High-Fidelity Preservation and Retrieval of Quantum Information in a High-Temperature Atomic Vapor Cell Device

Abstract

A quantum memory device and methods for storing and retrieving qubits from the quantum memory device are described. The quantum memory device includes a first optical component that converts an input qubit encoded into an arbitrary polarization state of photons into a spatial qubit propagating on a pair of parallel optical rails, an atomic vapor memory that stores the spatial qubit in atomic vapor, and a second optical component that converts the spatial qubit into an output qubit encoded into an arbitrary polarization state of photons when retrieved from the atomic vapor memory.

Description

Background Art

[0001] A quantum network enables the transmission of information in the form of qubits (“qubits”) between physically separated quantum processors or other quantum devices (e.g., quantum sensors). The quantum network can be used to enable optical quantum communication over long distances and can be implemented via standard telecommunication optical fibers through the transmission of single photons in which information is encoded (e.g., in polarization). Additional components may be required to enable reliable transmission of quantum information over any distance.

Summary of the Invention

[0002] The following is a non-limiting summary of some embodiments of the present application. Some aspects of the present application relate to a quantum memory device. The quantum memory device includes a first optical component configured to convert an input qubit encoded in any polarization state of a photon into a spatial qubit propagating in a pair of parallel optical rails, an atomic vapor memory coupled to the output of the first optical component and configured to store the spatial qubit in an atomic vapor, and a second optical component coupled to the output of the atomic vapor memory and configured to convert the spatial qubit into an output qubit encoded in any polarization state of a photon when retrieved from the atomic vapor memory.

[0003] In some embodiments, the first optical component and / or the second optical component is a Sagnac-like device comprising a polarization beam splitter (PBS), a first angle-variable mirror optically coupled to the first output of the PBS, and a second angle-variable mirror optically coupled to the second output of the PBS. In some embodiments, the first angle-variable mirror is disposed at a first angle with respect to the polarization beam splitter, the second angle-variable mirror is disposed at a second angle with respect to the polarization beam splitter, and the second angle is different from the first angle. In some embodiments, changing the first angle and / or the second angle causes a change in the separation between the optical rails of a pair of parallel optical rails. In some embodiments, the quantum memory device further comprises a Bragg grating filter optically coupled to the input of the first optical component.

[0004] In some embodiments, the quantum memory device further comprises a pair of flat etalon cavities optically coupled to the output of the second optical component. In some embodiments, the flat etalon cavities of the pair of flat etalon cavities are disposed at a small off-parallel angle such that the incident surfaces of the flat etalon cavities are slightly non-parallel. In some embodiments, the quantum memory device further comprises at least three mirrors configured to pass the output qubit through the pair of flat etalon cavities at least twice.

[0005] In some embodiments, the quantum memory device further comprises a pair of curved etalon cavities optically coupled to the output of the second optical component. Some aspects of the present application relate to methods for storing and retrieving qubits. The method includes receiving a qubit encoded in an arbitrary polarization state of a photon, using a first optical component comprising an optical element in a Sagnac-like configuration to convert the qubit into a spatial qubit propagating in a pair of parallel optical rails, storing the spatial qubit in an atomic vapor memory, retrieving and outputting the spatial qubit from the atomic vapor memory, using a second optical component comprising an optical element in a Sagnac-like configuration to reconvert the spatial qubit into a qubit encoded in an arbitrary polarization state, and outputting the qubit.

[0006] In some embodiments, the step of converting the qubit into a spatial qubit includes receiving the qubit at a polarization beam splitter (PBS), using the PBS to convert the qubit into a spatial qubit, directing the spatial qubit to pass back through the PBS using a first angle-variable mirror and a second angle-variable mirror, and outputting the spatial qubit from the PBS to a pair of parallel optical rails.

[0007] In some embodiments, the first angle-variable mirror is disposed at a first angle with respect to the PBS, the second angle-variable mirror is disposed at a second angle with respect to the PBS, and the method further includes changing the spacing between the rails of a pair of parallel optical rails by changing the first angle and / or the second angle.

[0008] In some embodiments, the method further includes changing the coherence time for storing the qubit by increasing the spacing between the rails of a pair of parallel optical rails and increasing the diameter of each rail of the pair of parallel optical rails incident on the atomic vapor memory.

[0009] In some embodiments, the method further includes coupling a pair of control field beams to individual rails of a pair of optical rails before storing the spatial qubit in the atomic vapor memory.

[0010] In some embodiments, the method further includes filtering the input control field beam using a Bragg grating filter. In some embodiments, after outputting the qubit, the method includes passing the qubit through two flat etalons in a first direction in a first instance, wherein the two flat etalons are arranged at an angle such that the incident surfaces of the two flat etalons are not exactly parallel, and passing the qubit through two flat etalons in the first direction in a second instance. In some embodiments, passing the qubit through two flat etalons in the second instance includes guiding the qubit using three or more mirrors. In some embodiments, in the first instance, the qubit passes through the two flat etalons at a first position on one side from the central axis of the two flat etalons, and in the second instance, the qubit passes through the two flat etalons at a second position on the other side from the central axis. In some embodiments, by passing the qubit through the two flat etalons in the first and second instances, an extinction ratio of 100 dB or more and 150 dB or less is achieved.

[0011] In some embodiments, after outputting the qubit, the method further includes passing the qubit through two curved etalons in a first direction, wherein the two curved etalons are arranged at an angle such that the incident surfaces of the two curved etalons are not exactly parallel. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings are not intended to be drawn to scale. In the drawings, the same or substantially the same components shown in the various figures are respectively represented by like numerals. For clarity, not all components are labeled in all of the drawings. The drawings are as follows.

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Figure 6A

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[0013] Room-temperature quantum memories are used in quantum network architectures to enable the temporary storage and synchronization of photonic qubits across the network. To perform such functions, a quantum memory receives any random input qubit and stores it coherently without degrading the information encoded on the qubit. The inventors have recognized and understood that qubits with any polarization can be stored by separating the vertical and / or horizontal polarization components of the qubit into independent optical beams ("rails"). The separated vertical and horizontal components can be recombined into a single optical beam after being retrieved from storage.

[0014] Accordingly, a dual-rail quantum memory system is described herein. The dual-rail quantum memory uses optical elements arranged in a Sagnac-like configuration with mismatched mirror angles to separate and then combine the vertical and horizontal polarization components of a qubit. In a conventional Sagnac configuration, the same overlapping optical path is provided for two split optical beams. The inventors have recognized and understood that these two beams can be separated rather than overlapping by a distance defined by the mirror angles in a Sagnac-like configuration. The beams can be separated by arranging the mirrors of the Sagnac-like configuration at two slightly different angles (e.g., the angles can have mismatches in the range of 0° to 1°, 2°, 5°, and / or 10°). This configuration allows for an adjustable separation between the two optical beams. This adjustability supports the use of different beam diameters and enables an adjustable coherence time for the memory.

[0015] Accordingly, the inventors have developed a quantum memory device configured to store and retrieve qubits from an atomic vapor memory. The quantum memory device includes a first optical component (e.g., a Sagnac-like configuration) configured to convert an input qubit encoded in an arbitrary polarization state of a photon into a spatial qubit propagating in a pair of parallel optical rails. The quantum memory device includes an atomic vapor memory (e.g., including one or more atomic vapor cells) coupled to the output of the first optical component and configured to store the spatial qubit within the atomic vapor. Thereafter, the spatial qubit can be retrieved from the atomic vapor memory and output to a second optical component configured to convert the spatial qubit into an output qubit, which is encoded in an arbitrary polarization state of a photon. The first optical component and / or the second optical component is a Sagnac-like device including a polarization beam splitter (PBS) and two angle-variable mirrors optically coupled to two outputs of the PBS.

[0016] The inventors have further developed an optical frequency filtering mechanism that achieves high extinction (>120 dB) of the control field laser beam after retrieving the qubit from the quantum memory. The optical filtering mechanism achieves this high extinction value by passing the light twice through a low-finesse flat etalon. By passing the light twice through the flat etalon cavity, the stability against thermal alignment perturbations and mechanical alignment perturbations of the quantum memory is improved.

[0017] The following is a more detailed description of various concepts and embodiments regarding techniques for performing dynamic polarization drift correction for a quantum teleportation system. It should be understood that the various aspects described herein can be implemented in any of a number of ways. Examples of specific embodiments are provided herein for illustrative purposes only. Additionally, the various aspects described in the following embodiments may be used alone or in any combination and are not limited to the combinations explicitly described herein.

[0018] FIG. 1 shows a schematic diagram of an optical device 100 according to some embodiments of the technology described herein. The optical device 100 forms a quantum memory layer that can store and retrieve on demand photons having qubits (e.g., of any polarization). In FIG. 1, input 101 is an input port through which qubits (e.g., encoded in the polarization of one or more photons) enter the device 100. The device 100 includes several waveplates 102, 103 configured to adjust the polarization of the qubits and / or control fields.

[0019] In some embodiments, the qubits move from input 101 to module 110. Module 110 is a mixed - angle Sagnac interferometer that converts received qubits encoded in the polarization state of photons into spatial qubits that propagate along parallel optical rails 111a, 111b. Module 110 can encode the spatial qubits in the amplitude and phase of a superposition of single photons that propagate along parallel optical rails 111a, 111b. For example, if the received qubit is

[0020]

Number

[0021] encoded in any polarization state of, the spatial qubit output by module 110 is

[0022]

Number

[0023] encoded in the spatial state of. Where |L> and |R> are the left and right rails respectively, and iθ is the phase. In some embodiments, module 110 includes a polarization beam splitter (PBS) 110a and two angle-variable mirrors 110b, 110c. The angle between the two angle-variable mirrors 110b, 110c defines the separation between the two optical rails 111a, 111b after the photons exit module 110. By changing the separation between the two optical rails 111a, 111b, the coherence time of the quantum memory changes. Preferably, the separation between the two optical rails 111a, 111b can be changed such that the coherence time of the quantum memory is maximized without spatially overlapping the two optical rails 111a, 111b.

[0024] In some embodiments, after a pair of spatial qubits exit module 110, they enter device 112. Device 112 is configured to redirect the spatial qubits to an optical-matter interface 113. In some embodiments, device 112 can be a polarization beam splitter (e.g., a Glan-Taylor polarizer). The optical-matter interface 113 includes one or more atomic vapor cells 113a configured to store the quantum information carried by the pair of spatial qubits. For example, the one or more atomic vapor cells 113a can contain the vapor of an isotope (e.g., 87 atoms of Rb, atoms of Cs, or atoms of any other suitable alkali metal) that can absorb and store the quantum information. The atomic vapor cell 113a can be enclosed within a temperature-controlled and magnetically shielded container (e.g., formed from Mu-metal). Although the illustration in FIG. 1 shows only a single atomic vapor cell 113a, it should be understood that aspects of the present technology are not limited in this regard, and device 100 can include multiple (e.g., two, three, four, etc.) atomic vapor cells 113a.

[0025] In some embodiments, after the qubit is stored in the atomic vapor cell 113a, the qubit can be removed from the atomic vapor cell 113a and directed to module 114 by the polarization beam splitter 122. Module 114 is a mixing angle Sagnac interferometer configured to map the spatial qubit to a polarization qubit (e.g., to a photon having a qubit encoded in any polarization state). The final set of mirrors directs the polarization qubit to the output port 115. Device 100 outputs the polarization qubit taken out at the output port 115.

[0026] In some embodiments, device 100 includes a control field input 120. The control field input 120 is an input port for a control field laser beam. The control field laser beam is configured to control the process of storing and removing the qubit from device 100. The control qubit is directed from the control field input 120 to module 121. Module 121 is also a mixing angle Sagnac interferometer similar to module 110. Module 121 is configured to split the received control field qubit into two identical but spatially separated control field beams. Device 112 combines the two control field beams received from module 121 with a pair of spatial qubits (e.g., optical rails 111a, 111b) before the qubit enters the light-matter interface 113. When the qubit is removed from the light-matter interface 113, the polarization beam splitter 122 separates the control field beam from the removed qubit with a success rate of about 50 dB. Most of the control beam is removed from the removed data qubit after the polarization beam splitter 122.

[0027] FIG. 2 is a schematic diagram of a device 200 configured to reduce noise present in a control field laser beam before the control field laser beam enters a quantum memory, according to some embodiments of the techniques described herein. The device 200 is configured to reduce broadband noise in the control field laser beam. In particular, the device 200 is configured to reduce broadband noise caused by amplified spontaneous emission (ASE) of the laser and Raman scattering that occurs when the control field laser beam propagates along an optical fiber entering the device 100.

[0028] In some embodiments, the device 200 includes an input 201 and an output 204. The control field laser beam is incident on the device 200 through the input 201 and exits the device 200 through the output 204. The output 204 of the device 200 can be coupled to the control field input 120 of the device 100 such that the device 200 provides the control field laser beam to the device 100. The output 204 can be optically coupled to the control field input 120, for example, by a short optical fiber link or through a free space optical connection.

[0029] In some embodiments, the device 200 includes a filter 202. The filter 202 can be a Bragg grating filter configured to reflect light within a narrow frequency band (e.g., about 20 GHz wide). In some embodiments, the filter 202 can be passively adjusted to maintain resonance with the center frequency mode of the control field laser beam, minimizing the transmission of any laser or Raman emission outside the transmission peak of the control field laser from the device 200.

[0030] In some embodiments, device 200 includes a cavity 203 optically coupled to the output of filter 202. Cavity 203 can be a passive filtering cavity (e.g., a Fabry - Perot etalon cavity, a cavity having a frequency bandwidth of about 500 MHz). Cavity 203 can be stabilized, for example, using a PID - controlled temperature controller that houses cavity 203. Cavity 203 is adjusted to pass the center - frequency mode of the control - field laser.

[0031] FIG. 3 shows a schematic diagram of a device 300 configured to filter an optical signal extracted from device 100 described in connection with FIG. 1 herein, according to some embodiments of the techniques described herein. Device 300 includes an input 301 and an output 304. Input 301 can be optically coupled (e.g., using an optical fiber or through free space) to output 115 of device 100.

[0032] In some embodiments, device 300 includes etalon cavities 302a, 302b. Etalon cavities 302a, 302b are flat etalon cavities with low finesse (e.g., having a finesse value of about 30), providing high robustness against temperature variations and significantly lower sensitivity to laser alignment compared to conventional curved etalons. In some embodiments, the light received by etalon cavities 302a, 302b can be received at a small angle of incidence. This angle of incidence eliminates the need for isolation between the two etalon cavities 302a, 302b. In some embodiments, this angle can be greater than 0° and less than 10°, 5°, 2°, and / or 1°.

[0033] In some embodiments, in the first example, after light passes through the etalons 302a, 302b, the light can be redirected to pass back through the etalons 302a, 302b for further filtering. For example, the three mirrors 303a, 303b, 303c can be used to redirect the light to pass back through the etalons 302a, 302b for further filtering. By having the light pass through the etalons 302a, 302b twice, the light can be effectively filtered by the four etalons. The repeated filtering compensates for the low finesse of the cavities and provides an extinction value in the range of 100 dB to 150 dB for the remaining control fields associated with the qubits. Thus, when the qubit exits the device 300 at the output 304, the qubit can have a signal-to-noise ratio (SNR) of 10 or more and 100 or less.

[0034] FIG. 4 shows a schematic diagram of a device 400 configured to filter an optical signal extracted from the device 100 described in connection with FIG. 1 herein, according to some embodiments of the techniques described herein. The device 400 includes an input 401 and an output 404. The input 401 can be optically coupled to the output 115 of the device 100 (e.g., using an optical fiber or through free space).

[0035] In some embodiments, device 400 includes etalon cavities 402a, 402b. The etalon cavities 402a, 402b can be curved etalon cavities. In some embodiments, the light received by the etalon cavities 402a, 402b can be received at a small angle of incidence. This angle of incidence eliminates the need for isolation between the two etalon cavities 402a, 402b. In some embodiments, this angle can be greater than 0° and less than 10°, 5°, 2°, and / or 1°. The etalon cavities 402a, 402b can provide an extinction value in the range of 100 dB to 150 dB with respect to the remaining control fields associated with the qubit. Thus, when the qubit exits device 400 at output 404, the qubit can have a signal-to-noise ratio (SNR) of 10 or more and 100 or less, as described herein in connection with FIG. 9.

[0036] FIG. 5 is a flowchart illustrating a process 500 for storing and retrieving a qubit according to some embodiments of the techniques described herein. Process 500 can be performed, for example, in some embodiments, using an optical device 100 as described herein in connection with FIG. 1.

[0037] Process 500 can begin with an operation 502 in which a qubit encoded in an arbitrary polarization state of a photon is received. For example, an arbitrary polarization state |ψ> of a photon can be

[0038]

Number

[0039] described by. Here, |H> and |V> are the horizontal polarization basis state and the vertical polarization basis state, and iθ is the phase of the photon.

[0040] In some embodiments, qubits can be received by an optical device via an optical fiber connection. For example, a qubit can be received via a long-distance communication optical fiber from a qubit source located a certain distance (e.g., several kilometers) away from the optical device. Alternatively, in some embodiments, a qubit can be received through free space from a qubit source disposed via an optical fiber or at the same location as the optical device (e.g., in the same room as the optical device, in the same facility as the optical device).

[0041] In some embodiments, after operation 502, process 500 can transition to operation 504. In operation 504, the received qubit can be converted into a spatial qubit propagating in a pair of parallel optical rails. The received qubit can be converted using a first optical component comprising optical elements in a Sagnac-like configuration. For example, the first optical component can be module 110 of optical device 100 as described in connection with FIG. 1 herein.

[0042] In some embodiments, converting a qubit into a spatial qubit can include receiving the qubit at a polarization beam splitter (PBS; e.g., PBS110a) and using the PBS to convert the qubit into a spatial qubit. For example, the PBS can encode any polarization state of the received qubit into a spatial qubit output encoded into the spatial state of

[0043]

Number

[0044] where |L> and |R> are the left rail and the right rail, respectively. In some embodiments, two beams can be physically separated into a pair of parallel optical rails by using first and second angle-variable mirrors (e.g., angle-variable mirrors 110b, 110c) to direct the spatial qubit back through the PBS. The degree of physical separation of the optical rails (e.g., the spacing between the optical rails) can be adjusted by changing the relative angles of the first angle-variable mirror and / or the second angle-variable mirror with respect to the PBS. After the parallel optical rails are separated, the spatial qubit can be output from the PBS and the first optical component.

[0045] In some embodiments, after operation 504, process 500 may transition to operation 506. In operation 506, the spatial qubit can be stored in an atomic vapor memory. For example, the spatial qubit can be stored using the light-matter interface 113 as described in connection with FIG. 1 herein. The atomic vapor memory can include one or more atomic vapor cells. One or more atomic vapor cells 113a can contain the vapor of an isotope (e.g., 87 rubidium atoms, cesium atoms, or atoms of any other suitable alkali metal) that can absorb and store quantum information. The atomic vapor cell 113a can be enclosed within a temperature-controlled and magnetically shielded container (e.g., formed from Mu-metal). In some embodiments, the parallel optical rails can be coupled with a pair of control field beams before the spatial qubit is stored in the atomic vapor memory.

[0046] In some embodiments, the coherence time for qubit storage can be adjusted. For example, the coherence time can be adjusted by increasing the spacing between the rails of a pair of parallel optical rails (e.g., by changing the angles of the first angle-variable mirror and / or the second angle-variable mirror with respect to the PBS of the first optical component). Alternatively or additionally, the coherence time can be adjusted by changing the diameter of each rail of the pair of parallel optical rails incident on the atomic vapor memory.

[0047] In some embodiments, after operation 506, process 500 may proceed to operation 508. In operation 508, the spatial qubit may be retrieved from the atomic vapor memory and output.

[0048] In some embodiments, after operation 508, process 500 may proceed to operation 510. In operation 510, the spatial qubit may be reconverted to a qubit encoded in an arbitrary polarization state of a photon. The spatial qubit may be reconverted using a second optical component comprising optical elements in a Sagnac-like configuration. For example, the second optical component may be module 114 of optical device 100 as described in connection with FIG. 1 herein.

[0049] In some embodiments, reconverting the spatial qubit to a qubit may include receiving the spatial qubit at a polarization beam splitter (PBS) and using the PBS to convert the spatial qubit to a qubit. Another pair of angle-variable mirrors may be used to recombine a pair of parallel optical rails into a single beam for output.

[0050] In some embodiments, after operation 510, process 500 may proceed to operation 512. In operation 512, the qubit may be output. For example, the qubit may be output to an optical fiber cable using an optical fiber connection. Alternatively, the qubit may be output to free space using a free-space optical connection.

[0051] In some embodiments, after the qubit is output, the qubit may be further filtered. For example, the qubit may pass through one or more etalon cavities. In some embodiments, the qubit may pass through two curved etalons. The two curved etalons may be arranged at an angle such that their incident surfaces are not exactly parallel.

[0052] Alternatively, in some embodiments, the qubit may be passed through the two flat etalons in a first instance along a first direction and passed through again in a second instance. In the first instance, the qubit may pass through the two flat etalons at a first position on one side from the central axis of the two flat etalons. In the second instance, the qubit may pass through the two flat etalons at a second position on the other side from the central axis. By passing the qubit through the two flat etalons in the first instance and the second instance, an extinction ratio of 100 dB or more and 150 dB or less may be achieved. In some embodiments, three or more mirrors may be used to direct the qubit to pass through the two flat etalons in the second instance. The two flat etalons may be arranged at an angle such that their incident surfaces are not slightly parallel.

[0053] Figure 6A is a plot showing the effect of beam diameter on the coherence time of qubit storage in an atomic vapor memory of an optical-matter interface according to some embodiments of the technology described herein. The plot shows normalized efficiency on the vertical axis and storage time in microseconds on the horizontal axis. Three curves 601, 602, and 603 are plotted. Each curve 601, 602, and 603 represents an exponential fit to the measured values of the storage efficiency of a quantum memory (such as the optical device 100 of FIG. 1 herein) at different storage times for different beam diameters used in parallel optical rails of the quantum memory. Curve 601 shows a fit to data obtained using a beam with half the normalized diameter. Curve 602 shows a fit to data obtained using a beam with the normalized diameter. Curve 603 shows a fit to data obtained using a beam with twice the normalized diameter. The data shows that both coherence times increase as the beam size increases from curve 601 to curve 603. This data shows that the coherence time of the quantum memory described herein may be adjusted based on the beam size of the parallel optical rails of the quantum memory.

[0054] FIG. 6B is a plot showing the effect of the vapor pressure of an atomic vapor cell on the coherence time of an atomic vapor memory of an optical-matter interface, according to some embodiments of the techniques described herein. The plot shows the normalized efficiency on the vertical axis and the storage time in microseconds on the horizontal axis. Three data sets 604, 605, and 606 are plotted. Each data set 604, 605, and 606 was obtained for a different value of the vapor pressure in the atomic vapor cell of the optical-matter interface of the quantum memory. The data sets 604, 605, and 606 were obtained at vapor pressures of 10 Torr (1333.22 Pa), 20 Torr (2666.45 Pa), and 30 Torr (3999.67 Pa), respectively. The data shows that the coherence time increases as the vapor pressure increases from data set 604 to 605. This data indicates that the coherence time of the quantum memory described herein can be adjusted based on the vapor pressure of the atomic vapor cell used in the quantum memory. Combining the effects of the beam size and the vapor pressure, a coherence time in the range of 0.5 ms to 10 ms can be achieved.

[0055] FIGS. 7A and 7B are plots showing the storage efficiency as a function of storage time for the left and right optical rails of a quantum memory, respectively, according to some embodiments of the techniques described herein. The plots show the storage efficiency on the vertical axis and the storage time in microseconds on the horizontal axis. Curve 701 is an exponential function fit to the data obtained from the left optical rail, and curve 702 is an exponential function fit to the data obtained from the right optical rail. The coherence time can be extracted from the fits of curves 701 and 702 and is 157 ± 8 μs for the left rail and 133 ± 6 μs for the right rail.

[0056] Figure 8 is a plot showing the classical fidelity of an atomic vapor memory of an optical - matter interface over time, according to some embodiments of the technology described herein. The plot shows classical fidelity on the vertical axis and time (in minutes) on the horizontal axis. Data points 801 were collected by measuring the fidelity of qubits stored in a quantum memory at different retrieval times, as described herein. The measured fidelity is greater than 99.4% up to 400 minutes after qubit storage.

[0057] Figure 9 is a plot showing the signal - to - noise ratio (SNR) of photons retrieved from an atomic vapor memory of an optical - matter interface, according to some embodiments of the technology described herein. The plot shows the measured photon amplitude on the vertical axis and time in microseconds on the horizontal axis. The left peak 901 is due to the input qubit to the quantum memory, and the right peak 902 is due to the retrieval of the qubit from the quantum memory after 5 μs of storage. The measured SNR is approximately 10 for a dual - rail quantum memory and approximately 20 for a single - rail quantum memory (e.g., when polarization need not be preserved). Such a high SNR results in a fidelity greater than 95%.

[0058] The various aspects of the above - described embodiments can be used alone, in combination, or in various configurations not specifically described in the foregoing embodiments, and thus, in their application, are not limited to the details and configurations of the components described in the foregoing description or shown in the drawings. For example, aspects described in one embodiment can be combined with aspects described in other embodiments in any manner.

[0059] The use of ordinal terms such as "first," "second," "third," etc. in a claim to modify an element of the claim in the claims scope does not, by itself, imply any particular precedence, priority, or order of one claim element over another claim element, or the temporal order in which acts of a method are performed, but is used merely as a label to distinguish an element of one claim having a certain name from another element having the same name (except for the use of the ordinal term).

[0060] Also, the expressions and terms used in this specification are for illustrative purposes and should not be regarded as limiting. The use of "including," "comprising," "having," "containing," "involving," and variations thereof in this specification is meant to encompass the listed items and their equivalents as well as additional items.

[0061] The term "exemplary" as used in this specification is used to mean serving as an example, instance, or illustration. Accordingly, any embodiment, implementation, process, feature, etc. described in this specification as exemplary should be understood to be for illustrative purposes only and, unless otherwise indicated, should not be understood to be a preferred or advantageous example.

[0062] Although some aspects of at least one embodiment have been described thus, it should be understood that various changes, modifications, and improvements will readily occur to those skilled in the art. Such changes, modifications, and improvements are intended to be part of this disclosure and are intended to be within the spirit and scope of the principles described herein. Accordingly, the foregoing description and drawings are for illustrative purposes only.

Claims

1. A quantum memory device, comprising: a first optical component configured to convert an input qubit encoded in an arbitrary polarization state of a photon into a spatial qubit propagating in a pair of parallel optical rails; an atomic vapor memory coupled to an output of the first optical component and configured to store the spatial qubit in an atomic vapor; a second optical component coupled to an output of the atomic vapor memory, the second optical component being configured to convert the spatial qubit into an output qubit when taken out from the atomic vapor memory, the output qubit being encoded in the arbitrary polarization state of a photon, the second optical component, wherein the first optical component and the second optical component are Sagnac-like devices having mismatched mirror angles, a quantum memory device.

2. The first optical component and / or the second optical component comprises a polarization beam splitter (PBS); a first angle-variable mirror optically coupled to a first output of the PBS; a second angle-variable mirror optically coupled to a second output of the PBS, the quantum memory device according to claim 1.

3. The first angle-variable mirror is arranged at a first angle with respect to the polarization beam splitter; The second angle-variable mirror is arranged at a second angle with respect to the polarization beam splitter, the second angle being different from the first angle, the quantum memory device according to claim 2.

4. By changing the first angle and / or the second angle, a change in the separation between the optical rails of the pair of parallel optical rails occurs, the quantum memory device according to claim 3.

5. The quantum memory device according to claim 1, further comprising a Bragg grating filter optically coupled to an input of the first optical component.

6. The quantum memory device according to claim 1, further comprising a pair of flat etalon cavities optically coupled to an output of the second optical component.

7. The flat etalon cavities of the pair of flat etalon cavities are arranged at an angle such that the incident surfaces of the flat etalon cavities are not exactly parallel, the quantum memory device according to claim 6.

8. The quantum memory device according to claim 6, further comprising at least three mirrors configured to pass the output qubit through the pair of flat etalons at least twice.

9. The quantum memory device according to claim 1, further comprising a pair of curved etalon cavities optically coupled to the output of the second optical component.

10. A method for storing and retrieving qubits, comprising: receiving a qubit encoded in an arbitrary polarization state of a photon; converting the qubit into a spatial qubit propagating along a pair of parallel optical rails using a first optical component comprising a Sagnac-like device with mismatched mirror angles; storing the spatial qubit in an atomic vapor memory; retrieving and outputting the spatial qubit from the atomic vapor memory; reconverting the spatial qubit into a qubit encoded in the arbitrary polarization state using a second optical component comprising a Sagnac-like device with mismatched mirror angles; outputting the qubit.

11. The step of converting the qubit into a spatial qubit comprises: receiving the qubit at a polarization beam splitter (PBS); converting the qubit into the spatial qubit using the PBS; directing the spatial qubit to pass back through the PBS using a first angle-variable mirror and a second angle-variable mirror; outputting the spatial qubit from the PBS to a pair of parallel optical rails. The method according to claim 10.

12. The first angle-variable mirror is arranged at a first angle with respect to the PBS, The second angle-variable mirror is arranged at a second angle with respect to the PBS, the second angle being different from the first angle, The method further comprises varying the distance between the rails of the pair of parallel optical rails by changing the first angle and / or the second angle. The method according to claim 11.

13. The coherence time regarding the storage of the qubit is increased by increasing the distance between the rails of the pair of parallel optical rails The method according to claim 12, further comprising a step of changing by increasing the diameter of each of the pair of parallel optical rails incident on the atomic vapor memory.

14. The method according to claim 11, further comprising a step of coupling a pair of control field beams to individual rails of the pair of parallel optical rails before storing the spatial qubit in the atomic vapor memory.

15. The method according to claim 14, further comprising a step of filtering an input control field beam using a Bragg grating filter.

16. After outputting the qubit, a step of passing the qubit through two flat etalons in a first direction in a first instance, wherein the two flat etalons are arranged at an angle such that the incident surfaces of the two flat etalons are not exactly parallel, The method according to claim 10, further comprising a step of passing the qubit through the two flat etalons in the first direction in a second instance.

17. The method according to claim 16, wherein the step of passing the qubit through the two flat etalons in the second instance includes guiding the qubit using three or more mirrors.

18. In the first instance, the qubit passes through the two flat etalons at a first position on one side from the central axis of the two flat etalons, The method according to claim 16, wherein in the second instance, the qubit passes through the two flat etalons at a second position on the other side from the central axis.

19. The method according to claim 16, wherein the step of passing the qubit through the two flat etalons in the first and second instances achieves an extinction ratio of 100 dB or more and 150 dB or less.

20. After outputting the qubit, The method according to claim 10, further comprising a step of passing the qubit through two curved etalons in a first direction, wherein the two curved etalons are arranged at an angle such that the incident surfaces of the two curved etalons are not exactly parallel.

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