Single microwave photon detection device comprising a plurality of quantum bits in series

Abstract

Device for detecting single microwave photons comprising a plurality of quantum bits in series The invention relates to a device (100, 100') for detecting a single microwave photon (2) comprising a plurality N of quantum bits (Q1, …, QN) at two energy levels coupled in series by a number (N-1) of bus resonators Rj, an input resonator (R1) coupled to the first quantum bit Q1, an output resonator (RN+1) coupled to the N-th quantum bit QN, a number N of readout systems (10) each coupled to one of the N quantum bits and 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 a resonator Ri and a pump tone of determined frequency can be converted into an excitation of the quantum bit Qi and a photon from the resonator Ri+1. Figure for abstract: Fig. 2

Description

Title of the invention: Single microwave photon detection device comprising a plurality of quantum bits in series Technical field

[0001] 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.

[0002] 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.

[0003] The invention also relates to a method for detecting single microwave photons implemented by such a device. Prior art

[0004] There are high-performance single-photon detection devices in the optical and infrared domains. These devices have very high detection efficiencies (close to unity), low operating noise and almost continuous operating modes. They are notably implemented in carrying out quantum optics experiments, for example to prepare non-classical states, to generate entanglement, or to produce remotely shared cryptographic keys.

[0005] 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 significant problem in various technological fields: we can cite telecommunications (in particular 3G, 4G, Wi-Fi technologies), radar technologies, medical imaging or even chemical analysis (in particular magnetic resonance).

[0006] The use of quantum sensors makes it possible to count single photons in the microwave domain and thus to 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.

[0007] The article "Cyclically Operated Microwave Single-Photon Counter with Sensitivity of 1-22 W / sqrt(Hz)", Balembois et al., PRApplied (2023) describes a single-quantum-bit microwave single-photon detection device.

[0008] Patent FR 3090891 B1 describes a device 1 for detecting a single microwave photon such as that shown in [Fig.l].

[0009] The detection device 1 is a superconducting circuit containing a superconducting quantum bit 10, or qubit, which can take two quantum states denoted lg> (for g round, i.e. the ground state) and le> (for excited, i.e. an excited state). The quantum bit 10 oscillates at a frequency fq. The energy difference between the states lg> and I e> is fq. h, where h is Planck's constant.

[0010] The quantum bit 10 is represented by a capacitor arranged in parallel to a Josephson junction in [Fig.l].

[0011] 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 fa and makes it possible to collect an incident photon 2.

[0012] 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 fb and allows an output photon 3 to be released after the detection of the incident photon 2.

[0013] The input and output resonators 12, 14 are each represented by a capacitor arranged in parallel with an inductor in [Fig.l].

[0014] A microwave pump tone 4, i.e. a coherent signal, oscillating at a frequency fp is applied to the qubit 10 to activate the detection process. The pump tone 4 is transmitted to the qubit 10 by a transmission line 20.

[0015] A detection of a microwave photon is carried out when the quantum bit 10 is excited, which is caused by the passage of the photon in the detection device.

[0016] Indeed, initially, the incident photon 2 enters the input resonator 12.

[0017] 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 lg> to the excited state le>, as well as into an output photon 3 emitted in the output resonator 14.

[0018] This four-wave mixing process is possible when the frequency of the parametric pumping satisfies the conservation of energy fp + fa = fq + fb. In other words, the energy of the incident photon 2 and a photon of the pump tone 4 must be equal to the energy of the excitation of the quantum bit 10 and an output photon 3.

[0019] 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.

[0020] Measuring the state of the quantum bit 10 makes it possible to determine whether it is in the ground state or in the excited state, which corresponds to the detection of an incident photon 2. If the 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.

[0021] However, it is possible that the quantum bit 10 is excited independently of the detection of a microwave photon. This can be caused for example by thermal noise, by a quasiparticle or by 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.

[0022] The main sources of dark noise are detector imperfections and thermal photons from the environment.

[0023] When the photon flux per unit time is very low, dark noise limits the sensitivity of photon counting.

[0024] Dark noise can be reduced by minimizing spontaneous excitations of the quantum bit, for example by improving the screening of the device, its thermalization or by improving the nano-fabrication techniques of the device. However, these precautions are not always sufficient.

[0025] 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.

[0026] 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 W.s1 / 2 and is defined for a photon counting device by the equation:

[0027] NEp = h_fa^lv

[0028] where h is Planck's constant, fa the operating frequency of the detector, a the dark count in Hz or s 1 and q the efficiency of the detector, i.e. the probability of obtaining a detection signal when a photon arrives at the detector.

[0029] There is a need for a single microwave photon detection device that minimizes dark noise while providing high efficiency.

[0030] The aim of the invention is to meet at least part of this need. Statement of the invention

[0031] To this end, the invention relates, in one of its aspects, to a device for detecting a single microwave photon comprising: - a number N greater than or equal to 2 of quantum bits Qb ..., 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 Q; having a frequency fq>i for i between 1 and N inclusive, - an input resonator Ri configured to receive a single incident microwave photon having a frequency fr 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 and having a frequency fr>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 frj , for j between 2 and N inclusive, - 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, - 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 R; and a pump tone of frequency fp>i = fq>i + fr>i+1 - fr>i can be converted into an excitation of the quantum bit Q; and a photon from the bus or output resonator Ri+i, for i between 1 and N inclusive.

[0032] The detection device is thus configured to convert a photon from the resonator Ri and a pump tone of frequency fp4 into an excitation of the quantum bit Q; and a photon from the resonator Ri+i, for i between 1 and N inclusive.

[0033] The conversion process is known per se. It is activated by the combination of the resonator photon R; and the pump tone of frequency fp>i and consists of a four-wave modulated interaction.

[0034] The pump tone of frequency fp>i is preferably applied to the quantum bit Q; but can also be applied to the resonator R; or to the resonator Ri+i.

[0035] Preferably, the detection device is configured so that the conversion of the photon from the resonator RN into an excitation of the quantum bit QN has a transfer rate rN less than or equal to the dissipation rate kw of the output resonator Rn+i.

[0036] The incident photon may be transmitted to the input resonator by a transmission line coupled to this resonator. The output photon from the output resonator may be discharged by a transmission line coupled to this resonator.

[0037] The frequency of the incident photon may be between 1 and 20 GHz. In particular, it may be between 4 and 8 GHz.

[0038] 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.

[0039] 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 quantum bits makes it possible to limit false positives caused by unwanted excitation of the quantum bits. The impact of an incident photon on the detection device triggers a cascade of excitation of the quantum bits. Consequently, 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 only trigger the excitation of one quantum bit or a minority of quantum bits can therefore be identified as dark noise and discarded.

[0040] 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.

[0041] According to an advantageous characteristic, the N quantum bits are superconducting quantum bits of the transmon type.

[0042] According to another advantageous characteristic, each reading system comprises a resonator.

[0043] 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.

[0044] Alternatively, the reading system coupled to the N-th quantum bit QN is distinct from the output resonator Rn+i.

[0045] Advantageously, the input resonator, the output resonator and the bus resonators are superconducting microwave resonators.

[0046] According to an advantageous characteristic, the input resonator is coupled to a source of microwave photons and the output resonator is coupled to a dissipative environment adapted to dissipate a microwave photon.

[0047] Advantageously, the input resonator comprises a SQUID device configured to tune the frequency of the input resonator.

[0048] 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 / apply to each quantum bit Q; a pump tone of frequency fp>i = fq>i + fr>i+1 -fr>i, for i between 1 and N inclusive; b / measure the state of each quantum bit Q;.

[0049] 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.

[0050] 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.

[0051] According to an advantageous characteristic, the method comprises, prior to step a / , a step a0 / of adjusting the bandwidth of the detection device in which the amplitude A; of the pump tones of frequency fp>i is determined so as to adjust said bandwidth to a desired value.

[0052] 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.

[0053] Preferably, the amplitudes A; of the pump tones are chosen so that the transfer rates F; of photons in the resonators R; are all equal to the same value T. The bandwidth of the device is then determined by the value of T.

[0054] Thus, to detect an incident microwave photon, the latter is firstly 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 following quantum bit and a photon. In the same way, all of 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. Brief description of the drawings

[0055] [Fig.l] [Fig.l] represents a device for detecting single microwave photons according to the prior art.

[0056] [Fig.2] [Fig.2] represents an embodiment of a device for detecting single microwave photons according to the invention.

[0057] [Fig.3] [Fig.3] represents a second embodiment of a device for detection of single microwave photons according to the invention.

[0058] [Fig.4] [Fig.4] is a graph illustrating the probability of detecting a excitation of the first quantum bit of the device of [Fig.3] in the presence of a probe signal.

[0059] [Fig.5] [Fig.5] is a graph illustrating the probability of detecting a excitation of the first quantum bit of the device of [Fig.3] in the absence of a probe signal.

[0060] [Fig.6] [Fig.6] is a graph illustrating the probability of detecting a excitation of the second quantum bit of the device of [Fig.3] in the presence of a probe signal.

[0061] [Fig.7] [Fig.7] is a graph illustrating the probability of detecting a excitation of the second quantum bit of the device of [Fig.3] in the absence of a probe signal.

[0062] [Fig.8] [Fig.8] is a graph illustrating the probability of simultaneous detection of an excitation of the first and second quantum bits of the device of [Fig.3] in the presence of a probe signal.

[0063] [Fig.9] [Fig.9] is a graph illustrating the probability of simultaneous detection of an excitation of the first and second quantum bits of the device of [Fig.3] in the absence of a probe signal.

[0064] [Fig. 10] [Fig. 10] illustrates the response of the quantum bits of the device of [Fig.3] to a first flux of photons.

[0065] [Fig. 11] [Fig. 11] illustrates the response of the quantum bits of the device of [Fig.3] to a second flux of photons.

[0066] [Fig. 12] [Fig. 12] illustrates the response of the quantum bits of the device of [Fig.3] to a third flux of photons.

[0067] [Fig. 13] [Fig. 13] represents a third embodiment of a detection device according to the invention. Detailed description

[0068] [Fig. 1] was described in the preamble and will not be commented on below.

[0069] For the sake of clarity, the same elements of a photon detection device Single microwaves according to the state of the art and a detection device according to the invention are designated by the same numerical references.

[0070] [Fig.2] illustrates a first embodiment of a device 100 for detecting single microwave photons according to the invention, comprising N quantum bits.

[0071] The device 100 as illustrated firstly comprises a photon transmission line 16 which is configured to transmit single incident microwave photons 2 to an input resonator RP. The transmission line 16 is coupled to the input resonator Ri with a rate Kb. For example, Kb / (2jt) may be of the order of 1 MHz.

[0072] The transmission line 16 can in particular couple a source of microwave photons to the input resonator Rb

[0073] The input resonator Ri is coupled to a first quantum bit Qi of the device.

[0074] The device 100 further comprises a transmission line 20 configured to transmit a pump tone 4 to the quantum bit Qb

[0075] Pump tone 4 generates parametric pumping whose frequency is determined to allow four-wave mixing transitioning the quantum bit into its excited state.

[0076] A read resonator 10 is also coupled to the quantum bit Qb. The read resonator 10 is configured to allow the reading, or measurement, of the state of the quantum bit Qi when a read signal is transmitted into this resonator.

[0077] The first quantum bit Qi is coupled to a successive quantum bit Q; via a bus resonator R configured to transmit an intermediate photon 5 from the quantum bit Qi to the quantum bit Q,.

[0078] The device 100 may comprise one or more intermediate quantum bits, each quantum bit Q; being coupled to the next quantum bit Qi+i by a bus resonator R; +i. Each quantum bit Q; is also coupled to a transmission line 20 allowing the transmission of a pump tone 4 to the quantum bit Q; and to a reading resonator 10 allowing the state of the quantum bit Qb to be read.

[0079] The quantum bit QN_b, the last of the intermediate quantum bits Q;, is coupled to the N-th quantum bit QN by a bus resonator RN.

[0080] The N quantum bits of the device are thus connected in series by the bus resonators.

[0081] 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.

[0082] It is also coupled to an output resonator RN+[ configured to transmit an output photon 3 to an output transmission line 18. The output resonator RN +i is coupled to the output transmission line 18 with a dissipation rate kw. A high dissipation rate compared to the transfer rate T, for example a rate kw / (2jt) of the order of 3 MHz for a conversion rate of the order of 200 kHz, makes the output resonator highly dissipative, which allows the photon created in the output resonator Rn+i to be transmitted quickly to the output transmission line 18. The reverse conversion process, which would cause the detection signal to disappear, is thus avoided.

[0083] The transmission line 18 may in particular couple the output resonator Rn+i to a dissipative environment. The dissipative environment makes it possible to dissipate the output photon and may in particular be a cold load, such as an object cooled to a cryogenic temperature.

[0084] The input, output, reading and bus resonators may in particular be superconducting microwave resonators.

[0085] The quantum bits Qb ..., QN can in particular be transmons.

[0086] In the context of the invention, a transmission line may in particular be a guide coplanar wave in particular made of a superconducting metal.

[0087] The couplings between two elements of the detection device are preferably capacitive couplings.

[0088] 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.

[0089] In a particularly advantageous manner, the amplitude A; of the pump tone of frequency fp i applied to the quantum bit Q; can be adjusted. Indeed, the bandwidth of the detection device according to the invention can be modified dynamically when the amplitudes A; 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.

[0090] The amplitudes A; of the pump tones can therefore be determined so as to allow the excitation of the quantum bits and to select a bandwidth.

[0091] The four-wave coupling Hamiltonian of the device, when the pumps are applied at the four-wave mixing frequencies, is

[0092] „ y A / t + j H = + riqri+ J

[0093] where p and q; are respectively the quantum destruction operators associated with the resonator R; and the qubit Q;, rj and q,: are respectively the quantum creation operators associated with the resonator R; and the qubit Q;, A; is the amplitude of the pump applied to the qubit Q; and g; is the four-wave mixing rate associated with the qubit Q;.

[0094] The four-wave mixing ratio g; depends on the coupling between the components of the device according to the relation

[0096] where Xj1 is the dispersive shift between the qubit Q; and the resonator Rj.

[0097] The transfer rate of a photon in each of the resonators can be calculated at from this Hamiltonian. The pump amplitude applied to the qubit Q; makes it possible to adjust the transfer rates r;, ri+i of a photon respectively in the resonators Ri and Ri+i according to the relation:

[0098]

[0099] In the case of the input resonator Ri and the output resonator Rn+i, this relationship becomes respectively:

[0100]

[0101] and

[0102] |g;VAv|2 = 1^,

[0103] Preferably, to obtain optimal transmission of the photon in the series of resonators, the pump amplitudes A are adjusted; so that the transfer rates r2, ..., rN are all equal to a value T.

[0104] The transfer rate and bandwidth of the device are then determined by r and the theoretical efficiency of the device is 1.

[0105] Preferably, for optimal operation of the device, the dispersive shifts of the device are determined so that, for i between 1 and N inclusive:

[0106]

[0107] For example, we choose a value of g;2 greater than 5 times, 10 times or 20 times the value of T.

[0108] The value of r can be adjusted while maintaining a theoretical efficiency of 1 as long as the relationship r < min(Kb, Kw) is respected.

[0109] Standard techniques can be implemented for the manufacture of a detection device according to the invention.

[0110] The device 100, 100' can be manufactured on a silicon wafer with a thickness of a few hundred micrometers covered with a niobium layer with a thickness of the order of 100 nm. The quantum bits and the input, output and bus resonators can be produced by lithography on the niobium layer. The lengths of the resonators are chosen according to the desired frequencies.

[0111] Detection of a single microwave photon by means of the detection device according to the invention takes place as follows.

[0112] First, an incident microwave photon 2 is received in the input resonator Ri of frequency fr> b. The reception is done with a rate Kb. The incident photon 2 can be detected if its frequency is in the interval (fr> i ± Ti / 2).

[0113] The incident photon 2 is then transmitted to the quantum bit Qb II 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 Qb

[0114] 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 Q; of the device where i is between 2 and N1 inclusive, the photon emitted in the bus resonator R; is converted with a rate T; into an excitation of the quantum bit Q; and an intermediate photon 5 from the bus resonator R; +1.

[0115] The intermediate photon 5 of the bus resonator RN is converted with a rate TN into an excitation of the quantum bit QN and an output photon 3 of the output resonator Rn+i.

[0116] The ground state and the excited state of a quantum bit Q; of the device are separated by an energy fq>i. h where h is Planck's constant.

[0117] For i between 1 and (N+1) inclusive, an input, output or bus resonator R; of the device has a frequency fr> b

[0118] Preferably, the dissipation rate kw of the output resonator RN+1 is greater than the transfer rate TN. This makes it possible to quickly transmit the output photon 3 to the transmission line 18 and therefore to effectively inhibit the reverse photon conversion process which would cause the quantum bit QN to transition from the excited state to the ground state.

[0119] 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.

[0120] 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. The resetting of 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 reading resonator.

[0121] Resetting the quantum bits can also be done by applying a microwave pulse to each quantum bit to be reset by the parametric pump transmission line, this pulse being determined so as to return the quantum bit from its excited state to its ground state.

[0122] It is also possible to expect natural relaxation of the qubits, although the transfer rates of an active reset are much higher than the relaxation rates of the qubits.

[0123] Thus, an incident photon 2 arriving in the detection device 100 generates the excitation of N successive qubits rather than that of a single qubit.

[0124] 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 pthN where pth represents the probability of a spontaneous excitation of a quantum bit.

[0125] The dark noise is therefore greatly reduced compared to a detection device of the prior art.

[0126] 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.

[0127] 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 note — (f _ 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 । _Q_ for < 1. This probability therefore increases significantly with the number of quantum bits.

[0128] For an odd number N, an advantageous detection method consists of only counting 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.

[0129] 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 a false positive will be of the order of / jn^- and that of detecting \ / th a false negative will be of the order of [_

[0130] As an 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 3^2( 1-7 / ) + soj( । _ ^2 when q tends towards 1, against for a detection at all the qubits, i.e. 1-3^ when q tends towards 1. Detection at the majority of qubits therefore significantly improves the efficiency of the detection device.

[0131] 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 / \3, or 3«2 when n tends towards 1, against p ,3 for a 3p ,2( 1-p, ) + 1-p, th detection of all qubits.

[0132] The probability of spontaneous excitation pth of a quantum bit is typically of the order of 103 to 10 4. In practice, it is therefore not necessary to achieve a probability of pth3 so as to reduce the probability of counting a detection in the absence of an incident photon. A probability of the order of 3pth2 offers satisfactory performance. Detection at the majority of qubits therefore maintains good performance with regard to dark noise.

[0133] Advantageously, implementing detection at the majority of the 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 during detection at all of 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.

[0134] Other detection methods may be considered. In particular, when all the quantum bits are not identical and some offer better performance with respect to dark noise or false negatives, a greater weighting may be placed on these quantum bits compared to the others.

[0135] [Fig. 3] illustrates a second embodiment of a device 100' for detecting single microwave photons according to the invention. In this example, the device 100' comprises two quantum bits Qb Q2.

[0136] 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 allows reading the state of the quantum bit Qb A bus resonator R2 connects the quantum bit Qi to the quantum bit Q2.

[0137] 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 makes it possible to convert the incident photon and the pump tone into an excitation of the quantum bit Qi and an intermediate photon of 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 makes it possible to convert the intermediate photon and the pump tone into an excitation of the quantum bit Q2 and an output photon of the output resonator R3.

[0138] Reading the state of the quantum bits makes it possible to determine whether a detection event should be counted or not. If both quantum bits are measured in their excited state, a detection event is counted.

[0139] Figures 4 to 9 are graphs illustrating measurements obtained with a device such as the device 100' shown in [Fig.3].

[0140] 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 q of 20%.

[0141] This efficiency is half that of a typical single-quantum-bit detection device due to the errors of the two quantum bits adding together.

[0142] A pump tone is applied to each of the two quantum bits. The frequencies of the pump tones of the two quantum bits are individually swept over a frequency interval. For each pair of pump frequencies (fp>b fp>2), i.e. (a>i / (2ir), œ2 / (2jr)), 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.

[0143] For each of figures 4 to 9, the frequency fp>2 of the pump tone of the second quantum bit Q2 is represented on the abscissa and the frequency fp>i of the pump tone of the first quantum bit Qi is represented on the ordinate.

[0144] [Fig.4] represents the probability of detecting a signal on the first quantum bit Qi when a probe signal is actually sent. It can be seen that this probability is high, of the order of 0.25, around a determined frequency fp4, which is the one fulfilling the condition fp4 = fq4 + fr> 2 - fr,i. The frequencies fq4, fr> 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 determined frequency fp>i. It can be seen that the frequency fp 2 of the pump tone of the second quantum bit has no effect on the detection process of the first quantum bit.

[0145] [Fig.5] represents the probability of detecting a signal on the first quantum bit Qi when no probe signal is sent. It can be seen that this probability is uniformly low, of the order of 3.103. A slightly higher noise is observed around the frequency fp>i realizing the condition fp>i = fq4 + fr>0 - fr,i. This is due to false positives of the first quantum bit caused by thermal noise in the input resonator.

[0146] [Fig.6] 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 Qb. The high probability zone therefore has a spot shape resulting from a crossing between the line visible in [Fig.4] and a vertical line corresponding to the frequency fp>2 realizing the condition fp>2 = fq>2 + fr> 3 - fr>2 . This zone represents the optimal operating space of the detection device.

[0147] [Fig.7] represents 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 significant and includes areas where the noise is more pronounced.

[0148] [Fig.8] represents the probability of detecting a signal on both the first quantum bit Qi and the second quantum bit 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 quantum bits.

[0149] [Fig.9] represents the probability of detecting a signal on both the first qubit Qi and the second qubit 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 qubit individually. The areas of relatively higher probability (of the order of 10 4) in the center of the graph are due to the dark noise caused by the residual thermal noise. The areas of relatively lower probability (of the order of 106) in the corners of the graph represent the intrinsic noise of the device, i.e. the dark noise when the detector efficiency is zero.

[0150] Figures 10 to 12 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.

[0151] In [Fig. 10], no photon flux is sent to the device.

[0152] In [Fig. 11], a flux of 11 photons per second is sent to the device.

[0153] In [Fig. 12], a flux of 46 photons per second is sent to the device.

[0154] The vertical lines represent the detection events respectively on the first quantum bit Qb on the second quantum bit Q2 and the simultaneous detection events for the two quantum bits Qi and Q2.

[0155] It is noted that the dark noise is very low when considering 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.

[0156] 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 [Fig. 13], 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 enables its frequency to be varied.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

1.

2.

3.

4.

5. Claims Device (100, 100') for detecting a single microwave photon (2) comprising: - a number N greater than or equal to 2 of quantum bits (Qb ..., 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 Q; having a frequency fq>i for i between 1 and N inclusive, - an input resonator (RJ configured to receive a single incident microwave photon (2) and having a frequency fr>b the input resonator being coupled to a first quantum bit Qi of the N quantum bits, - an output resonator (Rn+i) configured to discharge an output photon (3) and having a frequency fr>N+b 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 Q, । to the quantum bit Qj and each having a frequency frj, 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, so that a photon from an input or bus resonator R; and a pump tone of frequency fp>i = fq>i + fr>i+1 - fr>i can be converted into an excitation of the quantum bit Q;and a photon from the bus or output resonator Ri+i, for i between 1 and N inclusive.; Detection device according to claim 1, the N quantum bits being superconducting quantum bits of the transmon type. Detection device according to one of the preceding claims, each reading system comprising a resonator. Detection device according to one of the preceding claims, the reading system coupled to the N-th quantum bit QN comprising the output resonator Rn+i. Detection device according to one of the preceding claims, the input resonator, the output resonator and the bus resonators being superconducting microwave resonators.

6. A detection device according to one of the preceding claims, the input resonator being coupled to a microwave photon source and the output resonator being coupled to a dissipative environment adapted to dissipate a microwave photon.

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

8. Method for detecting a single microwave photon implemented by means of a detection device (100, 100') according to one of the preceding claims, comprising the steps of: a / applying to each quantum bit Q; a pump tone of frequency fP,i = fq,i + fr,i+i - fr,i, for i between 1 and N inclusive; b / measuring the state of each quantum bit Q;.

9. Detection method according to the preceding claim, 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.

10. Detection method according to claim 8, 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.

11. Detection method according to one of claims 8 to 10, comprising, prior to step a / , a step a0 / of adjusting the bandwidth of the detection device in which the amplitude A; of the pump tones of frequency fp>i is determined so as to adjust said bandwidth to a desired value.

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