System for providing an EPR quantum channel

Abstract

Disclosed is a system for providing an EPR quantum channel, comprising: • an emitter of entangled photons comprising a source configured to generate a pair of entangled photons, • a first receiver and a second receiver each comprising: - a unit for storing qubits, which is arranged to store the qubit carried by the received photon, - a photon switch arranged to send the received photon to the storing unit, - a detector of passage of photons, which is configured to control the photon switch and note the time of reception of the photon, • an information processor arranged to control the photon switch of the receiver, • a communication system linking the two receivers to each other, which is arranged to communicate the times of reception of the photons, in order to determine the pairs of entangled photons the two photons of which each reached the receiver to which they were sent.

Description

[0001] Description

[0002] Title: EPR quantum channel provision system

[0003] Technical field

[0004] The present invention relates to quantum communication methods, and more particularly those allowing the teleportation of quantum information.

[0005] Prior art

[0006] The transmission of information over long distances is currently reserved for classical information and not for quantum information, called Qubits (or sometimes Qubits). However, the transmission of Qubits over significant distances would make it possible to link quantum computers together. This would allow, for example, low-power computers to have their calculations performed by more powerful computers, while maintaining the confidentiality of the transmitted and calculated data. This would also make it possible to take advantage of a network of quantum computers to overcome the short lifespan of quantum memories, often on the order of a second, by working at the same time to populate a quantum database together which, grouped in a central computer, could be queried.

[0007] Over short distances, quantum teleportation would allow the implementation of quantum computers allowing, for example, the transfer of Qubits and the control of the transfer of Qubits to units allowing quantum operations to be carried out.

[0008] In quantum cryptography, it is known to use a pair of entangled photons to generate and transmit a code securely.

[0009] Entangled photons are photons whose quantum state of one photon in the pair, for example the polarization, depends on the quantum state of the other photon in the pair, regardless of the distance separating them. The phenomenon of quantum entanglement has been observed and demonstrated experimentally several times, for example in the article Shashi Prabhakar et al., Two-photon quantum interference and entanglement at 2.1 / um, Sci. Adv. 6, eaay 5195 (2020).

[0010] The article Wang, R. (2005), Superluminal telecommunication: an observable contradiction between quantum entanglement and relativistic causality, arXiv: Quantum Physics, describes a quantum communication system allowing superluminal information transmission thanks to a pair of entangled photons sent to two receivers wishing to communicate with each other. The photons are entangled, a linear polarization is measured for the photon arriving at the first receiver from different directions at 45° to each other. At the second receiver, the second photon is demultiplied and the average polarization of the demultiplied flux is measured to determine the direction of the measured polarization of the first photon. This system does not, however, allow the transfer of quantum states.

[0011] The polarization of a photon is not necessarily binary like that of a spin or even quaternary. It can notably be represented on the Jones sphere which characterizes the orientation and the ellipse of the polarization: a polarization can in fact be linear, the electric field always being parallel to an axis perpendicular to the direction of propagation of the photon, or circular, the electric field rotating around this axis, or between the two: the electric field traveling an ellipse around the propagation axis. In the case where the polarization is not circular, the orientation measures the direction of this axis, and the eccentricity measures the flattening of the ellipse. Patent FR3125658 exploits this property and makes it possible to transfer non-quantum information at speeds close to that of light, over large distances, for example more than 200km, using optical fibers or photon transmissions through space.

[0012] Furthermore, quantum information teleportation systems exist, such as the system for teleporting a Qubit between a photon and a matter atom described in the article Long distance multiplexed quantum teleportation from a telecom photon to a solid-state qubif Nature communications (2023). However, these systems are limited to short distances and the teleportation is not carried out between two matter atoms but between a photon carried by an optical fiber and a photon stored in a crystal.

[0013] Additionally, existing quantum information teleportation systems are limited by photon losses incurred during their transport and are limited in transmission speed by the photon transmission speed.

[0014] Finally, the article Nondestructive detection of photonic qubits, Journal Nature (2021), presents a photon detection method that does not destroy the detected photon, however this article does not describe its use to enable the teleportation of Qubits.

[0015] Disclosure of the invention There is a need to further improve quantum communication systems and methods, in particular those making it possible to provide an EPR (Einstein-Podolski-Rosen) quantum channel to users up to several hundred kilometers away if optical fibers are used, or even further, in particular if optical links through free space are used.

[0016] Summary of the invention

[0017] The invention aims to meet this need, and it achieves this, according to a first of its aspects, thanks to a system for providing an EPR quantum channel comprising:

[0018] • an emitter of entangled photons comprising a source configured to generate at least one pair of entangled photons comprising a first photon emitted on a first propagation path, and, preferably simultaneously, a second photon emitted on a second propagation path different from the first propagation path,

[0019] • a first receiver arranged on the first propagation path comprising:

[0020] - at least one Qubit storage unit arranged to store the Qubit carried by the photon received by the first receiver,

[0021] - a photon switch arranged to send the photon received by the first receiver to said storage unit, or optionally to another path which can lead to a storage unit or a photon discharge path,

[0022] - a photon passage detector configured to control the photon switch and note the time of reception by the first receiver of the photon, and optionally the identifier of said storage unit to which the photon is sent if applicable,

[0023] • a second receiver arranged on the second propagation path so as to be reached by the second photon, said second receiver comprising:

[0024] - at least one Qubit storage unit arranged to store the Qubit carried by the photon received by the second receiver,

[0025] - a photon switch arranged to send the photon received by the second receiver to said storage unit, or optionally to another path that can lead to a storage unit or a photon discharge path, - a photon passage detector configured to control the photon switch and note the time of reception by the second receiver of the photon, and optionally the identifier of said storage unit to which the photon is sent if applicable,

[0026] • at least one information processor included in the first and / or the second receiver, for example an electronic microprocessor, arranged to control the photon switch of said receiver,

[0027] • a communication system linking the two receivers together, said communication system being arranged in particular to communicate to said information processor the times of reception of the photons at the first and second receivers, in order to determine the pairs of entangled photons of which the two photons have each reached the receiver to which it was sent.

[0028] The system for providing an EPR quantum channel according to the invention makes it possible to transmit information over long distances, in particular distances greater than 100 km, for example distances between 100 km and 400 km using optical fibers, or more, in particular using optical communications through space.

[0029] Since the distance between the receivers and the transmitter is very large, photon losses during transmission are significant. In optical fibers, photon losses are approximately 0.2 dB / km. The Qubit storage units of the two receivers allow only the Qubits carried by the photons received by a first receiver to be stored for a waiting period, the waiting period corresponding to the time required to know whether the Qubit carried by the photon entangled with said photon has been received by the other receiver and stored there, added to a period during which said first receiver makes said Qubit received by the first receiver available to the user.

[0030] The system according to the invention thus makes it possible to transmit information with a minimized error rate, the majority of error sources being eliminated.

[0031] This system can enable quantum information to be transmitted with a latency independent of the distance between the two locations wishing to communicate, and using only photons, which requires little energy. The system is used, for example, in communication networks and / or quantum computer networks, on Earth, in the sky and / or in space, in particular to communicate with systems far from Earth, such as satellites or spacecraft.

[0032] The first and / or the second receiver may comprise at least one additional path, in particular downstream of the switch, which may lead to a storage unit or a photon evacuation path.

[0033] Each of the receivers preferably has at least one interface, for example electronic, allowing it in particular to communicate with the outside world and / or allowing it to display the presence, in one of its Qubit storage units, of a Qubit entangled with a Qubit carried by a storage unit of the other receiver.

[0034] Photon storage unit

[0035] A storage unit of the first and / or second receivers can be chosen from optical paths, in particular optical fibers or passages in space arranged using mirrors, or vectors of physical Qubits, in particular ions interacting indirectly with the incident photons.

[0036] A photon storage unit of the first and / or second receivers may be located upstream of the photon switch of the first and / or second receivers, respectively. One or more photon storage units of the first and / or second receivers may be located downstream of the photon switch of the first and / or second receivers, respectively.

[0037] At least one storage unit may be an optical fiber, the photon received in the optical fiber being stored therein for the time of its travel in said optical fiber. The length of the optical fiber is preferably small enough that there are few photon losses inside, in particular less than 1% photon losses, and large enough to be able to store the photon for a duration allowing a first receiver to determine whether the other receiver has received the entangled photon. The length of the optical fiber may be between 0.2 m and 500 m. The length of the fiber is for example substantially equal to 200 m, which makes it possible to store the photon for approximately 1 microsecond.

[0038] A storage unit of the first and / or second receivers may be a physical Qubit vector, for example an atom, in particular located in an optical cavity. A physical Qubit vector may be physically displaced to be replaced by another physical Qubit vector, preferably initialized before or after its displacement in a proper state allowing it to interact with a new incident photon. A physical Qubit vector may be stimulated to emit a photon with which it is entangled before being reset to be ready to interact with a new photon coming from the entangled photon emitter, said photon with which it is entangled interacting in particular with the control port of a CNOT gate. Said displacement or said stimulation of the physical Qubit vector is advantageously controlled by an external device.

[0039] A storage unit of the first and / or second receivers is preferably arranged to be made available to store a new Qubit, at the request of one of the information processors, or periodically, in particular if the Qubit storage unit requires manipulation to be able to memorize a new Qubit.

[0040] If the Qubit storage unit can detect the reception of a photon, the said Qubit storage unit can also detect the passage of photons.

[0041] The duration for which each storage unit is capable of retaining a Qubit is preferably greater than the time difference between the arrivals at the two receivers of the two entangled photons, to which are added the time taken by the communication system to communicate information on the arrival times of the entangled photons, the time required for the processors to decide on the presence of two entangled Qubits in one of the storage units of the receivers, and the time taken by the processors to inform the interfaces.

[0042] The communication system can be arranged to communicate information faster than the speed of photon transport in the optical fiber. This can reduce the number of storage units required to store each of the Qubits carried by the photons for the same reception rate of said photons. This can also reduce the time during which a Qubit is stored in a storage unit. This can therefore allow the use of optical fibers as storage units, and / or increase the fidelity of the stored Qubits.

[0043] The EPR quantum channel provision system can include a single Qubit storage unit per receiver.

[0044] Each Qubit storage unit can have an identifier. Photon switch

[0045] The photon switch of the first receiver and / or the second receiver may be an optical switch.

[0046] The photon switch of the first receiver and / or the second receiver may comprise a mirror whose direction of F axis may be mechanically controlled by a sensor or an electrically controlled device, or, alternatively, comprise a prism or a blade made of a material whose refractive index depends on an electric field: a Pockels cell, or a transparent material with a non-linear refractive index, another light flux, for example transverse and preferably of a wavelength different from that of the photon and not being able to generate photons of the same wavelength as that of the photon, varying the refractive index of said non-linear material and thus controlling the location and possibly the direction of exit of the photon from said material.

[0047] The photon switch of the first receiver and / or the second receiver may be an optical microresonator, i.e. a cavity formed of an optical medium surrounded by reflective surfaces, in particular configured to serve as an optical CNOT gate as described below, into which atoms to interact with each photon are successively brought. At least two holes may be formed in the cavity to allow at least two light rays to pass through, serving as optical tweezers and making it possible to bring and position the atom within the cavity. The use of optical tweezers makes it possible in particular for an atom to be moved after interacting with the first of two photons to perform a CNOT operation. The atom may in particular be moved to another cavity and replaced by a new atom, this making it possible to receive a new photon carrying a Qubit used as a control Qubit for another CNOT gate.

[0048] The entry and exit of photons into the microresonator is done in particular by optical fibers whose end is cut into the shape of a lens.

[0049] Alternatively, the photon switch of the first receiver and / or the second receiver is an acousto-optic deflector. The acousto-optic deflector may comprise an acousto-optic material belonging in particular to the group consisting of: quartz glass, lithium niobate, arsenic trisulfide, tellurium dioxide, more generally tellurium glasses, flint glass, calomel, lead (II) bromide. Alternatively, the acousto-optic material is water. The acousto-optic deflector may comprise electroacoustic transducers, in particular piezoelectric materials. The acousto-optic deflector has its refractive index varying as a function of the sound wave propagating in the deflector. The direction of transmission of the photon by the acousto-optic deflector depends on the refractive index of the deflector, and therefore on the sound wave propagating inside it.The acousto-optic deflector therefore makes it possible to send the photon to a predefined storage unit, using a suitable sound wave.

[0050] The photon switch of the first receiver and / or the second receiver advantageously has the possibility of directing a received photon towards a photon absorber.

[0051] Photon passage detector

[0052] The photon passage detector of the first and / or second receiver may comprise a non-destructive photonic Qubit detector, for example using parametric conversion of the incoming photon, which produces two photons of smaller frequencies from a single photon, where one of the produced photons announces the presence of a qubit and the other provides the information on said qubit, or, alternatively, a photon detector as described in the article Niemietz, D., Farrera, P., Langenfeld, S. et al., Nondestructive detection of photonic qubits, Nature 591, 570-574 (2021). The non-destructive photonic Qubit detector makes it possible to detect the passage of the photon without absorbing it, therefore without destroying its properties.

[0053] The non-destructive photonic Qubit detector can include a QED cavity in which a rubidium atom is trapped. This atom has two interesting energy levels defining observable values ​​and is placed in a quantum superposition state of these two values. When the photon is reflected by the cavity, it can modify the quantum superposition state of the atom by making it undergo a Pauli z gate and therefore leaves a measurable trace. The detection of this change of state can be done by applying a microwave pulse which rotates the quantum state of Pi / 2 to put it either in the observable state 1 if no Pauli z gate has been applied, or in the state 0 if a Pauli z gate has been applied.By measuring the change in state of the atom, we can then detect the passage of a photon without destroying it, the application of electromagnetic microwave pulses making it possible to return the atom to its initial superposition state after the modification of its superposition state has been detected.

[0054] Alternatively, the photon passage detector of the first and / or second receiver may comprise a device as described in the article Brekenfeld, M., Niemietz, D., Christesen, J.D. et al. A quantum network node with crossed optical fibre cavities. Nat. Phys. 16, 647-651 (2020), the device storing the Qubit transported by a photon incident on an atom of matter and making it possible to detect said storage.

[0055] The photon passage detector of the first receiver and the photon passage detector of the second receiver may each comprise a clock, the two clocks being synchronized with each other. The photon passage detectors of the first and second receivers may thus note the time of reception of the photons by the first receiver and the second receiver respectively.

[0056] Preferably, the clocks of the two receivers are configured to account for the phenomenon of different flow times at different locations, including different altitudes of each of the components.

[0057] The time accuracy of a photon detector can be improved by using several photon detectors to which the arriving photons are successively directed using a photon switch, another photon switch advantageously making it possible to return the photons having passed through the various photon detectors on the same optical path. A clock can, for example, control these switches to change the photon detector used at regular time intervals.

[0058] If, for example, the time precision of the photon detector is tl, tl corresponding to the inverse of its operating frequency, i.e. for example the frequency of verification of the quantum state of the atom if it is a cavity detector as described above, and the required time precision is t2 less than tl, it is possible to use tl / t2 photon detectors (or its higher integer) and to orient the photons towards a different photon detector every t2. This allows the photon detector to have the time tl to detect or not the passage of a photon. At the end of time tl, a photon having possibly been detected, the quantum state of the photon detector can be reset to allow the detection of a new photon.

[0059] Information processor

[0060] The two receivers may each have at least one information processor.

[0061] The or one of the information processors having identified a pair of photons of which the two entangled photons have each reached a receiver, can inform the interfaces of each of the receivers of the identifiers of the storage units memorizing the Qubits carried by said entangled photons.

[0062] The or one of the information processors having identified a pair of entangled photons of which at least one of the photons has not reached one of the two receivers, can make available for the memorization of a new Qubit the storage unit in which the Qubit carried by the other photons has been memorized and, if necessary, can command the operation allowing it to be made available.

[0063] The information processor(s) can command the receiver furthest from the photon emitter to direct the photon whose entangled photon was not received by the other receiver to a photon absorber.

[0064] Communication Unit

[0065] The photon passage detector of the first receiver may comprise a communication unit and the photon passage detector of the second receiver may comprise a communication unit, the two communication units preferably communicating with each other via the communication system.

[0066] The exchange of photon reception times allows each of the receivers having received a first photon to determine whether the other receiver has received the photon entangled with this first photon.

[0067] The communication system advantageously allows information to be transmitted between each of the receivers faster than the transit speed of photons between the transmitter and each of the receivers. The quantum communication system is, for example, that described in patent FR3125658. Alternatively, the communication system is an optical communication system, in particular by optical fiber, a radio communication system, or a power line communication system.

[0068] A high speed of the communication system can reduce the number of storage units of the receiver. In particular, if the time taken by the two receivers to communicate via the communication system, added to the time taken to control the switches, is less than the time separating the arrival of two photons at each of the receivers, the number of Qubit storage units can be reduced to one, the storage unit then being located upstream of the photon switch, making the connection between the photon passage detector and the photon switch.

[0069] If the information communication system does not allow information to be transmitted between each of the receivers faster than the transit speed of photons between the transmitter and each of the receivers, the system for providing an EPR quantum channel includes at least two Qubit storage units per receiver.

[0070] The transit time of the photon in said photon storage unit can be extended, in particular by using a storage unit with a high refractive index, or with a length greater than the distance separating the photon passage detector and the photon switch.

[0071] The communication units advantageously include means of exchanging classical information between the two receivers, allowing them in particular to exchange the classical bits necessary for the implementation of quantum teleportation.

[0072] Control unit

[0073] The photon passage detector of the first receiver may comprise a control unit, in particular configured to control the photon switch of the first receiver.

[0074] The photon passage detector of the second receiver may comprise a control unit, in particular configured to control the photon switch of the second receiver.

[0075] The control unit of the first and / or second receivers may comprise the information processor(s).

[0076] The control unit is, for example, configured to control the current sent to the electroacoustic transducers of the photon switch, in order to modify the refractive index of the acousto-optic material and thus the direction of transmission of the photon.

[0077] The control unit of the first receiver may be configured to command the photon switch of the first receiver to send the photon to a first vector of Qubits external to the system, if the photon passage detector of the first receiver receives the information that the second receiver has received the photon entangled with said photon.

[0078] The control unit of the first receiver may be configured to command the photon switch of the first receiver to send the photon to a photon escape path of the first receiver, if the photon passage detector of the first receiver receives information that the second receiver has not received the photon entangled with said photon.

[0079] The control unit of the second receiver may be configured to command the photon switch of the second receiver to send the photon to a second vector of Qubits external to the system, if the photon passage detector of the second receiver receives the information that the first receiver has received the photon entangled with said photon. The control unit of the second receiver may be configured to command the photon switch of the second receiver to send the photon to a photon escape path of the second receiver, if the photon passage detector of the second receiver receives the information that the first receiver has not received the photon entangled with said photon.

[0080] Anti-reflective coatings

[0081] To avoid the loss of photons, anti-reflection layers are preferably provided at the interfaces between adjacent transparent media of different indices crossed by the photons, as well as at the interfaces of prisms and birefringent plates crossed by photons, the anti-reflection layer being preferably adapted to the index(es) of the material and to the angle(s) of incidence and direction(s) of polarization as well as to the wavelength of the photon which must cross it.

[0082] Dichroic filters

[0083] The first and / or the second receiver preferably comprises one or more dichroic filters allowing only photons of a given wavelength to pass through, in particular a prism made of a transparent dispersive material, and this in particular if the refractive indices of the non-linear materials are modified by the application of powerful light fluxes.

[0084] Methods for providing an EPR quantum channel

[0085] The invention also relates to a method for providing an EPR quantum channel using the system for providing an EPR quantum channel defined above, comprising the steps of: (a) Generating a pair of entangled photons from a transmitter, the first photon of the pair being emitted towards a first receiver and the second photon of the pair being emitted, preferably simultaneously, towards a second receiver, the first and second photons being entangled,

[0086] (b) The possible detection of the arrival of a photon by a photon passage detector at each of the receivers,

[0087] (c) Communication of photon reception times between the two photon passage detectors of the first and second receivers,

[0088] If the communication between the two photon passage detectors is faster than the time taken by the photon received by said receiver to transit from the photon passage detector to the photon switch of this receiver, the method comprises:

[0089] (d) Transit of the photon from the photon passage detector to the photon switch through a single temporary storage unit of the receiver,

[0090] If the photon passage detector having received the photon receives the information that the other photon passage detector has received the photon entangled with said photon, the method comprises:

[0091] (dl) Sending the photon by the photon switch to a vector of Qubits outside the system,

[0092] If the photon passage detector having received the photon receives the information that the other photon passage detector has not received the photon entangled with said photon, the method comprises:

[0093] (d2) Sending the photon by the photon switch to a photon escape path,

[0094] If the communication between the two photon passage detectors is slower than the time taken by the photon received by said receiver to transit from the photon passage detector to the photon switch of this receiver, the method comprises:

[0095] (e) The transit of the photon from the photon passage detector to the photon switch and the sending by the photon switch of the photon to a Qubit storage unit, If the photon passage detector having received the photon receives the information that the other photon passage detector has received the photon entangled with said photon, the method comprises:

[0096] (el) Sending the photon by the switch of the Qubit storage unit to a Qubit vector outside the system if the Qubit storage unit is an optical device or, if the Qubit storage unit is not an optical device, making the Qubit contained in the Qubit storage unit available to a user, said making available being for example announced to the user by the receiver electronics,

[0097] If the photon passage detector having received the photon receives the information that the other photon passage detector has not received the photon entangled with said photon, the method comprises:

[0098] (e2) Sending the photon from the optical Qubit storage unit's photon switch to a photon escape path, or if the Qubit storage unit is not an optical device, resetting the Qubit storage unit so that it is ready to receive a new photon.

[0099] Transmission of

[0100] Photons can be counted as they arrive at a photon passage detector. If the counting reveals the arrival of more than one photon during a predefined time interval, at least some of these photons can be transmitted during this interval to the photon escape path.

[0101] Together for TV

[0102] The invention also relates, according to another of its aspects, to an assembly for the teleportation of quantum information comprising at least two systems for providing EPR quantum channels as described previously, chained one after the other, and comprising in particular at least one optical CNOT gate between each of the systems.

[0103] The set can include between 1 and 10 EPR quantum channel delivery systems as described above, chained one after the other.

[0104] The set may comprise between 1 and 30 EPR quantum channel provision systems as described above, chained one after the other, if said systems benefit from an error correction system.

[0105] The EPR quantum channel delivery systems in the chain are preferably chained together by identical devices operating according to the same process.

[0106] A first photon exiting a first EPR quantum channel provision system of the set is advantageously sent into the control port of said CNOT gate. A second photon exiting a second EPR quantum channel provision system of the set, following the first system, is advantageously sent into the controlled port of said CNOT gate.

[0107] The assembly may comprise one or more devices arranged to perform a Bell measurement on the first and second photons at the output of said CNOT gate, the first photon passing through a Hadamard gate at the output of the CNOT gate before its Bell measurement is performed. The results of this measurement are preferably sent, in particular using the communication units associated with the two EPR quantum channel provision systems, to a set of unitary quantum gates at the output of the last EPR quantum channel provision system in the chain.

[0108] The set of unitary quantum gates at the output of the last system in the chain is preferably formed of linear optical systems allowing the application to the photon leaving the last system in the chain, either no modification, or a NOT gate, or a succession of gate combinations, in particular a NOT gate followed by a Pauli Z gate.

[0109] The photon leaving the receiver of the last EPR quantum channel provision system in the chain, said receiver not being connected to any other receiver in another system, is advantageously sent into a unitary gate, the unitary gate being the product of the unitary quantum gates obtained by each of the Bell measurements at the interface of each of the EPR quantum channel provision systems in the chain.

[0110] The unitary quantum gates include:

[0111] If Di = 0 and D2 = 0, the unitary quantum gate is the identity,

[0112] If Di = 0 and D2 = 1, the unitary quantum gate is a Pauli z gate,

[0113] If Di = 1 and D2 = 0, the unitary quantum gate is a NOT gate,

[0114] If Di =1 and D2 =1, the unitary quantum gate is a NOT gate followed by a Pauli gate z,

[0115] Di being the measurement of the quantum state of the first photon leaving the CNOT gate through the port associated with the control photon and D2 being the measurement of the quantum state of the second photon leaving the CNOT gate through the port associated with the controlled photon, the measurement D of the quantum state of a photon being for example 1 if it is measured with a vertical polarization and 0 with a horizontal polarization.

[0116] CNOT Gate

[0117] The CNOT gate may include a cavity in which a neutral atom, in particular a Rubidium atom, is trapped.

[0118] The first photon, which is the so-called control photon, can in particular be linearly polarized upon entering the CNOT gate.

[0119] The second photon, which is the so-called controlled photon, is preferably linearly polarized at its entry into the CNOT gate. The second photon possibly passing through the cavity possibly undergoes a phase shift of Pi. The CNOT gate advantageously comprises a first polarization converter making it possible, for example, to transform the polarization of the second photon into circular polarization, in one direction if the photon is horizontally polarized, or in the other direction if it is vertically polarized before sending said photon towards the cavity.

[0120] The CNOT gate may include an orientator arranged to direct the first photon and the second photon towards a polarizing filter.

[0121] The polarizing filter may be arranged to send the first photon to a mirror, in particular if it is vertically polarized, or to a cavity comprising a Rubidium atom, in particular if it is horizontally polarized. The polarizing filter is for example arranged to send the second photon to the mirror if it is vertically polarized, or to the cavity comprising the Rubidium atom if it is horizontally polarized.

[0122] An external device can apply an electromagnetic pulse to the atom so that its quantum state changes by n / 2.

[0123] The polarizing filter can also be arranged to return the first photon and / or the second photon reflected by the mirror and / or the cavity towards the orientator, in particular with the vertical component of the polarization of the second photon without phase change and with the horizontal component shifted from TT, causing the circular polarization of the second photon to change direction.

[0124] The orientator can also be arranged to send the first photon to an output of the CNOT gate.

[0125] The CNOT gate advantageously includes a second polarization converter at the output of the orientator to convert the circular polarization of the second photon into linear polarization.

[0126] The distances between the polarizing filter and the mirror and between the polarizing filter and the cavity can be adjusted so that the optical path of the first and second photons is the same on return to the orientator regardless of where they are reflected, the mirror or the cavity.

[0127] The first and second polarization converters are, for example, quarter-wave plates adapted to the wavelength of the second photon, or quarter-wave plates interposed between two outputs of two multiplexers associated with the same wavelength, and adapted to said same wavelength.

[0128] The orientator can be made of electrically controlled elements such as liquid crystals or Pockels cells, in order to be able to control the trajectory of the photons according to the date of their passage.

[0129] The wavelengths of the two photons can be different. The orientator can include materials and / or dichroic filters.

[0130] Quantum information teleportation process

[0131] The invention also relates to a method for teleporting quantum information using an assembly for teleporting quantum information as defined above, comprising the transmission of a Qubit from a first receiver to a second receiver, the first receiver being called the input receiver and the second receiver being called the output receiver.

[0132] The set for quantum information teleportation advantageously assigns an identifier to each pair comprising the input photon, arriving at the input receiver, and the output Qubit vector, at the output of the output receiver, and makes this identifier accessible to the user of said set, for example by writing it into a memory accessible to users.

[0133] The output receiver may have an indicator to signal to its user(s) the availability of a vector of output Qubits linked to a photon arriving at the input receiver.

[0134] The input receiver advantageously has registers indicating for each input photon whose entangled photon has been identified as having reached the other receiver of the input system, the estimated time of exit from the input receiver of said input photon, so that the user of the input receiver can prepare for its possible manipulation.

[0135] The input receiver also advantageously indicates the time available to the user to manipulate said input photon before its entangled photon interacts with an external physical Qubit vector or leaves said input receiver. The manipulation is for example the passage of the photon in a controlled quantum NOT logic gate (i.e. CNOT) or a Toffoli quantum logic gate.

[0136] If the output Qubit vector is physical and can be consulted, in particular by optical excitation, the output system advantageously places in a memory that can be consulted by the user of the output receiver, the estimated time separating the optical reading command of the output Qubit vector and the arrival of the photon emitted by the output Qubit vector after its optical excitation, the path taken by this photon being able to be different depending on the location of the physical Qubit vector within the output receiver.

[0137] The teleportation can be made between first and second Qubits separated by an EPR quantum channel provision system as described above, or by an assembly for the teleportation of quantum information as described above, the vectors of the first and second Qubits being able in particular to be respectively a photon and a photon, a neutral atom of Rubidium 87and a photon, a neutral atom of Rubidium 87 and another neutral atom of Rubidium 87 .

[0138] Method of long-distance teleportation of an Oubit carried by a Rubidium atom on a photon

[0139] The invention also relates to a method for long-distance teleportation of a Qubit carried by a neutral atom, such as a Rubidium atom, on a photon, using the system for providing a quantum channel described above, comprising the steps consisting of:

[0140] Generating a pair of entangled photons from an emitter, the first photon of the pair being emitted towards a first receiver and the second photon of the pair being emitted, preferably simultaneously, towards a second receiver, the first and second photons being entangled, the photons being in particular of wavelength 780 nm, the first photon encoding a Qubit, in particular according to a linear polarization basis,

[0141] Send the first photon received to the first receiver in a CNOT gate comprising a cavity in which the neutral atom is trapped, in particular as described above,

[0142] Send pulses of electromagnetic waves in the microwave range onto the neutral atom, thus causing it to undergo a Hadamard gate,

[0143] Measure the excitation state of the neutral atom and measure the polarization of the first photon at the output of the CNOT gate,

[0144] Communicate said measurements to the second receiver,

[0145] Depending on the measurements of the states of the neutral atom and the direction of the polarization of the first photon, that is, the state 0 or 1 of the Qubit carried by each, modify the polarization of the second photon.

[0146] If the measurement of the Qubit carried by the neutral atom is 0 and that of the Qubit carried by the first photon is 0, i.e. for example it has a vertical polarization, the second photon does not undergo any operation. If the measurement of the Qubit carried by the neutral atom is 0 and that of the Qubit carried by the first photon is 1, i.e. for example it has a horizontal polarization, the second photon can be sent through a Pauli Z gate.

[0147] If the measurement of the Qubit carried by the neutral atom is 1 and that of the Qubit carried by the first photon is 0, i.e. for example it has a horizontal polarization, the second photon can be sent through a NOT gate.

[0148] If the measurement of the Qubit carried by the neutral atom is 1 and that of the Qubit carried by the first photon is 1, i.e. for example it has horizontal polarization, the second photon can be sent through a NOT gate followed by a Pauli Z gate.

[0149] The NOT gate, for example, consists of two birefringent prisms and two half-wave plates.

[0150] The Pauli Z gate, for example, consists of a half-wave plate.

[0151] Method of teleporting an Oubit carried by a first neutral atom onto a second neutral atom

[0152] The invention also relates to a method for long-distance teleportation of a Qubit carried by a first neutral atom, such as a Rubidium atom, onto a second neutral atom, such as a Rubidium atom, using the system for providing a quantum channel described above, comprising the steps consisting of:

[0153] Generating a pair of entangled photons from an emitter, the first photon of the pair being emitted towards a first receiver and the second photon of the pair being emitted, preferably simultaneously, towards a second receiver, the first and second photons being entangled, the photons being in particular of wavelength 780 nm, the first photon encoding a Qubit, in particular according to a linear polarization basis,

[0154] Excite the first neutral atom, located at the first receiver, until it emits a photon and direct the emitted photon to the control port of a first CNOT control gate before sending it to a first Hadamard gate,

[0155] Send the first photon of the entangled photon pair to the controlled port of the first CNOT gate, Measure the direction of polarization of the photon emitted by the first neutral atom at the output of the Hadamard gate and the direction of polarization of the first photon at the output of the CNOT gate,

[0156] Communicate said measurements to the second receiver,

[0157] According to the measurements of the states of the neutral atom and the direction of the polarization of the first photon, that is to say of the state 0 or 1 of the Qubit carried by each, modify the polarization of the second photon of the pair of entangled photons to the second, Send the modified second photon to the control port of a second CNOT gate before sending it to a second Hadamard gate,

[0158] Excite the second neutral atom, located at the second receiver, until it emits a photon and direct the emitted photon to the controlled port of the second CNOT gate,

[0159] Measure the direction of polarization of the photon emitted by the second neutral atom at the output of the second CNOT gate and the direction of polarization of the second photon at the output of the second Hadamard gate,

[0160] Sending at least one microwave electromagnetic pulse to the second neutral atom to cause it to undergo a unitary quantum gate, characterized in particular as follows: o If the measurement of the direction of the polarization of the photon emitted by the second neutral atom is 0 and that of the polarization of the second photon is 0, the second neutral atom does not undergo, for example, any operation, o If the measurement of the direction of the polarization of the photon emitted by the second neutral atom is 1 and that of the polarization of the second photon is 0, the second neutral atom can undergo a Pauli Z gate, o If the measurement of the direction of the polarization of the photon emitted by the second neutral atom is 0 and that of the polarization of the second photon is 1, the second neutral atom can undergo a NOT gate, o If the measurement of the direction of the polarization of the photon emitted by the second neutral atom is 1 and that of the polarization of the second photon is 1,the second neutral atom can undergo a NOT gate followed by a Pauli Z gate. Alternatively, the entangled photons emitted by the emitter are of a wavelength other than 780 nm and are converted into 780 nm photons before their arrival at the photon receivers by a frequency mixer, as for example described in the article "Entanglement between a trapped ion qubit and a 780-nm photon via quantum frequency conversion" published on October 26, 2022 in Physics review.,

[0161] Method for teleporting Oubits carried by hardware Oubit vectors, in parallel, allowing the implementation of an algorithm for correcting errors during a transmission of Oubits using the communication system

[0162] The invention also relates to a method for teleporting Qubits carried by hardware Qubit vectors, in parallel, allowing the implementation of an algorithm for correcting errors during a transmission of Qubits, the method using the EPR quantum channel provision system described above, the method comprising: the emission of pairs of entangled photons, the first photon of each pair being emitted towards a first receiver and the second photon of each pair being emitted, preferably simultaneously, towards a second receiver, each receiver containing at least one neutral atom, such as a Rubidium atom, for example nine neutral atoms, the neutral atom(s) being arranged to be able to emit photons and undergo unitary quantum gates, each of the atoms of a receiver being paired with a neutral atom of the other receiver.simultaneously, at each receiver, sending photons emitted by one of the neutral atoms of said receiver to a single photon passage detector of said receiver, the single photon passage detector being located upstream of a single CNOT gate of said receiver, until said neutral atom of said receiver has been entangled with the neutral atom to which it is paired in the other receiver, once each pair of neutral atoms is entangled, applying an error reduction algorithm, for example the Schor algorithm.

[0163] Brief description of the drawings

[0164] The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, and by examining the appended drawing, in which: [Fig 1] describes in a schematic and partial manner a system for providing EPR quantum channels according to the invention,

[0165] [Fig 2] schematically and partially describes another system for providing EPR quantum channels according to the invention,

[0166] [Fig 3A] schematically and partially describes an example of a CNOT gate adapted to linearly polarized photons,

[0167] [Fig 3B] schematically and partially describes an example of a CNOT gate adapted to circularly polarized photons,

[0168] [Fig 3C] schematically and partially describes an example of a CNOT gate between two unsynchronized photons having wavelengths close to 780 nm,

[0169] [Fig 3D] schematically and partially describes an example of storage of a Qubit carried by a photon in a Rubidium atom,

[0170] [Fig 3E] schematically and partially describes an example of extraction of a Qubit carried by a Rubidium atom towards a photon,

[0171] [Fig 4] schematically and partially describes a method for teleporting quantum information according to the invention, from a quantum memory consisting of a Rubidium Rb atom located in a first receiver, to a second receiver,

[0172] [Fig 5] schematically and partially describes a method of teleporting quantum information according to the invention, to a quantum memory consisting of a neutral atom,

[0173] [Fig 6] schematically and partially describes a set for the teleportation of quantum information according to the invention,

[0174] [Fig 7] schematically and partially describes a method of parallel teleportation of nine Qubits carried by hardware Qubit vectors, allowing the implementation of the Schor algorithm for error correction during a Qubit transmission using the system of figure 1,

[0175] [Fig 8] schematically and partially describes a process for generating entangled photons according to their polarization,

[0176] [Fig 9] schematically and partially describes a method of measuring a result of quantum information teleportation, [Fig 10] schematically and partially describes the transfer of a database stored in a computer memory to a quantum memory of the same computer, and

[0177] [Fig 11] schematically and partially describes an optical cavity with interchangeable atoms.

[0178] Detailed description

[0179] Figure 1 illustrates a system 1 for providing EPR quantum channels according to the invention. The system comprises an emitter 2 emitting a pair of entangled photons (P1, P2), the first photon P1 propagating on a propagation path C1 and the second photon C2 propagating on a propagation path C2 different from the path C1.

[0180] Due to the nature of light and the wave-particle duality of the photon, the terms "wave", "electromagnetic wave", "photon" and "particle" are used interchangeably in the following to designate the product emitted by emitter 2.

[0181] The system 1 comprises a first receiver 3, located on the propagation path C1 of the first photon P1, and a second receiver 4, located on the propagation path C2 of the second photon P2. The photons P1 and P2 are preferably emitted simultaneously and the paths C1 and C2 are preferably of the same length, that is to say that the photons P1 and P2 emitted by the transmitter 2 arrive entangled preferably simultaneously at the receivers 3 and 4.

[0182] In the example considered, the receiver 3 comprises a photon passage detector 31, a photon switch 32, photon storage units 33a to 33c, and a photon evacuation path 34.

[0183] The receiver 4 comprises a photon passage detector 41, a photon switch 42, photon storage units 43a to 43c, and a photon escape path 44.

[0184] The photon storage units 33a to 33c and 43a to 43c are, for example, optical fibers, preferably of the same length, in particular of a length between 1 mm and 200 m. The shorter the length of the optical fiber, the faster the photon reaches the user. The longer it is, the longer the time during which the user can be warned of the arrival of the photon. The transmission of the photons P1 and P2 between the transmitter and the first and second receivers respectively can be done in different ways, and in different media. The photons propagate, for example, in an optical fiber or a waveguide, or freely in space, the space being empty or filled with gas.

[0185] Photons can pass through several media, with different refractive indices. For example, anti-reflection plates can be inserted between two media crossed, if necessary, in order to avoid unwanted optical phenomena, in particular the Fresnel reflection of the waves emitted by emitter 2.

[0186] Optical correction devices, not shown, can advantageously be arranged upstream of the photon passage detectors 31 and 4L. The optical correction devices are preferably arranged to correct the distortions that the photons have undergone between the emitter 2 and said optical correction devices. Indeed, the photons may have undergone transformations on their polarization but also on their shape during their journey on the paths C1 and C2. The optical correction devices advantageously have photon phase and shape detectors, in particular used on photons that said detectors can absorb and which are therefore not transmitted to the photon passage detectors 31 or 4L. The photon phase and shape detectors operate for example intermittently on samples of incident photons.

[0187] The wavelength of the emitted wave is, for example, chosen according to the medium(s) to be crossed; for example, photons in the infrared range will preferably be used when they have to pass through the atmosphere or air.

[0188] For example, the transmitter 2 generates entangled photon pairs using the spontaneous parametric conversion (SPDC) method. Alternatively, the transmitter 2 can generate entangled photon pairs via a CNOT gate into which two photons having passed through an optical Hadamard gate are injected. For example, the transmitter 2 is configured to emit on average at least one pair per unit of time, for example every nanosecond, which corresponds to a photon transmission frequency of 1 GHz.

[0189] P1 and P2 photons are polarization-entangled. Alternatively, P1 and P2 photons can be wavelength-entangled, as described in C. Bernhard, B. Bessire, T. Feurer, A. Stefanov, 2013, Shaping frequency entangled qudits, Physical Review A. 88. P1 and P2 photons can also be time-entangled (so-called time-bin entangled photons).

[0190] When emitted, the photons P1 and P2 are, for example, rectilinearly polarized, that is to say that the corresponding electromagnetic wave has an electric field whose direction is perpendicular to their direction of propagation C1 or C2. The direction of polarization is preferably fixed, for example horizontal, so as to allow the devices 30 and 40 to detect and modify the disturbances brought to said polarization during the travel of the photons from the emitter 2.

[0191] The quantum state corresponding to the polarization of photons is sometimes undetermined until it has been measured or absorbed. Before being measured, the quantum state of the photon is therefore sometimes considered as the superposition of possible states, namely, in the example considered, as the superposition of a polarization at an angle of 45° and a polarization at an angle of -45°.

[0192] Alternatively, the emitted PI and P2 photons are circularly polarized, i.e. the direction of the corresponding electric field changes according to a rotational movement, while its norm remains constant.

[0193] In the example considered, the photon switch 32 or 42 is configured to send the photon PI or P2 received by the receiver 3 or 4 to a photon storage unit 33a, 33b or 33c, or 43a, 43b or 43c, the number of photon storage units per receiver not being limited, said storage unit being chosen by the controller 310 or 410.

[0194] The storage units 33a to 33c or 43a to 43c are configured to store the photon PI or P2, pending delivery of said photon to the user(s).

[0195] The evacuation path 34 or 44 is configured to evacuate from the receiver the photon PI or P2 received by said receiver when its entangled photon P2 or PI has not been received by the other receiver.

[0196] Each photon passage detector 31 and 41 comprises a control unit 310 and 410 respectively and a communication unit 311 and 411 respectively.

[0197] The photon passage detector 31 or 41 respectively detects the passage of the photon P1 or P2 and communicates this information respectively to the control units 310 or 410. The control units 310 and 410 each have a clock, not shown, said clocks preferably being synchronized. The control units 310 and 410 communicate with each other via the communication units 311 and 411.

[0198] The two communication units 311 and 411 communicate with each other via a communication system 5, in particular to exchange the reception times of the photons P1 and P2. The exchange of the reception times of the photons allows each of the receivers 3 and 4 having received a first photon to determine whether the other receiver has received the photon entangled with this first photon.

[0199] The communication system 5 is preferably a communication system as described in patent FR3125658 or, alternatively, an optical communication system, in particular by optical fiber, a radio communication system, or a communication system by power line carriers.

[0200] The control units 310 and 410 are in particular configured to control the photon switches 32 and 42. The control units 310 and 410 are for example configured to control the current sent to the electroacoustic transducers of the photon switches 32 and 42, in order to modify the refractive index of the acousto-optic material and thus the transmission direction of the photon.

[0201] The control unit 310 or 410 may be configured to command the photon switch 32 or 42 to send the photon to the photon escape path 34 or 44, if the photon passage detector 31 or 41 receives information that the other receiver has not received the photon entangled with said photon.

[0202] Alternatively, as illustrated in Figure 2, the control unit 310 or 410 may be configured to command the switch 32 or 42 to sequentially communicate the Qubit carried by the photon P1 or P2 to the Qubit vectors of a set 33 of Qubit vectors grouping the Qubit vectors 33a, 33b and 33c, or to a set 43 of Qubit vectors grouping the Qubit vectors 43a, 43b and 43c. Then, the control unit 310 or 410 may be configured to command systems, not shown, surrounding said Qubit vectors the procedure for transferring the Qubit carried by the photon communicated to said Qubit vector, onto said Qubit vector itself, for example as illustrated in Figure 3D. The communication units 311 and 411 having communicated together, the control units 310 and 410 can then establish that such Qubit carried by such vector of Qubits 33a to 33c and 43a to 43c is entangled with a Qubit of the other receiver.The control units 310 and 410 can then indicate to users the availability of such Qubits.

[0203] Controllers 35 and 45 can then, for example at the request of users, extract Qubits identified as having an entangled Qubit at the other receiver, for example by the procedure indicated in Figure 3E.

[0204] Figures 3A-3C illustrate examples of CNOT gates.

[0205] Figure 3A illustrates a CNOT gate adapted to linearly polarized photons, for Qubits encoded on photons on a linear polarization basis, transforming two Qubits QubitO and Qubit 1 into two other Qubits Qubit2 and Qubit3, where Qubit2=QubitO and Qubit3=Qubitl unless Qubit0=l, in which case Qubit3=0 if Qubitl=l and Qubit3=l if Qubitl=0.

[0206] A first incident photon Phi, which is the so-called control photon, linearly polarized, enters through an E0 input for control photon of a CFLIP flip control gate, notably as described in the article “A photon-photon quantum gate based on a single atom in an optical resonator” published by the Max Planck Institute in 2017.

[0207] A second photon Ph2 is transformed by a polarization converter Convl into a circularly polarized photon, in one direction if the photon Ph2 is horizontally polarized, or in the other direction if it is vertically polarized.

[0208] The control photon Phi emerging from the CFLIP gate is sent to an output S0. The CFLIP gate lets the second photon Ph2 out in a circularly polarized state unchanged if the Phi photon was vertically polarized, or circularly polarized in the other direction if the Phi photon was horizontally polarized.

[0209] The circularly polarized photon Ph2 is then converted into a linearly polarized photon by a Conv2 converter.

[0210] Figure 3B illustrates a CNOT gate adapted to circularly polarized photons, for Qubits encoded on photons on a circular polarization basis, transforming two Qubits QubitO and QubitL into two other Qubits Qubit2 and Qubit3, where Qubit2=QubitO and Qubit3=QubitL, except if Qubit0=L, in which case Qubit3=0 if QubitL=L and Qubit3=L if QubitL=0. A first incident photon Phi, which is the so-called control photon, circularly polarized, is converted into a linearly polarized photon by a polarization converter Conv3, then passes through a control photon input of a flip control gate CFLIP.

[0211] The control photon Phi coming out of the CFLIP gate is sent to the polarization converter Conv4 which converts it into a circularly polarized photon.

[0212] A second photon Ph2 is sent through a controlled input of the CFLIP gate from which it emerges circularly polarized in the same direction, or in the opposite direction depending on the polarization direction of the photon Phi having passed through the CFLIP gate.

[0213] Figure 3C illustrates a CNOT gate between two unsynchronized photons with wavelengths close to 780 nm, the photons carrying Qubits encoded according to a linear polarization (the said CNOT gate illustrated in Figure 3C being inspired by the article "Scalable Photonic Quantum Computation through Cavity-Assisted Interactions" published in March 2004 in the Physical Review Letter).

[0214] The first photon Phi, which is the so-called control photon, linearly polarized, enters at E0 in the CNOT gate. It is directed by an orientator Ot towards a polarizing filter FP which sends it towards a mirror Ma if it is vertically polarized, or towards a cavity Cv containing a Rubidium atom Rb if it is horizontally polarized.

[0215] An electromagnetic pulse is applied to the Rb atom so that its quantum state changes by TT / 2.

[0216] The photon Phi reflected by the mirror Ma or by the cavity Cv is then returned by the polarizing filter FP towards the orientator Ot which orients it towards an output S0.

[0217] The second photon Ph2 enters El in the CNOT gate. Its polarization is transformed by a polarization converter Convl into circular polarization, in one direction if the photon Ph2 is horizontally polarized, or in the other direction if it is vertically polarized.

[0218] The orientator Ot sends the circularly polarized photon Ph2 towards the polarizing filter FP which reflects the vertical component of the polarization without phase change but reflects the horizontal component with a phase shift of TT, causing the circular polarization of the photon Ph2 to change direction.

[0219] The circularly polarized photon Ph2 is then converted into a linearly polarized photon by a converter Conv2. The distances between the polarizing filter FP and the mirror Ma and between the polarizing filter FP and the cavity Cv are adjusted so that the optical path of the photons Phi and Ph2 is the same on return to the orientator Ot regardless of the location of their reflection, the mirror Ma or the cavity Cv.

[0220] The polarization converters Convl and Conv2 are, for example, quarter-wave plates adapted to the wavelength of the photon Ph2, or quarter-wave plates inserted between two outputs of two multiplexers associated with the same wavelength, and adapted to the said same wavelength.

[0221] The Ot orientator can be made of electrically controlled elements such as liquid crystals or Pockels cells, in order to be able to control the trajectory of the photons according to the date of their passage.

[0222] Since the wavelengths of the two photons Phi and Ph2 can be different, the Ot orientator can also include materials and / or dichroic filters.

[0223] Figure 3D illustrates a quantum storage method on a Rubidium 87 Rb atom of a Qubit carried by a photon. An incident photon Ph carrying a Qubit is sent to the control port 50 of a CNOT gate then to a Hadamard gate H at the output of the CNOT gate. After the photon Ph has passed through the CNOT gate, the Rb atom is excited by optical pumping by a laser Ls until a photon passage detector DP detects the passage of a photon. Said photon is then directed to the controlled port 60 of the CNOT gate. A Bell measurement is then carried out on the incident photon Ph and the photon Ph' controlled by the polarization detectors D and D'. One or more microwave electromagnetic pulses EM are then sent by an antenna 4 onto the Rb atom to subject it to a quantum gate. Said quantum gate is characterized as follows:

[0224] If D = 0 and D' = 0, the quantum gate is the identity,

[0225] If D = 0 and D' = 1, the quantum gate is a NOT gate,

[0226] If D = 1 and D' = 0, the quantum gate is a Pauli gate z,

[0227] If D = 1 and D' = 1, the quantum gate is a NOT gate followed by a Pauli z gate.

[0228] Figure 3E illustrates a method for extracting a Qubit carried by a Rubidium 87 Rb atom on a photon. The Rb atom is a quantum memory encoding Qubits on its states. The Rb atom is excited by optical pumping by a laser Ls until the photon passage detector DP detects a PhO photon. Said PhO photon is emitted at a wavelength of 780nm and is entangled with the Rb atom according to a linear polarization. A measurement of the state of the Rb atom can be carried out, possibly preceded by the application of a da Hadamard gate to said atom, at the request of the user and the result communicated to the user.

[0229] Figure 4 illustrates a method for providing an EPR quantum channel according to the invention, using system 1, and more particularly illustrates the long-distance teleportation of a Qubit of a quantum memory consisting of a Rubidium Rb atom located in receiver 3, on a photon.

[0230] The Rubidium atom is in a combination of the two Rydberg states F=l;mF=l and F=2;mF=2, representing respectively the associated low and high states of the atom on which a Qubit is encoded.

[0231] The system for providing a quantum channel EPR 1 sends from its receiver 3 a first photon PI of wavelength 780 nm coding a Qubit according to a linear polarization basis, entangled with another photon P2 on its way to the receiver 4. The receiver 3 signals to an orientator Ot the arrival of the photon PL

[0232] The photon PI passes through a polarization converter Convl which transforms a linear polarization of a photon into a circular polarization. The orientator Ot then directs the photon PI towards a polarizing filter FP which sends it towards a mirror Ma if it is vertically polarized, or towards a cavity Cv containing the Rubidium atom Rb if it is horizontally polarized.

[0233] The photon PI reflected by the mirror Ma or by the cavity Cv is then returned by the polarizing filter FP towards the orientator Ot which orients it towards a polarization converter Conv2 which transforms a circular polarization of a photon into a linear polarization.

[0234] One or more pulses of electromagnetic waves in the microwave range are sent to the Rubidium atom, causing it to undergo a Hadamard gate.

[0235] Then, a measurement of the excitation state of the Rubidium Rb atom is carried out as well as a measurement of the polarization of the photon by a photon detector DI. These measurements are communicated to the receiver 4, either by a direct communication line TL, or by a first line TL1, then the quantum channel provision system EPR 1, then a second line TL2. According to the measurements of the states of the Rubidium Rb atom and the direction of the polarization of the photon PI, that is to say of the state 0 or 1 of the Qubit carried by each, the polarization modifier MP actuates two switches Cml and Cm2 before the photon P2 from the second receiver 4 reaches them, passing the photon P2 through combinations of devices Dpi, Dp2 and Dp3 allowing operations to be carried out on the photon P2.

[0236] If the measurement of the Qubit carried by the Rubidium atom is 0 and that of the Qubit carried by the photon PI is 0, that is to say that it has a vertical polarization, the photon P2 does not undergo any operation.

[0237] If the measurement of the Qubit carried by the Rubidium atom is 0 and that of the Qubit carried by the PI photon is 1, that is, it has a horizontal polarization, the P2 photon is sent through a Pauli Z gate.

[0238] If the measurement of the Qubit carried by the Rubidium atom is 1 and that of the Qubit carried by the PI photon is 0, that is, it has a horizontal polarization, the P2 photon is sent through a NOT gate.

[0239] If the measurement of the Qubit carried by the Rubidium atom is 1 and that of the Qubit carried by the PI photon is 1, i.e. it has horizontal polarization, the P2 photon is sent through a NOT gate followed by a Pauli Z gate.

[0240] The NOT gate, for example, consists of two birefringent prisms and two half-wave plates.

[0241] The Pauli Z gate, for example, consists of a half-wave plate.

[0242] Figure 5 illustrates a method of providing an EPR quantum channel according to the invention, using system 1, and more particularly illustrates a method of teleporting quantum information to a quantum memory consisting of a neutral atom, using system 1.

[0243] A first rubidium atom Rbl is excited until a photon passage detector DPI detects an emitted photon Ph and directs it to the control port 50 of a first control gate CNOT1 as described above before sending it to a Hadamard gate H, then its polarization direction is measured by a photon detector DI. The receiver 3 of the system 1 sends a first photon PI of wavelength 780 nm to the controlled port 60 of the gate CNOT1 which modifies it, then the direction of the linear polarization of said photon PI is measured by the photon detector D2.

[0244] Receiver 4 of system 1 sends a second photon P2, entangled with the first photon PI, towards a phase modifier MP, which according to the measurements of the detectors DI and D2 changes the polarization of said second photon in the manner described above.

[0245] The second photon P2 is then directed to the control port 50 of a second gate CNOT2 operating at a wavelength of 780nm as described above, before passing through a Hadamard gate H and having the direction of its polarization measured by a photon detector D3.

[0246] A second Rb2 atom is excited until a second passage detector DP2 detects the passage of a photon Ph' which is then directed towards the controlled port 60 of the second gate CNOT2 before having the direction of its linear polarization measured by a photon detector D4.

[0247] One or more microwave electromagnetic pulses are sent to the second Rb2 atom to cause it to undergo a unitary quantum gate characterized as follows:

[0248] If D3 = 0 and D4 = 0, the unitary quantum gate is the identity,

[0249] If D3 = 0 and D4 = 1, the unitary quantum gate is a Pauli z gate,

[0250] If D3 = 1 and D4 = 0, the unitary quantum gate is a NOT gate,

[0251] If D3 = 1 and D4 = 1, the unitary quantum gate is a NOT gate followed by a Pauli z gate.

[0252] Alternatively, the entangled photons emitted by photon emitter 2 are of a wavelength other than 780 nm and are converted into 780 nm photons before their arrival at photon receivers 3 and 4 by a frequency mixer, as for example described in the article "Entanglement between a trapped ion qubit and a 780-nm photon via quantum frequency conversion" published on October 26, 2022 in Physics review.

[0253] Several EPR 1 quantum channel delivery systems can be used in succession to form a set for the teleportation of quantum information, allowing Qubits to be transported over very long distances, for example over 1000 km using optical fibers. Figure 6 illustrates such an EPR quantum channel delivery set comprising two EPR 1a and 1b quantum channel delivery systems.

[0254] A first system for providing EPR quantum channels provides receivers 3a and 4a with two entangled photons Pla and P2a.

[0255] The photon from P2a is directed to the control port 50 of a CNOT gate as described above operating for example in a linear polarization base.

[0256] Said photon P2a exits the CNOT gate to pass through a Hadamard gate H before having the direction of its polarization measured by a photon detector Da.

[0257] The controlled port 60 of the CNOT gate is supplied by a third photon Pib coming from the receiver 3b, itself entangled with a fourth photon P2b coming out of the receiver 4b and made available by a second quantum channel provision system EPR 1b.

[0258] The third photon P2b is modified in the CNOT gate before having its polarization direction measured by a Db photon detector.

[0259] The measurement results are communicated to a polarization modifier MPb, either by a direct data transmission line TL, or via the transmission line TL1, the second system 1b and the transmission line TL2.

[0260] The fourth photon P2b leaves the receiver 4b to go towards the polarization modifier MPb.

[0261] According to the measurements of the polarization directions in Da and Db, and therefore of the state 0 or 1 of each of the Qubits carried by the photons P2a and Pib, the polarization modifier MPb actuates two switches Cml and Cm2 before the photon P2b from the second receiver 4b reaches them, making the photon P2b pass through combinations of devices Dpi, Dp2, Dp3 allowing operations to be carried out on the photon P2b.

[0262] If the measurement P2a is 0 and that of Pib is 0, the photon P2b undergoes no operation.

[0263] If the measurement P2a is 0 and that of Pib is 1, the photon P2b is sent through a Pauli Z gate.

[0264] If the measurement P2a is 1 and that of Pib is 0, the photon P2b is sent through a CNOT gate. If the measurement P2a is 1 and that of Pib is 1, the photon P2b is sent through a CNOT gate and then through a Pauli Z gate.

[0265] The system thus formed therefore provides users of the 3a receiver and at the output of the MPb polarization modifier with two entangled photons Pla and P2b.

[0266] More than two EPR quantum channel provision systems according to the invention can be linked one after the other to form a quantum transmission chain making available to users at each of its ends photons entangled with photons having reached the other end of said chain.

[0267] Figure 7 shows the transmitter 2 of entangled photons sending pairs of entangled photons PI and P2 to the receivers 3 and 4, the latter also having a direct transmission line TL allowing them to exchange classical information.

[0268] Each receiver 3 and 4 contains, for example, nine Rubidium 87 Rb atoms arranged to be able to emit photons and undergo unitary quantum gates.

[0269] Each of the Rb atoms of one receptor 3 or 4 is paired with an Rb atom of the other receptor 4 or 3.

[0270] An optical switch, not shown, present at each of the receivers 3 and 4 makes it possible to direct the photons emitted by any one of the Rb atoms of the receiver towards a single photon passage detector, not shown, upstream of the single CNOT gate, not shown, arranged in each of the two receivers.

[0271] Said optical switches of the two receivers 3 and 4 are actuated simultaneously to, within each receiver, send the photons emitted by one of the Rb atoms towards the photon passage detector until said Rb atom has been entangled with the Rb atom to which it is paired in the other receiver.

[0272] Once each of the nine pairs of Rb atoms has been entangled, an error reduction algorithm, for example the Schor algorithm, can be applied, with both receivers 3 and 4 being able to use the same classical information transmission medium TL as that used by the communication system.

[0273] Figure 8 illustrates a method for generating entangled photons. A vertically linearly polarized photon emitter 2 is stimulated to emit a first photon PI. This first photon PI passes through a Hadamard gate H, such as a half-wave plate, before arriving at the input of a CNOT gate as described above, as a control photon. A second vertically linearly polarized photon P2 is emitted by the photon emitter 2, said second photon P2 being directed towards the controlled port 60 of the CNOT gate.

[0274] The PI and P2 photons exiting at S0 and SI are entangled and can be used as entangled photons in system 1.

[0275] Figure 9 illustrates a method for measuring the result of quantum information teleportation. The method makes it possible to instantly measure the probability that the Qubit carried by a photon has of being read in a 0 or 1 state. A particle, for example a Rubidium 87 Rb atom or an ytterbium ion, on which for example a Qubit has been teleported, is stimulated in order to emit a photon P which is entangled with it. Said photon P is multiplied by an optical amplifier 13 preserving its polarization, for example an Erbium-doped fiber amplifier to form a photon flux F. The polarization of the photon flux F is measured by a conventional polarization detector D as described above.

[0276] For example, a horizontal polarization denotes that the Qubit carried by the particle was in state 0 while a vertical polarization denotes that it was in state 1, and an intermediate polarization denotes that the Qubit was in an undetermined state.

[0277] Figure 10 illustrates the transfer of a classical database whose classical data is distributed over 3 separate computers 7, 8 and 9, to a quantum computer 10.

[0278] The computers 7, 8 and 9 respectively transfer their classical data to a respective quantum memory 71, 81 and 91, then the quantum memories of the quantum computers 7, 8 and 9 are transferred by a quantum information teleportation method according to the invention, to Qubit vectors forming a quantum memory 11 of the quantum computer 10.

[0279] The quantum computer 10 can then query the reconstructed database in its quantum memory 11.

[0280] The photon switch 32 or 42 may be an optical microresonator 12, i.e. a cavity formed of an optical medium surrounded by reflective surfaces 13, in particular configured to serve as an optical CNOT gate, as illustrated in FIG. 11. Atoms to interact with each photon are brought successively into the cavity. At least two holes 14 are formed in the cavity to allow at least two light rays 15 emitted by a laser 16 to pass through and serving as optical tweezers, the optical tweezers making it possible to bring and position the atom within the cavity. The use of optical tweezers makes it possible in particular for an atom to be moved after interacting with the first of two photons to perform a CNOT operation. The atom may in particular be moved to another cavity and replaced by a new atom, this making it possible to receive a new photon carrying a Qubit used as a control Qubit of another optical CNOT gate.The cavity includes a hole 17 allowing the passage of the carrier photon of the incident Qubit. The cavity also includes an inlet / outlet 18 for the atoms and the photons emitted by the atoms in the cavity, the atoms moving along an axis 19.

Claims

Claims 1. System for providing an EPR quantum channel comprising: • an emitter of entangled photons comprising a source configured to generate at least one pair of entangled photons comprising a first photon emitted on a first propagation path, and simultaneously a second photon emitted on a second propagation path different from the first propagation path, • a first receiver arranged on the first propagation path comprising: - at least one Qubit storage unit arranged to store the Qubit carried by the photon received by the first receiver, - a photon switch arranged to send the photon received by the first receiver to said storage unit, - a photon passage detector configured to control the photon switch and note the time of reception by the first photon receiver, • a second receiver arranged on the second propagation path so as to be reached by the second photon, said second receiver comprising: - at least one Qubit storage unit arranged to store the Qubit carried by the photon received by the second receiver, - a photon switch arranged to send the photon received by the second receiver to said storage unit, - a photon passage detector configured to control the photon switch and note the time of reception by the second photon receiver, • at least one information processor included in the first and / or the second receiver, for example an electronic microprocessor, arranged to control the photon switch of said receiver, • a communication system linking the two receivers together, said communication system being arranged in particular to communicate to said information processor the times of reception of the photons at the first and second receivers, in order to determine the pairs of entangled photons of which the two photons have each reached the receiver to which it was sent.

2. System according to the preceding claim, a storage unit of the first and / or second receivers being chosen from optical paths, in particular optical fibers or passages in space arranged using mirrors, or vectors of physical Qubits, in particular ions interacting indirectly with the incident photons.

3. System according to one of claims 1 and 2, the duration during which each storage unit is capable of retaining a Qubit being greater than the time difference between the arrivals at the two receivers of the two entangled photons, to which are added the time taken by the communication system to communicate the information on the arrival times of the entangled photons, the time necessary for the processors to decide on the presence of two entangled Qubits in one of the storage units of the receivers, and the time taken by the processors to inform the interfaces of the receivers thereof.

4. System according to any one of the preceding claims, the photon switch of the first receiver and / or of the second receiver being an optical microresonator, that is to say a cavity formed of an optical medium surrounded by reflective surfaces into which atoms are successively brought to interact with each photon, at least two holes being in particular formed in the cavity to allow at least two light rays to pass through, serving as optical tweezers and making it possible to bring and position the atom within the cavity.

5. System according to any one of the preceding claims, the photon passage detector of the first and / or second receiver comprising a non-destructive photonic Qubit detector, for example using parametric conversion of the incoming photon which produces two photons of lower frequencies from a single photon, where one of the photons produced announces the presence of a qubit and the other provides the information on said qubit.

6. System according to any one of claims 1 to 4, the photon passage detector of the first and / or second receiver comprising a Qubit detector non-destructive photonics, the non-destructive photonic Qubit detector comprising a QED cavity in which a rubidium atom is trapped.

7. Method for providing an EPR quantum channel using the system for providing an EPR quantum channel according to any one of the preceding claims, comprising the steps of: (a) Generating a pair of entangled photons from an emitter, the first photon of the pair being emitted to a first receiver and the second photon of the pair being emitted simultaneously to a second receiver, the first and second photons being entangled, (b) The possible detection of the arrival of a photon by a photon passage detector at each of the receivers, (c) Communication of photon reception times between the two photon passage detectors of the first and second receivers, If the communication between the two photon passage detectors is faster than the time taken by the photon received by said receiver to transit from the photon passage detector to the photon switch of this receiver, the method comprises: (d) Transit of the photon from the photon passage detector to the photon switch through a single temporary storage unit of the receiver, If the photon passage detector having received the photon receives the information that the other photon passage detector has received the photon entangled with said photon, the method comprises: (dl) Sending the photon by the photon switch to a vector of Qubits outside the system, If the photon passage detector having received the photon receives the information that the other photon passage detector has not received the photon entangled with said photon, the method comprises: (d2) Sending the photon by the photon switch to a photon escape path, If the communication between the two photon passage detectors is slower than the time taken by the photon received by said receiver to transit from the photon passage detector to the photon switch of this receiver, the method comprises: (e) Transit of the photon from the photon passage detector to the photon switch and sending by the photon switch of the photon to a Qubit storage unit, If the photon passage detector having received the photon receives the information that the other photon passage detector has received the photon entangled with said photon, the method comprises: (el) Sending the photon by the switch of the Qubit storage unit to a Qubit vector outside the system if the Qubit storage unit is a device or, if the Qubit storage unit is not an optical device, making the Qubit contained in the Qubit storage unit available to a user, said making available being for example announced to the user by the electronics of the receiver, If the photon passage detector having received the photon receives the information that the other photon passage detector has not received the photon entangled with said photon, the method comprises: (e2) Sending the photon from the optical Qubit storage unit's photon switch to a photon escape path, or if the Qubit storage unit is not an optical device, resetting the Qubit storage unit so that it is ready to receive a new photon.

8. Assembly for the teleportation of quantum information comprising at least two systems for providing EPR quantum channels as described in any one of claims 1 to 6, chained one after the other, the assembly comprising at least one optical CNOT gate between each of the systems, a first photon exiting a first EPR quantum channel provision system of the set being sent into the control port (50) of said CNOT gate, a second photon exiting a second EPR quantum channel provision system of the set, following the first system, being sent into the controlled port (60) of said CNOT gate.

9. Assembly according to the preceding claim, a Bell measurement being carried out on the first and second photons at the output of said CNOT gate, the first photon passing through a Hadamard gate at the output of the CNOT gate before its Bell measurement is carried out, the results of this measurement being sent to a set of unitary quantum gates at the output of the last EPR quantum channel provision system in the chain.

10. Assembly according to the preceding claim, the unitary quantum gate being such that: If Di = 0 and D2 = 0, the unitary quantum gate is the identity, If Di = 0 and D2 = 1, the unitary quantum gate is a Pauli z gate, If Di = 1 and D2 = 0, the unitary quantum gate is a NOT gate, If Di = 1 and D2 = 1, the unitary quantum gate is a NOT gate followed by a Pauli gate z, Di being the measurement of the quantum state of the first photon leaving the CNOT gate through the port associated with the control photon and D2 being the measurement of the quantum state of the second photon leaving the CNOT gate through the port associated with the controlled photon.

11. Assembly according to one of claims 9 and 10, the photon leaving the receiver of the last EPR quantum channel provision system in the chain, said receiver not being connected to any other receiver in another system, being sent into a unitary gate, the unitary gate being the product of the unitary quantum gates obtained by each of the Bell measurements at the interface of each of the EPR quantum channel provision systems in the chain.

12. Assembly according to any one of claims 8 to 11, comprising between 1 and 10 systems for providing EPR quantum channels chained together one after the other, or comprising between 1 and 30 systems for providing EPR quantum channels chained together one after the other if said systems benefit from an error correction system.

13. Assembly according to any one of claims 8 to 12, the CNOT gate comprising a cavity in which a neutral atom, in particular a Rubidium atom, is trapped.

14. A method for teleporting quantum information using the assembly for teleporting quantum information as defined in any one of claims 8 to 13, comprising transmitting a Qubit from a first receiver to a second receiver.

15. A method for long-distance teleportation of a Qubit carried by a neutral atom onto a photon, using the system for providing a quantum channel described in any one of claims 1 to 6, comprising the steps of: o Generating a pair of entangled photons from a transmitter, the first photon of the pair being emitted to a first receiver and the second photon of the pair being emitted, preferably simultaneously, to a second receiver, the first and second photons being entangled, the photons being in particular of wavelength 780 nm, the first photon encoding a Qubit, in particular according to a linear polarization basis, o Sending the first photon received to the first receiver in a CNOT gate comprising a cavity in which the neutral atom is trapped, o Sending pulses of electromagnetic waves in the microwave domain onto the neutral atom, thus subjecting it to a Hadamard gate,o Measure the excitation state of the neutral atom and measure the polarization of the first photon at the output of the CNOT gate, o Communicate said measurements to the second receiver, o Depending on the measurements of the states of the neutral atom and the direction of the polarization of the first photon, i.e. state 0 or 1 of the Qubit carried by each, modify the polarization of the second photon.

16. A method for long-distance teleportation of a Qubit carried by a first neutral atom onto a second neutral atom, using the system for providing a quantum channel described in any one of claims 1 to 6, comprising the steps of: a. Generating a pair of entangled photons from an emitter, the first photon of the pair being emitted to a first receiver and the second photon of the pair being emitted, preferably simultaneously, to a second receiver, the first and second photons being entangled, the photons being in particular of wavelength 780 nm, the first photon encoding a Qubit, in particular according to a linear polarization basis, b. Exciting the first neutral atom, located at the first receiver, until it emits a photon and directing the emitted photon to the control port of a first CNOT control gate before sending it to a first Hadamard gate, c.Send the first photon of the entangled photon pair to the controlled port of the first CNOT gate, d. Measure the direction of polarization of the photon emitted by the first neutral atom at the output of the Hadamard gate and the direction of polarization of the first photon at the output of the CNOT gate, e. Communicate said measurements to the second receiver, f. According to the measurements of the states of the neutral atom and the direction of polarization of the first photon, i.e., the state 0 or 1 of the Qubit carried by each, modify the polarization of the second photon of the entangled photon pair to the second, g. Send the modified second photon to the control port of a second CNOT gate before sending it to a second Hadamard gate,. h. Exciting the second neutral atom, located at the second receiver, until it emits a photon and directing the emitted photon towards the controlled port of the second CNOT gate, i. Measuring the direction of the polarization of the photon emitted by the second neutral atom at the output of the second CNOT gate and the direction of the polarization of the second photon at the output of the second Hadamard gate, j. Sending at least one microwave electromagnetic pulse to the second neutral atom to make it undergo a unitary quantum gate, notably characterized in the following way: i. If the measurement of the direction of the polarization of the photon emitted by the second neutral atom is 0 and that of the polarization of the second photon is 0, the second neutral atom does not undergo for example any operation, ii.If the measure of the direction of polarization of the photon emitted by the second neutral atom is 1 and that of the polarization of the second photon is 0, the second neutral atom can undergo a Pauli Z gate, iii. If the measure of the direction of polarization of the photon emitted by the second neutral atom is 0 and that of the polarization of the second photon is 1, the second neutral atom can undergo a NOT gate, iv. If the measure of the direction of polarization of the photon emitted by the second neutral atom is 1 and that of the polarization of the second photon is 1, the second neutral atom can undergo a NOT gate followed by a Pauli Z gate.

17. Method for teleporting Qubits carried by hardware Qubit vectors, in parallel, allowing the implementation of an algorithm for correcting errors during a transmission of Qubits, the method using the EPR quantum channel provision system described in any one of the preceding claims, the method comprising: a. the emission of pairs of entangled photons, the first photon of each pair being emitted towards a first receiver and the second photon of each pair being emitted, preferably simultaneously, to a second receiver, each receiver containing at least one neutral atom, for example nine neutral atoms, the neutral atom(s) being arranged to be able to emit photons and undergo unitary quantum gates, each of the neutral atoms of one receiver being paired with a neutral atom of the other receiver. b. simultaneously, at each receiver, sending photons emitted by one of the neutral atoms of said receiver to a single photon passage detector of said receiver, the single photon passage detector being located upstream of a single CNOT gate of said receiver, until said neutral atom of said receiver has been entangled with the neutral atom to which it is paired in the other receiver, c. once each pair of neutral atoms is entangled, applying an error reduction algorithm, for example the Schor algorithm.

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