System and method for multiplexing quantum signals in a quantum network node

Abstract

The present invention relates to a system for multiplexing quantum signals in a quantum network node, wherein the system comprises: - a central waveguide; - at least one coherent light source that is configured to emit light that is coupled to the central waveguide; - at least one modulator that is operatively coupled to the at least one laser and is configured to change the frequency and / or amplitude of optical signals from the light emitted by the coherent light source; - a plurality of add-drop filters that are coupled to the central waveguide, each of the add-drop filters being configured to in-couple and out-couple optical signals in a predetermined frequency band to and from the central waveguide respectively, wherein each predetermined frequency band of the add-drop filters differs from the predetermined frequency bands from the other add-drop filters; - a plurality of qubit systems, each qubit system being operatively coupled to a respective add-drop filter, wherein each qubit system comprises at least one optically interfaced networking qubit, OINQ, configured to emit quantum signals; and - a controller that is operatively coupled to the at least one modulator and is configured to sequentially change the frequency of the optical signals in the central waveguide to a frequency that falls within any of the predetermined frequency bands of the add-drop filters to sequentially address the plurality of qubits systems.

Description

[0001] SYSTEM AND METHOD FOR MULTIPLEXING QUANTUM SIGNALS IN A QUANTUM NETWORK NODE

[0002] The present invention relates to a system for multiplexing quantum signals in a quantum network node. The present invention further relates to a method for multiplexing quantum signals in a quantum network node.

[0003] Remote entanglement between qubits, when arranged into a network, may be an object for novel applications such as remote sensing and coordination tasks. For achieving this remote entanglement, a variety of types of matter qubits, including solid-state emitters, have been used. One way to realize remote entanglement over large distances requires sending quantum signals that are entangled with the solid-state emitter spin-state (a long-lived matter qubit encoding). The most practical way of sending the quantum signals is using photonic states in a low-loss telecom band. This spin-photonic entangled state can for example be generated via short, resonant optical excitation of a spin-selective transition of the emitter and subsequently collecting the spontaneously emitted photon into a well-defined mode. Alternatively, spin-photon entanglement can be generated via a spin-dependent interaction of a photonic qubit, such as a single photon or weak coherent state, with an optical medium, such as an optical cavity.

[0004] Quantum Frequency Conversion (QFC) may be used to convert the wavelength of the quantum signals from the natural wavelength of the emitter to the telecom band. QFC may be described as a nonlinear frequency conversion process applied on quantum light that can retain the quantum information including entanglement encoded on the initial light. The low probability of generating the spin-photon entangled state per qubit due to the physical properties and practical losses during the process of generation, and the chance of losing the spin-photon entangled state during transmission, limits the current rate at which entanglement can be generated at scale.

[0005] To reach distances and entanglement generation rates beyond lab demonstrations and proof-of-principle links, multiple orders of magnitude increase in entanglement generation rate need to be realized. Therefore, it is an object to improve the rate at which quantum signals can be sent. To this end, it may be desired to multiplex the quantum signals.

[0006] US 2023 / 0344516 Al describes the spectral and temporal multiplexing of a number of emitters within an inhomogeneous distribution of frequencies that are coupled to a single optical waveguide. To this end, a frequency of a laser may be modulated to address different defect centres at different times. In this way, photons may be spectrally multiplexed in a single spatial mode. These modes are converted to telecom- wavelengths. Afterwards, a IxM switch directs the telecomwavelength signals to other nodes in the network.

[0007] A downside of this way of multiplexing the photons is that all the emitters are simultaneously exposed to applied optical fields. To avoid degrading the quality of the spin-photonentanglement, each operable emitter must have a distinct optical frequency that is far detuned from all the other emitters in the spatial mode (compared to the applied optical Rabi frequency for an emitter on resonance). As frequency ‘collisions’ occur statistically within the inhomogeneous distribution of emitters, high performance across many (>10) emitters requires a large inhomogeneous distribution (typically arising from local strain fluctuations within the host material) and mechanisms for further tuning of subsets of the emitters, for example by the application of localized strain fields. The scale of strain required to achieve a significant degree of multiplexing (100, or 1000 emitters, or more) is stated to be > 100 GHz, which is both technically challenging to achieve and can degrade emitter properties such as the optical addressability of the spin state.

[0008] It is an object for the present invention to obviate or at least reduce the abovementioned problems. In particular, it is an object for the present invention to provide a system that is able to efficiently multiplex quantum signals in a quantum network, preferably while shielding nontargeted emitters from optical control fields to maintain high-quality spin-photon and spin-spin entanglement.

[0009] This object is achieved by a system for multiplexing quantum signals in a quantum network node, wherein the system comprises:

[0010] a central waveguide;

[0011] at least one coherent light source that is configured to emit light that is coupled to the central waveguide;

[0012] at least one modulator that is operatively coupled to the at least one coherent light source and is configured to change the frequency and / or amplitude of optical signals generated by the at least one coherent light source;

[0013] a plurality of add-drop filters that are coupled to the central waveguide, each of the add-drop filters being configured to in-couple and out-couple optical signals in a predetermined frequency band to and from the central waveguide respectively, wherein each predetermined frequency band of the add-drop filters differs from the predetermined frequency bands from the other add-drop filters;

[0014] a plurality of qubit systems, each qubit system being operatively coupled to a respective add-drop filter, wherein each qubit system comprises at least one optically interfaced networking qubit, OINQ, that is configured to generate a quantum signal; and

[0015] a controller that is operatively coupled to the at least one modulator and is configured to sequentially change the frequency of the optical signals in the central waveguide to a frequency that falls within any of the predetermined frequency bands of the add-drop filters to sequentially address the plurality of qubits systems.A first advantage of the system is that by changing the frequency, which can be changed quickly as the modulator may modulate the frequency in the order of nanoseconds, different qubit systems and thus different optically interfaced networking qubits (OINQs) can be addressed. In this way, a train of quantum signals, each emitted or scattered by a different OINQ, can be obtained. This increases the number of quantum signals which can be obtained in the central waveguide, thereby increasing the rate of successful entanglement events. The waveguide may be configured to send optical signals either to other nodes in the network or to intermediate ‘mid-points’ at which the optical signals can be interfaced with the signals originating from a second node.

[0016] A second advantage of the system is that the add-drop filters shield non-addressed qubit systems from optical signals targeted at other qubit systems. In this way, the qubit decoherence induced by those control pulses on OINQs in the non-addressed qubit systems which have already generated entanglement, or which are storing quantum states, is significantly reduced.

[0017] Compared to a system in which all qubit systems are operatively coupled, a third advantage is that this additional shielding reduces multi-photon emission events associated with interaction of the optical signal with non-targeted OINQs, thus improving the quality of the generated entanglement. In this setting, a fourth advantage is that the switching array also provides additional shielding, which enables the re-use of OINQ frequencies for temporal multiplexing without decohering qubit systems that have already generated entanglement, or which are storing quantum states..

[0018] When the at least one OINQ receives an optical signal, the interaction of that optical signal with the at least one OINQ generates entanglement between the at least one OINQ and a quantum signal, preferably a photonic qubit. The qubit system is configured to transmit the quantum signal from the OINQ back towards the central waveguide. Note that the optical signal may itself be either a strong pulse or a quantum signal, such as a photonic qubit.

[0019] An OINQ is a stationary qubit which can be optically interfaced, meaning that the qubit can generate a state-dependent optical response to an impinging control field, such as emitting a quantum signal that can be transmitted through an optical fibre to form a network. In general, the emitted quantum signal will be a photonic qubit. Examples of an OINQ may be a diamond with a colour centre, for example a tin-vacancy centre. Other examples may be colour centres in semiconductor or wide-bandgap materials such as silicon or silicon carbide, rare-earth-ion dopants in crystalline hosts, trapped ions, trapped atoms, optically active quantum dots, and so forth.

[0020] Each optical signal has a specific frequency or frequency band. Therefore, optical signals may have one or more frequencies. The controller is configured to sequentially change the one or more frequencies of the optical signals in the central waveguide to one or more frequencies that fall within any of the predetermined frequency band of the add-drop filters.

[0021] The coherent light source may be a laser.The emitted light may be conceptually divided into optical signals. These discrete optical signals may arise naturally from the coherent light source, for example if the coherent light source is a pulsed-laser or a single-photon source. The optical signals may also be generated by the modulator from continuous wave emission by the changing of the frequency and / or amplitude of the emitted light.

[0022] In an embodiment each qubit system of the plurality of qubit systems comprises a plurality of OINQs and an optical switch array, wherein the optical switch array is provided between the add-drop filter and the plurality of OINQs, the optical switch array being configured to selectively direct an optical signal to a subset of the plurality of OINQs and to transmit a subsequently emitted quantum signal from the subset of OINQs back towards the central waveguide, wherein the controller is operatively coupled to optical switch array, and wherein the controller is configured to (sequentially) reconfigure the optical switch array to sequentially address the plurality of OINQs.

[0023] The OINQs in said subset might be addressed individually with optical signals at different frequencies that fall within the frequency band of the add-drop filters. The subset of OINQs may be a part of the plurality of OINQs. The subset of OINQs may be a single OINQ or multiple OINQs. For example, the subset may be a N number of OINQs, wherein N is a natural number, for example, 1, 2, 3, etc.

[0024] The optical switch array may be configured to selectively direct an optical signal to a subset of the plurality of OINQs and to transmit a subsequent quantum signal from the OINQ together with the at least one modular. In other words, due to the configuration of the optical switch array in conjunction with the at least one modulator an optical signal may be selectively directed and a subsequently emitted quantum signal may be transmitted back.

[0025] This embodiment combines the fast modulation of the coherent light source frequency to address different qubit systems via passive routing elements, namely add-drop filters, together with active routing elements, namely the optical switch array. Compared to a system comprising only a central waveguide and a single optical switch array, a first advantage of this approach is that the rate at which qubits can be addressed (‘addressing rate’) is first set by the coherent light source modulation speed, rather than the switching speed of the optical switch array, which is typically slower. Accordingly, the addressing rate can in principle be increased by a factor equal to the number of incorporated add-drop filters, or by a factor equal to the ratio of the coherent light source modulation frequency to the optical switch array reconfiguration frequency, whichever factor is smaller.

[0026] Compared to a system comprising only a central waveguide and add-drop filters, the second advantage of this embodiment is that the total number of addressable OINQs that can be addressed for a given number of add-drop filters is increased by a factor equal to the number of switching array outputs. This is beneficial because only using frequency modulation without theoptical switch array could result in a lossy method. The add-drop filters are all operatively coupled to the central waveguide, which also transmits the obtained quantum signals. The transmission of an emitted quantum signal is equal to the add-drop transmission factor exponentiated by the number of subsequently encountered add-drop filters. Thus, an increasing number of add-drop filters results in an increasing average loss within the node. Furthermore, the maximum number of add-drop filters is set by the frequency distribution of the emission line of the OINQs, which has a finite spread. Using the switching array allows for more OINQs per add-drop filter.

[0027] A third advantage of this approach is that the add-drop filters may be bulky, or costly. The use of optical switch arrays reduces the number of needed add-drop filters, thereby reducing the footprint or costs of the system as a whole.

[0028] Compared to an approach in which all the addressable OINQs of the qubit system are hosted in a single spatial mode, a fourth advantage of this approach is that the optical switch array provides additional suppression of addressing light to non-targeted OINQs within each qubit system.

[0029] The optical switch array may comprise a plurality of 2x2 switches which can take different configurations such that the input and output may be rerouted as desired. In this typical approach, the average degree of suppression offered by the optical switch array scales as k ceil(log2(n)), where k is the suppression offered by each 2x2 switch (‘on / off’ ratio), n is the number of OINQs and ceil is the ceiling function. The on-off ratio of each 2x2 switch may be more than 10 dB, 20 dB, or 30 dB.

[0030] The optical switch array can also be denoted as optical switch matrix.

[0031] In an embodiment the switches are micro-electromechanical systems based photonic integrated circuit or electro-optical switches.

[0032] In an embodiment each of the OINQs are tuned to an emission frequency, preferably by strain tuning.

[0033] By tuning the emission frequency of the OINQs the OINQs can be configured to be receptive of the frequencies associated with the add-drop filters.

[0034] In an embodiment the system further comprises an OINQ tuning device that dynamically tunes the emission frequency of the OINQs.

[0035] In an embodiment the system further comprises an add-drop tuning device that dynamically tunes the predetermined frequency band of the add-drop filters.

[0036] In an embodiment the system further comprises a quantum frequency conversion device that is configured to change the frequency of the photonic modes in the central waveguide to a converted frequency, wherein the quantum frequency conversion device is preferably positioned downstream the add-drop filters.In an embodiment the quantum frequency conversion device further is configured to remove the frequency differences of the quantum signals entangled with the OINQs.

[0037] The invention further relates to an assembly for multiplexing quantum signals in a quantum network node, comprising:

[0038] a plurality of systems according to the invention, wherein each of the converted frequency of the systems are different from each other; and

[0039] a wavelength division multiplexer that is coupled to each of the central waveguides from the plurality of systems and is configured to multiplex the central waveguides modes into a single spatial mode.

[0040] The wavelength division multiplexing may be the third multiplexing step. An advantage of the wavelength division multiplexer is that multiple optical signals of different systems can be combined into an optical fibre to increase the number of quantum signals sent over the optical fibre. This increases the number of successful entanglements which can be achieved.

[0041] The invention further relates to an optical quantum network, comprising:

[0042] a plurality of nodes, each node comprising a system according to the invention or an assembly according to the invention; and

[0043] at least one mid-point that is operatively coupled to at least two nodes of the plurality of nodes, wherein the at least one mid-point is configured to receive spatial modes from at least two nodes of the plurality of nodes, wherein the at least one mid-point comprises an interfacing device that is configured to interface the spatial modes from at least two of the nodes, wherein the interfacing device preferably comprises a Bellstate measurement device.

[0044] Optionally, the Bell-state measurement device can be an asynchronous Bell-state measurement device.

[0045] In an embodiment the optical quantum network further comprises an assembly according to the invention, wherein the mid-point further comprises a wavelength division demultiplexer system that is configured to demultiplex the modes received from the nodes such that each resulting signal can be linked with the corresponding signal from another node.

[0046] The invention further relates to a reflection based optical quantum network, comprising a plurality of nodes, each node comprising a system according to the invention or an assembly according to the invention; and

[0047] at least one photon state analysing device that is coupled to at least one node of the plurality of nodes and is configured to herald spin-spin entanglement between the OINQs of the at least one node via measurement of the quantum signals that were sent from the nodes of the network.A quantum signal may be generated at a first node of the plurality of nodes. The quantum signal may interact with a first OINQ at the first node. The first OINQ may become entangled with the quantum signal (via a spin-dependent interaction, such as a spin-dependent phase induced by an interaction with an optical medium such as an optical cavity containing an OINQ). The quantum signal may be sent over a transmission line towards a second node. Said quantum signal then interacts with (at least) a second OINQ at the second node, where the quantum signal also becomes entangled with the second OINQ. The quantum signal can interact with more than two OINQs. Finally, the quantum signal may be analysed in a photon state analysing device. Spin-spin entanglement may be heralded by the outcome of the qubit analysis measurement.

[0048] The invention further relates to an optical quantum network, comprising:

[0049] a plurality of nodes, each node comprising a system according to the invention or an assembly according to the invention; and

[0050] at least one mid-point that is operatively coupled to at least two nodes of the plurality of nodes, wherein the at least one mid-point is configured to receive spatial modes from at least two nodes of the plurality of nodes, wherein the at least one mid-point comprises an interfacing device that is configured to interface the spatial modes from at least two of the nodes, wherein the interfacing device preferably comprises a Bellstate measurement device, or

[0051] at least one photon state analysing device that is coupled to at least one node of the plurality of nodes and is configured to herald spin-spin entanglement between the OINQs of the at least one node via measurement of the quantum signals that were sent from the nodes of the network.

[0052] The invention further relates to a method for multiplexing quantum signals in a quantum network, comprising:

[0053] providing:

[0054] o at least one coherent light source that is configured to emit light that is coupled a central waveguide;

[0055] o at least one modulator that is operatively coupled to the at least one coherent light source and is configured to change the frequency and / or amplitude of optical signals from the light emitted by the coherent light source within the central waveguide, each of the add-drop filters being configured to in-couple and out-couple optical signals in a predetermined frequency band to and from the central waveguide respectively, wherein each predetermined frequency band of the add-drop filters differs from the predetermined frequency bands from the other add-drop filters;o a plurality of qubit systems each being coupled to a respective add-drop filter, wherein each qubit system comprises at least one optically interfaced networking qubit, OINQ, configured to generate a quantum signal; and o a controller that is operatively coupled to the frequency modulator, further comprising,

[0056] emitting, by at the at least one coherent light source, light that couples to the central waveguide;

[0057] changing, by the controller, the frequency of the optical signals in the central waveguide to a frequency that falls within any of the predetermined frequency bands of the add-drop filters to sequentially address the plurality of qubits systems. The method has similar effects and advantages as described for the system.

[0058] In an embodiment each qubit system of the plurality of qubit systems comprises a plurality of OINQs and an optical switch array, wherein the optical switch array is provided between the add-drop filter and the plurality of OINQs, the optical switch array being configured to selectively direct an optical signal to a subset of OINQs of the plurality of OINQs and to transmit a subsequently emitted quantum signal from the adressed OINQ back towards the central waveguide, wherein the controller is operatively coupled to the optical switch array, further comprising:

[0059] reconfiguring, by the controller, the optical switch array to sequentially address the plurality of OINQs.

[0060] The subset of OINQs may be a single OINQ or multiple OINQs. For example, the subset may be a N number of OINQs, wherein N is a natural number, for example, 1, 2, 3, etc.

[0061] In an embodiment the system further comprises an OINQ tuning device, wherein the controller is operatively coupled to the tuning device, further comprising:

[0062] dynamically tuning, by the controller, the emission frequency of the OINQs. In an embodiment the system further comprises an add-drop tuning device, wherein the controller is operatively coupled to the add-drop tuning device, further comprising:

[0063] dynamically tuning, by the controller, the predetermined frequency band of the adddrop filters.

[0064] In an embodiment the method further comprises providing a quantum frequency conversion device, wherein the controller is operatively coupled to the quantum frequency conversion device, further comprising:

[0065] receiving the quantum signals in the quantum frequency conversion device; providing, by the quantum frequency conversion device, a tuneable pump field that interacts with the quantum signals to remove the frequency differences of the quantum signals emitted by the OINQs; and

[0066] changing the frequency of the quantum signals to a converted frequency.In an embodiment the method further comprises:

[0067] inputting a plurality of modes from a plurality of central waveguides according to any of the aforementioned methods into a wavelength division multiplexer; and multiplexing, by the wavelength division multiplexer, the modes from the central waveguides into a single spatial mode.

[0068] In an embodiment the method further comprises:

[0069] demultiplex, at a mid-point by a demultiplexer, the modes received from the nodes such that each resulting signal can be linked with the corresponding signal from another node.

[0070] The invention further relates to a second method for multiplexing quantum signals in a quantum network, comprising:

[0071] executing a first multiplexing step that sequentially addresses qubit systems, comprising:

[0072] o outputting, by at least one coherent light source, light into a central waveguide; o positioning a plurality of add-drop filters on the central waveguide, each of the add-drop filters being configured to in-couple and out-couple optical signals in a predetermined frequency band to and from the central waveguide respectively, wherein each predetermined frequency band of the add-drop filters differs from the predetermined frequency bands from the other add-drop filters, and wherein each add-drop filter is coupled to a qubit system that comprises a plurality of individual optically interfaced networking qubits, OINQs; and

[0073] o modulating the frequenc(ies) and / or amplitude of the light emitted by the at least one coherent light source to obtain a train of temporally spaced optical signals, each temporally spaced optical signal sequentially addressing one or more different add-drop filter;

[0074] executing a second multiplexing step, comprising:

[0075] o reconfiguring an optical switch array from a first configuration to a second configuration, such that an optical signal received in the second configuration addresses a different subset of OINQs compared to the first configuration, wherein the optical switch array is provided in each qubit system; and executing a third multiplexing step, comprising:

[0076] o inputting a plurality of quantum signals obtained from the temporal multiplexing step and the first multiplexing step into a wavelength division multiplexer; ando combining, by the wavelength division multiplexer, the plurality of quantum signals into a single spatial mode.

[0077] The first multiplexing step and the second multiplexing step may both be denoted as spectral-temporal. The third multiplexing step may be denoted as spectral.

[0078] The plurality of quantum signals may comprise a plurality of streams of quantum signals. The wavelength division multiplexer may combine the plurality of streams of quantum signals into a single spatial mode.

[0079] It is noted that all other embodiment disclosed in the present disclosure may be combined with the second method. The second method has similar effects and advantages as described for the system and the first method.

[0080] Further advantages, features and details are elucidated on the basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, wherein:

[0081] figure 1 depicts schematic view of a system according to the invention;

[0082] figure 2 depicts a schematic view of a quantum frequency conversion system; and figure 3 depicts a schematic view of demultiplexing at a central node.

[0083] System 2 (figure 1) comprises coherent light source 4 which is coupled to modulator 6. Coherent light source 4 may be a laser. Modulator 6 may be a frequency and / or amplitude modulator. Modulator 6 is configured to modulate the light coming from coherent light source 4. Modulator 6 may generate optical signals from the light that is emitted by coherent light source 4. The optical signals that are generated by modulator 6 are transferred into central waveguide 8. Central waveguide 8 is in this illustrated embodiment an optical waveguide that transfers the optical signals generated by modulator 6 in guiding direction D. Central waveguide 8 may for example be an optical fibre or an on-chip waveguide.

[0084] A plurality of add-drop filters 10a, 10b, 10c are coupled to central waveguide 8. Each adddrop filter 10a, 10b, 10c can out-couple optical signals of a predetermined frequency band from central waveguide 8 to transmission line 12a, 12b, 12c. Each add-drop filter 10a, 10b, 10c is also configured to in-couple optical signals of said predetermined frequency band from transmission line 12a, 12b, 12c to central waveguide 8. In others words, add-drop filters 10a, 10b, 10c can pick of optical signals of certain frequencies of central waveguide 8 and input optical signals of said certain frequencies into central waveguide 8. It is clear for the skilled person that instead of one transmission line 12a, 12b, 12c for each add-drop filter 10a, 10b, 10c, it is also possible to provide two transmission lines per add-drop filter, wherein a first transmission line of the two transmission lines is configured to out-couple optical signals from the add-drop filter and a second transmission line of the two transmission lines is configured to in-couple optical signals to the add-drop filter.

[0085] Each add-drop filter 10a, 10b, 10c is coupled to its respective transmission line 12a, 12b, 12c. Transmission line 12a, 12b, 12c extends between add-drop filter 10a, 10b, 10c and opticalswitch array 14a, 14b, 14c and is configured to transmit optical signals from add-drop filter 10a, 10b, 10c towards optical switch array 14a, 14b, 14c and vice versa. Each optical switch array 14a, 14b, 14c is operatively coupled to an associated plurality of optically interfaced networking qubits (OINQs) 16a, 16n. Each optical switch array 14a, 14b, 14c is configured to selectively direct an optical signal from central waveguide 8 to an individual OINQ of plurality of OINQs 16a, 16n, and to transmit a subsequent quantum signal from the individual OINQ back towards central waveguide 8.

[0086] Optical switch array 14a, 14b, 14c can have different configurations. Each configuration is associated with a different subset of OINQs of the plurality of OINQs 16a, 16n which are colocated in the same light-guiding structure. The subset of OINQs may be a single OINQ or multiple OINQs. For example, the subset may be a N number of OINQs, wherein N is a natural number, for example, 1, 2, 3, etc. Optical switch arrays 14a, 14b, 14c may thus be set to a specific configuration to address a specific subset of OINQs of the plurality of OINQs 16a, 16n. Qubit systems 18 comprise optical switch array 14 and plurality of OINQs 16a, 16n. In this illustrated embodiment three qubit systems are shown, namely qubit system 18a comprising optical switch array 14a and its associated plurality of OINQs 16a, 16n, qubit system 18b comprising optical switch array 14b and its associated plurality of OINQs 16a, 16n, and qubit system 18c comprising optical switch array 14c and its associated plurality of OINQs 16a, 16n.

[0087] Each add-drop filter 10a, 10b, 10c is tuned to a different frequency or frequency band. For example, add-drop filter 10a is tuned to frequency fl, add-drop filter 10b is tuned to frequency f2, and add-drop filter 10a is tuned to frequency f3. Frequencies fl, f2 and f3 may also be frequency bands. For example, frequency fl may be 5 GHz, or may be frequency band of 4-6 GHz.

[0088] Frequency tuning device 20 is configured to tune add-drop filters 10a, 10b, 10c to a specific frequency. Frequency tuning device 20 can in this way change the frequency which is incoupled and out-coupled by the add-drop filters, or ensure that the frequency remains the same by compensating an undesired change of the frequency of the add-drop filters. It is clear for the skilled person that frequency tuning device 20 is optional, meaning that system 2 may also function without frequency tuning device 20.

[0089] Modulator 6 is configured to generate optical signals of different frequencies such that the add-drop filters 10a, 10b, 10c can sequentially be addressed. For example, modulator 6 generates a first optical signal of frequency fl to address add-drop filter 10a, subsequently generate a second optical signal of frequency f2 to address add-drop filter 10b, and subsequently generate a third optical signal of frequency f3 to address add-drop filter 10c. In this way, by modulating the frequency of the optical signals, the system can subsequently address different add-drop filters.

[0090] It is noted that in this illustrated embodiment only three add-drop filters 10a, 10b, 10c and three qubit systems 18a, 18b, 18c are shown. It is clear for the skilled person that in practice thenumber of add-drop filters and qubit systems may be any natural number. For example, the number of add-drop filters and qubit systems may be 10, 100, or 1,000, depending on the desired amount of multiplexing. Furthermore, it is also clear for the skilled person that the number of OINQs 16a, 16n can also be any natural number. For example, the number of OINQs 16a, 16n may be 10, 100 or 1000, depending on the desired amount of multiplexing.

[0091] Switch controller 22 is operatively coupled to each of the optical switch arrays 14a, 14b, 14c. Switch controller 22 is configured to set the configuration of the optical switch arrays 14a, 14b, 14c. For example, switch controller 22 may send a first control signal to put optical switch array 14a in a first configuration in which an optical signal coming from transmission line 12a is routed towards first set of OINQs 16a. At some time later, switch controller 22 may send a second control signal to put optical switch array 14a in a second configuration in which an optical signal coming from transmission line 12a is routed towards second set of OINQs 16n. The first and second configuration also provide that an emitted quantum signal from first set of OINQs 16a and second set of OINQs 16n, respectively, are sent back towards central waveguide 8. In this way, switch controller 22 may set to which set of OINQs optical signals from central waveguide 8 are routed. It is noted that although first set of OINQs 16a and second set of OINQs 16n may be illustrated as a single element, they both may comprise multiple OINQs.

[0092] Emission tuning device 24, which can also be noted as OINQ tuning device, is configured to dynamically tune at least one of the OINQs from quantum systems 18a, 18b, 18c. Preferably, the optical transmission frequency of quantum systems 18a, 18b, 18c are tuned. Preferably, emission tuning device 24 tunes the OINQs by strain tuning. By tuning at least one of the OINQs from quantum systems 18a, 18b, 18c, the emission frequency of the OINQs can be changed. It is clear for the skilled person that emission tuning device 24 is optional, meaning that system 2 may also function without emission tuning device 24.

[0093] Controller 26 comprising frequency tuning device 20, switch controller 22 and emission tuning device 24. Controller 26 is operatively coupled to laser 4 and modulator 6. Controller 26 is configured to sequentially change the frequency of the optical signals in the central waveguide by controlling modulator 6. The frequency of the optical signals may be changed to a frequency that falls within any of the predetermined frequency bands fl, f2, f3 of add-drop filters 10a, 10b, 10c to sequentially address the plurality of qubits systems 18a, 18b, 18c.

[0094] Controller 26 is in this illustrated embodiment shown as a single controller which comprises frequency tuning device 20, switch controller 22 and emission tuning device 24. It is clear for the skilled person that controller 26 may also be distributed in the system. For example, switching controller 22 may be arranged at a location remote from controller 26, but still be operatively coupled thereto.In a first multiplexing step, controller 26 sequentially changes the frequency of the optical signals generated by modulator 6. In this way, each time the frequency of the optical signal is changed a different add-drop filter 10a, 10b, 10c and thus a different qubit system 18a, 18b, 18c can be addressed. As this optical signal is then in-coupled back to the central waveguide, each time a frequency is changed, a different quantum signal (for example a photonic qubit) may be inserted into central waveguide 8.

[0095] In a second multiplexing step, switching controller 22 of controller 26 configures optical switch arrays 14a, 14b, 14c such that a different OINQ is addressed each time the frequency of the optical signals is changed by the controller 26 and modulator 6.

[0096] The switching time of optical switch arrays 14a, 14b, 14c may be in the order of milliseconds, or possibly faster. The temporal mode of a quantum signal from the OINQ may be in the order of 10 nanoseconds. Therefore, only using optical switch arrays 14a, 14b, 14c cannot obtain an optical signal wherein all possible temporal modes are occupied by the quantum signals emitted from the OINQ. To this end, the first multiplexing step is combined with the second multiplexing step which uses optical switch arrays 14a, 14b, 14c.

[0097] A possible way of viewing the combination of the first multiplexing step and the second multiplexing step may be as follows. Modulator 6 changes the frequency of a first optical signal that is generated to a first frequency fl . Optical signal with frequency fl is out-coupled of central waveguide 8 by add-drop filter 10a and sent through transmission line 12a. First optical switch array 14a is set to a first configuration which addresses first set of OINQs 16a. First set of OINQs 16a emits a first quantum signal which is transferred back to add-drop filter 10a and in-coupled to central waveguide 8. Then modulator 6 changes the frequency of a second optical signal that is generated to a second frequency f2, such that second add-drop filter 10b and second qubit systems 18b is addressed, resulting in a second quantum signal that is in-coupled to central waveguide 8 by add-drop filter 10b. Similar to the first and second optical signal, a third optical signal with frequency f3 is generated by modulator 6, coupled to add-drop filter 10c and in-coupled back as third quantum signal.

[0098] In the meantime, controller 26 has set first optical switch array 14a from the first configuration to a second configuration that routes optical signals to second set of OINQs 16n. Then, modulator 6 generates a fourth optical signal, again with frequency fl . Then fourth optical signal is out-coupled by add-drop filter 10a to second set of OINQs 16n which results in a fourth quantum signal that is transmitted back to central waveguide 8. This process may continue with again subsequently addressing second and third qubit systems 18b, 18c by changing the frequency of the fifth and sixth optical signals to f2 and f3, respectively. The skilled person understands that this example is for illustrative purposes only, and that in practice many more qubit systems may be addressed before returning to first qubit system 18 a.A first advantage of the system and method is that many more quantum signals can be obtained in the central waveguide than only using the optical switch array. A second advantage is that the add-drop filters shield non-addressed qubit systems from optical signals targeted at other qubit systems.

[0099] Additionally, using the optical switch array has the additional advantage that the number of addressable OINQs that can be addressed for a given number of add-drop filters is increased by at least a factor equal to the number of switching array outputs Only using frequency modulation without the optical switch array could result in a lossy method. Next to this, the optical switch array provides additional suppression of addressing light to non-targeted OINQs within each qubit system.

[0100] The quantum signal in central waveguide 8 may be send to quantum frequency conversion system 128. The quantum signal in central waveguide 8 may undergo a third multiplexing step. The third multiplexing step will be further explained in figures 2 and 3.

[0101] Quantum frequency conversion device 128 (figure 2) comprises tunable pump field 130. Once the temporal modes are fully occupied, and the amount of time it takes to address OINQs emitting onto central waveguide 8 exceed the round-trip-time (the time it takes for a photon to travel to and back from the midpoint), one can employ the third multiplexing method to increase the rate of quantum signals. By removing the frequency differences between the emission of the OINQs in for example system 108a, using a Quantum Frequency Conversion (QFC) process, the resulting quantum signals in the telecom band can be combined onto a single optical fibre using Wavelength Division Multiplexer (WDM) 140. The QFC process can remove the spectral differences by tuning the tunable pump field 130 to have the same energy difference as the quantum signal with respect to the central target wavelengths. By repeating this multiplexing for different waveguides 108a, 108b, 108n that use separate quantum frequency conversion devices 132, 134, 136 tuned to a different central target wavelength, the stream of quantum signals can be multiplexed onto the same optical fibre. After the quantum frequency conversion devices 132, 134, 136 the quantum signals may be routed by a (1x2) WDM LongPass 138. Choosing a special target wavelength one can route the photons to local (MxM) switching matrix 142 that can perform the Bell state measurement for local entanglement. From (Mxl) WDM 140 the quantum signals may be sent to central node 144 in which a Bell-state measurement may be performed with quantum signals from another node.

[0102] The connections to de-multiplex the signals are shown in figure 3. The quantum signals that are incoming from different nodes 146a, 146b, 146c first pass through frequency / phase shifters 148a, 148b, 148c, respectively. Afterwards, the quantum signals are split by a (IxM) wavelength demultiplexer 150a, 150b, 150c. The signals separated in wavelength can be combined with the corresponding signals from different nodes that match their wavelength. For example, first signals152a, 152b, 152c of first wavelength fl may be combined in Bell state measurement device 164. Second signals 154a, 154b, 154c of first wavelength f2 may be combined in Bell state measurement device 166. Third reference signals 156a, 156b, 156c, which are configured to stabilize the frequency and / or phase of the optical signal, may be combined in beat generator 168.

[0103] Active feedback can be realized using dedicated signal 162 from reference laser 160 at a chosen spectral location to perform joint stabilization of the wavelength to within the linewidth of the emitters. This allows for multiplexed detection of heralding signals allowing for entanglement generation between multiple pairs of nodes.

[0104] The first multiplexing step and the second multiplexing step may both be denoted as spectral-temporal. The third multiplexing step may be denoted as spectral.

[0105] The present invention is by no means limited to the above described preferred embodiments thereof. The rights sought are defined by the following clauses within the scope of which many modifications can be envisaged.

Claims

CLAIMS1. System for multiplexing quantum signals in a quantum network node, comprising:a central waveguide;at least one coherent light source that is configured to emit light that is coupled to the central waveguide;at least one modulator that is operatively coupled to the at least one coherent light source and which is configured to change the frequency and / or amplitude of optical signals generated by the at least one coherent light source;a plurality of add-drop filters that are coupled to the central waveguide, each of the add-drop filters being configured to in-couple and out-couple optical signals in a predetermined frequency band to and from the central waveguide respectively, wherein each predetermined frequency band of the add-drop filters differs from the predetermined frequency bands from the other add-drop filters;a plurality of qubit systems, each qubit system being operatively coupled to a respective add-drop filter, wherein each qubit system comprises at least one optically interfaced networking qubit, OINQ, configured to generate a quantum signal; and a controller that is operatively coupled to the at least one modulator and is configured to sequentially change the frequency of the optical signals in the central waveguide to a frequency that falls within any of the predetermined frequency bands of the add-drop filters to sequentially address the plurality of qubits systems.

2. System according to claim 1 , wherein each qubit system of the plurality of qubit systems comprises a plurality of OINQs and an optical switch array, wherein the optical switch array is provided between the add-drop filter and the plurality of OINQs, the optical switch array being configured to selectively direct an optical signal to a subset of the plurality of OINQs and to transmit a subsequently emitted quantum signal from the subset of OINQs back towards the central waveguide, wherein the controller is operatively coupled to optical switch array, and wherein the controller is configured to reconfigure the optical switch array to sequentially address the plurality of OINQs.

3. System according to claim 2, wherein the optical switch array comprises microelectromechanical systems based photonic integrated circuits or electro-optical switches.

4. System according to any one of claims 1, 2, or 3, wherein each of the OINQs are tuned to an emission frequency, preferably by strain tuning.

5. System according to claim 4, wherein the system further comprises an OINQ tuning device that dynamically tunes the emission frequency of the OINQs.

6. System according to any one of the foregoing claims, wherein the system further comprises an add-drop tuning device that dynamically tunes the predetermined frequency band of the add-drop filters.

7. System according to any one of the foregoing claims, further comprising a quantum frequency conversion device that is configured to change the frequency of the photonic modes in the central waveguide to a converted frequency, wherein the quantum frequency conversion device is preferably positioned downstream the add-drop filters.

8. System according to claim 7, wherein the quantum frequency conversion device further is configured to remove the frequency differences of the quantum signals emitted by the OINQs.

9. Assembly for multiplexing quantum signals in a quantum network node, comprising: a plurality of systems according to claim 7 or 8, wherein each of the converted frequency of the systems are different from each other; anda wavelength division multiplexer that is coupled to each of the central waveguides from the plurality of systems and is configured to multiplex the central waveguides modes into a single spatial mode.

10. Optical quantum network, comprising:a plurality of nodes, each node comprising a system according to any one of the claims 1-8 or an assembly according to claim 9; andat least one mid-point that is operatively coupled to at least two nodes of the plurality of nodes, wherein the at least one mid-point is configured to receive spatial modes from at least two nodes of the plurality of nodes, wherein the at least one mid-point comprises an interfacing device that is configured to interface the spatial modes from at least two of the nodes, wherein the interfacing device preferably comprises a Bellstate measurement device.

11. Optical quantum network according to claim 10, comprising an assembly according to claim 9, wherein the mid-point further comprises a wavelength division demultiplexersystem that is configured to demultiplex the modes received from the nodes such that each resulting signal can be linked with the corresponding signal from another node.

12. Reflection based optical quantum network, comprisinga plurality of nodes, each node comprising a system according to any one of the claims 1-8 or an assembly according to claim 9; andat least one photon state analysing device that is coupled to at least one node of the plurality of nodes and is configured to herald spin-spin entanglement between the OINQs of the at least one node via measurement of the quantum signals that were sent from nodes of the network.

13. Method for multiplexing quantum signals in a quantum network, comprising:providing:o at least one coherent light source that is configured to emit light that is coupled a central waveguide;o at least one modulator that is operatively coupled to the at least one coherent light source and is configured to change the frequency and / or amplitude of optical signals from the light emitted by the coherent light source, a plurality of add-drop filters that are coupled to the central waveguide, each of the adddrop filters being configured to in-couple and out-couple optical signals in a predetermined frequency band to and from the central waveguide respectively, wherein each predetermined frequency band of the add-drop filters differs from the predetermined frequency bands from the other add-drop filters; o a plurality of qubit systems each being coupled to a respective add-drop filter, wherein each qubit system comprises at least one optically interfaced networking qubit, OINQ, configured to generate a quantum signal; and o a controller that is operatively coupled to the at least one modulator, further comprising,emitting, by at the at least one coherent light source, light that couples to the central waveguide;changing, by the controller, the frequency of the optical signals in the central waveguide to a frequency that falls within any of the predetermined frequency bands of the add-drop filters to sequentially address the plurality of qubits systems.

14. Method according to claim 13, each qubit system of the plurality of qubit systems comprises a plurality of OINQs and an optical switch array, wherein the optical switcharray is provided between the add-drop filter and the plurality of OINQs, the optical switch array being configured to selectively direct an optical signal to an individual OINQ of the plurality of OINQs and to transmit a subsequent quantum signal from the OINQ back towards the central waveguide, wherein the controller is operatively coupled to the optical switch array, further comprising:reconfiguring, by the controller, the optical switch array to sequentially address the plurality of OINQs.

15. Method according to claim 13 or 14, wherein the system further comprises an OINQ tuning device, wherein the controller is operatively coupled to the tuning device, further comprising:dynamically tuning, by the controller, the emission frequency of the OINQs.

16. Method according to any one of claims 13-15, wherein the system further comprises an add-drop tuning device, wherein the controller is operatively coupled to the add-drop tuning device, further comprising:dynamically tuning, by the controller, the predetermined frequency band of the adddrop filters.

17. Method according to any one of claims 14-16, further comprising a quantum frequency conversion device, wherein the controller is operatively coupled to the quantum frequency conversion device, further comprising:receiving the quantum signals in the quantum frequency conversion device; providing, by the quantum frequency conversion device, a tuneable pump field that interacts with the quantum signals to remove the frequency differences of the quantum signals emitted by the OINQs; andchanging the frequency of the quantum signals to a converted frequency.

18. Method according to any one of the foregoing claims, 13-17, further comprising:inputting a plurality of modes from a plurality of central waveguides according to any of the aforementioned methods into a wavelength division multiplexer; and multiplexing, by the wavelength division multiplexer, the modes from the central waveguides into a single spatial mode.

19. Method according to claim 18, further comprising:demultiplex, at a mid-point by a demultiplexer, the modes received from the nodes such that each resulting signal can be linked with the corresponding signal from another node.

20. Method for multiplexing quantum signals in a quantum network, comprising:executing a first multiplexing step that sequentially addresses qubit systems, comprising:o outputting, by at least one coherent light source, light into a central waveguide; o positioning a plurality of add-drop filters on the central waveguide, each of the add-drop filters being configured to in-couple and out-couple optical signals in a predetermined frequency band to and from the central waveguide respectively, wherein each predetermined frequency band of the add-drop filters differs from the predetermined frequency bands from the other add-drop filters, and wherein each add-drop filter is coupled to a qubit system that comprises a plurality of individually addressable optically interfaced networking qubits, OINQs; ando modulating the frequency and / or amplitude of the light emitted by the at least one coherent light source to obtain a train of temporally spaced optical signals, each temporally spaced optical signal sequentially addressing a different adddrop filter;executing a second multiplexing step, comprising:o reconfiguring an optical switch array from a first configuration to a second configuration, such that an optical signal received in the second configuration addresses a different subset of OINQs compared to the first configuration, wherein the optical switch array is provided in each qubit system; and executing a third multiplexing step, comprising:o inputting a plurality of optical signals obtained from the first multiplexing step and the second multiplexing step into a wavelength division multiplexer; and o combining, by the wavelength division multiplexer, the plurality of optical signals into a single spatial mode.

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