Compact quantum memory device

Abstract

A quantum memory device for storing a signal field is provided. The quantum memory device comprises an optical cavity comprising a first reflective surface and a second reflective surface; a first optical element and a second optical element arranged within the optical cavity; an atomic system situated between the first optical element and the second optical element, the atomic system configured to store the signal field via an atomic transition; and one or more magnets configured to apply an axial magnetic field across the atomic system.

Description

Compact Quantum Memory DeviceTechnical Field

[0001] The present disclosure relates to quantum memory devices. More particularly, the present application relates to quantum memory devices that utilise atomic systems to temporarily store a signal (optical) field, for example a single photon.Background

[0002] A quantum memory is an important part of a quantum information processing system such as a quantum computer or a quantum repeater. When implemented using photons, a quantum memory provides an interface between light and matter that allows for storage and retrieval of photonic quantum information e.g. the quantum state of stored photons. In particular, a quantum state of light (often referred to as a signal or signal field) may be stored by controlled conversion into a matter excitation and retrieved by controlled conversion of the matter excitation back into a quantum state of light. In atomic ensemble-based quantum memories for example, the signal may be converted to a spin-wave excitation by applying a control laser pulse (often referred to as a control field or simply a control), the control pulse timed to reach the atomic ensemble contemporaneously with the signal, and having a frequency tuned to excite a two -photon electric dipole transition. However, such a simple explanation hides the many technical challenges that inhibit efficient quantum memory devices from being widely adopted. For example, many quantum memory devices require multiple lasers beyond those that may be required to generate the signal field and the control field, for example further laser systems may typically be required to optically pump the atomic ensemble to ensure it is in the correct initial state. Furthermore, many quantum memory devices are relatively large and bulky, which may make them challenging to manufacture at scale.

[0003] An example of a quantum memory device is provided in EP3469594B1, which relies upon the off -resonant cascaded absorption (ORCA) protocol. The standard ORCA memory may be implemented in a single pass configuration on a relatively long vapour cell of length between 5 to 10 cm containing an atomic ensemble. A signal pulse (representing the information to be stored) is sent into the vapour cell from a first direction, and at the same time a control pulse is sent into the vapour cell from a second direction opposite to the first direction, such that together an atomic transition is driven to place one or more of the atoms in the atomic ensemble in a higher energy state thereby storing the signal. The control pulse energy needed for efficient memory operation is on the order of InJ, which requires a high-powered pulsed laser system for the memory device to function. The ensemble is generally prepared in a specific ground state to have an effective three-level atomic system required for an exponential response. This state preparation may require additional lasers with CW power on the order of a few 100 milli-Watts.

[0004] The number of required laser systems and optical power levels, complex beam orientations, and large physical dimensions of the sub -components may make a quantum memory device based on such a single-pass implementation of a memory protocol challenging to manufacture reliably at scale.Summary

[0005] According to an aspect of the present disclosure, a quantum memory device for storing a signal field is provided. The quantum memory device comprises an optical cavity comprising a first reflective surface and asecond reflective surface; a first optical element and a second optical element arranged within the optical cavity; an atomic system situated between the first optical element and the second optical element, the atomic system configured to store the signal field via an atomic transition; and one or more magnets configured to apply an axial magnetic field across the atomic system. The magnetic field acts to separate degenerate energy levels of the atomic system such that a transition energy for the atomic transition corresponds to a combination of: a polarization state and a frequency of the signal field propagating through the atomic system; and a polarization state and a frequency of a control field propagating through the atomic system. The first reflective surface of the optical cavity is arranged to: receive, from outside the optical cavity, the signal field and the control field; and transmit the signal field and the control field into a transverse mode of the optical cavity. Each optical element is configured to change a polarization state of a signal field or control field transmitted therethrough such that: (i) the signal field propagating in a first direction through the atomic system has a first polarization state; (ii) the signal field propagating in a second direction opposite to the first direction through the atomic system has a second polarization state orthogonal to the first polarization state; (iii) the control field propagating in the first direction through the atomic system has a third polarization state; and (iv) the control field propagating in the second direction through the atomic system has a fourth polarization state orthogonal to the third polarization state.

[0006] A quantum memory device as described herein provides several advantages over those known in the art. For example, by providing an axial magnetic field across the atomic system, the degenerate discrete energy states of the atomic system are separated based on their magnetic quantum numbers. The atomic transition characterising the memory may accordingly be associated with a particular polarization and frequency combination of the signal and control fields, and noise from unintentionally driving a transition into a different energy state is reduced, leading to an improved memory performance. Furthermore, the first and second optical elements change the polarization of the signal and control fields propagating in the atomic system such that the signal (control) field propagating in a first direction through the atomic system has an orthogonal polarization to the signal (control) field propagating in the opposite direction. Rotating the polarization of the fields in this way inhibits intensity standing waves from being formed in the cavity, which improves memory efficiency. Furthermore, the particular atomic transition that characterises the memory may accordingly be associated with the signal and control either propagating in the same direction or in opposite directions, as may be more advantageous for the particular atomic system of the system. For example, when the atomic system has a “ladder-like” energy structure, it may be beneficial for the atomic transition to be linked to the signal field and control field propagating in opposite directions, while when the atomic system has a “lambda-like” energy structure, it may be beneficial for the atomic transition to be linked to the signal and control field propagating in the same direction. By providing a cavity, both the signal and control may be provided from the same direction, which also enables a more compact and scalable design with less complex beam geometry than single-pass quantum memory device designs. Furthermore, the control pulse energy may be greatly reduced in comparison to other quantum memory device designs, with the power reduction proportional to the cavity finesse. Accordingly, the quantum memory device described herein may be more efficient, more compact and may use fewer components (e.g. there may be no need for any additional lasers for optical pumping) than those quantum memory devices known in the art.

[0007] The atomic system may comprise atomic valence electrons having a first energy state, a second energy state and a third energy state. The atomic transition may comprise an atomic transition between the first energy state and the third energy state. The signal field may have a frequency configured to couple the first energy stateand the second energy state in the atomic system. The signal field may be near-resonant, for example the signal field may be detuned from the second energy level but capable of coupling the first and second energy states. The control field may have a frequency configured to couple the second energy state and the third energy state in the atomic system. The control field may also be near-resonant or slightly off-resonant, but have an energy such that in combination with the signal field the atomic transition from the first energy state to the third energy state may be driven.

[0008] The use of off-resonant transitions between the first and second energy states, and the second and third energy states (owing to the choices of the frequencies of the signal field and the control field), may reduce noise by helping to remove fluorescence noise. This is because the frequency of the control field may be chosen to be (e.g. far) outside the linewidth of the transition between the first and second states. The use of off -resonant transitions between the first and second energy states, and the second and third energy states, thus allowing off- resonant storage of the signal field, may allow the signal field and the control field to have a relatively large bandwidth. This in turn may help to enable the signal field and / or the control field to comprise short pulses. Using short pulses may improve the operation time of the quantum memory device.

[0009] In some examples, the second energy state may have a higher energy than the first energy state, and the third energy state may have a higher energy than the second energy state. Such a configuration of energy states may characterise the atomic system as having a “ladder-like” energy structure. An example of such a ladder-like energy structure can be found in rubidium, in which the first energy state may be correspond to the 5S state, the second energy state may correspond to the 5P state and the third energy state may correspond to a magnetic substate of the 5D state, although different energy levels of rubidium may be utilised. In some examples, the transition energy for the atomic transition may correspond to a combination of the signal field and the control field propagating in opposite directions. Driving the atomic transition with counterpropagating signal and control fields may be particularly beneficial in ladder-like atomic systems. For example, counter-propagating fields may reduce (e.g. cancel out) doppler shift effects.

[0010] In other examples, the second energy state may have a higher energy than the first energy state, and the third energy state may have a lower energy than the second energy state. Such a configuration of energy states may characterise the atomic system as having a “lambda-like” energy structure. In some examples, the transition energy for the atomic transition may correspond to a combination of the signal field and the control field propagating in the same direction. Driving the atomic transition with co-propagating signal and control fields may be particularly beneficial in lambda-like atomic systems.

[0011] The skilled person will appreciate that atomic systems having a different energy structure (e.g. different configurations of energy levels) may be used.

[0012] In some examples, the quantum memory device may include one or more birefringent elements to enable tuning of the cavity resonances.

[0013] In some examples, the first polarization state may be orthogonal to the third polarization state. The signal field and the control field may accordingly have orthogonal polarization states when co-propagating through the atomic system. For some choices of the atomic system, the first polarization state being orthogonal to the third polarization state may be advantageous.

[0014] In some examples, the control field may be in an orthogonal polarization state to the signal field on entry to and / or exit from the cavity. For example, the signal field may have a fifth polarization state outside of thecavity, the control field may have a sixth polarization state outside of the cavity, and the fifth polarization state may be orthogonal to the sixth polarization state. The first reflective surface of the optical cavity may accordingly be arranged to: receive, from outside the optical cavity, the signal field having the fifth polarization state and the control field having the sixth polarization state; and transmit the signal field and the control field into a transverse mode of the optical cavity. Advantageously, if the signal field sent to or retrieved from the quantum memory device is orthogonally polarized with respect to the control field, then the signal and control may be easily combined or separated with polarization-sensitive components such as a polarization beam splitter. Accordingly, the optical set-up for sending information to or retrieving information from the optical memory may be less lossy and easier to implement.

[0015] The first optical element or the second optical element may comprise a quarter wave plate or similar.

[0016] The atomic system may comprise a room-temperature atomic system.

[0017] The atomic system may comprise alkali atoms. For example, the atomic system may comprise rubidium atoms or may comprise caesium atoms.

[0018] The atomic system may be comprised within a vapour cell. Advantageously, the quantum memory devices described herein may be significantly more compact than single-pass quantum memory devices. For example, the atomic system may be comprised within a vapour cell having a size of less than ten millimetres.

[0019] In some examples, the vapour cell may comprise the optical cavity. In other words, the optical cavity may be integrated into the vapour cell. In such circumstances, the atomic system is situated between the first optical element and the second optical element, but atoms may also be found elsewhere inside (and possibly outside) of the cavity.

[0020] At least a part of the quantum memory device may be implemented in optical fibre. For example, at least a part of the system may be implemented in a hollow -core fibre.

[0021] At least a part of the quantum memory device may be implemented in a photonic integrated circuit. For example, the atomic system may be comprised in a MEMS vapour cell integrated with photonic integrated circuits with reflective surfaces and optical elements fabricated using micro -fabrication techniques.

[0022] According to an aspect of the present disclosure, a quantum memory system is provided. The quantum memory system comprises a plurality of quantum memory cells and one or more magnets configured to apply a magnetic field across the memory cells. Each quantum memory cell comprises: an optical cavity comprising a first reflective surface and a second reflective surface; a first optical element and a second optical element arranged within the optical cavity, wherein each optical element is configured to change a polarization state of a signal field or control field transmitted therethrough; and an atomic system situated between the first optical element and the second optical element, the atomic system configured to store a signal field via an atomic transition. The one or more magnets are configured to apply an axial magnetic field across the atomic systems of the memory cells to separate degenerate energy levels of each atomic system such that a transition energy for the atomic transition corresponds to a combination of: a polarization state and a frequency of the signal field propagating through the atomic system; and a polarization state and a frequency of a control field propagating through the atomic system. The optical elements of each quantum memory cell are arranged such that: the signal field propagating in a first direction through the atomic system of the quantum memory cell has a respective first polarization state; and the signal field propagating in a second direction opposite to the first direction through the atomic system of the quantum memory cell has a respective second polarization state orthogonal to the first polarization state; thecontrol field propagating in the first direction through the atomic system of the quantum memory cell has a respective third polarization state; and the control field propagating in the second direction through the atomic system has a respective fourth polarization state orthogonal to the third polarization state.

[0023] Advantageously, a quantum memory system as described herein arranges a plurality of quantum memory cells in a compact manner such that as few as one magnet may be used for all the memory cells.

[0024] According to another aspect of the present disclosure, a method is disclosed of storing and retrieving an electromagnetic signal field in a quantum memory device (as described herein). The method comprises storing, at least partially, the signal field in the quantum memory device by causing a control field to be input into the optical cavity contemporaneously with the signal field, the control field having a polarization state and frequency selected to complement the signal field to drive the atomic transition in the atomic system. The method further comprises retrieving, at least partially, the at least partially stored signal field from the quantum memory device by causing a second control field to be input into the optical cavity, the second control field having a polarization state and frequency selected to drive the atomic transition in the atomic system. In this context, “partially storing” refers to placing the signal field in a superposition of being stored and not stored in the quantum memory device.

[0025] The extent to which the signal field is stored or retrieved may be controlled by selecting the energy of the control field (e.g. by adjusting the power of the control field), which may be achieved through e.g. directly controlling the power of a laser producing the control field or via optical modulation techniques. Advantageously, by controlling the energy of the control field, the signal field may be at least partially stored in or at least partially retrieved from the quantum memory device, and accordingly the quantum memory device may perform a beamsplitter-like operation on the signal field in time.

[0026] Many modifications and other embodiments set out herein will come to mind to a person skilled in the art in light of the teachings presented herein. Therefore, it will be understood that the disclosure herein is not to be limited to the specific embodiments disclosed herein. Moreover, although the description provided herein provides example embodiments in the context of certain example combinations of elements, steps and / or functions, it will be appreciated that different combinations of elements, steps and / or functions may be provided by alternative embodiments without departing from the spirit or scope of the disclosure.Brief description of the Drawings

[0027] Illustrative embodiments of the present disclosure will now be described by way of example only, with reference to the accompanying figures.

[0028] Fig. 1 shows an illustration of a quantum memory device according to an example.

[0029] Fig. 2 shows a first energy level diagram in the absence of a magnetic field and a second energy level diagram in the presence of a magnetic field according to an example.

[0030] Fig. 3 shows a fibre-optic apparatus for coupling a signal field and a control field into or out of a quantum memory device such as that of Fig. 1.

[0031] Fig. 4 shows an illustration of a quantum memory device according to an example.

[0032] Fig. 5 shows a flowchart of a method for storing and retrieving a signal from a quantum memory device.

[0033] The features shown in the figures are not drawn to scale. Throughout the description and the drawings, like reference numerals refer to like parts.Detailed Description

[0034] Embodiments of the disclosure are described with reference to the accompanying drawings. However, it should be appreciated that the disclosure is not limited to the embodiments, and all changes and / or equivalents or replacements thereto also belong to the scope of the disclosure. The same or similar reference denotations may be used to refer to the same or similar elements throughout the specification and the drawings.

[0035] As used herein, the terms “have”, “may have”, “include”, or “may include” a feature (e.g. a number, function, operation, or a component such as a part) indicate the existence of the feature and do not exclude the existence of other features. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0036] As used herein, the terms “A or B”, “at least one of A and / or B”, or “one or more of A and / or B” may include all possible combinations of A and B. For example, “A or B”, “at least one of A or B”, “at least one of A and B” may indicate all of (1) including at least one A, (2) including at least one B, or (3) including at least one A and at least one B.

[0037] As used herein, the terms “first” and “second” may modify various components regardless of importance and do not limit the components. These terms are only used to distinguish one component from another. For example, reference to a first component and a second component may indicate different components from each other regardless of the order or importance of the components.

[0038] It will be understood that when an element (e.g. a first element) is referred to as being (physically, operatively or communicatively) “coupled with / to”, or “connected with / to” another element (e.g. a second element), it can be coupled with / to the other element directly or via a third element. In contrast, it will be understood that when an element (e.g. a first element) is referred to as being “directly coupled with / to” or “directly connected with / to” another element (e.g. a second element), no element (e.g. a third element) intervenes between the element and the other element.

[0039] The terms as used herein are provided merely to describe some embodiments thereof, but not to limit the scope of other embodiments of the disclosure. It is to be understood that the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. All terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of the disclosure belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealised or overly formal sense unless expressly so defined herein.

[0040] The terms “signal”, “signal pulse” or “signal field” may be used herein to refer to the optical signal to be stored. For example, the signal field may comprise a single photon. The terms “control”, “control pulse” or “control field” may be used herein to refer to an optical field that, when incident on an atomic system at the sametime as the signal field, may drive an atomic transition in the atomic system, thereby in effect storing the state of the signal field.

[0041] References to orthogonality of polarization states of a field (e.g. a signal field or control field) as used herein refer to the polarization of the field relative to a fixed coordinate system and not necessarily the propagation direction of the field. For example, a signal field propagating in a first direction through an atomic system may be polarised e.g. horizontally while a signal field propagating in a second direction through the atomic system opposite to the first direction (that is, in the reverse direction) may be vertically polarized, and accordingly the first polarization state of the signal field propagating in the first direction is orthogonal to the second polarization state of the signal field propagating in the reverse (second) direction. For example, a signal field propagating in a first direction through an atomic system may be e.g. left circularly polarised with respect to the direction of propagation (the first direction) while a signal field propagating in a second direction through the atomic system opposite to the first direction (that is, in the reverse direction) may be e.g. left circularly polarised with respect to the direction of propagation (the second direction) which may be thought of as right circularly polarised with respect to the first direction through the atomic system, and accordingly the first polarization state of the signal field propagating in the first direction is orthogonal to the second polarization state of the signal field propagating in the reverse (second) direction. The fixed coordinate system may be defined with respect to the direction of the magnetic field applied across the atomic system.

[0042] Fig. 1 shows an illustration of a quantum memory device 100 according to an example. The quantum memory device 100 is configured to store a signal field 170 (indicated with a dashed arrow in the figure) by using a control field 180 (indicated with a solid arrow in the figure). In this example, the signal field 170 is linearly polarised (indicated with (||) in Fig. 1) and the control field 180 is linearly polarised in a direction orthogonal to the signal field 170 (indicated with (-L) in Fig. 1). The signal field may comprise, for example, a temporal pulse or a single photon.

[0043] The quantum memory device 100 comprises an optical cavity 110, 120, first and second optical components 130, 140 arranged within the cavity, an atomic system 150 and one or more magnets 160 for applying an axial magnetic field 190 across the atomic system 150.

[0044] The optical cavity comprises a first reflective surface 110, which in this example is a partially reflective mirror, and a second reflective surface 120, which in this example is a fully reflective mirror. The labels “first” and “second” have been used in relation to the reflective surfaces 110, 120 only to distinguish them from one another. The cavity is sized such that the frequency of the signal field 170 is resonant with the optical cavity and that the frequency of the control field 180 is also resonant with the cavity. The first reflective surface 110 is positioned such that the signal field 170 and the control field 180 may both enter the cavity through the same input path, in other words the first reflective surface 110 is configured to receive both the signal field 170 and control field 180 and transmit both the signal field 170 and control field 180 into a transverse mode of the cavity.

[0045] For improved performance, the control fields and the signal field may resonate in the cavity. This can be achieved for ladder type memories by, for example, temperature tuning since a broad range of intermediate state detuning allows efficient memory operation. Temperature dependence on the refractive index for ground state resonant signal transition may differ from the control transition which can also be used for resonance tuning. For a lambda type memory an additional birefringent element may be used to tune the cavity resonances.

[0046] The first optical element 130 and second optical element 140 are arranged within the optical cavity. The optical elements 130, 140 are arranged to rotate a polarization state of any light passing therethrough (e.g. a signal field or control field passing through the optical element 130, 140). The labels “first” and “second” have been used in relation to the optical elements 130, 140 only to distinguish them from one another, for example the first optical element 130 in Fig. 1 is the optical element positioned closest to the first reflective surface 110, but in other examples may be the optical element positioned closest to the second reflective element 120.

[0047] In this example, each of the first and second optical elements 130, 140 comprises a quarter wave plate. Accordingly, when the signal field 170 having linear polarization (||) enters the cavity via the first reflective surface 110, on the first pass through the cavity the first optical element 130 rotates the polarization state of the signal field 170 from linear (||) to a circular polarization that may contribute to a “cr transition in the atomic system 150. On reaching the second optical element 140, the polarization state of the signal field 170 is changed from circular to linear polarization. After reflection on the second reflective surface 120, the second optical element 140 rotates the polarization state of the signal field 170 from linear to a circular polarization that may contribute to a “cr +“ transition in the atomic system 150. Accordingly, the first and second optical elements 130, 140 are arranged such that a polarization state of the signal field 170 propagating through the atomic system 150 in a first direction is orthogonal (with respect to a fixed coordinate system) to a polarization state of the signal field 170 propagating through the atomic system 150 in a second direction opposite to the first direction. Similarly, when the control field 180 having perpendicular linear polarization (-L) enters the cavity via the first reflective surface 110, on the first pass through the cavity the first optical element 130 rotates the polarization state of the control field 180 from perpendicular linear (-L) to a circular polarization that may contribute to a “cr +“ transition in an atomic system 150. At the second optical element 140, the polarization state of the control field 180 is changed from circular to linear. After reflection on the second reflective surface 120, the second optical element 140 rotates the polarization state of the control field 180 from linear to a circular polarization that may drive a “cr transition in the atomic system 150. Accordingly, a polarization state of the control field 180 propagating through the atomic system 150 in a first direction is orthogonal to a polarization state of the control field 180 propagating through the atomic system 150 in a second direction opposite to the first direction. In the quantum memory device 100 of Fig. 1 the polarization state of the signal field 170 is orthogonal to the polarization state of the control field 180, but the skilled person will appreciate that this need not be the case.

[0048] The first and second optical elements 130, 140 together prevent the fields from forming standing waves in the cavity. A standing wave mode is not desirable for interaction with a spatially extended atomic ensemble at room temperature since the atoms are in constant motion and therefore experience a time -varying field as they move between the nodes and anti-nodes of the standing wave, which may significantly reduce the lifetime of a quantum memory. Moreover, resonant enhancement for both signal and control fields in a cavity with standing waves results in two-photon absorption for both co- and counter- propagating beam geometries, which further reduces memory efficiency. This is because most memory protocols work efficiently only for one of these geometries due to phase-matching and effective Doppler cancellation. Accordingly, the first and second optical elements in the cavity improve the efficiency of the memory.

[0049] The atomic system 150 is situated between the first optical element 130 and the second optical element 140. The atomic system 150 in this example comprises an ensemble of atoms in a glass-blown vapour cell at roomtemperature, the vapour cell and accordingly the atomic system situated fully between the first and second optical elements. For example, the atoms may be rubidium atoms or caesium atoms. The vapour cell may have a length of less than 10mm, for example less than 5mm.

[0050] The atomic system in this example has a “ladder-like” energy structure. In other words, the atomic system 150 comprises atomic valence electrons having a first energy state 210, a second energy state 220 and a third energy state 230 (see Fig. 2). The second energy state 210 has a greater energy than the first energy state 210. The third energy state 230 has a greater energy than the second energy state 220. The first energy state 210 may correspond for example to the 5S state of rubidium, the second energy state 220 may correspond for example to the 5P state of rubidium and the third energy state 230 may correspond for example to a magnetic sub-state of the 5D state of rubidium.

[0051] The signal field 170 in this example has a frequency configured to couple an off-resonant transition 240 between the first energy state 210 and the second energy state 220 in the atomic system 150. The detuning from the intermediate state 220 is indicated by a capital Delta in the figure. The control field 180 in this example has a frequency, and therefore a corresponding energy 250, that when combined with the energy of the signal field 170, may drive a two-photon transition between the first energy state 210 and the third energy state 230. By driving a transition from the first energy state 210 to the third energy state 230, the signal field 170 may be stored in the quantum memory device 100.

[0052] The skilled person will appreciate that the energy level diagrams illustrated in Fig. 2 are representative only. An atomic system may have a different energy level structure to those depicted. In particular, the first energy state or second energy state may also be magnetic substates.

[0053] The one or more magnets 160 are configured to provide an axial magnetic field 190. In this example, there is one permanent magnet 160 having a hollow cylindrical (ring-shaped) profile, with the cavity (110, 120), optical elements (130, 140) and atomic system 150 located within the ring-shaped magnet. The magnet may comprise for example a ring-shaped Neodymium permanent magnet.

[0054] In Fig. 2, a first illustrative energy level diagram 200a illustrates energy levels in the absence of the axial magnetic field 190. The third energy state 230 is one energy state amongst a group 280 of degenerate energy levels having different magnetic quantum numbers. In the absence of the magnetic field 190, such degeneracy may lead to noise in the quantum memory 100. For example, when driving a transition from the first energy state 210 to the third energy state 230 in the absence of the magnetic field 190, one may inadvertently drive a transition from the first energy state to a different energy state in the group 280 of energy states, In the second illustrative energy level diagram 200b of Fig. 2, energy levels in the presence of the axial magnetic field 190 are illustrated. The group 280 of energy levels 230, 260, 270 has been separated such that different transition energies are required to couple the first energy state 210 to each of the third energy state 230, fourth energy state 260 and fifth energy state 270, and accordingly different control frequencies. For clarity in explanation, any degeneracies of the first state 210 or second state 220 have been ignored in the figure. The magnetic field 190 may be strong enough that the atomic system 150 is placed in the Paschen Back regime. As each energy level 230, 260, 270 is associated with a different magnetic quantum number, each transition corresponds to a different polarization combination of the signal field 170 and control field 180. For example, as shown in the energy level diagram 200b a transition between the first energy state 210 and the third energy state 230 may be driven using a combination of the signal field and control field both having circular polarization driving "cr - " transitions in the atoms. Transitionsbetween the first energy state 210 and the other magnetic sub-states in the group 280 of energy states may be driven by signal field and control field with appropriate polarizations.

[0055] The atomic transition that enables the signal field 170 to be stored (the transition between the first energy state 210 and the third energy state 230) accordingly corresponds to a combination of a polarization state and a frequency of the signal field propagating through the atomic system and a polarization state and a frequency of a control field propagating through the atomic system. With reference again to the specific example of Fig. 1, the atomic transition between the first energy state 210 and the third energy state 230 accordingly corresponds to the signal field 170 propagating through the atomic system 150 in a direction from the first optical element 130 towards the second optical element 140 and the control field 180 propagating through the atomic system 150 in the opposite direction. It will be appreciated that owing to the counter-propagation of the signal field 170 and the control field 180 incident on the atomic system 150, any Doppler shifts introduced by the incidence of the signal field 170 and control field 180 on the atomic system 150 that may otherwise cause dephasing of the coherent excitation between the first energy state 210 and the third energy state 230 and thus of the stored signal field may be reduced e.g. substantially cancelled out. Reducing such Doppler shifts may thus help to avoid having to cool the atomic ensemble and help to maintain the coherent excitation of the first, second and third states, thereby helping to maximise and exploit the storage time of the signal field 170 in the atomic system 150.

[0056] The skilled person will appreciate that while in this example, the atomic transition characterising the memory may correspond to counter-propagating fields in the atomic ensemble, this need not be the case. For example, by changing the polarization states of the signal field 170 and control field 180 provided to the cavity, and by adjusting the frequency of the control field 180, the atomic transition that stores the signal field may be e.g. the transition between the first energy state 210 and the fourth energy state 260, which would correspond to co-propagating fields in the atomic system 150. The user may accordingly have a choice as to which energy state is classed as the third energy state. In some examples, in which the ladder-like atomic system is replaced with a lambda-like energy structure, co-propagating fields may be better than counter-propagating fields.

[0057] The parameters or sizes for the quantum memory device 100 may be chosen in any suitable way. For example, in the example quantum memory device 100 described in relation to Fig. 1 and Fig. 2 with a rubidium atomic system, an example suitable set of parameters is as follows: the first reflective surface 110 has a reflectivity of ~56%, the second reflective surface 120 has a reflectivity of -100%, the reflective surfaces are spaced ~10mm apart, and the vapour cell may have a length of ~8mm. The magnetic field may approximately 165 to 175 mTesla and may be applied by a ring-shaped neodymium magnet. The skilled person will appreciate that other parameters and dimensions may be used.

[0058] The signal field and control field may be produced in any suitable way depending on the context in which the quantum memory device is being used. For example, the signal field may be generated by an external cavity diode laser. For example, the control field may be generated by a control laser, for example a continuous-wave fibre laser. The skilled person will also appreciate that pulse carvers (e.g. EOMs) and amplifiers may also be utilised.

[0059] Furthermore, the skilled person will appreciate that many parameters relating to the signal field and control field may also be varied to improve the memory efficiency. For example, the control pulse energy, pulse width, signal pulse width, temporal overlap of the signal and control fields, and the frequencies of the signal and control fields may all be tuned to improve the performance of the quantum memory device.

[0060] Fig. 3 illustrates an extension of the quantum memory device 100 of Fig. 1. In particular, the quantum memory apparatus 300 shown in Fig. 3 comprises a quantum memory device 310 (which may correspond to the quantum memory device of Fig. 1) comprises mode-matching optics 320, a fiber polarization beam splitter (PBS) 330 and an optical circulator 340. The mode-matching optics 320, PBS 330 and optical circulator 340 are fibre- coupled. In use, the signal field 170 is provided to a first port of the optical circulator 340 and output from a second port towards the PBS 330. The control field 180 and signal field 170 are passed to the mode-matching optics 320 which may comprise, for example, one or more lenses, that direct the fields through the first reflective surface of device 310 to store the signal 170 in an atomic system of the device 310. To read the signal out, a second control pulse is provided to the device 310. The resulting signal is routed from the device via the PBS 330 towards the second port of the circulator 340, whereby it is directed out of a third port of the circulator 340 to downstream photonic circuitry. Such single in-out fibre integration of the form shown in Fig. 3 is possible due to the characteristics of the device 310 described above.

[0061] The skilled person will appreciate that the quantum memory device 100 described herein is a representative example and that many variations, modifications or adaptations of the quantum memory device 100 may be made without departing from the spirit or scope of the invention. A few such modifications are referred to below.

[0062] For example, in the discussion above in relation to Fig. 1, the signal field and the control field are orthogonal to one another both inside the cavity and outside of the cavity. The skilled person will appreciate that this need not be the case. For example, in the discussion above, co -propagating signal and control fields are shown as being in orthogonal polarization states, however, in other examples the signal field may be in a polarization state that is not orthogonal to a polarization state of the control field. In such circumstances, certain polarization components of each field may contribute to the atomic transition.

[0063] In the examples described above, the atomic system comprises an ensemble of rubidium atoms. However, the skilled person will appreciate that the ensemble may comprise other atoms, for example caesium atoms.

[0064] The skilled person will further appreciate that while the atomic system herein has been described as having a ladder-like energy structure, other atomic systems may be utilised providing different energy structures. For example, an atomic system having a lambda energy structure may be utilised in which the third energy state has a lower energy than the second energy state. Such lambda-like energy structures may be found in, for example, alkali atoms such as rubidium or caesium, or alkaline earth atoms such as calcium or strontium.

[0065] While in the discussion herein the quantum memory device has been described in terms of bulk optics, the skilled person will appreciate that this need not be the case. For example, at least a part of the quantum memory device may be implemented in optical fibre, such as a hollow -core fibre. In another example, the quantum memory device may be wholly or partly implemented as part of a photonic integrated circuit. For example, the atomic system may be comprised in a MEMS vapour cell integrated with photonic integrated circuits with reflective surfaces and optical elements fabricated using micro -fabrication techniques.

[0066] Furthermore, in Fig. 1, the atomic system 150 is located entirely within the cavity 110, 120. However, other designs such as that shown in Fig. 4 may be utilized. In Fig. 4, Similar to the quantum device 100, the quantum device 400 comprises an optical cavity 410, 420, first and second optical components 430, 440 arranged within the cavity, an atomic system 450 and one or more magnets 460 for applying an axial magnetic field across the atomic system 150. The quantum memory device 400 is configured to store a signal field 170 (indicated witha dashed arrow in the figure) by using a control field 180 (indicated with a solid arrow in the figure). In this example, the signal field 170 is linearly polarised (indicated with (||) in Fig. 4) and the control field 180 is linearly polarised in a direction orthogonal to the signal field 170 (indicated with (-L) in Fig. 4). In this example, the magnet 460 is an annular permanent magnet configured to apply an axial magnetic field across the atomic system 450.

[0067] In the quantum device 400, the cavity 410, 420 and the optical elements 430, 440 are comprised within a container (e.g. a vapour cell) with the atomic system 450. Accordingly, the atomic system is situated between the first optical element and the second optical element but atoms may also be found elsewhere within the cavity or even in this example outside of the cavity. A quantum memory device such as that in Fig. 4 may be formed in any of a number of ways. As one example, the first and second reflective surfaces 410, 420 and the optical elements 430, 440 may be held in a lens tube, and the lens tube may be placed inside a vacuum chamber which is evacuated and filled with e.g. rubidium. As another example, a lens tube holding the cavity 410, 420 and optical elements 430, 440 may be provided in a specially manufactured vapour cell.

[0068] Functionally, the quantum memory device 400 operates in a similar manner to the quantum memory device 100 as set out above. The quantum memory device 400 may be adapted or modified in any of the ways described above in relation to the quantum memory device 100.

[0069] Fig. 5 shows a flowchart of a method for using a quantum memory device such as that of Fig. 1 or Fig. 4. As described above, the quantum memory device utilizes an atomic transition that corresponds to a polarization state and a frequency of a signal field and a polarization state and frequency of a control field. In some examples, the method of Fig. 5 may performed by an electronic controller (for example a configured Field Programmable Gate Array, and ASIC, or a CPU) coupled to a control laser for producing the control field. The skilled person would appreciate that the described method is compatible with all variations of the quantum memory devices described herein.

[0070] At 510, the method comprises storing, at least partially, a signal field 170 in the quantum memory device (100, 400). The signal field 170 may be stored by causing a control field 180 to be input into the optical cavity contemporaneously with the signal field 170. The control field is configured to have, when propagating through the atomic system, a suitable polarization state and frequency selected to complement the signal field 170 co- or counter-propagating through the atomic system. The control field 180 and signal field 170 may accordingly together drive the atomic transition, thereby at least partially storing the signal field.

[0071] At 520, the method comprises retrieving, at least partially, the at least partially stored signal field 170 from the quantum memory device (100, 400) by causing a second control field 180 to be input into the optical cavity.

[0072] The extent to which the signal field is stored (or retrieved) may be controlled by adjusting the photon flux of the first control field (or second control field in the case of retrieval). For example, the power of the control laser that produces the control field may be directly controlled to adjust the power (and therefore the energy) of the control field. For example, an optical modulator (for example an electro -optical modulator) may be configured to control the energy of the control field. The first control field and the second control field may have different photon fluxes (e.g. different energies).

[0073] Each feature disclosed in this specification (including any accompanying claims, abstract or drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic seriesof equivalent or similar features. The disclosure is not restricted to the details of any foregoing embodiments. The disclosure extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

CLAIMS1. A quantum memory device for storing a signal field, the quantum memory device comprising: an optical cavity comprising a first reflective surface and a second reflective surface; a first optical element and a second optical element arranged within the optical cavity; an atomic system situated between the first optical element and the second optical element, the atomic system configured to store the signal field via an atomic transition; and one or more magnets configured to apply an axial magnetic field across the atomic system to separate degenerate energy levels of the atomic system such that a transition energy for the atomic transition corresponds to a combination of: a polarization state and a frequency of the signal field propagating through the atomic system; and a polarization state and a frequency of a control field propagating through the atomic system; wherein the first reflective surface of the optical cavity is arranged to: receive, from outside the optical cavity, the signal field and the control field; and transmit the signal field and the control field into a transverse mode of the optical cavity; wherein each optical element is configured to change a polarization state of a signal field or control field transmitted therethrough, such that: the signal field propagating in a first direction through the atomic system has a first polarization state; and the signal field propagating in a second direction opposite to the first direction through the atomic system has a second polarization state orthogonal to the first polarization state; the control field propagating in the first direction through the atomic system has a third polarization state; and the control field propagating in the second direction through the atomic system has a fourth polarization state orthogonal to the third polarization state.

2. A quantum memory device according to claim 1, wherein the transition energy for the atomic transition corresponds to a combination of the signal field and the control field propagating in opposite directions.

3. A quantum memory device according to claim 1, wherein the transition energy for the atomic transition corresponds to a combination of the signal field and the control field propagating in the same direction.

4. A quantum memory device according to any preceding claim, wherein: the atomic system comprises atomic valence electrons having a first energy state, a second energy state and a third energy state; the atomic transition comprises an atomic transition between the first energy state and the third energy state; the signal field has a frequency configured to couple the first energy state and the second energy state in the atomic system; andthe control field has a frequency configured to couple the second energy state and the third energy state in the atomic system.

5. A quantum memory device according to claim 4, wherein: the second energy state has a higher energy than the first energy state; and the third energy state has a higher energy than the second energy state.

6. A quantum memory device according to claim 4, wherein: the second energy state has a higher energy than the first energy state; and the third energy state has a lower energy than the second energy state.

7. A quantum memory device according to any preceding claim, wherein the first polarization state is orthogonal to the third polarization state.

8. A quantum memory device according to any preceding claim, wherein the first reflective surface of the optical cavity is arranged to: receive, from outside the optical cavity, the signal field having a fifth polarization state and the control field having a sixth polarization state; and transmit the signal field and the control field into a transverse mode of the optical cavity; and wherein the fifth polarization state is orthogonal to the sixth polarization state.

9. A quantum memory device according to any preceding claim, wherein the first optical element or the second optical element comprises a quarter wave plate.

10. A quantum memory device according to any preceding claim, wherein the atomic system is a roomtemperature atomic system.

11. A quantum memory device according to any preceding claim, wherein the atomic system comprises alkali atoms.

12. A quantum memory device according to any preceding claim, wherein the atomic system is comprised within a vapour cell.

13. A quantum memory device according to claim 12, wherein the vapour cell has a size less than or equal to ten millimetres.

14. A quantum memory device according to any preceding claim, wherein the optical cavity is comprised within the vapour cell.

15. A quantum memory device according to any of claims 1 to 11, wherein at least a part of the quantum memory device is implemented in optical fibre.

16. A quantum memory device according to any of claims 1 to 11, wherein at least a part of the quantum memory device is implemented in a photonic integrated circuit.

17. A quantum memory system comprising: a plurality of quantum memory cells, each memory cell comprising: an optical cavity comprising a first reflective surface and a second reflective surface; a first optical element and a second optical element arranged within the optical cavity, wherein each optical element is configured to change a polarization state of a signal field or control field transmitted therethrough; an atomic system situated between the first optical element and the second optical element, the atomic system configured to store a signal field via an atomic transition; and one or more magnets configured to apply an axial magnetic field across the atomic systems of the memory cells to separate degenerate energy levels of each atomic system such that a transition energy for the atomic transition corresponds to a combination of: a polarization state and a frequency of the signal field propagating through the atomic system; and a polarization state and a frequency of a control field propagating through the atomic system; wherein the optical elements of each quantum memory cell are arranged such that: the signal field propagating in a first direction through the atomic system of the quantum memory cell has a respective first polarization state; and the signal field propagating in a second direction opposite to the first direction through the atomic system of the quantum memory cell has a respective second polarization state orthogonal to the first polarization state; the control field propagating in the first direction through the atomic system of the quantum memory cell has a respective third polarization state; and the control field propagating in the second direction through the atomic system has a respective fourth polarization state orthogonal to the third polarization state.

18. A method of storing and retrieving an electromagnetic signal field in a quantum memory device, wherein the quantum memory device comprises: an optical cavity comprising a first reflective surface and a second reflective surface, the first reflective surface configured to receive the signal field from outside the cavity and transmit the signal field into a transverse mode of the optical cavity; a first optical element and a second optical element arranged within the optical cavity; an atomic system situated between the first optical element and the second optical element, the atomic system configured to store the signal field via an atomic transition; andone or magnets configured to apply an axial magnetic field across the atomic system to separate degenerate energy levels of the atomic system such that a transition energy for the atomic transition corresponds to a combination of (i) a polarization state and a frequency of the signal field propagating through the atomic system, and (ii) a polarization state and a frequency of a control field propagating through the atomic system; wherein the method comprises: storing, at least partially, the signal field in the quantum memory device by causing a control field to be input into the optical cavity contemporaneously with the signal field, the control field having a polarization state and frequency selected to complement the signal field to drive the atomic transition in the atomic system; and retrieving, at least partially, the at least partially stored signal field from the quantum memory device by causing a second control field to be input into the optical cavity, the second control field having a polarization state and frequency selected to drive the atomic transition in the atomic system.

Images