qudit brought into the second level of excitation;

[0090] [Fig 8b] [Fig 8c] [Fig 8d] Figures 8b to 8d are graphs illustrating the measurements made on the qudit in its second excitation level;

[0091] [Fig 8e] Figure 8e illustrates, schematically, the measurements carried out on the qudit in its first level of excitation;

[0092] [Fig 8f] [Fig 8g] [Fig 8h] Figures 8f to 8h are graphs illustrating the measurements made on the qudit in its first level of excitation;

[0093] [Fig 9a] Figure 9a illustrates, schematically, a qudit brought into the second excited level;

[0094] [Fig 9b] [Fig 9c] [Fig 9d] Figures 9b to 9d are graphs illustrating the measurements made on the qudit in its second excited level;

[0095] [Fig 9e] Figure 9e illustrates, schematically, the qudit at its fundamental level;

[0096] [Fig 9f] [Fig 9g] [Fig 9h] Figures 9f to 9h are graphs illustrating measurements of the qudit in its ground state.

[0097] [Fig 10] Figure 10 represents a particular case of a device for detecting single microwave photons according to the invention;

[0098] [Fig 11] Figure 11 represents a variant of the particular case of the single microwave photon detection device illustrated in Figure 10;

[0099] [Fig 12] Figure 12 is a graph illustrating the probability of detecting an excitation of the first quantum bit of the device of Figure 11 in the presence of a probe signal.

[0100] [Fig 13] Figure 13 is a graph illustrating the probability of detecting an excitation of the first quantum bit of the device of Figure 11 in the absence of a probe signal. [Fig 14] Figure 14 is a graph illustrating the probability of detecting an excitation of the second quantum bit of the device of Figure 11 in the presence of a probe signal.

[0101] [Fig 15] Figure 15 is a graph illustrating the probability of detecting an excitation of the second quantum bit of the device of Figure 11 in the absence of a probe signal.

[0102] [Fig 16] Figure 16 is a graph illustrating the probability of simultaneous detection of an excitation of the first and second quantum bits of the device of Figure 11 in the presence of a probe signal.

[0103] [Fig 17] Figure 17 is a graph illustrating the probability of simultaneous detection of an excitation of the first and second quantum bits of the device of Figure 11 in the absence of a probe signal.

[0104] [Fig 18] Figure 18 illustrates the response of the quantum bits of the device of Figure 11 to a first flux of photons.

[0105] [Fig 19] Figure 19 illustrates the response of the quantum bits of the device of Figure 11 to a second photon flux.

[0106] [Fig 20] Figure 20 illustrates the response of the quantum bits of the device of Figure 11 to a third photon flux.

[0107] [Fig 21] Figure 21 represents a third embodiment of a detection device according to the invention.

[0108] Detailed description

[0109] Figure 1 was described in the preamble and will not be commented on below.

[0110] For the sake of clarity, the same elements of a single microwave photon detection device according to the state of the art and of a detection device according to the invention are designated by the same numerical references.

[0111] Figure 2 schematically represents a device 100 for detecting a single microwave photon according to the invention, comprising a quantum system with N qudit(s) Qd,i to Qd.N, with N greater than or equal to 1. The quantum system is capable of operating a number X greater than or equal to 2 of energy transitions between energy levels, these transitions being distributed between the N qudit(s), and corresponding to respective frequencies fi, .. fx. In order to operate the X energy transitions, the device 100 comprises N subsystem(s), each comprising a qudit QI ...QN a set of resonators R defining as a whole at least two electromagnetic modes M, at least one transmission line 20 configured to transmit a number k between 1 and X pump tone(s) 4 to the qudit(s) concerned, each pump tone 4 making it possible to carry out parametric pumping whose frequency f pis chosen to allow a four-wave mixing transitioning the qudit Qi concerned from one energy state to another, at least one reading resonator 10 to detect the state of the qudit in question.

[0112] As shown, each subsystem i is coupled to the next subsystem i+1 by a shared electromagnetic mode Mi.

[0113] The first subsystem is distinguished from the following ones in that it is coupled to a photon transmission line 16 which is configured to transmit incident microwave photons 2 to the first electromagnetic mode Mi, in this case housed in an input resonator Ri. The transmission line 16 is coupled to the input resonator Ri with a rate Kb. For example, Kb / (2K) can be of the order of 1 MHz. The transmission line 16 can in particular couple a microwave photon source to the input resonator Ri.

[0114] An operation of the device can be as follows.

[0115] The number X of desired energy transitions is previously defined. The frequencies of the pump tones 4 to be applied to each subsystem are also previously defined so that fwa + f P ,i = fi + fM,i+i, with fwa the frequency of the electromagnetic mode Mi, f p ,i, the frequency of the pump tone Pi, fi the frequency of the energy transition i and fwa+i, the frequency of the electromagnetic mode Mi+i.

[0116] When starting device 100 all 4 pump tones are applied.

[0117] In the case where an incident microwave photon reaches the first resonator Ri, the application of the first pump tone causes a first energy transition of the first qudit Qd.i which results in the generation of a first resulting photon which propagates in the second resonator R2. The application of the second pump tone causes a second energy transition which generates a second resulting photon which propagates in the third resonator, so that the X energy transitions can take place.

[0118] The N ieme subsystem is coupled to the output resonator Rx+i, configured to transmit an output photon 3 to an output transmission line 18. The output resonator Rx+i is coupled to the output transmission line 18 with a dissipation rate K W The dissipation rate is high compared to the transfer rate T, with for example a rate K w / (27t) of the order of 3 MHz for a conversion rate of the order of 200 kHz, which makes the output resonator highly dissipative, and allows the photon created in the output resonator Rx+i to be transmitted quickly to the transmission line 18. The reverse conversion process, which would cause the detection signal to disappear, is thus avoided.

[0119] The transmission line 18 can thus couple the output resonator Rx+i to a dissipative environment, which can be a cold load, such as an object cooled to a cryogenic temperature.

[0120] The X energy transitions are distributed over the N qudit(s). Thus, a qudit can undergo between one and X state changes, depending on the number of pump tones applied to it.

[0121] Each subsystem may include a readout resonator 10, as illustrated, for determining the state of the qudit concerned.

[0122] Thus, once all the pump tones have been applied, for a predefined duration, they are switched off in order to allow the reading system to read the states of the qudit(s). The device then determines, on the basis of the information provided by this reading system, whether an incident photon has indeed passed through the device.

[0123] For example, the detection of a majority of energy transitions compared to the number of predefined energy transitions leads to the conclusion that an incident photon has indeed passed through the device. Conversely, the detection of a minority of energy transitions compared to the number of predefined energy transitions leads to the conclusion that there is no incident photon. It is understood that this is only an example and that any other threshold can be applied. For example, the device can be configured to conclude the presence of an incident photon only if all the energy transitions have actually taken place.

[0124] Figure 3 illustrates an example of a detection device 100 according to the invention, in which the system comprises a single three-state qudit Q. In this case, the operation of the device is as follows.

[0125] We first determine that we expect X=2 energy transitions. We then determine two pump tones whose frequencies are such that: fwa + f P ,i = fi + flU.2 and fw,2 + fp,2 = f2 + fM,3.

[0126] When the device is started, the pump tones of the respective frequencies fi, fz are applied.

[0127] If an incident photon 2 is present in the input resonator Ri, then the qudit Q undergoes, by parametric pumping, a first energy transition ii from its ground state to a first excited state, generating a first resultant photon PRI which propagates in the second resonator R2. The qudit in its first excited state also undergoes, by parametric pumping, a second energy transition i2 from its first excited state to its second excited state, generating a second resultant photon PR2 which propagates in a third resonator R3 which forms an output resonator. Once the pump tones are switched off, the readout resonator 10 is used to determine the state of the qudit Q. If the qudit Q is in its second excited state, then it is concluded that an incident photon was indeed present. If the qudit Q remained in its ground state it is concluded that no incident photon was present.Finally, if the qudit Q is in its first excited state, it is concluded that it is most likely dark noise, and that no incident photon was actually present.

[0128] By "use of the readout resonator" is meant here sending a microwave pulse into the readout resonator, this pulse probably containing several photons. The pulse is reflected with an additional phase dependent on the state of the qudit.

[0129] Figures 4 and 5 schematically illustrate a variant of the device comprising two qudits QI, Q2. Four energy transitions are desired, so that four pump tones of respective frequencies fl, f2, f3, f4 are applied to the system.

[0130] As illustrated in Figure 5, the second resulting photon PR2 obtained as described with reference to Figure 3, propagating in the third resonator R3 is here transformed, by parametric pumping during a third energy transition i3 into a third resulting photon PR3 which propagates in a fourth resonator R4, this third energy transition corresponding to a first energy transition of the second qudit Q2 which passes from the ground state to the first excited level. A fourth energy transition i4 can take place on the second qudit Q2, this fourth energy transition corresponding to a second energy transition of the second qudit Q2 which passes from the first excited level to the second excited level. This fourth transition of the global quantum system generates a fourth resulting photon PR4 which propagates in a fifth resonator R5 which forms, in this case, the output resonator.

[0131] The pump tones are then deactivated to allow the states of the two qubits Qi, Q2 to be read and to conclude on the presence or absence of the incident photon. For example, if we detect that the two qubits are each in their second excited state, it is possible to conclude on the presence of an incident photon.

[0132] Figure 6 illustrates another variant of the device 100 according to the invention in which the system comprises a first qudit Q' 1 with three states and a second qudit with two states Q'2, also called "qubit". This corresponds to N=2 (2 qudits in total), X=3 (three transitions in total), k=3 (3 levels or 2 transitions) and K=2 (2 levels or one transition) in the diagram of Figure 2.

[0133] In this example, it is desired that three transitions be made in total in the system in the event of detection of an incident photon, and pump tones at three frequencies f' 1, f'2, f'3 are previously defined, the first and second pump tones f' 1, f'2 being applied by a first transmission line 20 and the third pump tone being applied by a second transmission line 21.

[0134] The pump tones sent through the transmission lines 20, 21 are activated to allow, in the presence of an incident photon, the energy transitions to be carried out. In this case, the first energy transition corresponds to a first transition of the first qudit Q' 1, which passes from the ground state to the first excited level, the second energy transition corresponds to a second transition of the first qudit Q' 1, which passes from the first excited state to the second excited state and the third energy transition corresponds to a first transition of the second qudit Q' 2, which passes from the ground state to the first excited state.

[0135] Once the pump tones are switched off, the reading system is used to determine the states of the qudits Q' 1, Q' 2 of the system and conclude on the presence or absence of incident photon.

[0136] Figures 7a to 7h illustrate an experimental realization implementing 4-wave mixing for a detector having a single three-state qudit. Figures 7b to 7d and 7e to 7h represent the probabilities as a function of the frequency values ​​(first pump frequency on the ordinate and second pump frequency on the abscissa). The qudit is initialized in its ground state 0, a single pump signal is applied (Figures 7a to 7d) or not (Figures 7e to 7h) so that the incident photon propagating in the first resonator Ri is converted, or not, into a first resulting photon PRI propagating in the second resonator R2.

[0137] Figures 7b and 7f illustrate the probability of finding the qudit in its ground state, respectively, when a pump tone of frequency fl is applied and when no pump tone is applied. Note that the probability is very high and homogeneous for all frequencies in Figure 7f, while a band indicates a very low probability at frequency fl in Figure 7b. Figures 7c and 7g illustrate the probability of finding the qudit in its first excited state 1, respectively, when a pump tone of frequency fl is applied and when no pump tone is applied. Note that the probability is homogeneous and very low for all frequencies in Figure 7g, while a band indicates a very high probability at frequency fl in Figure 7c.It can thus be deduced from these four figures that the application of the pump tone fl allows the energy transition of the qudit from its ground state to its first excited state. Figures 7d and 7h illustrate the probability of finding the qudit in its second excited state, respectively, when a pump tone of frequency fl is applied and when no pump tone is applied. It is noted that the probability is homogeneous and very low for all frequencies in both figures. A band appears slightly in Figure 7d, linked to inaccuracies in the control and reading of the system. Figures 8a to 8h illustrate an experimental realization implementing 4-wave mixing for a detector having a single three-state qudit. Figures 8b to 8d and 8e to 8h represent the probabilities as a function of the frequency values ​​(first pump angular frequency on the ordinate and second pump angular frequency on the abscissa).

[0138] The qudit is initialized in its state “1”, a single pump signal is applied (figures 8a, 8b, 8c, 8d) or not (figures 8e, 8f, 8g, 8h) so that the photon present in the intermediate mode defined by the resonator R2 is converted, or not, into the output mode defined by the resonator R3.

[0139] Figure 8a illustrates the 2 nd conversion mechanism, the qudit ending in its state "2". In this experiment, the levels of the qudit can each be read individually. The experiment consists of a scan of the 2 nd pump frequency (CÛ2 / 2H), at fixed amplitudes, knowing that the first is off, i.e. at zero amplitude (and at frequency ccq / 2TT)). We measured the probability of the qudit being measured in its state "0" (figures 8b, 8f), in its state "1" (figures 8c, 8g) or in its state 2 (figures 8d, 8h).

[0140] Figures 8b, 8c and 8d show that we observe at a single frequency of the 2nd pumps a population transfer from its level 1 (depletion D) to its state 2 (vertical line L). The population in state 0 is almost zero.

[0141] Figures 8f, 8g, 8h confirm that the 1 - 2 transfer of the state of the qudit is indeed due to the application of 2 nd pump by setting its intensity to zero while sending photons into the intermediate resonator.

[0142] Figures 9a to 9h illustrate an experimental realization implementing 4-wave mixing for a detector having a single three-state qudit. Figures 9b to 9d and 9e to 9h represent the probabilities as a function of the frequency values ​​(first pump angular frequency on the ordinate and second pump angular frequency on the abscissa).

[0143] The qudit is initialized in its state 0, two pump signals are applied (figures 9a, 9b, 9c and 9d) or not (figures 9e, 9f, 9g and 9h) so that the photon presented at the input (input mode defined by the resonator Ri) is converted, or not, into the output mode defined by the resonator R3.

[0144] Figure 9a illustrates the overall conversion mechanism, with the qudit ending in its second excited state “2”. In this experiment, each of the qudit levels can be read out individually. The experiment consists of a sweep of the pump frequencies (CÛ1 / 2TI and CÛ2 / 2H), at fixed amplitudes. The probability of the qudit being measured in its “0” state corresponds to Figures 9b, 9f, in its “1” state to Figures 9c, 9g or in its “2” state to Figures 9d, 9h.

[0145] In Figures 9b, 9c and 9d, we observe at a single frequency of the l erepumps a population transfer from its state 0 (depletion D) to its state 1 (horizontal line L). The population in state 1 itself has a depletion at a single 2 nd pump frequency, meaning the transfer between the l-> 2 states of the qudit (bright central area C in Figure 9d). The non-zero population in state 1 results from error induced by the decoherence of the 2~> I transition. Figures 9f, 9g and 9h confirm that the O-^ 2 transfer of the qudit state is indeed due to the application of the pumps by setting their intensities to zero, while sending photons to be detected at the detector input.

[0146] Figures 10 and 11 illustrate a special case in which the system consists exclusively of qubits, i.e. quantum subsystems capable of performing exactly one energy transition each. Then, the number X of energy transitions is equal to the number N of qubits in the quantum system.

[0147] For example, in Figure 10, the first qubit Qi is coupled to a successive qubit Qi via a bus resonator Ri configured to transmit an intermediate photon 5 from the quantum bit Qi to the quantum bit Qi.

[0148] The device 100 may comprise one or more intermediate quantum bits, each quantum bit Qi being coupled to the next quantum bit Qi+i by a bus resonator Ri+i. Each quantum bit Qi is also coupled to a transmission line 20 allowing the transmission of a pump tone 4 to the quantum bit Qi and to a reading resonator 10 allowing the state of the quantum bit Qi to be read.

[0149] The QN-I quantum bit, the last of the intermediate Qi quantum bits, is coupled to the N-th QN quantum bit by a bus resonator RN.

[0150] The N quantum bits of the device are thus connected in series by the bus resonators.

[0151] The N-th quantum bit QN is coupled to a transmission line 20 allowing the transmission of a pump tone 4 to the quantum bit QN as well as to a reading resonator 10.

[0152] It is also coupled to an RN+I output resonator

[0153] The input, output, read and bus resonators can notably be superconducting microwave resonators.

[0154] Quantum bits Qi, ..., QN can notably be transmons.

[0155] In the context of the invention, a transmission line may in particular be a coplanar waveguide, in particular made of a superconducting metal.

[0156] The couplings between two elements of the detection device are preferably capacitive couplings. The output resonator RN+I can advantageously be used as a read resonator, in which case it is not necessary to have a specific read resonator for the last quantum bit QN.

[0157] Particularly advantageously, the amplitude Ai of the pump tone of frequency f p ,i applied to the quantum bit Qi can be adjusted. Indeed, the bandwidth of the detection device according to the invention can be dynamically modified when the amplitudes Ai of the pump tones are modified. It is thus possible to adjust the width of the bandwidth so as to integrate more or less input signal. This makes it possible in particular to control the signal-to-noise ratio of the input signal.

[0158] The amplitudes Ai of the pump tones can therefore be determined to allow the excitation of the quantum bits and to select a bandwidth.

[0159] The four-wave coupling Hamiltonian of the device, when the pumps are applied at the four-wave mixing frequencies, is where n and qi are respectively the quantum destruction operators associated with the resonator Ri and the qubit Qi, n 1 ' and q, : are respectively the quantum creation operators associated with the resonator Ri and the qubit Qi, Ai is the amplitude of the pump applied to the qubit Qi and gi is the four-wave mixing rate associated with the qubit Qi.

[0160] The four-wave mixing ratio gi depends on the coupling between the device components according to the relation where / j' is the dispersive shift between the qubit Qi and the resonator Rj.

[0161] The transfer rate of a photon in each of the resonators can be calculated from this Hamiltonian. The pump amplitude applied to the qubit Qi makes it possible to adjust the transfer rates Ei, Ti+i of a photon respectively in the resonators Ri and Ri+i according to the relation:

[0162] I^AI 2 = r f r f+1

[0163] In the case of the input resonator Ri and the output resonator RN+I, this relationship becomes respectively: l^iAi l 2 = K b r2et

[0164] Preferably, to obtain optimal photon transmission in the series of resonators, the pump amplitudes Ai are adjusted so that the transfer rates Ti, . . ., TN are all equal to a value T.

[0165] The transfer rate and bandwidth of the device are then determined by r and the theoretical efficiency of the device is 1.

[0166] Preferably, for optimal operation of the device, the dispersive shifts of the device are determined so that, for i between 1 and N inclusive: g 2 = xixi+i » r

[0167] For example, we choose a value of gi 2 greater than 5 times, 10 times or 20 times the value of T.

[0168] The value of T can be adjusted while maintaining a theoretical efficiency of 1 as long as the relation T < min(Kb, K W ) is respected.

[0169] Standard techniques may be implemented for the manufacture of a detection device according to the invention.

[0170] The device 100, 100' can be fabricated on a silicon wafer a few hundred micrometers thick covered with a niobium layer about 100 nm thick. The quantum bits and the input, output and bus resonators can be fabricated by lithography on the niobium layer. The lengths of the resonators are chosen according to the desired frequencies.

[0171] A detection of a single microwave photon using the detection device as shown in Figures 10 and 11 proceeds as follows.

[0172] First, an incident microwave photon 2 is received in the input resonator Ri of frequency f r ,i . Reception is done with a rate Kb. The incident photon 2 can be detected if its frequency is in the interval (f r ,i ± T i / 2).

[0173] The incident photon 2 is then transmitted to the quantum bit Qi. H is converted with a rate T i into an excitation of the quantum bit and an intermediate photon 5 of the bus resonator R2 thanks to the parametric pumping of the quantum bit Qi.

[0174] The intermediate photon 5 from the bus resonator R2 is transmitted to the next quantum bit Q2 and the same conversion process is repeated. For a quantum bit Qi of the device where i is between 2 and Nl inclusive, the photon emitted in the bus resonator Ri is converted with a rate Ti into an excitation of the quantum bit Qi and an intermediate photon 5 from the bus resonator Ri+i.

[0175] The intermediate photon 5 from the bus resonator RN is converted with a rate TN into an excitation of the quantum bit QN and an output photon 3 from the output resonator RN+I.

[0176] The ground state and the excited state of a quantum bit Qi of the device are separated by an energy f q,i . h where h is Planck's constant.

[0177] For i between 1 and (N+1) inclusive, an input, output or bus resonator Ri of the device has a frequency f r ,i.

[0178] Preferably, the dissipation rate K W of the output resonator RN+I is greater than the transfer rate TN. This allows the output photon 3 to be transmitted quickly to the transmission line 18 and thus effectively inhibits the reverse photon conversion process that would cause the quantum bit QN to transition from the excited state to the ground state.

[0179] The state of each quantum bit is determined by a reading of this quantum bit by the associated reading resonator 10. The step of reading the quantum bits can be carried out in parallel with the steps of receiving and converting the photons or after these steps. In the latter case, the reading step can be carried out in the absence of parametric pumping, which makes it possible to use the output resonator 14 as a reading resonator.

[0180] A step of resetting the quantum bits to their ground state may be performed after reading their state in order to prepare the device for receiving a new incident photon 2. Resetting the quantum bits may be performed by exciting the bus resonators with a microwave pulse chosen to enable the conversion of the excitation of a quantum bit into a propagating photon. Preferably, the pumping frequency is chosen so that the excitation of a quantum bit and the pulse on the associated bus resonator are converted into a photon of the associated read resonator.

[0181] Resetting the quantum bits can also be done by applying a microwave pulse to each quantum bit to be reset through the parametric pump transmission line, this pulse being determined so as to return the quantum bit from its excited state to its ground state.

[0182] It is also possible to expect natural relaxation of the quantum bits, although the transfer rates of an active reset are much higher than the relaxation rates of the quantum bits. Thus, an incident photon 2 arriving in the detection device 100 causes the excitation of N successive qubits rather than that of a single qubit.

[0183] According to a first alternative, the detection of an incident photon is counted when the N quantum bits are measured in their excited state. Since the spontaneous excitation events of the quantum bits are independent, the probability of detecting a false positive corresponds to the probability of simultaneously measuring the N quantum bits in their excited state, i.e. to a probability pth N where pth represents the probability of spontaneous excitation of a quantum bit.

[0184] The dark noise is therefore greatly reduced compared to a detection device of the prior art.

[0185] Advantageously, a more sophisticated method for detecting an incident photon can be implemented in order to minimize the inefficiency of the detection device, i.e. the false negatives corresponding to the probability of not counting an event when an incident photon 2 reaches the detection device.

[0186] The main process contributing to the inefficiency of the device is the spontaneous de-excitation of a quantum bit. These events are independent for each of the quantum bits. We denote by fj = (1 — 77) the probability of a spontaneous de-excitation of a quantum bit. The probability of having at least one spontaneous de-excitation among the N quantum bits is then 1 — (1 Nfj for fj « 1. This probability therefore increases significantly with the number of quantum bits.

[0187] For an odd number N, an advantageous detection method is to only count a detection if a majority of quantum bits are measured in their excited state. For an even number N greater than 3, a detection can only be counted if half N / 2 or a majority N / 2 + 1 of the quantum bits are measured in their excited state.

[0188] We are therefore not limited to the case where all quantum bits are measured in their excited state. In this way, the probabilities of false positives and false negatives can both be kept arbitrarily low: for odd N, the probability of detecting For example, for N=3, the probability of counting a detection at the majority of qubits when an incident photon is transmitted to the detector is 3î 2 (l — 77) + *2 ^"2

[0189] T], i.e. 1 — 3î when r tends towards 1, against 7 for detection at all qubits, i.e. 1 — 3rj when r tends towards 1. Detection at the majority of qubits therefore significantly improves the efficiency of the detection device.

[0190] For N=3 still, the probability of counting a detection at the majority of qubits when no incident photon is transmitted to the detector is 3p th 2 (1 — p th ) + (1 — p t h) 3 , or 3pt 2 when r] tends towards 1, against p t / l 3 for detection of all qubits.

[0191] The probability of spontaneous excitation pth of a quantum bit is typically of the order of 10' 3 at 10' 4 . In practice, it is therefore not necessary to achieve a probability of pth 3so as to reduce the probability of counting a detection in the absence of an incident photon. A probability of the order of 3pth 2 offers satisfactory performance. Detection at the majority of qubits therefore maintains good performance with regard to dark noise.

[0192] Advantageously, implementing detection at the majority of qubits makes the detection device easier to operate. Indeed, the detection is then more robust to spontaneous de-excitation or thermal excitation of an individual quantum bit than when detecting all the quantum bits. It is possible to partially relax certain operating constraints of the device likely to increase the probability of spontaneous de-excitation without impacting the proper operation of the detection device.

[0193] Other detection methods can be considered. In particular, when not all qubits are identical and some offer better performance with respect to dark noise or false negatives, a greater weight can be placed on these qubits compared to others.

[0194] Figure 11 illustrates an embodiment of a device 100' for detecting single microwave photons according to the invention. In this example, the device 100' comprises two quantum bits Qi, Q2.

[0195] The input resonator Ri is coupled to the quantum bit Qi and the output resonator R3 is coupled to the quantum bit Q2. The output resonator R3 is also used as a read resonator to read the state of the quantum bit Q2. The read resonator 10 is used to read the state of the quantum bit Qi. A bus resonator R2 connects the quantum bit Qi to the quantum bit Q2.

[0196] The presence of an incident photon in the input resonator Ri and the application of a pump tone 4 to the first quantum bit Qi converts the incident photon and the pump tone into an excitation of the quantum bit Qi and an intermediate photon from the bus resonator R2. Similarly, the presence of an intermediate photon in the bus resonator R2 and the application of a pump tone 4 to the second quantum bit Q2 converts the intermediate photon and the pump tone into an excitation of the quantum bit Q2 and an output photon from the output resonator R3.

[0197] Reading the state of the qubits determines whether a detection event should be counted or not. If both qubits are measured in their excited state, a detection event is counted.

[0198] Figures 12 to 17 are graphs illustrating measurements obtained with a device such as the device 100' shown in Figure 11.

[0199] In this example, the frequency of the probe signal applied to the input of the detection device is 9 GHz. The amplitude of the probe signal and that of the pump tones is fixed. The detection device has an efficiency r] of 20%.

[0200] This efficiency is half that of a typical single-quadbit detection device due to the errors of the two qubits adding together.

[0201] A pump tone is applied to each of the two qubits. The frequencies of the pump tones of the two qubits are individually swept over a frequency interval. For each pair of pump frequencies (f P ,i , f P,2), or (coi / (27t), 002 / (271)), the probability that a signal is actually detected on one or the other of the quantum bits is measured on the one hand when a probe signal is sent into the input resonator, and on the other hand when no probe signal is sent.

[0202] For each of figures 12 to 17, the frequency f p ,2 of the pump tone of the second quantum bit Q2 is represented on the abscissa and the frequency f p ,i of the pump tone of the first quantum bit Qi is represented on the ordinate.

[0203] Figure 12 represents the probability of detecting a signal on the first quantum bit Qi when a probe signal is actually sent. We note that this probability is high, of the order of 0.25, around a frequency f p ,i determined, which is the one realizing the condition f p ,i = f q ,i + f r ,2 - f r ,i. The frequencies f q j, f r ,i and fr ,2 are in fact constant during the measurement: the four-wave mixing allowing the conversion of the incident photon is therefore only permitted for this frequency f p ,i determined. We note that the frequency f p ,2 of the pump tone of the second qubit has no effect on the detection process of the first qubit.

[0204] Figure 13 represents the probability of detecting a signal on the first quantum bit Qi when no probe signal is sent. We note that this probability is uniformly low, of the order of 3.10' 3 . A slightly higher noise is observed around the frequency f p ,i realizing the condition f p ,i = f q ,i + f r ,o - f r ,i. This is due to false positives of the first quantum bit caused by thermal noise in the input resonator.

[0205] Figure 14 represents the probability of detecting a signal on the second quantum bit Q2 when a probe signal is actually sent. A detection on the second quantum bit Q2 is only allowed in the presence of a photon in the bus resonator coupling the two quantum bits, i.e. when a detection is carried out on the first quantum bit Qi. The high probability zone therefore has a spot shape resulting from a crossing between the line visible in Figure 12 and a vertical line corresponding to the frequency f p ,2 realizing the condition f p ,2 = f q ,2 + fr,3 ​​- fr,2. This area represents the optimal operating space of the detection device.

[0206] Figure 15 shows the probability of detecting a signal on the second quantum bit Q2 when no probe signal is sent. It can be seen that the noise of the second quantum bit is relatively large and has areas where the noise is more pronounced.

[0207] Figure 16 shows the probability of detecting a signal on both the first qubit Qi and the second qubit Q2 when a probe signal is actually sent. This result corresponds to the frequency space in which the four-wave mixing conditions are verified on both qubits.

[0208] Figure 17 shows the probability of detecting a signal on both the first quantum bit Qi and the second quantum bit Q2 when no probe signal is sent. It can be seen that this probability is much lower than that of detecting a signal on the first or second quantum bit individually. The areas of relatively higher probability (of the order of 10' 4 ) in the center of the graph are due to dark noise caused by residual thermal noise. The areas of relatively lower probability (of the order of 10' 6 ) in the corners of the graph represent the intrinsic noise of the device, i.e. the dark noise when the detector efficiency is zero.

[0209] Figures 18 to 20 represent the response of the quantum bits of the same detection device to three different fluxes of incident photons over a time interval of one second.

[0210] In Figure 18, no photon flux is sent to the device.

[0211] In Figure 19, a flux of 11 photons per second is sent to the device.

[0212] In Figure 20, a flux of 46 photons per second is sent to the device.

[0213] The vertical lines represent the detection events respectively on the first quantum bit Qi, on the second quantum bit Q2 and the simultaneous detection events for both quantum bits Qi and Q2.

[0214] We note that the dark noise is very low when we consider the two quantum bits simultaneously (we measure 4 false positives per second) in comparison to the dark noise measured individually on each of the two quantum bits.

[0215] Other variants and improvements may be envisaged without departing from the scope of the invention. In particular, the detection device according to the invention may also comprise means for filtering and / or tuning the resonant frequency of the input resonator and / or the bus resonators and / or the output resonator. In particular, as shown in the embodiment of Figure 21, the detection device may comprise a SQUID device integrated into the input resonator Ri so as to enable the frequency of this resonator to be tuned. A SQUID device comprises two Josephson junctions arranged in parallel. By inducing a magnetic field in the SQUID, the inductance of the input resonator Ri can be adjusted, which makes it possible to vary its frequency.To induce the magnetic field in the SQUID 11, a current is passed near it in a fast flux line, i.e., a transmission line short-circuited to ground by a loop.

Claims

Claims 1. A single microwave photon detection device (100; 100'), comprising: a multi-state quantum system comprising N qudit(s), N being greater than or equal to 1, the system being capable of operating, in response to the reception of an incident microwave photon, a number X greater than or equal to 2 of energy transitions between energy levels distributed between the N qudit(s), these energy transitions corresponding to respective frequencies fi, ..., fx, a set of resonators (Ri, R2, ...) defining X+l electromagnetic modes of respective frequencies fw,i to fw.x+1, a parametric pumping system for applying X pump tones of frequencies f p , i respective, verifying fM,i + f P,i = fi + fM,i+i with each integer i ranging from 1 to X, so as to convert into frequency, by parametric pumping, the excitation contained in an electromagnetic mode i into an excitation of the electromagnetic mode i+1 and a transition i of the qudit concerned, a system for reading the energy levels of the qudit(s) of the quantum system providing information on the reception or not of an incident microwave photon.

2. Detection device according to claim 1, the set of resonators comprising X+l resonators defining the X+l electromagnetic modes.

3. Detection device according to one of the preceding claims, the quantum system comprising a qudit on which the X energy transitions take place.

4. Detection device according to claim 1, the quantum system comprising at least two spatially distinct qudits, between which the X energy transitions are distributed.

5. Detection device according to claim 4, at least one of the two qudits being a qubit.

6. Detection device according to claim 4, the two qudits being qubits.

7. Detection device according to claim 4, at least one of the two qudits having at least three energy levels.

8. Detection device according to claim 1 or 2, the quantum system comprising a qubit on which one of the energy transitions takes place and a qubit with at least three levels on which at least one other of the energy transitions takes place.

9. Detection device according to one of the preceding claims, the qud(s) being transmon-type superconductor qud(s).

10. Detection device according to any one of the preceding claims, the reading system measuring the state of each qudit.

11. Detection device according to any one of the preceding claims, the reading system comprising at least one reading resonator, the reading resonator(s) being coupled to one or more corresponding qudits.

12. Detection device according to any one of the preceding claims, comprising at least as many reading resonator(s) as qudit(s).

13. A detection device according to any preceding claim, the system comprising an input resonator coupled to a microwave photon source and an output resonator coupled to a dissipative environment adapted to dissipate a microwave photon.

14. Device according to the preceding claim, the input resonator comprising a SQUID device (11) configured to tune the frequency of the input resonator.

15. Method for detecting a single incident microwave photon using a microwave photon detection device, in particular according to one of the preceding claims, the device comprising a multi-state quantum system comprising N qudit(s), N being greater than or equal to 1, the system being capable of operating, in response to the reception of an incident microwave photon, a number X greater than or equal to 2 of energy transitions between energy levels distributed between the N qudit(s), these energy transitions corresponding to respective frequencies fi, ..., fx, the method comprising the following steps: a / applying X pump tones of frequencies f p ,i respective, verifying fwa + f P ,i = fi + fM,i+i with each integer i ranging from 1 to X, so as to convert into frequency, by parametric pumping, the excitation contained in an electromagnetic mode i into an excitation of the electromagnetic mode i+1 and a transition i of the qudit concerned, b / read energy levels of the qudit(s) of the quantum system and deduce information from them on whether or not an incident microwave photon is received by the detector.

16. Detection method according to the preceding claim, at least one of the energy transitions being a transition of a qubit from a ground state to an excited state.

17. Detection method according to one of claims 15 or 16, at least one of the energy transitions being a transition of a qudit from a ground state to an excited state or between two successive excited states.

18. Detection method according to claim 16, each energy transition being a transition of a qudit between two successive energy states.

19. Detection method according to one of claims 15 to 18, the step of reading the energy levels of the qudit(s) consisting of detecting whether a majority of energy transitions have been carried out in the quantum system.

20. Method according to the preceding claim, the quantum system comprising an odd number X of energy transitions.

21. Detection method according to one of claims 15 to 18, the step of reading the states of the qudit(s) consisting of detecting whether the X energy transitions have actually been carried out.

22. Detection method according to one of claims 15 to 21, comprising, prior to step a / , a step of adjusting the bandwidth of the quantum system in which the amplitude Ai of the frequencies of the pump tones is determined so as to adjust said bandwidth to a desired value.

23. Device (100, 100') for detecting a single microwave photon (2) comprising: a number N greater than or equal to 2 of quantum bits (Qi, ..., QN) at two energy levels of which the ground state and an excited state are controllable and detectable, the transition between the ground and excited states of each quantum bit Qi having a frequency f q ,i for i between 1 and N inclusive, an input resonator (Ri) configured to receive a single incident microwave photon (2) and having a frequency f r .i, the input resonator being coupled to a first quantum bit Qi of the N quantum bits, an output resonator (RN+I) configured to evacuate an output photon (3) and having a frequency f r.N+i, the output resonator being coupled to an N-th quantum bit QN of the N quantum bits, a number (Nl) of bus resonators Rj configured to transmit an intermediate photon from the quantum bit Qj-i to the quantum bit Qj and each having a frequency f r j, for j between 2 and N inclusive, at least one reading system (10), coupled to the N quantum bits so as to detect their energy levels, in particular N or less than N reading systems, at least one parametric pumping transmission line (20), in particular N or less than N transmission lines, configured to transmit a pump tone to one of the N quantum bits and / or to one of the input, output or bus resonators, so that a photon from an input or bus resonator Ri and a pump tone of frequency fp,i = fq,i + fr,i+i - fr,i can be converted into an excitation of the quantum bit Qi and a photon from the bus or output resonator Ri+i, for i between 1 and N inclusive.

24. Device according to claim 23, comprising a number N of reading systems (10), each coupled to one of the N quantum bits, so as to detect the state of the quantum bit.

25. Device according to one of claims 23 and 24, comprising a number N of parametric pump transmission lines (20) each configured to transmit a pump tone to one of the N quantum bits and / or to one of the input, output or bus resonators, so that a photon from an input or bus resonator Ri and a pump tone of frequency f p ,i = f q ,i + f r ,i+i - fr,i can be converted into an excitation of the quantum bit Qi and a photon of the bus or output resonator Ri+i, for i between 1 and N inclusive.

26. Device (100, 100') for detecting a single microwave photon (2) comprising: - a number N greater than or equal to 2 of quantum bits (Qi, ..., QN) at two energy levels whose ground state and an excited state are controllable and detectable, the transition between the ground and excited states of each quantum bit Qi having a frequency f q ,i for i between 1 and N inclusive, - an input resonator (Ri) configured to receive a single incident microwave photon (2) and having a frequency f r .i, the input resonator being coupled to a first quantum bit Qi of the N quantum bits, - an output resonator (RN+I) configured to evacuate an output photon (3) and having a frequency f r ,N+i, the output resonator being coupled to an N-th quantum bit QN of the N quantum bits, - a number (N- 1) of bus resonators Rj configured to transmit an intermediate photon from the quantum bit Qj-i to the quantum bit Qj and each having a frequency f rj, for j between 2 and N inclusive, - a number N of reading systems (10) each coupled to one of the N quantum bits so as to detect the state of said quantum bit, - a number N of parametric pump transmission lines (20) each configured to transmit a pump tone to one of the N quantum bits and / or to one of the input, output or bus resonators, such that a photon from an input or bus resonator Ri and a pump tone of frequency f p ,i = f q ,i + f r ,i+i - fr,i can be converted into an excitation of the quantum bit Qi and a photon of the bus or output resonator Ri+i, for i between 1 and N inclusive.

27. Detection device according to claim 26, the N quantum bits being superconducting quantum bits of the transmon type.

28. Detection device according to one of claims 26 and 27, each reading system comprising a resonator.

29. Detection device according to one of claims 26 to 28, the reading system coupled to the N-th quantum bit QN comprising the output resonator RN+I.

30. Detection device according to one of claims 26 to 29, the input resonator, the output resonator and the bus resonators being superconducting microwave resonators.

31. Detection device according to one of claims 26 to 30, the input resonator being coupled to a source of microwave photons and the output resonator being coupled to a dissipative environment adapted to dissipate a microwave photon.

32. Detection device according to one of claims 26 to 31, the input resonator comprising a SQUID device (11) configured to tune the frequency of the input resonator.

33. Method for detecting a single microwave photon implemented by means of a detection device (100, 100') according to one of claims 26 to 32, comprising the steps of: a / applying to each quantum bit Qi a pump tone of frequency f p ,i = f q ,i + f r ,i+i - f r ,i, for i between 1 and N inclusive; b / measure the state of each quantum bit Qi.

34. Detection method according to claim 33, the detection device comprising an odd number of quantum bits, the method comprising, after step b / , a detection step c / in which it is determined whether at least a majority (N+1) / 2 of the quantum bits is measured in the excited state, in which case a detection event is counted.

35. Detection method according to claim 33, the method comprising, after step b / , a detection step cl / in which it is determined whether all the quantum bits are measured in the excited state, in which case a detection event is counted.

36. Detection method according to one of claims 33 to 35, comprising, prior to step a / , a step a0 / of adjusting the bandwidth of the detection device in which the amplitude Ai of the pump tones of frequency f p ,i is determined so as to adjust said bandwidth to a desired value.

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