Quantum memory device

Abstract

This invention provides a quantum memory device, and a method of operating the quantum memory device, the quantum memory device comprising: a structure defining a plurality of vapour containment channels extending through the structure, each vapour containment channel containing vapour; and a controller configured to selectively: couple a first optical signal to a first vapour containment channel of the plurality of vapour containment channels, wherein the first optical signal contains 10 information, and control storage of the information of the first optical signal in the first vapour containment channel.

Description

[0001] A36148

[0002] QUANTUM MEMORY DEVICE

[0003] Field of the Invention

[0004] The present invention relates to a quantum memory device and a method of operating a 5 quantum memory device.

[0005] Background

[0006] It is desirable to contain an atomic vapour, such as a vapour of alkali and alkali earth metals, for use in a variety of quantum technologies, such as an atomic receiver or atomic clock, and a variety of optical technologies, such as an optical reference or gas laser. Typically, a containment of atomic vapour is provided as a vessel with an optical coupling to allow an optical signal to interact with the contained atomic vapour.

[0007] A vessel may be coupled to a plurality of optical signals which each interact with a distinct 15 volume of contained atomic vapour within the vessel. Each interaction between an optical signal and a distinct volume of contained atomic vapour may be part of a unique application of a quantum or optical technology, thereby increasing the usefulness of the device. For example, each interaction between an optical signal and a distinct volume of contained atomic vapour may store information as an Electromagnetically Induced 20 Transparency (EIT) based quantum memory, such that the plurality of interactions of the plurality of optical signals increases the information storage capacity of the vessel. However, such a vessel may experience crosstalk between neighbouring volumes of contained atomic vapour due to leakage of an optical signal between neighbouring volumes and / or motion of the atomic vapour between neighbouring volumes.

[0008] Summary of the Invention

[0009] According to a first aspect of the invention, there is provided a quantum memory device comprising: a structure defining a plurality of vapour containment channels extending through the structure, each vapour containment channel containing vapour; and a 30 controller configured to selectively: couple a first optical signal to a first vapour containment channel of the plurality of vapour containment channels, wherein the first optical signal contains information, and control storage of the information of the first optical signal in the first vapour containment channel.A36148

[0010] The structure may be constructed of a material that prevents leakage of the first optical signal and / or vapour between channels of the plurality of channels. That is, the material may be impermeable to the vapour and / or opaque to the first optical signal.

[0011] 5 The controller may be further configured to selectively: couple a second optical signal to a second vapour containment channel of the plurality of vapour containment channels, wherein the second optical signal contains information, and control storage of the information of the second optical signal in the second vapour containment channel.

[0012] The information of the first optical signal and the information of the second optical signal may be stored contemporaneously.

[0013] The controller may be further configured to selectively: couple a third optical signal to the first vapour containment channel, and control an interaction between the third optical 15 signal and vapour contained in the first vapour containment channel so as to control storage of the information of the first optical signal in the first vapour containment channel.

[0014] The controller may be further configured to selectively: couple the third optical signal to 20 the second vapour containment channel, and control an interaction between the third optical signal and the vapour contained in the second vapour containment channel so as to control storage of the information of the second optical signal in the second vapour containment channel.

[0015] Each vapour containment channel may comprise a Hollow Core Fibre, HCF, extending therethrough.

[0016] The quantum memory device may further comprise a housing defining an interior section, wherein: the structure is positioned within the interior section of the housing, the interior 30 section of the housing contains vapour, and a first end of the first vapour containment channel is open to the interior section of the housing for fluidic communication of the vapour between the interior section of the housing and the first vapour containment channel.A36148

[0017] A second end of the first vapour containment channel may be open to the interior section of the housing for fluidic communication of the vapour between the interior section of the housing and the first vapour containment channel.

[0018] 5 The controller may be configured to control storage of the information so as to store the information or to retrieve the information.

[0019] The third optical signal may have a frequency for inducing Electromagnetically Induced Transparency, EIT, in the first vapour containment channel, and the controller is may be 10 configured to selectively: control an intensity of the third optical signal in the first vapour containment channel during passage of the first optical signal so as to reduce EIT in the first vapour containment channel to the first optical signal such that the information of the first optical signal is stored in the first vapour containment channel, and control the intensity of the third optical signal in the first vapour containment channel so as to 15 increase EIT in the first vapour containment channel to the first optical signal such that the stored information of the first optical signal is retrieved from the first vapour containment channel.

[0020] The intensity of the third optical signal may be reduced so as to cease EIT in the first 20 vapour containment channel such that the information of the first optical signal is stored in the first vapour containment channel, and the intensity of the third optical signal may be increased so as to induce EIT in the first vapour containment channel to the first optical signal such that the information of the first optical signal is retrieved from the first vapour containment channel.

[0021] 25

[0022] The quantum memory device may further comprise a generator for generating an electromagnetic field, wherein the controller may be configured to selectively control the electromagnetic field generated by the generator so as to control storage of the information of the first optical signal in the first vapour containment channel.

[0023] 30

[0024] According to a second aspect of the invention, there is provided a method of operating a quantum memory device, the quantum memory device comprising a structure defining a plurality of vapour containment channels extending through the structure, each vapour containment channel containing vapour, the method comprising the steps of: selectively 35 coupling a first optical signal to a first vapour containment channel of the plurality ofA36148

[0025] vapour containment channels; and selectively controlling storage of the information of the first optical signal in the first vapour containment channel.

[0026] The method may further comprise the steps of: selectively coupling a second optical 5 signal to a second vapour containment channel of the plurality of vapour containment channels; and selectively controlling storage of the information of the second optical signal in the second vapour containment channel.

[0027] The information of the first optical signal and the information of the second optical signal may be stored contemporaneously.

[0028] The method may further comprise the steps of: selectively coupling a third optical signal to the first vapour containment channel, and selectively controlling an interaction between the third optical signal and vapour contained in the first vapour containment 15 channel so as to control storage of the information of the first optical signal in the first vapour containment channel.

[0029] The method may further comprise the steps of: selectively coupling the third optical signal to the second vapour containment channel, and selectively controlling an 20 interaction between the third optical signal and vapour contained in the second vapour containment channel so as to control storage of the information of the second optical signal in the second vapour containment channel.

[0030] The third optical signal may have a frequency for inducing Electromagnetically Induced Transparency, EIT, in the first vapour containment channel, and the method further comprises the steps of: selectively controlling an intensity of the third optical signal in the first vapour containment channel during passage of the third optical signal so as to reduce EIT in the first vapour containment channel to the first optical signal such that the information of the first optical signal is stored in the first vapour containment channel, 30 and selectively controlling the intensity of the third optical signal in the first vapour containment channel so as to increase EIT in the first vapour containment channel to the first optical signal such that the stored information of the first optical signal is retrieved from the first vapour containment channel.According to a third aspect of the invention, there is provided a computer program comprising instructions which, when the program is executed by a quantum memory device, cause the quantum memory device to carry out the steps of the second aspect of the invention. The computer program may be stored on a computer readable carrier 5 medium.

[0031] Brief Description of the Figures

[0032] In order that the present invention may be better understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which:

[0033] Figure 1 is a schematic view of a Rydberg cell array;

[0034] Figure 2 is an end-view of a structure of the Rydberg cell array of Figure 1 ;

[0035] Figure 3 is a schematic view of a first system comprising the Rydberg cell array of Figure 15 1 ;

[0036] Figure 4 is a flow diagram of a method of storing information using the first system of Figure 3;

[0037] Figure 5 is a flow diagram of a method of retrieving information using the first system of Figure 3;

[0038] 20 Figure 6 is a schematic view of a second system comprising the Rydberg cell array of Figure 1 ;

[0039] Figure 7 is a flow diagram of a method of manufacturing the Rydberg cell array of Figure 1 ;

[0040] Figure 8 is a schematic diagram of an alternative Rydberg cell array; and

[0041] 25 Figure 9 is a flow diagram of a method of operating a quantum memory device.

[0042] Detailed Description

[0043] Figure 1 is a side view of a Rydberg cell array 100. The Rydberg cell array 100 comprises a structure 110 and a housing 120. The structure 110 and housing 120 are both cylindrical. The structure 110 is positioned in an interior section of the housing 120, such as by being affixed to an interior wall of the housing 120 (e.g. by adhesive). The structure 110 is therefore enclosed within the housing 120.

[0044] The housing 120 comprises a first end face 121 and a second end face 123 that are both 35 optically transparent and comprise anti-reflective coatings. A length of the housing 120(i.e. a distance along a major axis of the housing 120) is greater than a length of the structure 110 (i.e. a distance along a major axis of the structure 110). A first volume is therefore defined as a portion of the housing 120 between the first end face 121 of the housing 120 and a first end face 118 of the structure 110. A second volume is also 5 defined as a portion of the housing 120 between the second end face 123 of the housing 120 and a second end face 119 of the structure 110. All interior surfaces of the first and second volumes (including interior surfaces of the first and second end faces 121, 123 of the housing 120, the end faces 118, 119 of the structure 110, the interior surface of the cylindrical section of the housing 120 between the first end face 121 of the housing 120 and the first end face 118 of the structure 110, and an interior surface of the cylindrical section of the housing 120 between the second end face 123 of the housing 120 and the second end face 119 of the structure 110) comprise an anti-absorption coating (e.g. paraffin).

[0045] 15 Figure 1 further illustrates a first burr 125 and second burr 127. During manufacturing, the first burr 125 is initially an opening or nozzle acting as an interface for fluidic communication between an exterior environment of the Rydberg cell array 100 and the first volume of the housing 120 so as to enable filling and / or evacuation of the first volume of housing 120. Once manufactured, the opening / nozzle is sealed so as to create the 20 first burr 125. Similarly, the second burr 127 is initially an opening or nozzle acting as an interface for fluidic communication between the exterior environment of the Rydberg cell array 110 and the second volume of the housing 120.

[0046] Figure 2 is an end-view of the structure 110 illustrating the first end face 118 of the 25 structure 110. The structure 110 comprises a substrate 111 defining a plurality of channels 113. An open end of each channel of the plurality of channels 113 is shown in Figure 2. The channels extend in parallel (or at least substantially in parallel) to the major axis of the structure 110. The substrate 111 is enclosed in an inner housing 115 having an outer diameter that is equal (or substantially equal) to the inner diameter of the housing 120 of the Rydberg cell array 100.

[0047] A Hollow Core Fibre (HCF) extends through each channel of the plurality of channels 113. Each channel therefore has an inner diameter that is equal (or substantially equal) to an outer diameter of the HCF extending therethrough. An outer wall of the HCF may 35 be fixed to an inner wall of the channel by adhesive. As shown in the inset of Figure 2,the HCF comprises a fibre 117a, a hollow core 117b, and a plurality of inner circular structures 117c surrounding the hollow core 117b. The HCF is an anti-resonant HCF. Figure 2 provides example diameters of the hollow core 117b and fibre 117a. The diameter of the hollow core 117b of the HCF may be in the range of 0.005mm (inclusive) 5 to 0.05mm (inclusive) and the diameter of the fibre 117a may be in the range of 0.08mm (inclusive) to 0.250mm (inclusive).

[0048] An interior of the Rydberg cell array 100 - being the first volume of the housing 120, second volume of the housing 120, and the respective hollow core 117b of the HCF extending along each channel of the plurality of channels 113 - is filled with an atomic vapour, such as an alkali or alkali earth metallic vapour (e.g. Rubidium, Caesium or Strontium). The construction material of at least one of the substrate 111 of the structure 110 and the fibre 117a of the HCF is / are selected to prevent (or at least substantially reduce) leakage of light and atomic vapour between HCFs. Each HCF therefore forms 15 a distinct Rydberg cell, such that the HCFs of the plurality of channels form an array of Rydberg cells. That is, leakage of light may be prevented through the use of waveguide structures to contain the light within the channel. Opaque material may also be used to further prevent any leakage of light. The construction material may be any material that does not react with the atomic vapour. If a material that reacts with the atomic vapour is 20 used, then the filling time may be increased to compensate for reactions between the atomic vapour and the material.

[0049] As noted above, the end faces 121, 123 of the housing 120 are optically transparent. The housing 120 therefore permits passage of one or more optical signals through the 25 housing 120. The or each optical signal may couple with a particular Rydberg cell and thereafter be guided by the HCF of that Rydberg cell and excite the atomic vapour contained in that Rydberg cell. As described in more detail below, the Rydberg cell array 100 may be used as part of a quantum and / or optical technology in which each Rydberg cell of the Rydberg cell array 100 may act independently of each other Rydberg cell of the Rydberg cell array 100. The design of each HCF, such as its dimensions and construction material, may be selected so as to guide an optical signal of a particular frequency, depending on the application of the Rydberg cell array 100.

[0050] The Rydberg cell array 100 provides a number of advantages over a conventional array 35 (as described in the Background section above). For example, the Rydberg cell arrayA36148

[0051] 100 minimises (or at least reduces) crosstalk between Rydberg cells by minimising (or at least reducing) transfer of the optical signal and / or vapour between Rydberg cells. In other words, the optical signal and vapour are constrained along the whole length of the Rydberg cell. This is achieved at least by the physical barrier between neighbouring 5 Rydberg cells provided by the substrate 111 of the structure 110 and / or the fibre 117a of the HCF in each channel. Furthermore, in the conventional array, the Rydberg cells (each being a particular interaction between an optical signal and a distinct volume of contained atomic vapour) should be a sufficient distance apart to reduce this crosstalk issue. Therefore, the Rydberg cell array 100 achieves both a benefit of crosstalk reduction and a reduction in distance between Rydberg cells. In other words, the Rydberg cell array 100 may enjoy a greater density of Rydberg cells relative to the conventional array.

[0052] Additionally, the first and second volumes contain atomic vapour that act as a source of 15 vapour to the one or more hollow cores 117b of the HCFs. Therefore, the Rydberg cell array 100 may be manufactured in a relatively short amount of time (relative to a conventional vapour filled HCF) due to the vapour source in the first and second volumes continuing to fill the HCFs post-manufacture. Furthermore, the first and second volumes may replenish vapour in one or more hollow cores 117b of the HCFs due to vapour 20 absorption (e.g. by the fibre 117a). The first and second volumes of the housing 120 therefore extend the useful lifetime of the Rydberg cell array 100.

[0053] Figure 3 illustrates a first system 1000 utilising the Rydberg cell array 100 as an Electromagnetically Induced Transparency (EIT) based quantum memory. The first system 1000 comprises a coupling laser 1010, a first prism 1020, a first Spatial Light Modulator (SLM) 1030, a first lens array 1040, the Rydberg cell array 100, a second lens array 1050, a second SLM 1060, a second prism 1070, a signal laser 1080, and a detector 1090. Three channels of the plurality of channels are illustrated in Figure 3 by a dashed outline.

[0054] 30

[0055] The first system 1000 further comprises a controller 1100 configured to control other entities of the first system 1000 via respective connections (not shown). The controller 1100 comprises a timer for timed control of other entities in the first system 1000.A36148

[0056] In the following description of the first system 1000 shown in Figure 3, the term Rydberg cell” is used to describe an optical transmission medium of a particular channel of the plurality of channels 113 (i.e. the hollow core 117b of the HCF of that particular channel).

[0057] 5 The coupling laser 1010 is configured to generate a coupling beam that is split into a plurality of coupling beams (each illustrated as a solid arrowed line) using a diffraction grating (not shown) or similar optical component. Each coupling beam of the plurality of coupling beams is directed, by the first prism 1020, to a particular Rydberg cell of the Rydberg cell array 100. A propagation axis of the coupling beam aligns with a major axis of the HCF of the Rydberg cell, such that the coupling beam enters the HCF of the Rydberg cell and is thereafter guided by the HCF along its length. In other words, the first prism 1020 directs and optically couples each coupling beam of the plurality of coupling beams with a particular Rydberg cell of the Rydberg cell array 100. The coupling beam has a coupling frequency that is configured such that an energy of the 15 coupling beam matches the energy difference between a first ground state and an excited state of the atomic vapour contained in the Rydberg cell. Each coupling beam, following its passage of a Rydberg cell, is directed by the second prism 1070 towards, for example, a sensor or an absorbing power sink.

[0058] 20 The first prism 1020 may act in cooperation with the first lens array 1040 to direct and optically couple each coupling beam with a particular Rydberg cell of the Rydberg cell array 100. That is, the first lens array 1040 may focus each coupling beam so as to achieve optimal optical coupling with the HCF of that Rydberg cell (or at least improved optical coupling relative to the first prism 1020 alone). The first SLM 1030 may also 25 cooperate with the first prism 1020 and / or first lens array 1040 to direct and optically couple each coupling beam with the HCF, such as by applying a phase change to the coupling beam.

[0059] The first SLM 1030 may operate to selectively allow or prevent passage of each coupling signal of the plurality of coupling signals. The first SLM 1030 may therefore be operated to selectively allow passage of a first set of the plurality of coupling signals (the first set comprising zero, one, or more coupling signals of the plurality of coupling signals), each coupling signal of the first set of the plurality of coupling signals being allowed passage to a respective Rydberg cell of the Rydberg cell array 100 so as to excite the atomic 35 vapour of the Rydberg cell to a Rydberg state.A36148

[0060] The first SLM 1030 is further operated to selectively prevent passage of a second set of the plurality of coupling signals (such that the atomic vapour in the / each respective Rydberg cell is not excited to a Rydberg state). The second set of the plurality of coupling 5 signals may comprise zero, one or more coupling signals of the plurality of coupling signals. This may be achieved, for example, by the first SLM 1030 reducing an amplitude of each coupling signal of the second set of coupling signals to zero (or a negligible amount). In contrast, the first SLM 1030 does not apply an amplitude reduction to one or more coupling signals of the first set of coupling signals (or each coupling signal of the first set of coupling signals still has sufficient amplitude to excite the atomic vapour within the respective Rydberg cell following any amplitude reduction).

[0061] The signal laser 1080 is configured to generate a signal beam (illustrated as a dot-dash arrowed line) that contains information (as a quantum state of the signal beam) to be 15 stored. The signal beam is directed, by the second prism 1070 and second SLM 1060, towards a particular Rydberg cell of the Rydberg cell array 100. The second SLM 1060 is therefore able, by directing the signal beam to a particular Rydberg cell (e.g. by application of a phase change to the signal beam), to selectively allow passage of the signal beam to that particular Rydberg cell. A propagation axis of the signal beam aligns 20 with the major axis of the HCF of the Rydberg cell, such that the signal beam enters the HCF of the Rydberg cell and is thereafter guided by the HCF along its length. The signal beam, following its passage of the Rydberg cell, is directed by the first prism 1020 towards the detector 1090.

[0062] 25 The second prism 1070 and second SLM 1060 may act in cooperation with the second lens array 1050 to direct and optically couple the signal beam with a particular Rydberg cell. That is, the second lens array 1050 may focus the signal beam so as to achieve optimal optical coupling with the Rydberg cell (or at least improved optical coupling relative to the second prism 1070 and / or second SLM 1060 alone).

[0063] The signal beam has a signal frequency that is configured such that an energy of the signal beam matches the energy difference between a second ground state and the excited state of the atomic vapour contained in the Rydberg cell. If the atomic vapour is not excited by the control beam, then the signal beam will be absorbed (at least partially) 35 by the atomic vapour. However, if the atomic vapour is excited by the control beam, thenA36148

[0064] absorption of the signal beam is greatly reduced. This phenomenon is experienced when the control and signal beams have a frequency difference that is close to a frequency of a Raman transition between the first and second ground states of the atomic vapour.

[0065] 5 As the control and signal beams have different frequencies, the first and second prisms 1020, 1060 additionally act to spatially separate the control and signal beams as they propagate along each Rydberg cell.

[0066] As noted above, the plurality of coupling beams are selectively allowed / prevented passage to their corresponding HCFs of the plurality of channels 113. The atomic vapour contained in a Rydberg cell that is coupled to a coupling beam of the first set of the plurality of coupling beams will be excited by that coupling signal such that the atomic vapour becomes transparent to the signal beam. The atomic vapour contained in a Rydberg cell that is not coupled to a coupling beam of the first set of the plurality of 15 coupling beams (that is, that Rydberg cell is associated with a coupling beam of the second set of the plurality of coupling beams that is prevented passage to that Rydberg cell by the first SLM 1030) does not become transparent to the signal beam.

[0067] The first system 1000 may be controlled to store information contained in the signal beam 20 by implementing an Electromagnetically Induced Transparency (EIT) based quantum memory process. A general description of the EIT quantum memory process can be found, for example, in paper, “Optical quantum memory” A. I. Lvovsky et al., Nature Photonics 3, 706 - 714 (2009). The first system 1000 implements the EIT quantum memory process by the controller 1100 controlling the control laser 1010, first prism 25 1020, first SLM 1030 and first lens 1040 to pass a control beam of the plurality of control beams through a particular Rydberg cell of the Rydberg cell array 100. Whilst the control beam is interacting with the atomic vapour contained in that Rydberg cell, the controller 1100 controls the signal laser 1080, second prism 1070, second SLM 1060 and second lens 1050 to pass a signal beam through that Rydberg cell. Whilst the signal beam propagates along the Rydberg cell (i.e. after it has entered the HCF of that Rydberg cell and before it has exited the HCF of that Rydberg cell), the controller 1100 controls the first SLM 1030 so as to prevent passage of the control beam to the Rydberg cell (e.g. by the first SLM 1030 reducing an intensity of the control beam to zero). The controller 1100 and first SLM 1030 are configured for rapid reconfiguration so as to switch between 35 states in the timeframe of the signal beam’s transit of the Rydberg cell. The atomicA36148

[0068] vapour contained in the Rydberg cell is then no longer transparent to the signal beam (in other words, the EIT window is collapsed) and the quantum state of the signal beam (i.e. the information) is stored in that Rydberg cell. When it is desired to retrieve that information, the controller 1100 controls the control laser 1010, first prism 1020, first SLM 5 1030 and first lens 1040 to again pass a control beam through that Rydberg cell. The signal beam then resumes propagation along the Rydberg cell towards the detector 1090 for detection of the information contained in the signal beam.

[0069] The first system 1000 further benefits from being able to contemporaneously store information of a plurality of signal beams. That is, the system 100 may be controlled as described in the immediately above paragraph such that information of a first signal beam is stored in a first Rydberg cell of the Rydberg cell array 100. Whilst that information is stored (that is, before the first system 1000 is controlled to retrieve the information of the first signal beam, as described above), the first system 1000 may be 15 further controlled such that information of a second signal beam is stored in a second Rydberg cell of the Rydberg cell array 100. The first system 1000, comprising the Rydberg cell array 100, may therefore enable contemporaneous storage of information of a plurality of signal beams by:

[0070] • selectively coupling a first coupling beam to a first Rydberg cell of the Rydberg 20 cell array 100,

[0071] • selectively coupling a second coupling beam to a second Rydberg cell of the Rydberg cell array 100,

[0072] • selectively coupling a first signal beam to the first Rydberg cell whilst the first coupling beam interacts with the atomic vapour of the first Rydberg cell, 25 • selectively coupling a second signal beam to the second Rydberg cell whilst the second coupling beam interacts with the atomic vapour of the second Rydberg cell,

[0073] • whilst the first signal beam propagates along the first Rydberg cell, preventing passage of the first coupling beam to the first Rydberg cell so as to store information of the first signal beam in the first Rydberg cell, and • whilst the second signal beam propagates along the second Rydberg cell 113 and whilst information of the first signal beam is stored in the first Rydberg cell, preventing passage of the second coupling beam to the second Rydberg cell so as to store information of the second signal beam in the second Rydberg cell.

[0074] 35A36148

[0075] The first system 1000 may therefore be operated such that information of a plurality of signals beams can be stored, in parallel, in the Rydberg cell array 100. The Rydberg cell array 100 may store information in each Rydberg cell of the Rydberg cell array 100. The first system 1000 realises this benefit without experiencing any crosstalk (due to 5 leakage of light and / or atomic vapour between Rydberg cells) which would otherwise allow quantum information to drift between Rydberg cells.

[0076] Furthermore, the first system 1000 benefits from the replenishment of atomic vapour in one of more Rydberg cells by the atomic vapour in one or both of the first and second 10 volumes. The replenishment may be beneficial when atomic vapour in one or more Rydberg cells reacts with surrounding material, which would otherwise reduce the performance of the EIT quantum information process in those Rydberg cells.

[0077] A method of storing information of a signal beam will now be described with reference to 15 Figure 4. The first system 1000 is initially in a first state in which all coupling beams of the plurality of coupling beams are allowed passage to a respective Rydberg cell the Rydberg cell array 100. In other words, all coupling beams are members of the first set of the plurality of coupling beams. Therefore, the atomic vapour of each Rydberg cell of the Rydberg cell array 100 is transparent to a signal beam.

[0078] 20

[0079] In the following description, a first coupling beam of the plurality of coupling beams is associated with a first Rydberg cell of the Rydberg cell array 100, a second coupling beam of the plurality of coupling beams is associated with a second Rydberg cell of the Rydberg cell array 100, etc.

[0080] 25

[0081] In step S101, the first system 1000 identifies a trigger event for storing information contained in a first signal beam. In response, in step S103, the controller 1100 identifies a Rydberg cell for storage of the information contained in the first signal beam (e.g. a first Rydberg cell of the Rydberg cell array 100). In step S105, the controller 1100 30 selectively couples (e.g. by control of the second SLM 1060, second prism 1070 and / or second lens array 1050) the first signal beam with the first Rydberg cell. Whilst the first signal beam propagates along the first Rydberg cell (i.e. after it enters the HCF of the first Rydberg cell but before it exits the HCF of the first Rydberg cell), in step S107, the controller 1100 prevents passage of the first coupling beam to the first Rydberg cell (e.g.

[0082] 35 by control of the first SLM 1030 to reduce an amplitude of the first coupling beam toA36148

[0083] zero). In other words, the first coupling beam is no longer a member of the first set of the plurality of coupling beams and becomes a member of the second set of the plurality of coupling beams). The information contained in the first signal beam is therefore stored in the first Rydberg cell.

[0084] 5

[0085] Steps S101 to S107 may then be repeated so as to store information contained in a second signal beam in a second Rydberg cell.

[0086] A method of retrieving information of a signal beam will now be described with reference 10 to Figure 5. The first system 1000 is in a state in which a first Rydberg cell of the Rydberg cell array 100 stores information of a first signal beam (such that the first control beam is a member of the second set of the plurality of coupling signals and is therefore prevented passage to the first Rydberg cell) and a second Rydberg cell of the Rydberg cell array 100 stores information of a second signal beam (such that the second control 15 beam is a member of the second set of the plurality of coupling signals and is therefore prevented passage to the second Rydberg cell). In step S201, the first system 1000 identifies a trigger event for retrieving information of the first signal beam. In response, in step S203, the controller 1100 selectively couples (e.g. by control of the first SLM 1030 to increase the amplitude of the first coupling beam) the first control beam with the first 20 Rydberg cell. In other words, the first coupling beam is no longer a member of the second set of the plurality of coupling beams and becomes a member of the first set of the plurality of coupling beams). The first signal beam is therefore released from the first Rydberg cell and propagates to the detector 1090.

[0087] 25 The first system 1000 therefore provides for controlled storage of quantum information.

[0088] The skilled person will understand that the first system 1000 may be employed in a variety of quantum technologies, such as by replicating the information storage function of a classical computing architecture (e.g. Dynamic Random Access Memory (DRAM), and / or board register) in a quantum computing environment.

[0089] 30

[0090] Additionally, the first system 1000 may be employed in a quantum computing environment to provide a function that does not have a clear analogue in classical computing. In a first example, a Quantum Key Distribution (QKD) protocol may require an interferometric measurement of two photons thus requiring the two photons to arrive 35 at the detector at the same time. Therefore, the first system 1000 may be employed tostore and release one or both photons in the event of dissimilar link lengths or incorrectly timed release of the photons. These synchronisation challenges are proportional to the density of the QKD network, such that the first system 1000 may be an enabling technology for the QKD network. In a second example, the first system 1000 may be 5 part of a photonic quantum computer design having a probabilistic photon source in which the first system 1000 releases a sufficient number of photons required for a given computation.

[0091] The Rydberg cell array 100 is described as having a vapour storage volume at both ends of the housing 120. This provides a benefit of an improved filling (or replenishment) rate due to a differential pressure between the two vapour storage volumes. However, this is non-essential. Figure 7 illustrates a further design of the Rydberg cell array 100 in which the housing 120 defines a single vapour storage volume at a first end of the housing 120.

[0092] 15

[0093] Furthermore, the Rydberg cell array 100 may be constructed without vapour storage volumes. The Rydberg cell array 100 may therefore comprise the structure 110 only, such that the housing 115 of the structure 110 encloses the Rydberg cell array 100.

[0094] 20 Furthermore, it is non-essential that a system operates in series such that a first Rydberg cell of the Rydberg cell array 100 completes its interaction with one or more optical signals before a second Rydberg cell of the Rydberg cell array 100 begins its interaction with one or more optical signals. That is, interactions with a plurality of Rydberg cells of the Rydberg cell array 100 may occur in parallel. For example, the first system 1000 may store and retrieve quantum information in parallel by the timed control of the signal and control beams. Quantum information may be communicated to different Rydberg cells of the Rydberg cell array 100 by separate signal beams (from separate signal lasers) or by multiplexing the quantum information on a single signal beam.

[0095] 30 Furthermore, a role of a laser in a system may be dynamic. For example, in the first system 1000, the quantum information may be stored or retrieved in a particular Rydberg cell based on the timed control of the signal beam and control beam from either direction.A36148

[0096] Furthermore, it is non-essential that the quantum information is in the form of single photons. Collective photonic excitations, spatial modes of light and / or other transduced equivalents of a quantum state or system may be used instead.

[0097] 5 It is also non-essential for an HCF to be threaded through each channel of the plurality of channels 113 of the structure 110. The channel itself may be configured as a waveguide to guide the optical signals along its length. Alternatively, the channel may be sufficiently wide to allow the control / signal beam to pass through the channel without being guided.

[0098] 10

[0099] Furthermore, it is also non-essential that the first system 1000 implements an EIT -based quantum memory scheme. That is, the first system 1000 may implement parallel storage and / or retrieval of information from a plurality of Rydberg cells in the Rydberg cell array 100 via alternative quantum memory schemes, such as via fast ladder memory (FLAME), 15 off resonant cascaded absorption (ORCA), gradient echo memory (GEM), or controlled reversible inhomogeneous broadening (CRIB). The skilled person will understand that the FLAME and ORCA quantum memory schemes may use a substantially similar setup to the first system 1000 described above in which the controller controls an interaction between the control beam and the vapour of a Rydberg cell of the Rydberg cell array 20 100 (e.g. reducing an intensity of the control beam to a negligible amount) so as to control storage of the information of the signal beam in that Rydberg cell (which may be performed contemporaneously in a plurality of Rydberg cells). In the GEM and CRIB based quantum memory schemes, the first system 1000 may be modified so as to further comprise a generator for applying an electromagnetic field to a Rydberg cell of the 25 Rydberg cell array 100. The controller may control the generator so as to selectively apply the field to a particular Rydberg cell so as to store information of a signal beam (by dephasing the polarisation of the atomic vapour contained in the Rydberg cell) or to retrieve information of the signal beam (by rephasing the polarisation of the atomic vapour contained in the Rydberg cell), which again may be performed 30 contemporaneously in a plurality of Rydberg cells of the Rydberg cell array 100.

[0100] A method of manufacturing the Rydberg cell array 100 will now be described with reference to Figure 8. In a first step, S301, a Photonic Crystal Fibre (PCF) preform is assembled comprising a set of hollow glass tubes arranged inside a hollow glass cylinder 35 such that the glass tubes extend in parallel (or substantially parallel) to a major axis ofA36148

[0101] the hollow glass cylinder. In step S303, the PCF preform is drawn such that the set of glass tubes become the plurality of channels 113 of the structure 110 and the hollow glass cylinder becomes the inner housing of the structure 110. Pressure is applied to ensure the set of glass tubes remain open during the drawing process.

[0102] 5

[0103] The set of glass tubes are controlled (e.g. during the drawing process and by selection of the dimensions of the glass tubes) such that an inner diameter of the resulting channel of the plurality of channels 113 of the structure 110 matches an outer diameter of the HCF to be threaded through the channel. In step S305, the drawn structure is cleaved to create at least one structure 110.

[0104] In step S307, the structure 110 is secured (e.g. by adhesive) inside the housing 120, creating the Rydberg cell array 100. In step S309, a vacuum system is used to evacuate the interior of the Rydberg cell array 100 (e.g. via a first and / or second opening), and a 15 bake out is performed to ensure no water or other contaminants remain inside the Rydberg cell array 100. In step S311, the atomic vapour is released into the Rydberg cell array 100 via the first opening. Differential pressure may be applied between the first and second openings to accelerate the filling process. One filled, the first and second openings are sealed, forming the first and second burrs 125, 127 respectively.

[0105] 20

[0106] The above process enables the Rydberg cell array 100 to be manufactured in a scalable manner in which the number of channels and a length of the structure 110 may be tailored to a particular application. The length of the structure 110 may be, for example, from 0.5cm to 10cm. However, the skilled person will understand that the above manufacturing method is non-essential, and the Rydberg cell array 100 may be manufactured in other ways, such as by a metal or glass structure manufacturing process that can withstand low pressures.

[0107] Figure 9 is a flow diagram illustrating a method of operating a quantum memory device, 30 comprising the steps of: selectively coupling a first optical signal to a first vapour containment channel of the plurality of vapour containment channels (S401); and selectively controlling storage of the information of the first optical signal in the first vapour containment channel (S403).A36148

[0108] The skilled person will understand that any combination of features is possible within the scope of the invention, as claimed.

Claims

A36148CLAIMS1. A quantum memory device comprising:a structure defining a plurality of vapour containment channels extending 5 through the structure, each vapour containment channel containing vapour; anda controller configured to selectively:couple a first optical signal to a first vapour containment channel of the plurality of vapour containment channels, wherein the first optical signal contains information, andcontrol storage of the information of the first optical signal in the first vapour containment channel.

2. A quantum memory device as claimed in Claim 1 , wherein the controller is further configured to selectively:15 couple a second optical signal to a second vapour containment channel of the plurality of vapour containment channels, wherein the second optical signal contains information, andcontrol storage of the information of the second optical signal in the second vapour containment channel.

203. A quantum memory device as claimed in Claim 2, wherein the information of the first optical signal and the information of the second optical signal are stored contemporaneously.

4. A quantum memory device as claimed in any one of the preceding claims, wherein the controller is further configured to selectively:couple a third optical signal to the first vapour containment channel, and control an interaction between the third optical signal and vapour contained in the first vapour containment channel so as to control storage of the information of the 30 first optical signal in the first vapour containment channel.

5. A quantum memory device as claimed in Claim 4 as dependent on Claim 2, wherein the controller is further configured to selectively:couple the third optical signal to the second vapour containment channel, andA36148control an interaction between the third optical signal and the vapour contained in the second vapour containment channel so as to control storage of the information of the second optical signal in the second vapour containment channel.5 6. A quantum memory device as claimed in any one of the preceding claims, wherein each vapour containment channel comprises a Hollow Core Fibre, HCF, extending therethrough.

7. A quantum memory device as claimed in any one of the preceding claims, further 10 comprising a housing defining an interior section, wherein:the structure is positioned within the interior section of the housing,the interior section of the housing contains vapour, anda first end of the first vapour containment channel is open to the interior section of the housing for fluidic communication of the vapour between the interior section of the 15 housing and the first vapour containment channel.

8. A quantum memory device as claimed in Claim 7, wherein a second end of the first vapour containment channel is open to the interior section of the housing for fluidic communication of the vapour between the interior section of the housing and the first 20 vapour containment channel.

9. A quantum memory device as claimed in any one of the preceding claims, wherein the controller is configured to control storage of the information so as to store the information or to retrieve the information.2510. A quantum memory device as claimed in Claim 4 or any one of Claims 5 to 9 as dependent on Claim 4, wherein the third optical signal has a frequency for inducing Electromagnetically Induced Transparency, EIT, in the first vapour containment channel, and the controller is configured to selectively:30 control an intensity of the third optical signal in the first vapour containment channel during passage of the first optical signal so as to reduce EIT in the first vapour containment channel to the first optical signal such that the information of the first optical signal is stored in the first vapour containment channel, andcontrol the intensity of the third optical signal in the first vapour containment 35 channel so as to increase EIT in the first vapour containment channel to the first opticalA36148signal such that the stored information of the first optical signal is retrieved from the first vapour containment channel.

11. A quantum memory device as claimed in any one of the preceding claims, 5 further comprising a generator for generating an electromagnetic field, wherein the controller is configured to selectively control the electromagnetic field generated by the generator so as to control storage of the information of the first optical signal in the first vapour containment channel.

12. A method of operating a quantum memory device, the quantum memory device comprising a structure defining a plurality of vapour containment channels extending through the structure, each vapour containment channel containing vapour, the method comprising the steps of:selectively coupling a first optical signal to a first vapour containment channel of 15 the plurality of vapour containment channels; andselectively controlling storage of the information of the first optical signal in the first vapour containment channel.

13. A method as claimed in Claim 12, further comprising the steps of:20 selectively coupling a second optical signal to a second vapour containment channel of the plurality of vapour containment channels; andselectively controlling storage of the information of the second optical signal in the second vapour containment channel.25 14. A method as claimed in Claim 13, wherein the information of the first optical signal and the information of the second optical signal are stored contemporaneously.

15. A method as claimed in any one of Claims 12 to 14, further comprising the steps of:selectively coupling a third optical signal to the first vapour containment channel, andselectively controlling an interaction between the third optical signal and vapour contained in the first vapour containment channel so as to control storage of the information of the first optical signal in the first vapour containment channel.22A3614816. A method as claimed in Claim 15 as dependent on Claim 13, further comprising the steps of:selectively coupling the third optical signal to the second vapour containment channel, and5 selectively controlling an interaction between the third optical signal and vapour contained in the second vapour containment channel so as to control storage of the information of the second optical signal in the second vapour containment channel.

17. A method as claimed in either Claim 15 or Claim 16, wherein the third optical signal has a frequency for inducing Electromagnetically Induced Transparency, EIT, in the first vapour containment channel, and the method further comprises the steps of:selectively controlling an intensity of the third optical signal in the first vapour containment channel during passage of the third optical signal so as to reduce EIT in the first vapour containment channel to the first optical signal such that the information 15 of the first optical signal is stored in the first vapour containment channel, and selectively controlling the intensity of the third optical signal in the first vapour containment channel so as to increase EIT in the first vapour containment channel to the first optical signal such that the stored information of the first optical signal is retrieved from the first vapour containment channel.

18. A computer program comprising instructions which, when the program is executed by a quantum memory device, cause the quantum memory device to carry out the steps of any one of Claims 12 to 17.25 19. A computer readable carrier medium comprising the computer program of Claim18.

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