Quantum transducer and associated quantum network

Abstract

The quantum transducer can be used as a quantum network element, and have a microwave cavity being magnetically-sensitive and connected to an end node of a quantum network, an optical cavity, and a mechanical resonator having a resonance mode spatially distributed between a first portion and a second portion, an inductive mediator at the first portion, the first portion magnetomechanically coupled to the microwave cavity, and the second portion optomechanically coupled to the optical cavity.

Description

QUANTUM TRANSDUCER AND ASSOCIATED QUANTUM NETWORKTECHNICAL FIELD

[0001] This specification relates to transducers for converting quantum information from one form to another.BACKGROUND

[0002] Quantum computing, and quantum networks, which have a demonstrated track record in laboratories across the world, are getting closer to becoming the new standard for computing and telecommunications. However, various hurdles still need to be overcome. One of these hurdles is quantum transduction, of quantum information, between different frequencies. Indeed, quantum information can be carried by electromagnetic radiation (photons) at different frequencies of the electromagnetic spectrum, and different pieces of equipment may operate in different frequency ranges, requiring transduction between the two pieces of equipment. While converting frequencies may seem trivial to an unskilled reader, there are very significant challenges when converting from frequencies of one portion of the electromagnetic spectrum to another, since the physical properties of the electromagnetic waves and their interaction with the environment can be very different. In practice, very different equipment is used with microwaves than with radio waves, X-rays, or with light, for example and it is often not straightforward at all to find components which will perform a same function in such different portions of the electromagnetic spectrum (e.g., emitting, detecting, waveguiding, etc.).SUMMARY

[0003] In accordance with one aspect, there is provided a quantum network element communicatively connected to an end node of a quantum network, the quantum network element having : a microwave cavity being magnetically-sensitive and connected to the end node, an optical cavity, and a mechanical resonator having a resonance mode spatially distributed between a first portion and a second portion, an inductive mediatorat the first portion, the first portion magnetomechanically coupled to the microwave cavity, and the second portion optomechanically coupled to the optical cavity.

[0004] In accordance with another aspect, there is provided a method of communicating quantum information comprising: propagating one or more microwave photons into a microwave cavity being magnetically-sensitive, the one or more microwave photons having quantum information in the form of a superposition of states; transducing the one or more microwave photons in the microwave cavity into one or more phonons in a mechanical resonator, via magnetomechanical coupling between the microwave cavity and an inductive mediator at a first portion of the mechanical resonator; distributing the one or more phonons between the first portion of the mechanical resonator and a second portion of the mechanical resonator; transducing the one or more phonons in the mechanical resonator into one or more optical photons in an optical cavity via optomechanical coupling between the mechanical resonator and the optical cavity; and propagating the one or more optical photons away from the optical cavity, the one or more optical photons having the quantum information in the form of the superposition of states.

[0005] In accordance with another aspect, there is provided a quantum transducer comprising : a microwave cavity being magnetically-sensitive; an optical cavity; and a mechanical resonator having a resonance mode spatially distributed between a first portion and a second portion, an inductive mediator at the first portion, the first portion magnetomechanically coupled to the microwave cavity, and the second portion optomechanically coupled to the optical cavity.

[0006] The quantum transducer may be used as an element of a quantum network. The quantum transducer may be integrated to a node of a quantum network.

[0007] In the context of this specification, microwave frequencies can be defined herein as extending between 300 MHz and 300 GHz, whereas optical frequencies can be defined herein as extending between 300 GHz and 30 000 THz.

[0008] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.DESCRIPTION OF THE FIGURES

[0009] In the figures,

[0010] Fig. 1 is a block diagram presenting elements of a quantum network in accordance with an embodiment;

[0011] Fig. 2 is a block diagram of an example of a quantum transducer in accordance with an embodiment;

[0012] Figs 3A and 3B are schematic view of the optomechanical coupling between a portion of a mechanical resonator and an optical cavity involving displacement of a boundary of the optical cavity and displacement of a dielectric within the optical cavity, respectively;

[0013] Figs 4A and 4B are schematic view of microwave cavities which are magnetically sensitive, namely a microwave circuit integrating a SQUID and a microwave circuit integrating a high inductance element, respectively;

[0014] Figs 5A, 5B and 5C are schematic views of inductive mediators which can be used in the conversion of microwave photons to phonons, or vice-versa, namely a moving / deforming magnet, a moving / deforming superconductor under external magnetic field, and a deforming element of a microwave circuit under external magnetic field, or wiggly arm, respectively;

[0015] Fig. 6 is a schematic view of a quantum transducer embodied as an assembly of components and where the mechanical resonator is a membrane;

[0016] Fig. 7 is a schematic view of a quantum transducer embodied as an integrated circuit and having an inductive mediator in the form of a deforming element of a microwave circuit;

[0017] Fig. 8 is a schematic view of a quantum transducer embodied as an integrated circuit and having an inductive mediator in the form of a moving / deforming magnet, moving / deforming superconductor, or deforming magnetoelastic element;

[0018] Fig. 9 is a block diagram of two quantum transducers integrated in a cascade configuration, forming an example of a quantum converter in accordance with an embodiment;

[0019] Fig. 10 is a block diagram of two quantum transducers integrated in a cascade configuration, forming an example of a quantum converter in accordance with an embodiment; and

[0020] Fig. 11 is a schematic view of a quantum transducer integrated to a single or n- photon source instead of being integrated or coupled to an end node of a quantum network;DETAILED DESCRIPTION

[0021] Fig. 1 shows an example configuration of elements of a quantum network 10. A quantum network 10 has several quantum end nodes which are configured to output or receive quantum information, e.g., information encoded in quantum properties of fields or particles, such as in the form of a superposition of states in one or more photon. The quantum end nodes can operate in the microwave frequencies (e.g. encode or decode information in superposition of states in one or more photon in the microwave frequencies), and be connected to a microwave transmission line, which can be said to form part of a microwave subnetwork. Each quantum end node can have one or more channel, and a typical embodiment may involve numerous channels.

[0022] The quantum network 10 can be configured to allow the quantum end nodes to communicate with one another, which can involve a routing function (e.g., router), or another networking function. The router may operate in the optical (e.g. telecom) frequencies. Typical optical telecom equipment may operate between 1 and 800 THz, notably at the infrared wavelength of 1550 nm. The router can be connected to optical fibers for instance, which may be referred to as an optical subnetwork. One or moretransducers 12 may be embodied as one or more elements of the quantum network, such as integrated to an end node, to convert the microwave photons which encode the information in the form of a superposition of states and which may travel in the microwave subnetwork into optical photons, which also encode the information in the form of a superposition of states and which may travel in the optical subnetwork and be operated upon by the router, and / or vice-versa. In Fig. 1 , groups of one or more quantum transducers 12 connected to different channels of a same quantum end node may collectively be referred to as quantum transducing units.

[0023] The nature of the quantum end nodes can vary from one embodiment to another. For instance, in distributed quantum computing, the quantum end nodes can be quantum processors. In many quantum computers, the qubits operate at microwave frequencies between 4 and 9 GHz, for instance. In quantum information processing, such as in distributed quantum sensing or distributed quantum metrology, the quantum end nodes can be quantum sensors.

[0024] In other embodiments, a quantum memory, such as one or more qubits, can be used instead of a quantum router in a configuration otherwise such as shown in Fig. 1 , and more generally, quantum transducers can have uses and applications outside quantum networks, with quantum networking being presented here solely as one example family of use cases.

[0025] It is typically much more difficult to transduce quantum signals, which involves preserving the information encoded as a superposition of states (and thus avoiding decoherence), than to transduce classical signals. Indeed, in contrast to classical transduction, which can rely on amplification or signal replication without loss of information, quantum transduction can involve enabling a coherent mapping between quantum states of distinct physical modes. According to the no-cloning theorem, an arbitrary quantum state cannot be copied or measured without disturbing its superposition. Consequently, quantum transducing involves preserving both the amplitude and phase coherence of the input state while avoiding the introduction of noise or decoherence that would render the process effectively classical. This requirement makes quantum signal transduction intrinsically more challenging than classical signalconversion, as it precludes amplification and monitoring during operation and demands high efficiency and minimal added noise.

[0026] Technically, the transduction of quantum signals can involve the communication of quantum information between one or more first photons at a first energy level and one or more second photons at a second energy level. Photon energies are related to photon frequency in accordance with E = hf where E is energy, h is Planck's constant, and f is frequency, and inversely correlated to wavelength in accordance with E = hc / A, where c is the speed of light, and A is wavelength. Transduction of quantum signals may require an intermediary which carries the quantum information between the one or more first photons and the one or more second photons. Indeed, while transferring quantum information between photons of different energies may appear trivial to the non-initiate, the photon energy entails a number of real-world effects which make this non-trivial. For instance, some materials are transparent to photons of some wavelengths, while being non-transparent to photons of other wavelengths. One can think of LIV filters on sunglasses as an example, which are transparent to visible light while being opaque to electromagnetic radiation in the LIV portion of the electromagnetic spectrum.

[0027] Different parameters may be taken into consideration when performing an assessment of a transducer. These parameters can include transduction efficiency, noise, and bandwidth. Transduction efficiency factors in both coupling coefficient between the one or more first photons and the intermediary and coupling coefficient between the intermediary and the one or more second photons. Transduction efficiency can be related to noise generation, and noise generation can prevent the transmission of quantum information. A better transduction efficiency allows to operate with a smaller number of photons, which can reduce the noise. A technology may provide better transduction efficiency but reduced bandwidth, whereas another technology may offer greater bandwidth but lower transduction efficiency. Finally, some technologies may be efficient in one portion of the electromagnetic spectrum (or band), while not being efficient in one or more other portions of the electromagnetic spectrum, and may be unusable simply for not being adapted to the correct band.

[0028] A Hamiltonian derived from quantum optomechanics, describing the quantum interactions of the optical and microwave fields with a common mechanical resonator, is:where flc, and fl are the resonant frequencies of the optical, microwave, and mechanical resonators, respectively, goandgO lare the radiation pressure coupling constants for the optical and microwave fields, and Eoand E^ are the classical driving field amplitudes for the optical and microwave resonators with carrier frequencies co0and oo . The optical (microwave) field annihilation operator is a(c), with ^representing the annihilation operator for the mechanical resonator.

[0029] An ideal transfer function for microwaves to optical modes is given by t(co) =go.nT^V’correspond to the number of optical and microwave photons respectively.

[0030] The success probability psfor a microwave-optical conversion is given by ps=easier to create single-photon states in the microwave domain than in the optical, this might be a viable path to deterministic optical single-photon provided the mechanics is cooled to its ground state. As it can be understood based on the above, the vacuum optomechanical coupling g and gO flcan constitute key variables to optimize when considering performance.

[0031] The optical and microwave fields can become entangled via the mechanism of two-mode squeezing. This entanglement can be used as the basis of the teleportation scheme between optical and microwave degrees of freedom. The Hamiltonian of such mechanism is given byco, and A = flp- GO are the optical and microwave detunings between the photon and the resonance frequencies.

[0032] Within the interaction picture at opposite detuning Ao= — A^ = ft and, within the rotating wave approximation in a frame rotating at the frequency fl, the approximate interaction Hamiltonian becomes Ha= hg a b ++ ch).

[0033] The second term of the latter equation describes nondegenerate parametric amplification and is responsible for entangling the microwave resonator with the mechanical resonator. The first term corresponds to frequency conversion. When the optical and microwave resonators become entangled, this platform provides a resource for a continuous variable teleportation protocol.

[0034] The frequency shift per meter is quantified by the general first-order dn (x optomechanical coupling strength G, where G =, with ncbeing the resonance frequency of a cavity.

[0035] The optomechanical coupling strength is an important and widely applicable parameter for various type of optomechanical platforms. From it, we define the vacuum optomechanical coupling rate g0= Gxzpm, where xzpmis the zero point fluctuation of the mechanical resonator which has unit of rad / s or Hz. For instance, the optomechanical coupling for the specific case of a Fabry-Perot cavity of length LFPat the first order, meaning x « L, is given

[0036] One approach to quantum transduction between microwave and optical frequencies is to use a mechanical resonator as an intermediary, the mechanical resonator having a resonance mode spatially distributed between two portions which may arbitrarily referred to herein as the first portion and the second portion. The first portion can be electromechanically (capacitively) coupled to a microwave cavity, and the second portion can be optomechanically coupled to an optical cavity.

[0037] Since the resonance frequency of a microwave cavity is given= 1 / VLC, the optomechanical coupling can be expressed in term of the dependence of C and L over x such that

[0038] In the case of a microwave cavity with a mechanically compliant capacitance with gap size d, i.e. coupled via charge-mediated coupling, the optomechanical coupling of a parallel-plate capacitor is limited by

[0039] Even for highly optimized devices, it is challenging to increase gO llbeyond 300 Hz with this approach, due to the challenge to reduce the gap size below 50 nm for parallel plate capacitor. Still, this charge-mediated coupling is one of the most used approaches to perform microwave-to-optical conversion, amplification and teleportation. Accordingly, while this approach can be satisfactory to a certain degree, there can remain room for improvement, namely in terms of improving the coupling efficiency between the mechanical resonator and the microwave cavity.

[0040] Another approach to making a quantum transducer 12, presented in Fig. 2, is to use a mechanical resonator 14 having a resonance mode spatially distributed between, and shared across, two portions 16, 18 which may be arbitrarily referred to herein as the first portion 16 and the second portion 18. The first portion 16 can be magnetomechanically (inductively) coupled to a microwave cavity 20, whereas the second portion 18 can be optomechanically coupled to an optical cavity 22. The microwave cavity 20 can be magnetically sensitive, i.e., its resonance frequency may vary based on changes in magnetic field or magnetic flux. There can be an inductive mediator 24 at the first portion 16, which can be more specifically magnetically coupled to the microwave cavity 20 to drive the change in resonance frequency, e.g. due to the dependency between its movement (i.e., displacement or deformation) and the spatially distributed mechanical resonance mode. Movement such as mechanical displacement of the position, or change in geometry (deformation), can translate into a change in inductance. In some embodiments, the microwave cavity 20 can be communicatively coupled to a microwave transmission line 26, whereas the optical cavity 22 can be communicatively coupled to an optical fiber 28, for example. It was found that thisapproach could significantly improve coupling efficiency over the former approach, at least in some embodiments, potentially being useful in single-photon coupling.

[0041] The microwave cavity 20 can have a resonance frequency between 300 MHz and 300 GHz, between 1GHz and 100 GHz, or between 4 and 9 GHz for instance, whereas the optical cavity can have a resonance frequency between 300 GHz and 30 000 THz, between 1 and 800 THz, or at wavelengths in the infrared region (e.g., at or around 1550 nm) or in the visible region (e.g., between 400 and 700nm) for instance.

[0042] Inductive coupling between a microwave cavity 20 and a mechanical resonator 14 offers a way to increase g04lbeyond 300 Hz. Here, the vacuum oprtomechanical coup rling a may be g aiven

[0043] To achieve inductive coupling, considering the termthe resonance frequencyof the microwave cavity 20 should be sensitive to / dependant on the flux (e.g., magnetically sensitive) or the magnetic field density B, i.e.as is the case for flux or field-dependant inductance L(O, B). Moreover, considering the term p , a displacement in the mechanical position or a change in geometry is to translate into a change in inductance.

[0044] There are various ways in which the quantum transducer 12, and more specifically its elements, namely the mechanical resonator 14, microwave cavity 20, inductive mediator 24, and optical cavity 22, can be embodied, some of which may lead to better performance than others. A first example type of approach is assembly. A second example type of approach is integration. In the assembly type, two or more of the elements, such as the mechanical resonator 14, the microwave cavity 20 and the optical cavity 22, can be manufactured independently from one another as components and subsequently assembled into a hybrid assembly. The assembly type of approach can be used in embodiments where the dimension of the mechanical resonator 14 is in the range of tens or hundreds of microns, for instance. It will be noted that the inductive mediator 24 may be integrated to the mechanical resonator 14 or to the microwave cavity 20 in an assembly type of approach. In the integrated type of approach, a singlecomponent can include the elements. For instance, an integrated circuit can constitute a component which includes several elements. In such an integrated type of approach, the fabrication of a quantum transducer 12 can involve deposition of several layers on a substrate, for instance. In some embodiments, the assembly approach can allow more modularity in terms of optimizing the fabrication of individual components of the quantum transducer 12 independently from one another. In some other embodiments, the integrated approach, when feasible, may allow more efficient, and cost-effective, industrial manufacturing.

[0045] Let us now turn to certain specific examples.

[0046] Firstly, there are various ways of embodying the mechanical resonator 14, including cantilever, membranes (drumheads, trampolines, soft-clamped membranes, phononic crystal membranes), bulk acoustic wave resonators, micro-ring resonators, micro-pillars, optomechanical photonic crystal zipper cavity and optomechanical photonic crystal nanobeam, to name some examples. Some types of mechanical resonators with spatially distributed resonance modes are more adapted to integrated approaches, whereas some others are more adapted to assembly approaches. For instance, Flowers- Jacobs, N.E., Hoch, S.W., Sankey, J.C., Kashkanova, A., Jayich, A.M., Deutsch, C., Reichel, J., and Harris, J. G.E. (2012), Fiber-cavity-based optomechanical device, Applied Physics Letters 101 , 221109 describes an example of an assembly where the mechanical resonator is a membrane which is coupled to an optical cavity. Moreover, Hill, J.T., Safavi-Naeini, A.H., Chan, J., and Painter, O. (2012), Coherent optical wavelength conversion via cavity optomechanics, Nat Commun 3, 1196, describes an example of an integrated embodiment.

[0047] Figs. 3A and 3B present example embodiments of quantum transducers where the mechanical resonator 14 is embodied as a membrane 114A, 114B, of which only the portion coupled to the optical cavity 122A, 122B is shown. In Fig. 3A, the membrane 114A is optomechanically coupled to the optical cavity 122A by way of forming a boundary (e.g. mirror) of the optical cavity 122A in a manner that the resonance of the mechanical resonator 14 moves the boundary of the optical cavity 122A. In Fig. 3B, the membrane 122B is optomechanically coupled to the optical cavity 122B by wayof being made of a dielectric material which moves within the optical cavity 122B. The two latter examples of optical cavities 122A, 122B can have a contact or connector 130A, 130B adapted to connecting to a network element operating in the optical domain (i.e., optical portion of the electromagnetic spectrum).

[0048] There are various ways of embodying a microwave cavity 20 which is sensitive to flux or magnetic field. Fig. 4A presents a first example of a microwave cavity 120A which involves a Superconducting Quantum Interference Device (SQUID). Fig. 4B presents a second example of a microwave cavity 120B which involves high kinetic inductance materials. The two latter examples can alternately be referred to as microwave circuits, and can have a contact or connector 132A, 132B adapted to connecting to a GHz end point. Additional examples may involve nanowires, for instance.

[0049] There are various ways of embodying an inductive mediator 24 at a corresponding portion of the mechanical resonator 14. The corresponding portion of the mechanical resonator 14 can be the portion which is coupled to the microwave cavity 20. A first example, illustrated in Fig. 5A, is to equip the portion 134A with a magnetic material, acting as a magnet 136A. The magnet 136A can be positioned close to the magnetically sensitive microwave cavity 20 in a manner to be magnetically coupled thereto. The movement of the magnet 136A moves its magnetic field, which can change the resonance frequency of the microwave cavity 20.

[0050] A second example, illustrated in Fig. 5B, is to equip the portion 134B with a superconductor 136B, and to apply an external magnetic field. Indeed, the movement of the superconductor 136B, in the presence of the magnetic field, can change the resonance frequency of the microwave resonator 20 based on the Meissner effect. Through the Meissner effect, the superconductor 136B generates an opposite magnetic field which can be similar in function to the moving magnet approach explained in relation with Fig. 5A. The superconductor 136B can be positioned close to the microwave cavity in a manner to be magnetically coupled thereto. The two former embodiments can be said to have a magnetically functionalized mechanical resonator 14. In the first case, the inductive mediator 14 generates a magnetic field, whereas in the second, the inductive mediator 14 alters the magnetic field.

[0051] A third example, illustrated in Fig. 5C, is to equip the portion 134C of the mechanical resonator with a conductor segment 138 of a microwave circuit 136C (in which case the conductor segment may be referred to as a “wiggly arm” of the microwave circuit), in a manner for the resonance mode of the corresponding portion 134C of the mechanical resonator 20 to deform the conductor segment 138 when it resonates. The deformation of the conductor segment 138 can be configured in a manner to change the resonance frequency of the microwave circuit 136C, for instance.

[0052] In yet another approach, an example of which will be detailed below with reference to Fig. 8, magneto elasticity (i.e. a magnetoelastic material) may be used. Indeed, in a process similar to piezoelectricity, a material can generate a magnetic field when displaced, deformed or put under stress. Magnetoelasticity is sometimes alternately referred to as magnetostriction and can involve the Villary effect. Accordingly, a varying magnetic field can be generated via mechanical coupling between the mechanical resonator and a magnetoelastic component.

[0053] Accordingly, the inductive mediator 24 can alternately be an elastomagnetic material (e.g., Terfenol-D, Galfenol, Nickel, Cobalt, or Cobalt- Iron alloys, Metglas ®).

[0054] In some embodiments, an example of which is presented in Fig. 6, a quantum transducer 212 can have a mechanical resonator 14 embodied as a membrane 214. The membrane 214 can have two portions 234A, 234B, including a first portion 234B which is positioned in the microwave cavity 220, and a second portion 234A which is positioned in the optical cavity 222.

[0055] In such embodiments, or others, an underlying portion of the mechanical resonator 214 can be made of an optically transparent dielectric material such as silicon nitride, or a sufficiently thin layer of an otherwise non-transparent dielectric material to give some examples, and the inductance mediator 224 (e.g., magnet or superconductor) can be applied as a coating or otherwise adhered thereto. The coating may be applied on a face of the portion 234B which faces the microwave cavity for instance, or on the back face (e.g., in some embodiments where the mechanical resonator is thin). Components such as a mechanical resonator 214 can be manufactured independentlyfrom a microwave cavity 220 (e.g., such as shown in Fig. 4A and 4B), and independently from an optical cavity 222 (e.g., such as shown in Fig. 3A and 3B), and be assembled to one another. In such an embodiment, the manufacturing process of each component may be optimized independently of the manufacturing process of the other components.

[0056] In some other embodiments, an example of which is presented in Fig. 7, a quantum transducer 312 can be embodied in the form of an integrated circuit. In the example presented in Fig. 7, the quantum transducer 312 is has an optomechanical photonic crystal zipper cavity. More specifically, a SQUID 340 is integrated to a microwave cavity 320, the microwave cavity 320 itself is coupled to a RF coplanar waveguide 342, and to an optical cavity 322. The mechanical resonator embodied as a phononic crystal, and photonic crystals are also integrated, together with a coupling waveguide 344. In this embodiment, the inductive mediator 324 is of the wiggly arm type, and more specifically in the form of a portion of the microwave circuit 320 which is mechanically deformable by action of the mechanical resonator 314.

[0057] Fig. 8 presents another example of a quantum transducer 412. In this example, the quantum transducer 412 is embodied as an integrated circuit and can be said to be of an optomechanical photonic crystal cavity type. The inductive mediator 424 can take the form of an integrated magnet, superconductor, or magnetoelastic portion. A phononic and photonic crystal can offer a spatially extended mechanical mode 450 extending, on the one hand, to the inductive mediator 424 and on the other hand, to an integrated, colocalized, photonic and phononic cavity 452, based on a photonic and phononic crystal 454. A coupling waveguide 444 with a photonic crystal 446 can also be integrated. An embodiment such as presented in Fig. 8 can be a way of achieving an integrated equivalent to an assembly such as the one presented in Fig. 6, for instance, or as a way of integrating a magnetoelastic inductive mediator, for instance.

[0058] In some other embodiments, operability may be achieved by using a membrane equipped with an elastomagnetic non-metal.

[0059] Quantum transducers such as described above and exemplified in relation with Figs. 2 to 8 may have applications outside quantum networking.

[0060] For instance, with reference to Figs. 9 and 10, two, or more than two, quantum transducers 12, 112, 212, 312, 412 such as described above can be combined in a cascade configuration to realize a quantum converter 500, 600 (e.g., instead of being coupled to an end point of a quantum network). In some embodiments, a quantum converter 500, 600 can transfer the quantum information from an optical input to an optical output in the terahertz frequency range, for example.

[0061] In another example, presented in Fig. 11 , a quantum transducer 12 can be used in the context of a single-photon, or n-photon source 700, such as to transduce an output of a 1-photon or n-photon qubit 712 (which can be in the GHz frequency range for example), in which quantum information is in encoded in the form of a superposition of states, to a single-photon or n-photon output 762 (e.g., in the optical frequency range), in a manner to preserve the quantum information. In such an embodiment, the qubit 760 can be used for state-preparation of one or more photon in the microwave frequency, for instance. Such a single-photon, or n-photon source 700 may find uses in photonic quantum computing or in quantum cryptography, for instance. In another example embodiment, a quantum transducer 12 such as presented above can be integrated to a single or n-photon source such as shown in Fig. 11 to produce an output in the terahertz frequency range.

[0062] In still another example, a quantum transducer such as described above and exemplified in relation with Figs. 2 to 8 may be coupled to a quantum memory, and used for storing a qubit, for example.

[0063] As can be understood, the examples described above and illustrated are intended to be exemplary only. For instance, while the microwave cavity can operate at frequencies between 4 and 9 GHz in some embodiments, which can be the case of superconducting qubits for instance, other embodiments may operate within other frequencies. Notably, atomic qubits may operate in visible wavelengths, between 400 and 700 nm for instance. More broadly, microwave cavities in different embodiments may operate between 1 GHz and 100 GHz, or even between 0.3 GHz and 300 GHz. Moreover, while the optical cavity may operate at telecom frequencies of 1550 nm, it may more broadly operate between 1 THz and 800 THz, or even more broadly between300 GHz and 30 000 THz. Moreover, different embodiments can use different types of materials or different types of components. For instance, Silicon Nitride (Si3N4) and Silicon (Si) are commonly used for high-performance optomechanical resonators due to their high Q-factors and compatibility with photonic systems. Diamond, Gallium Arsenide (GaAs), and Aluminum Nitride (AIN) are used for specialized applications requiring specific optical or mechanical properties. Graphene and other 2D materials are popular in experimental settings for their unique properties. The scope is indicated by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A quantum network element communicatively connected to an end node of a quantum network, the quantum network element having: a microwave cavity being magnetically sensitive and connected to the end node, an optical cavity, and a mechanical resonator having a resonance mode spatially distributed between a first portion and a second portion, an inductive mediator at the first portion, the first portion magnetomechanically coupled to the microwave cavity, and the second portion optomechanically coupled to the optical cavity.

2. The quantum network element of claim 1 wherein the quantum network element is communicatively connected between the end node and a router, wherein the optical cavity is connected to the router.

3. The quantum network element of claim 2 wherein the optical cavity and the router operate at frequencies between 1THz a 800 THz.

4. The quantum network element of claim 2 wherein the optical cavity and the router operate at a wavelength of 1550 nm.

5. The quantum network element of claim 1 or 2 wherein the end node and the microwave cavity operate at frequencies between 4 and 9 GHz.

6. A method of communicating quantum information comprising: propagating one or more microwave photons into a microwave cavity, the microwave cavity being magnetically sensitive, the one or more microwave photons carrying quantum information in the form of a superposition of states; transducing the one or more microwave photons in the microwave cavity into one or more phonons in a mechanical resonator, viamagnetomechanical coupling between the microwave cavity and an inductance mediator at a first portion of the mechanical resonator; distributing the one or more phonons between the first portion of the mechanical resonator and a second portion of the mechanical resonator; transducing the one or more phonons in the mechanical resonator into one or more optical photons in an optical cavity via optomechanical coupling between the mechanical resonator and the optical cavity; and propagating the one or more optical photons away from the optical cavity, the one or more optical photons then carrying the quantum information in the form of the superposition of states.

7. A quantum transducer comprising: a microwave cavity being magnetically sensitive; an optical cavity; and a mechanical resonator having a resonance mode spatially distributed between a first portion and a second portion, an inductive mediator at the first portion, the first portion magnetomechanically coupled to the microwave cavity, and the second portion optomechanically coupled to the optical cavity.

8. The quantum transducer of claim 7 wherein the microwave cavity, optical cavity and mechanical resonator are individual components assembled to one another.

9. The quantum transducer of claim 8 wherein the mechanical resonator is a membrane.

10. The quantum transducer of claim 9 wherein the membrane has dimensions in the hundreds of microns.11 . The quantum transducer of claim 9 or 10 wherein the membrane is made of silicon nitride.

12. The quantum transducer of any one of claims 9 to 11 wherein the inductive mediator is a layer of magnetic material covering the first portion of the membrane.

13. The quantum transducer of any one of claims 9 to 11 wherein the inductive mediator is a layer of superconducting material covering the first portion of the membrane.

14. The quantum transducer of claim 7 wherein the optical cavity and the mechanical resonator are elements of an integrated circuit, the mechanical resonator forming part of a photonic and phononic crystal.

15. The quantum transducer of claim 14 wherein the microwave cavity is also an element of the integrated circuit in the form of a microwave circuit, wherein the photonic and phononic crystal is mechanically coupled to a portion of the microwave cavity, the mechanical resonator mechanically coupled to a deformable portion of the microwave circuit, the deformable portion acting as the inductive mediator.

16. The quantum transducer of claim 14 wherein the inductive mediator is an element of the integrated circuit and is provided in the form of one of a magnet, a superconductor, and a magnetoelastic element.

17. The quantum transducer of claim 7 wherein the inductive mediator is one of a magnet and a superconductor, and the inductive mediator is moved by movement of the mechanical resonator.

18. The quantum transducer of any one of claims 7 to 17 wherein the microwave cavity has a superconducting quantum interference device (SQUID).

19. The quantum transducer of any one of claims 7 to 17 wherein the microwave cavity has a portion made of high kinetic inductance material magnetomechanically coupled to the first portion of the mechanical resonator.

20. The quantum transducer of any one of claims 7 to 19 wherein the second portion of the mechanical resonator is configured to move within the optical cavity.21 . The quantum transducer of any one of claims 7 to 19 wherein the second portion of the mechanical resonator forms a boundary of the optical cavity.

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