Multi-degree-of-freedom optical fiber integrated on-chip quantum memory

Abstract

The invention belongs to the technical field of quantum information science, and particularly relates to a multi-degree-of-freedom optical fiber integrated on-chip quantum memory. A plurality of optical waveguides on a rare earth doped crystal are used as storage media, each waveguide is used for preparing a multi-channel atomic frequency comb, a plurality of time domain mode declaration type single photon sources are stored, and a multi-degree-of-freedom multiplexing quantum storage scheme of a space domain, a frequency domain and a time domain is provided. All components for preparing the quantum memory on the optical fiber integrated chip can be made of mature materials and optoelectronic devices, and assembly preparation and practical development of the large-scale optical fiber-based quantum internet are facilitated. The optical fiber integrated quantum memory has the characteristics of miniaturization, integration and ultra-large multi-mode capacity, and has a crucial significance for promoting the development of quantum networks containing quantum repeaters.

Description

Technical Field

[0001] This invention belongs to the field of quantum information science and technology, specifically relating to a multi-degree-of-freedom fiber-optic integrated on-chip quantum memory. Background Technology

[0002] Quantum memories are the core components for building long-distance quantum networks. They work by storing photonic quantum states through the interaction of light and matter. Currently, physical systems for quantum storage include single atoms, atomic ensembles, rare-earth-doped solid-state ensembles, ion traps, NV / SiV color centers, quantum dots, and optomechanical oscillators. Among these, rare-earth-doped solid-state ensembles possess large uniformity and broadening, as well as long coherence times, making them suitable for quantum storage with large bandwidth, long storage times, and multimode capacity. For practical quantum networks, developing multi-degree-of-freedom parallel quantum memories and expanding their multimode capacity is an important development direction. Currently, rare-earth-doped solid-state quantum memories have achieved mode multiplexing in multiple degrees of freedom, including time, frequency, spatial, and orbital angular momentum, significantly improving the multimode capacity of a single memory node. For example, based on atomic frequency combs and gradient echo storage protocols, multi-frequency domain multiplexing can be achieved over a wide bandwidth; time-domain multimode storage can be achieved through multi-pulse sequences or time-slice structures; and multi-spatial mode or multi-channel structures can further expand parallel storage capabilities. However, existing quantum storage schemes that reuse multiple degrees of freedom mostly rely on bulk materials or free-space optical paths, which not only result in large system size, limited stability and scalability, but also often have complex coupling between different degrees of freedom, making it difficult to achieve a high-density, programmable, and scalable integrated architecture.

[0003] The miniaturization and integration of quantum memories using rare-earth-doped optical waveguides is of great significance for the construction of large-scale, scalable quantum networks. Current research on on-chip quantum memories mainly focuses on single-waveguide, single-space-channel structures, whose multimode capacity primarily depends on temporal or frequency degrees of freedom, while spatial degrees of freedom have not been fully explored. This limits the application potential of on-chip quantum memories in terms of parallel processing capabilities, throughput, and interfacing with multi-channel fiber optic networks. Therefore, there is an urgent need for an integrated quantum storage scheme that can simultaneously introduce multiple degrees of freedom (space, frequency, and time) on a chip scale and is highly compatible with fiber optic systems. Summary of the Invention

[0004] To address the problems of existing technologies, this invention proposes a multi-degree-of-freedom fiber-integrated on-chip quantum memory. This invention achieves spatial multiplexing by fabricating multiple parallel waveguides on the same rare-earth-doped crystal and combining this with an all-fiber encapsulation method. Furthermore, a frequency-domain multiplexing atomic frequency comb structure is fabricated in each waveguide, and time-domain multimode characteristics of this structure are used to achieve time-frequency multiplexing. Finally, multi-degree-of-freedom parallel fiber-integrated on-chip quantum storage in the spatial, frequency, and time domains is realized on a single chip. This scheme effectively utilizes the high integration characteristics of on-chip devices, significantly improves the multimode capacity of the quantum memory, and is compatible with existing fiber optic communication systems, demonstrating good practical application value.

[0005] The technical solution of this invention is as follows:

[0006] A multi-degree-of-freedom fiber-integrated on-chip quantum memory includes, in sequence, a pump laser source 1, an intensity modulator 2, a first phase modulator 3, a second phase modulator 4, an acousto-optic modulator 5, a tunable optical attenuator 6, a polarization controller 7, a fiber beam splitter group 8, a first optical switch array, a fiber-integrated rare-earth-doped optical waveguide array module 13, and a second optical switch array.

[0007] The first optical switch array consists of multiple dual-input single-output optical switches. The two input ends of each optical switch are respectively connected to a linear sweep pump light and a corresponding signal light output from the fiber beam splitter group 8.

[0008] The fiber-integrated rare-earth-doped optical waveguide array module 13 consists of a first optical fiber array 14, a rare-earth-doped optical waveguide array 15, and a second optical fiber array 16. The first optical fiber array 14 is connected to the output port of the optical switch in the first optical switch array via optical fiber, and the second optical fiber array 16 is connected to the input port of the optical switch in the second optical switch array via optical fiber. The rare-earth-doped optical waveguide array 15 is a multi-parallel waveguide array fabricated on the same rare-earth-doped crystal. After being connected to the first optical fiber array 14 and the second optical fiber array 16 respectively, multiple parallel waveguide paths are obtained. Utilizing the selective spectral hole-burning effect of pump light in the rare-earth ion ensemble, each waveguide can be used to fabricate an atomic frequency comb structure for time-frequency multiplexed quantum storage. Specifically, within a set time period, the optical switch in the first optical switch array is controlled to input linearly swept pump light into the corresponding waveguide within a set first time period. After a set interval time, the optical switch in the first optical switch array is controlled to input signal light into the corresponding waveguide within a set second time period, thereby achieving effective temporal separation between the atomic frequency comb fabrication process and the signal light storage process.

[0009] Corresponding to the first optical switch array, the second optical switch array is composed of multiple single-input dual-output optical switches. The input end of the optical switch in the second optical switch array is connected to the second fiber array 16. When an atomic frequency comb is fabricated in the waveguide, the optical switch in the corresponding second optical switch array outputs residual pump light or fluorescence noise from the first output end during the first time and interval, and outputs transmitted signal light and readout signal light from the second output end during the second time.

[0010] The pump laser source 1 is used to provide pump light for the preparation process of atomic frequency combs in quantum memory;

[0011] The intensity modulator 2 and the first phase modulator 3 are used to prepare an optical frequency comb based on pump light and output it to the second phase modulator 4;

[0012] The second phase modulator 4 is used to realize the frequency shift of the optical frequency comb, so as to obtain linear sweep pump light output to the acousto-optic modulator 5;

[0013] The acousto-optic modulator 5 is used to modulate the pulse width, duty cycle, and on / off state of the input linear sweep pump light.

[0014] The adjustable optical attenuator 6 adjusts the power of the linear sweep pulse pump light output by the acousto-optic modulator 5.

[0015] The polarization controller 7 adjusts the polarization state of the linear sweep pulse pump light output by the tunable optical attenuator 6;

[0016] The fiber beam splitter group 8 splits the linear sweep pulse pump light output by the polarization controller 7 and outputs it through multiple output ports.

[0017] Furthermore, the pump laser source 1 is a solid-state laser, a semiconductor laser, or a dye laser. The pump laser source 1 can generate continuous pump light with a center wavelength that matches the absorption wavelength of the rare earth ion ensemble in the rare earth doped optical waveguide array 15, covering a range of 400 nm to 1600 nm.

[0018] Furthermore, the intensity modulator 2 is an intensity modulator based on the electro-optic effect of lithium niobate crystal, with a bandwidth of GHz, capable of modulating optical pulses with pulse widths on the order of nanoseconds.

[0019] Furthermore, the first phase modulator 3 and the second phase modulator 4 are phase modulators based on KDP crystals or phase modulators based on lithium niobate crystals, with a bandwidth greater than or equal to 20 GHz.

[0020] Furthermore, the operating wavelength of the acousto-optic modulator 5 is matched with the operating wavelength of the quantum memory, and the switching time is approximately 15 ns.

[0021] Furthermore, the fiber optic beam splitter 8 is a fiber optic beam splitter group composed of multiple ordinary 50:50 fiber optic beam splitters, which realizes the average division of the power of one input light into multiple paths, and the optical power of multiple output ports is the same.

[0022] Furthermore, the optical switches in the first and second optical switch arrays are mechanical optical switches with a switching time of approximately 0.5 ms.

[0023] Furthermore, the rare-earth-doped optical waveguide array 15 is fabricated using semiconductor technology, which comprises at least one of laser direct writing, proton exchange, precision machining, ultraviolet lithography, electron beam exposure, and plasma etching.

[0024] Furthermore, the rare-earth-doped optical waveguide array 15 is a rare-earth-doped waveguide with non-uniform broadening on the GHz scale, such as erbium-doped lithium niobate waveguide, thulium-doped lithium niobate waveguide, neodymium-doped yttrium vanadate waveguide, etc.

[0025] The beneficial effects of this invention are as follows: This invention provides a multi-degree-of-freedom fiber-integrated on-chip quantum storage scheme. Multiple waveguides are fabricated on the same rare-earth-doped crystal, and each waveguide can be used to fabricate an atomic frequency comb structure for time-frequency multiplexed quantum storage, thereby achieving multi-degree-of-freedom fiber-integrated on-chip quantum storage with spatial, frequency, and time domain multiplexing. The pump light for each waveguide originates from the frequency chirped light of multiple frequency segments prepared by intensity and phase modulation of a continuous laser. The pump light is split into multiple pump lights of equal power after passing through an optical fiber beam splitter group 8. These are then selectively injected into a single optical waveguide or multiple optical waveguides simultaneously via switches in the first optical switch array. Utilizing the selective spectral hole-burning effect of the pump light in the rare-earth ion ensemble, atomic frequency combs with multiple channels are fabricated. Combined with their time-domain multimode storage characteristics, quantum storage with spatial, frequency, and time domain multiplexing is achieved. This scheme utilizes on-chip integration technology with multiple optical waveguides, combined with optical frequency combs and frequency chirping techniques, to achieve on-chip quantum storage of arbitrary wavelengths across time, frequency, and space, laying the foundation for high-capacity quantum information transmission. Simultaneously, by employing all-fiber encapsulation coupling technology between fiber arrays and optical waveguides, this multi-degree-of-freedom multiplexed on-chip quantum memory is fully encapsulated in optical fibers, ensuring compatibility with existing optical fiber communication systems. The multi-degree-of-freedom fiber-integrated on-chip quantum memory provided by this invention features high bandwidth and ultra-large multimode capacity. The rare-earth-doped optical waveguides used in this invention have the advantages of low loss and on-chip integration, while the remaining components can all be derived from mature optoelectronic devices, facilitating system assembly, fabrication, and practical application, and greatly promoting the construction of practical, large-scale, and scalable quantum networks. Attached Figure Description

[0026] Figure 1This is a schematic diagram of a multi-degree-of-freedom fiber-optic integrated on-chip quantum storage scheme according to an embodiment of the present invention;

[0027] Figure 2 This is a schematic diagram of a declared single-photon source structure used for testing the performance of a multi-degree-of-freedom fiber-optic integrated on-chip quantum storage according to an embodiment of the present invention.

[0028] Figure 3 This is a schematic diagram of a single-photon coincidence counting device used to test the performance of a multi-degree-of-freedom fiber-optic integrated on-chip quantum storage scheme according to an embodiment of the present invention.

[0029] Explanation of icon numbers:

[0030] Pump laser source 1, intensity modulator 2, first phase modulator 3, second phase modulator 4, acousto-optic modulator 5, tunable optical attenuator 6, polarization controller 7, fiber optic beam splitter group 8, first optical switch 9, second optical switch 10, third optical switch 11, fourth optical switch 12, fiber-integrated rare-earth-doped optical waveguide array module 13, first fiber array 14, rare-earth-doped optical waveguide array 15, second fiber array 16, fifth optical switch 17, sixth optical switch 18, seventh optical switch 19, eighth optical switch 20, second Semiconductor continuous laser 21, second intensity modulator 22, erbium-doped fiber amplifier 23, second tunable optical attenuator 24, second polarization controller 25, second fiber beam splitter group 26, first fiber integrated periodically polarized lithium niobate waveguide module 27, second fiber integrated periodically polarized lithium niobate waveguide module 28, third fiber integrated periodically polarized lithium niobate waveguide module 29, fourth fiber integrated periodically polarized lithium niobate waveguide module 30, first dense wavelength division multiplexer 31, second dense wavelength division multiplexer 32, third dense wavelength division multiplexer 33, and fourth dense wavelength division multiplexer 34. Multiplexer 33, fourth dense wavelength division multiplexer 34, first fiber Bragg grating 35, second fiber Bragg grating 36, third fiber Bragg grating 37, fourth fiber Bragg grating 38, fifth fiber Bragg grating 39, sixth fiber Bragg grating 40, seventh fiber Bragg grating 41, eighth fiber Bragg grating 42, third fiber polarization controller 43, fourth fiber polarization controller 44, fifth fiber polarization controller 45, sixth fiber polarization controller 46, seventh fiber polarization controller 47, eighth fiber polarization controller 48, ninth fiber polarization controller 49, tenth fiber polarization controller 50, first superconducting nanowire single-photon detector 51, second superconducting nanowire single-photon detector 52, third superconducting nanowire single-photon detector 53, fourth superconducting nanowire single-photon detector 54, fifth superconducting nanowire single-photon detector 55, sixth superconducting nanowire single-photon detector 56, seventh superconducting nanowire single-photon detector 57, eighth superconducting nanowire single-photon detector 58, time-to-digital converter 59, computer 60. Detailed Implementation

[0031] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0032] Example:

[0033] like Figure 1 As shown, in this example of the quantum memory, both the first and second optical switch arrays are equipped with four optical switches. Therefore, this example includes, in sequence, a pump laser source 1, an intensity modulator 2, a first phase modulator 3, a second phase modulator 4, an acousto-optic modulator 5, a tunable optical attenuator 6, a polarization controller 7, an optical fiber beam splitter group 8, a first optical switch array, an optical fiber integrated rare-earth-doped optical waveguide array module 13, and a second optical switch array. The first optical switch array specifically includes a first optical switch 9, a second optical switch 10, a third optical switch 11, and a fourth optical switch 12. The second optical switch array specifically includes a fifth optical switch 17, a sixth optical switch 18, a seventh optical switch 19, and an eighth optical switch 20.

[0034] The first input ports of the first optical switch 9, the second optical switch 10, the third optical switch 11, and the fourth optical switch 12 are respectively connected to the output ports of the fiber optic beam splitter group 8. The second input ports of the first optical switch 9, the second optical switch 10, the third optical switch 11, and the fourth optical switch 12 respectively input signal light, which includes signal light 1, signal light 2, signal light 3, and signal light 4.

[0035] The fiber-integrated rare-earth-doped optical waveguide array module 13 is composed of a first optical fiber array 14, a rare-earth-doped optical waveguide array 15, and a second optical fiber array 16. The first optical fiber array 14 is connected to the output ports of the first optical switch 9, the second optical switch 10, the third optical switch 11, and the fourth optical switch 12 via optical fibers. The second optical fiber array 16 is connected to the input ports of the fifth optical switch 17, the sixth optical switch 18, the seventh optical switch 19, and the eighth optical switch 20 via optical fibers.

[0036] The first optical switch 9, the second optical switch 10, the third optical switch 11, and the fourth optical switch 12 are used to select whether to allow pump light or signal light to enter the fiber-integrated rare-earth-doped optical waveguide array module 13.

[0037] The rare-earth-doped optical waveguide array 15 is a medium for storing multiple degrees of freedom quantum light. The pump light of the selected input waveguide interacts with the rare-earth ion ensemble, and an atomic frequency comb is prepared for storing signal light.

[0038] In this embodiment, the pump laser source 1 is a semiconductor continuous laser used to output tunable narrow linewidth continuous pump light with a center wavelength of 1532.05 nm and a linewidth of approximately 10 kHz.

[0039] The intensity modulator 2 is a lithium niobate intensity modulator with a bandwidth of 20 GHz. The first phase modulator 3 is a lithium niobate phase modulator with a bandwidth of 20 GHz. The continuous laser output from the pump laser source 1 is modulated in intensity and phase by the intensity modulator 2 and the first phase modulator 3 to output an optical frequency comb with five teeth. Specifically, a 15 GHz microwave signal is generated using a microwave source, and the microwave signal is split into two. One path is passed through a microwave amplifier and then loaded onto the intensity modulator 2, while the other path is passed through a phase shifter and then a microwave amplifier and then loaded onto the first phase modulator 3. The microwave signal power, the phase shift amount of the phase shifter, and the amplification factor of the two microwave amplifiers are adjusted in a coordinated manner to make the intensity of the five teeth of the optical frequency comb approximately equal. The five teeth of the optical frequency comb are spaced 15 GHz apart, and the center wavelengths corresponding to each optical frequency comb tooth are 1531.69 nm, 1531.81 nm, 1531.93 nm, 1532.05 nm, and 1532.17 nm, respectively.

[0040] The second phase modulator 4 is a lithium niobate phase modulator with a bandwidth of 20 GHz. The optical frequency comb output from the first phase modulator 3 enters the second phase modulator 4, which performs chirped frequency shifting on each tooth of the input optical frequency comb. Based on the voltage signal applied to it, the second phase modulator 4 performs time-varying phase modulation on each tooth of the input optical frequency comb, achieving linear frequency sweeping of each tooth within a range of -2 GHz to 2 GHz from the center frequency. The prepared five-segment linearly swept pump laser serves as the pump light for the five-channel atomic frequency comb, with a center frequency interval of 15 GHz and each segment having a linear sweep range of 4 GHz. Specifically, under the influence of the applied modulation signal, the refractive index of the electro-optic crystal changes, causing the light input to the phase modulator to acquire a time-varying additional phase, thereby achieving frequency shifting.

[0041] The acousto-optic modulator 5 is a lithium niobate acousto-optic modulator with a working center wavelength around 1532 nm, an extinction ratio of 50 dB, and a switching time of approximately 15 ns. The linearly swept pump light output from the second phase modulator 4 is input to the acousto-optic modulator 5. The acousto-optic modulator 5 modulates the input continuous pump light into an optical pulse with a period of 500 ms and a pulse width of 200 ms, based on an electrical pulse signal with a pulse width of 200 ms and a pulse width of 500 ms applied to it.

[0042] The adjustable optical attenuator 6 is an optical fiber adjustable attenuator. The linear sweep frequency pulse pump light output from the acousto-optic modulator 5 is input to the adjustable optical attenuator 6 to realize the adjustment of pump light power.

[0043] The polarization controller 7 is an optical fiber loop polarization controller. The linear sweep pulse pump light output from the adjustable optical attenuator 6 is input to the polarization controller 7 to adjust the polarization state of the pump light.

[0044] The fiber beam splitter group 8 is a group of multiple ordinary 50:50 fiber beam splitters, with one input port and multiple output ports. The linearly swept frequency pulse pump light output by the polarization controller 7 is output from multiple ports after passing through the fiber beam splitter group 8, and the output optical power is the same. The linearly swept frequency pump light output from each output port is used as the pump light for preparing a five-channel atomic frequency comb in each waveguide.

[0045] The first optical switch 9, the second optical switch 10, the third optical switch 11, and the fourth optical switch 12 each have two input ports and one output port, with an usable wavelength range of 1240-1640 nm and an extinction ratio of 75 dB. Each output port of the fiber optic beam splitter group 8 outputs linearly swept pump light corresponding to the first input ports of the first optical switches 9, 10, 11, and 12, while the second input ports of these switches receive signal light. Under the action of a loaded electrical pulse control signal, the first optical switches 9, 10, 11, and 12 selectively output pump light or signal light. The optical signals from the output ports of these switches are coupled into designated waveguides in the fiber-integrated rare-earth-doped optical waveguide array module 13 to achieve selective fabrication of a five-channel atomic frequency comb. Specifically, taking the first optical switch 9 as an example, the linearly swept pump light from the first output port of the fiber beam splitter group 8 is input to the first input port of the first optical switch 9, and its second input port is used to input signal light 1. When it is necessary to prepare a five-channel atomic frequency comb in the first waveguide of the rare-earth-doped optical waveguide array 15 corresponding to the rare-earth-doped optical waveguide array module 13 integrated with optical fiber, the first optical switch 9, under the control of an electrical pulse signal with a pulse width of 220 ms and a period of 500 ms, outputs linearly swept pump light for the first 200 ms within every 500 ms period, then has no output for the next 20 ms, and outputs signal light 1 for the remaining 280 ms. Through the aforementioned timing control, the fabrication process of the five-channel atomic frequency comb and the storage process of signal light 1 are effectively separated in time. This avoids the introduction of noise by the linearly swept pump light during the quantum storage stage and prevents the pump light from entering the single-photon detection stage in subsequent solid-state quantum storage applications, thus avoiding damage to the single-photon detector. When the fabrication of the five-channel atomic frequency comb in this waveguide is not required, the first optical switch 9 outputs only signal light 1 under a continuous low-level control signal. Since the linearly swept pump light does not enter the waveguide, the atomic frequency comb is not fabricated, and signal light 1 will be directly absorbed and transmitted by rare-earth ions in the waveguide without a storage process. By changing the electrical pulse control signals applied to the first optical switch 9, the second optical switch 10, the third optical switch 11, and the fourth optical switch 12, each optical switch can independently select whether to output linearly swept pump light to the corresponding waveguide, thereby realizing the selective fabrication of the five-channel atomic frequency comb in different waveguides and achieving programmable multiplexed quantum storage with spatial degrees of freedom.

[0046] The fiber-integrated rare-earth-doped optical waveguide array module 13 consists of a first fiber array 14, a rare-earth-doped optical waveguide array 15, and a second fiber array 16. Each input port of the first fiber array 14 is connected to the output ports of the first optical switch 9, the second optical switch 10, the third optical switch 11, and the fourth optical switch 12 via optical fibers. Each output port of the second fiber array 16 is connected to the input ports of the fifth optical switch 17, the sixth optical switch 18, the seventh optical switch 19, and the eighth optical switch 20 via optical fibers. Both the first fiber array 14 and the second fiber array 16 are composed of multiple single-mode fibers operating in the optical communication band. The mode field diameter of each single-mode fiber is approximately 10 μm, matching the guided mode of each waveguide in the rare-earth-doped optical waveguide array 15 to achieve efficient coupling between the fiber and the waveguide. The first fiber array 14 and the second fiber array 16 are respectively located at the input and output ends of the rare-earth-doped optical waveguide array 15 and are fixed together using low-temperature adhesive to ensure mechanical and optical coupling stability under low-temperature operating conditions. Specifically, taking the first waveguide in the rare-earth-doped optical waveguide array 15 as an example, the optical signal output by the first optical switch 9 is coupled into the first rare-earth-doped optical waveguide in the corresponding rare-earth-doped optical waveguide array 15 via the first single-mode optical fiber in the first optical fiber array 14. After being transmitted in the waveguide, it is then coupled to the first single-mode optical fiber in the second optical fiber array 16 and output, thus realizing the transmission of the optical signal in the fiber-to-waveguide-to-fiber structure.

[0047] The rare-earth-doped optical waveguide array 15 is an erbium-doped lithium niobate optical waveguide array fabricated using femtosecond laser direct writing technology, with an erbium ion doping concentration of 0.1 mol%. The rare-earth-doped optical waveguide array 15 is placed in the low-temperature environment required for solid-state quantum memory operation, with an operating temperature of approximately 15 mK. Under this condition, the non-uniform broadening of erbium ions is approximately 180 GHz, suitable for fabricating multi-channel atomic frequency comb structures to achieve frequency-domain multiplexing quantum storage functionality. To achieve low-loss coupling between the linearly swept pump light output from the first optical switch array and the fundamental mode of the femtosecond laser-written rare-earth-doped optical waveguide array 15, the polarization state of the pump light needs to be adjusted to the optimal polarization state. The polarization controller 7, located in the pump light path, can adjust the polarization state of the pump light to the optimal state matching the waveguide fundamental mode.

[0048] A linearly swept pump light coupled into the rare-earth-doped optical waveguide array 15 is used to fabricate a five-channel atomic frequency comb in the erbium ion ensemble of the waveguide through a selective spectral hole-burning process. Each channel has a bandwidth of 4 GHz and a channel spacing of 15 GHz, corresponding to center wavelengths of 1531.69 nm, 1531.81 nm, 1531.93 nm, 1532.05 nm, and 1532.17 nm. Since the storage performance of the atomic frequency comb is affected by the pump light power, the pump light power needs to be set to an optimal value to obtain the best storage performance. This is achieved by adjusting the pump light power using an adjustable optical attenuator 6, thus enabling precise control of the atomic frequency comb fabrication process.

[0049] The signal light coupled into the rare-earth-doped optical waveguide array 15 interacts coherently with the atomic frequency comb, is stored in the rare-earth-doped optical waveguide, and is read out at a specific time, which is determined by the comb tooth spacing of the atomic frequency comb. The residual pump light, the transmitted signal light, and the stored readout light are output from the rare-earth-doped optical waveguide array 15 and coupled into the second fiber array 16 for output. Specifically, taking the first waveguide in the rare-earth-doped optical waveguide array 15 as an example, when a five-channel atomic frequency comb is fabricated in this waveguide, within a 500 ms period, the first single-mode fiber corresponding to the second fiber array 16 outputs residual pump light or fluorescence noise caused by the spectral hole burning process for the first 220 ms, followed by the output of transmitted signal light and readout signal light within the next 280 ms; when the atomic frequency comb is not fabricated in this waveguide, the first single-mode fiber corresponding to the second fiber array 16 only outputs transmitted signal light throughout the entire period.

[0050] The fifth optical switch 17, sixth optical switch 18, seventh optical switch 19, and eighth optical switch 20 each have one input port and two output ports, with an usable wavelength range of 1240-1640 nm and an extinction ratio of 75 dB. Optical signals from the second fiber array 16 are input to the fifth optical switch 17, sixth optical switch 18, seventh optical switch 19, and eighth optical switch 20, respectively. Under the action of an electrical pulse control signal, the second optical switch array performs channel selection on the input optical signals, allowing the transmitted signal light and readout signal light to be output from the second output port as needed. When the corresponding optical waveguide does not have an atomic frequency comb, the second output port is not selected and no optical signal is output. Specifically, taking the fifth optical switch 17 as an example, the optical signal output from the first single-mode fiber of the second fiber array 16 is input to the fifth optical switch 17. When a five-channel atomic frequency comb is fabricated in the first waveguide of the rare-earth-doped optical waveguide array 15, the fifth optical switch 17, under the control of an electrical pulse with a pulse width of 220 ms and a period of 500 ms, outputs residual pump light or fluorescence noise from the first output port for the first 220 ms within each 500 ms period to prevent this portion of light from entering the second output port and damaging the subsequently connected detection device. Then, for the next 280 ms, the transmitted signal light and readout signal light are output from the second output port of the fifth optical switch 17. When an atomic frequency comb is not fabricated in the corresponding waveguide, the fifth optical switch 17, under the action of a continuous high-level control signal, outputs the transmitted light of the photons to be stored from the first output port, and the second output port does not output any optical signal.

[0051] After completing such Figure 1 After the quantum memory shown is fabricated, it is used... Figure 2 The announced single-photon source system inputs signal light to the quantum memory and, through... Figure 3 The single-photon counting device shown detects the stored readout signal light, thereby verifying the multimode multiplexing capability of the quantum memory in the spatial, frequency, and time domains.

[0052] Figure 2The announced single-photon source system shown includes a second semiconductor continuous laser 21, a second lithium niobate intensity modulator 22, an erbium-doped fiber amplifier 23, a second tunable optical attenuator 24, a second fiber polarization controller 25, a second fiber beam splitter group 26, a first fiber-integrated periodically polarized lithium niobate waveguide module 27, a second fiber-integrated periodically polarized lithium niobate waveguide module 28, a third fiber-integrated periodically polarized lithium niobate waveguide module 29, a fourth fiber-integrated periodically polarized lithium niobate waveguide module 30, a first dense wavelength division multiplexer 31, a second dense wavelength division multiplexer 32, a third dense wavelength division multiplexer 33, and a fourth dense wavelength division multiplexer 34. The second semiconductor continuous laser 21, the second lithium niobate intensity modulator 22, the erbium-doped fiber amplifier 23, the second tunable optical attenuator 24, and the second fiber polarization controller 24 are connected sequentially. The output port of the second fiber polarization controller 24 is connected to the input port of the second fiber beam splitter group 26. Each output port of the second fiber beam splitter group 26 is connected to the input ports of the first fiber-integrated periodically polarized lithium niobate waveguide module 27, the second fiber-integrated periodically polarized lithium niobate waveguide module 28, the third fiber-integrated periodically polarized lithium niobate waveguide module 29, and the fourth fiber-integrated periodically polarized lithium niobate waveguide module 30, respectively. The output ports of the periodically polarized lithium niobate waveguide module 27, the second fiber-integrated periodically polarized lithium niobate waveguide module 28, the third fiber-integrated periodically polarized lithium niobate waveguide module 29, and the fourth fiber-integrated periodically polarized lithium niobate waveguide module 30 are respectively connected to the input ports of the first dense wavelength division multiplexer 31, the second dense wavelength division multiplexer 32, the third dense wavelength division multiplexer 33, and the fourth dense wavelength division multiplexer 34. The two output ports of the first dense wavelength division multiplexer 31, the second dense wavelength division multiplexer 32, the third dense wavelength division multiplexer 33, and the fourth dense wavelength division multiplexer 34 output the signal light and idler light of the announced single-photon source, respectively. Signal light 1, signal light 2, signal light 3, and signal light 4 are respectively input... Figure 1 The second input ports of the first optical switch 9, the second optical switch 10, the third optical switch 11, and the fourth optical switch 12, and the inputs of idler frequency optical switches 1, 2, 3, and 4 are shown. Figure 3 The single-photon counting device shown is illustrated.

[0053] Figure 3The single-photon counting device shown includes, in sequence, a first fiber Bragg grating 35, a second fiber Bragg grating 36, a third fiber Bragg grating 37, a fourth fiber Bragg grating 38, a fifth fiber Bragg grating 39, a sixth fiber Bragg grating 40, a seventh fiber Bragg grating 41, an eighth fiber Bragg grating 42, a third fiber polarization controller 43, a fourth fiber polarization controller 44, a fifth fiber polarization controller 45, a sixth fiber polarization controller 46, a seventh fiber polarization controller 47, an eighth fiber polarization controller 48, a ninth fiber polarization controller 49, a tenth fiber polarization controller 50, a first superconducting nanowire single-photon detector 51, a second superconducting nanowire single-photon detector 52, a third superconducting nanowire single-photon detector 53, a fourth superconducting nanowire single-photon detector 54, a fifth superconducting nanowire single-photon detector 55, a sixth superconducting nanowire single-photon detector 56, a seventh superconducting nanowire single-photon detector 57, an eighth superconducting nanowire single-photon detector 58, a time-to-digital converter 59, and a computer 60.

[0054] The second semiconductor continuous laser 21 outputs a tunable narrow-linewidth continuous pump laser with a center wavelength of 1540.6 nm and a linewidth of approximately 10 kHz.

[0055] The continuous laser output from the second semiconductor continuous laser 21 is input to the second lithium niobate intensity modulator 22. The second lithium niobate intensity modulator 22 modulates the input continuous laser into a pulsed laser according to the applied electrical pulse control signal. To enable the storage of multiple time-domain modes, the pulsed laser parameters can be set to generate multiple pulses within one cycle. For example, with a cycle of 1 μs, 330 optical pulses are generated per cycle, with a full width at half maximum (FWHM) of 300 ps for each pulse and a spacing of 600 ps between adjacent pulses.

[0056] The pulsed laser output from the second lithium niobate phase modulator 22 is input to the erbium-doped fiber amplifier 23 for optical power amplification.

[0057] The pulsed laser output from the erbium-doped fiber amplifier 23 is input to the second fiber attenuator 24 for optical power adjustment.

[0058] The pulsed laser output from the second fiber attenuator 24 is input to the second fiber polarization controller 25 for polarization adjustment.

[0059] The pulsed laser output from the second fiber polarization controller 25 is input to the second fiber beam splitter group 26 and output from each output port of the second fiber beam splitter group 26, with each port outputting the same optical power.

[0060] The pulsed laser output from each port of the second fiber beam splitter group 26 is input to the periodically polarized lithium niobate waveguide module 27, the periodically polarized lithium niobate waveguide module 28, the periodically polarized lithium niobate waveguide module 29, and the periodically polarized lithium niobate waveguide module 30 integrated with the first and fourth fibers. Through cascaded second harmonic generation and spontaneous parametric down-conversion processes, a broadband announced single-photon source is generated. Depending on the pulsed laser parameters, an announced single-photon source generating multiple time-domain modes within one cycle can be created, for example, 330 time-domain modes within a 1 μs cycle. Signal beams 1, 2, 3, and 4 of this announced single-photon source are input to a pre-prepared multi-channel atomic frequency comb to achieve time-domain multiplexing quantum storage functionality.

[0061] The announced single-photon sources output from the first fiber-integrated periodically polarized lithium niobate waveguide module 27, the second fiber-integrated periodically polarized lithium niobate waveguide module 28, the third fiber-integrated periodically polarized lithium niobate waveguide module 29, and the fourth fiber-integrated periodically polarized lithium niobate waveguide module 30 are input to the first dense wavelength division multiplexer 31, the second dense wavelength division multiplexer 32, the third dense wavelength division multiplexer 33, and the fourth dense wavelength division multiplexer 34. Each of the first dense wavelength division multiplexer 31, the second dense wavelength division multiplexer 32, the third dense wavelength division multiplexer 33, and the fourth dense wavelength division multiplexer 34 has one input port and two output ports. The first output ports of the first dense wavelength division multiplexer 31, the second dense wavelength division multiplexer 32, the third dense wavelength division multiplexer 33, and the fourth dense wavelength division multiplexer 34 output signal light 1, signal light 2, signal light 3, and signal light 4 of the announced single-photon source, respectively. The second output ports output idler light 1, idler light 2, idler light 3, and idler light 4 corresponding to the announced single-photon source, respectively. The filtering bandwidth of the first dense wavelength division multiplexer 31, the second dense wavelength division multiplexer 32, the third dense wavelength division multiplexer 33, and the fourth dense wavelength division multiplexer 34 is 100 GHz, the filtering center of the signal light is 1532 nm, and the filtering center of the idler light is 1549 nm. The bandwidth coverage of the signal light and the idler light is... Figure 1The bandwidth range of the five-channel atomic frequency comb prepared in the process is such that it can simultaneously store the signal light of the announced single-photon source to achieve frequency-domain multiplexing quantum storage. To ensure optimal mode matching between the pulsed laser and the periodically polarized lithium niobate waveguide modules 27, 28, 29, and 30 (integrated with the first, second, and third fibers), and to maintain the non-classical characteristics of the announced single-photon source, the power and polarization state of the pulsed laser are adjusted by the second fiber attenuator 24 and the second fiber polarization controller 25. This ensures that the coincidence count rate of the signal light and idler light generated by the announced single-photon source in the first, second, third, and fourth fiber integrated periodically polarized lithium niobate waveguide modules 27, 28, 29, and 30 reaches its maximum value, and the second-order cross-correlation function is not less than 20.

[0062] Signal light 1, signal light 2, signal light 3, and signal light 4 output from the first output ports of the first dense wavelength division multiplexer 31, the second dense wavelength division multiplexer 32, the third dense wavelength division multiplexer 33, and the fourth dense wavelength division multiplexer 34 are used as the optical signals to be stored in the atomic frequency comb and are respectively input to... Figure 1 The second input ports of the first optical switch 9, the second optical switch 10, the third optical switch 11, and the fourth optical switch 12 are shown. Since the spectral bandwidth of each signal light covers five atomic frequency comb channels, a single signal light can be simultaneously stored in parallel across the five atomic frequency comb channels. After storage and readout, the transmitted signal light and the readout signal lights 1, 2, 3, and 4 read from each channel are respectively... Figure 1The outputs from the second output ports of the fifth optical switch 17, the sixth optical switch 18, the seventh optical switch 19, and the eighth optical switch 20 are shown, and input to the corresponding first fiber Bragg grating 35, second fiber Bragg grating 36, third fiber Bragg grating 37, and fourth fiber Bragg grating 38. The filter centers of the first fiber Bragg grating 35, the second fiber Bragg grating 36, the third fiber Bragg grating 37, and the fourth fiber Bragg grating 38 can be tuned by temperature control, and are used to selectively filter out signal light from a specified channel from multi-channel readout signal light, thereby realizing the differentiation and analysis of readout signal light from different atomic frequency comb channels. The idler light 1, idler light 2, idler light 3, and idler light 4 output from the second output ports of the first dense wavelength division multiplexer 31, the second dense wavelength division multiplexer 32, the third dense wavelength division multiplexer 33, and the fourth dense wavelength division multiplexer 34 serve as announcement signals for the corresponding readout signal light 1, signal light 2, signal light 3, and signal light 4, respectively. Successful detection of the idler light is used to announce the existence of the signal light successfully read from the atomic frequency comb quantum memory. Idle light corresponding to signal light 1, signal light 2, signal light 3, and signal light 4 are selected one-to-one from idler light 1, idler light 2, idler light 3, and idler light 4 through the fifth fiber Bragg grating 39, the sixth fiber Bragg grating 40, the seventh fiber Bragg grating 41, and the eighth fiber Bragg grating 42, respectively. The readout signal of the nth channel in the signal light corresponds to the nth channel with the same number in the idler light. The filtering bandwidth of the first fiber Bragg grating 35, the second fiber Bragg grating 36, the third fiber Bragg grating 37, the fourth fiber Bragg grating 38, the fifth fiber Bragg grating 39, the sixth fiber Bragg grating 40, the seventh fiber Bragg grating 41, and the eighth fiber Bragg grating 42 is all 4 GHz, consistent with the bandwidth of a single channel in the five-channel atomic frequency comb. Their filtering centers are tuned according to the center wavelength of each channel in the five-channel atomic frequency comb. Specifically, fiber Bragg gratings 35 and 39 are taken as examples. When selected in... Figure 1When a five-channel atomic frequency comb is fabricated in the first optical waveguide of the rare-earth-doped optical waveguide array 15, signal light 1 is simultaneously stored and read out in the five channels. The readout signal light from different channels is input into fiber Bragg grating 35, and the corresponding idler light 1 is input into fiber Bragg grating 39. By adjusting the control temperature of the fiber Bragg gratings, the filter centers of fiber Bragg gratings 35 and 39 are tuned to the following corresponding wavelength combinations: 1531.69 nm and 1549.60 nm, 1531.81 nm and 1549.48 nm, 1531.93 nm and 1549.37 nm, 1532.05 nm and 1549.25 nm, and 1532.17 nm and 1549.13 nm, respectively. Therefore, the signal light read from each channel in the five-channel atomic frequency comb and the idler light of the corresponding channel are selected respectively. The two satisfy the energy conservation relationship, and based on this, the coincidence count and second-order correlation function between the read signal light and the idler light of the corresponding channel are analyzed.

[0063] The corresponding channel readout signal light and idler light are selected from the first fiber Bragg grating 35, the second fiber Bragg grating 36, the third fiber Bragg grating 37, the fourth fiber Bragg grating 38, the fifth fiber Bragg grating 39, the sixth fiber Bragg grating 40, the seventh fiber Bragg grating 41, and the eighth fiber Bragg grating 42, respectively. After passing through the third fiber polarization controller 43, the fourth fiber polarization controller 44, the fifth fiber polarization controller 45, the sixth fiber polarization controller 46, the seventh fiber polarization controller 47, the eighth fiber polarization controller 48, the ninth fiber polarization controller 49, and the tenth fiber polarization controller 50, they are respectively input to the first superconducting nanowire single-photon detector 51, the second superconducting nanowire single-photon detector 52, the third superconducting nanowire single-photon detector 53, the fourth superconducting nanowire single-photon detector 54, the fifth superconducting nanowire single-photon detector 55, the sixth superconducting nanowire single-photon detector 56, the seventh superconducting nanowire single-photon detector 57, and the eighth superconducting nanowire single-photon detector 58 for detection. Since the first superconducting nanowire single-photon detector 51, the second superconducting nanowire single-photon detector 52, the third superconducting nanowire single-photon detector 53, the fourth superconducting nanowire single-photon detector 54, the fifth superconducting nanowire single-photon detector 55, the sixth superconducting nanowire single-photon detector 56, the seventh superconducting nanowire single-photon detector 57, and the eighth superconducting nanowire single-photon detector 58 are sensitive to the polarization state of the incident light, the polarization state of the incident light is adjusted by the third fiber polarization controller so that the single-photon wave packet is in the polarization state with the highest detection efficiency of the corresponding detector, thereby improving the single-photon detection efficiency.

[0064] The time-to-digital converter 59 has multiple input ports for receiving electrical pulse signals generated by the first superconducting nanowire single-photon detector 51, the second superconducting nanowire single-photon detector 52, the third superconducting nanowire single-photon detector 53, the fourth superconducting nanowire single-photon detector 54, the fifth superconducting nanowire single-photon detector 55, the sixth superconducting nanowire single-photon detector 56, the seventh superconducting nanowire single-photon detector 57, and the eighth superconducting nanowire single-photon detector 58 after detecting a single photon. The time-to-digital converter 59 performs time correlation analysis on the electrical pulse signals input from any pair of corresponding signal and idler optical channels to achieve delayed coincidence counting and obtain coincidence count values ​​under different relative delay conditions. The obtained relative delay data and corresponding coincidence count values ​​are transmitted to a computer in digital signal form. The computer processes the coincidence count data and normalizes it to its maximum value, thereby obtaining the normalized coincidence count distribution under different relative delays, which is then visualized. Specifically, taking superconducting nanowire single-photon detectors 51 and 55 as examples, when a five-channel atomic frequency comb is fabricated in the first waveguide of the rare-earth-doped optical waveguide array 15, the signal light read from any channel and its corresponding idler light are selected by fiber Bragg gratings 35 and 39, respectively, and input to superconducting nanowire single-photon detectors 51 and 55 for detection. The electrical pulse signals output by the two detectors are input to a time-to-digital converter 59 for coincidence count analysis. After the computer processes the obtained data, it obtains the relationship curve of the coincidence count between the channel readout signal light and the corresponding idler light as a function of relative delay.

[0065] The femtosecond laser direct-write lithium erbium niobate waveguide used in this embodiment of the invention is used to fabricate a multi-degree-of-freedom, all-fiber integrated on-chip quantum memory, which features a compact structure, low loss, high stability, large multimode capacity, and easy expansion and integration.

[0066] This invention integrates multiple rare-earth-doped optical waveguides on a single chip and fabricates multi-channel atomic frequency comb structures within each waveguide, enabling multiplexing of quantum storage across multiple degrees of freedom in the spatial, frequency, and time domains. This effectively enhances the multimode storage capacity and parallel processing capability of quantum memories. The key components of the quantum memory involved in this invention can all be implemented based on mature optical fiber communication band materials and optoelectronic devices, exhibiting good engineering implementation conditions and system compatibility. By introducing a multi-degree-of-freedom multiplexing mechanism, this invention provides a feasible technical solution for constructing high-capacity, high-throughput on-chip integrated quantum storage nodes. It can be widely applied to quantum repeaters, quantum network nodes, and quantum state caching and scheduling in optical fiber quantum communication systems, and is of great significance for promoting the practical development of communication band quantum memories and the construction of a large-scale quantum internet.

Claims

1. A multi-degree-of-freedom fiber-optic integrated on-chip quantum memory, characterized in that, The system comprises, in sequence, a pump laser source (1), an intensity modulator (2), a first phase modulator (3), a second phase modulator (4), an acousto-optic modulator (5), a tunable optical attenuator (6), a polarization controller (7), an optical fiber beam splitter group (8), a first optical switch array, an optical fiber integrated rare-earth-doped optical waveguide array module (13), and a second optical switch array. The first optical switch array consists of multiple dual-input single-output optical switches. The two input ends of each optical switch are respectively connected to a linear sweep pump light and a corresponding signal light output from the fiber beam splitter group (8). The fiber-integrated rare-earth-doped optical waveguide array module (13) consists of a first optical fiber array (14), a rare-earth-doped optical waveguide array (15), and a second optical fiber array (16). The first optical fiber array (14) is connected to the output port of the optical switch in the first optical switch array via optical fiber, and the second optical fiber array (16) is connected to the input port of the optical switch in the second optical switch array via optical fiber. The rare-earth-doped optical waveguide array (15) is a multi-parallel waveguide array fabricated on the same rare-earth-doped crystal, which is connected to the first optical fiber array (14) and the second optical fiber array (16) respectively. Multiple parallel waveguide paths were then obtained. Utilizing the selective spectral hole-burning effect of pump light in the rare-earth ion ensemble, each waveguide can be used to fabricate an atomic frequency comb structure for time-frequency multiplexed quantum storage. Specifically, within a set time period, the optical switches in the first optical switch array are controlled to input linearly swept pump light into the corresponding waveguide within a set first time period. After a set interval, the optical switches in the first optical switch array are controlled to input signal light into the corresponding waveguide within a set second time period, thereby achieving effective temporal separation between the atomic frequency comb fabrication process and the signal light storage process. Corresponding to the first optical switch array, the second optical switch array is composed of multiple single-input dual-output optical switches. The input end of the optical switch in the second optical switch array is connected to the second optical fiber array (16). When an atomic frequency comb is prepared in the waveguide, the optical switch in the corresponding second optical switch array outputs residual pump light or fluorescence noise from the first output end during the first time and interval, and outputs transmitted signal light and readout signal light from the second output end during the second time. The pump laser source (1) is used to provide pump light for the preparation process of atomic frequency combs in quantum memory; The intensity modulator (2) and the first phase modulator (3) are used to prepare an optical frequency comb based on pump light and output it to the second phase modulator (4). The second phase modulator (4) is used to realize the frequency shift of the optical frequency comb to obtain linear sweep pump light output to the acousto-optic modulator (5). The acousto-optic modulator (5) is used to modulate the pulse width, duty cycle, and on / off state of the input linear sweep pump light; The adjustable optical attenuator (6) adjusts the power of the linear sweep pulse pump light output by the acousto-optic modulator (5); The polarization controller (7) adjusts the polarization state of the linear sweep pulse pump light output by the tunable optical attenuator (6); The fiber beam splitter group (8) splits the linear sweep pulse pump light output by the polarization controller (7) and outputs it through multiple output ports.

2. The multi-degree-of-freedom fiber-optic integrated on-chip quantum memory according to claim 1, characterized in that, The pump laser source (1) is a solid-state laser, a semiconductor laser, or a dye laser. The pump laser source (1) generates continuous pump light, and the laser wavelength matches the absorption wavelength of the rare earth ion ensemble in the rare earth doped optical waveguide array (15) for the preparation of an atomic frequency comb quantum memory.

3. The multi-degree-of-freedom fiber-optic integrated on-chip quantum memory according to claim 1, characterized in that, The intensity modulator (2) is an intensity modulator based on the electro-optic effect of lithium niobate crystal.

4. The multi-degree-of-freedom fiber-optic integrated on-chip quantum memory according to claim 1, characterized in that, The first phase modulator (3) and the second phase modulator (4) are phase modulators based on KDP crystals or phase modulators based on lithium niobate crystals, with a bandwidth greater than or equal to 20 GHz.

5. The multi-degree-of-freedom fiber-optic integrated on-chip quantum memory according to claim 1, characterized in that, The operating wavelength of the acousto-optic modulator (5) is matched with the operating wavelength of the quantum memory, and the switching time is 15 ns.

6. The multi-degree-of-freedom fiber-optic integrated on-chip quantum memory according to claim 1, characterized in that, The polarization controller (7) is a waveplate polarization controller or an optical fiber ring polarization controller.

7. The multi-degree-of-freedom fiber-optic integrated on-chip quantum memory according to claim 1, characterized in that, The fiber optic beam splitter group (8) consists of multiple ordinary fiber optic beam splitters, enabling one optical input and multiple optical outputs.

8. The multi-degree-of-freedom fiber-optic integrated on-chip quantum memory according to claim 1, characterized in that, The switching time of the optical switches in the first and second optical switch arrays is 0.5 ms.

9. A multi-degree-of-freedom fiber-optic integrated on-chip quantum memory according to claim 1, characterized in that, The rare-earth-doped optical waveguide array (15) is fabricated using semiconductor technology, which consists of at least one of laser direct writing, proton exchange, precision machining, ultraviolet lithography, electron beam exposure, and plasma etching.

10. A multi-degree-of-freedom fiber-optic integrated on-chip quantum memory according to claim 1, characterized in that, The rare earth-doped optical waveguide array (15) is a rare earth-doped waveguide with non-uniform broadening on the GHz scale, including erbium-doped lithium niobate waveguide, thulium-doped lithium niobate waveguide, and neodymium-doped yttrium vanadate waveguide.

Images