System and method for detecting correlation between two photons

Abstract

A technique for reducing the number of detectors required to perform measurements related to quantum entanglement and quantum communication using optical qubits is described herein. This technique involves delaying the arrival of photons to the detector using delay lines between beam-splitter optical elements, thereby allowing multiple photons to be detected for correlation measurements using a shared detector, with the detections occurring at different times. This halves the hardware requirements for the detector and substantially reduces the cost and complexity of the system for detecting and processing qubits.

Description

Background Art

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[0003]

[0001] A quantum network facilitates 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 a standard communication optical fiber 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 embodiments provide a system for detecting a correlation between two photons, the system comprising a non-polarizing beam splitter configured to receive a first photon from a first entanglement source at a first input of the non-polarizing beam splitter and a second photon from a second entanglement source at a second input of the non-polarizing beam splitter; a polarizing beam splitter configured to receive the first output of the non-polarizing beam splitter at a first input of the polarizing beam splitter and to direct light having a first polarization to a first output of the polarizing beam splitter and light having a second polarization to a second output of the polarizing beam splitter; a delay line configured to receive the second output of the non-polarizing beam splitter and output light to a second input of the polarizing beam splitter; a first photon detector configured to receive an output corresponding to the first polarization; and a second photon detector configured to receive an output corresponding to the second polarization.

[0003] In some embodiments, the delay line has a length corresponding to a time delay greater than the temporal bandwidth of the first photon and the second photon or the jitter of the detector.

[0004] In some embodiments, the delay line provides a delay of 20 to 40 ns. In some embodiments, the delay line is an optical fiber cable. In some embodiments, the unpolarized beam splitter is an unpolarized beam splitter cube.

[0005] In some embodiments, the polarizing beam splitter is a GranTailor prism polarizer. In some embodiments, the first detector and the second detector are detectors of the same type.

[0006] In some embodiments, the same type of detector is a superconducting nanowire single-photon detector. In some embodiments, the first and second photon detectors are configured to detect four Bell parameters based on four polarization measurement results, the first two polarization measurement results being detected based on a first output of an unpolarized beam splitter, and the second two polarization measurement results being detected based on a second output of an unpolarized beam splitter.

[0007] In some embodiments, an additional detector having a non-polarizing beam splitter, a delay line, and a polarizing beam splitter is provided, the additional detector being configured to detect a correlation between a third photon from a third entanglement source and a fourth photon from a fourth entanglement source.

[0008] Some embodiments provide a system for detecting correlation between two photons, the system comprising: an unpolarized beam splitter configured to receive a first photon from a first entanglement source at a first input of the unpolarized beam splitter and a second photon from a second entanglement source at a second input of the unpolarized beam splitter; and a polarized beam splitter having a first input and a second input, configured to receive a first output of the unpolarized beam splitter at the first input of the polarized beam splitter, directing light having a first polarization to the first output of the polarized beam splitter and directing light having a second polarization The system comprises a polarizing beam splitter configured to lead to a second output of an optical beam splitter; a first delay line configured to receive the second output of the unpolarizing beam splitter and output light to the second input of the polarizing beam splitter; a second delay line configured to receive the second output of the polarizing beam splitter having a second polarization and output light to a shared beam path, the shared beam path configured to receive the first output of the polarizing beam splitter and the outputs of the second delay line; and a photon detector, the photon detector configured to receive photons at different times depending on the path between the unpolarizing beam splitter and the photon detector.

[0009] In some embodiments, the first delay line has a longer optical path length than the second delay line. In some embodiments, the second delay line has an optical path length approximately equal to the time bandwidth of the first and second photons, and the first delay line has an optical path length approximately equal to twice the time bandwidth of the first and second photons.

[0010] In some embodiments, the first photon detector is a superconducting nanowire single-photon detector. In some embodiments, the first photon detector is configured to detect four Bell parameters based on four polarization measurement results, the four polarization measurement results including a first polarization measurement result detected based on a first output of a non-polarizing beam splitter, a second polarization measurement result detected based on a first output and a second delay line of the non-polarizing beam splitter, a third polarization measurement result detected based on a second output and a first delay line of the non-polarizing beam splitter, and a fourth polarization measurement result detected based on a second output, a first delay line, and a second delay line of the non-polarizing beam splitter.

[0011] Some embodiments provide a method for detecting a quantum state based on the detection of two photons, the method comprising: receiving a first photon from a first beam path and a second photon from a second beam path; splitting the first and second beam paths into a first shared beam path and a second shared beam path; splitting the first shared beam path into a first polarized beam path and a second polarized beam path having polarization orthogonal to the first polarized beam path using a polarized beam splitter; delaying the second shared beam path; detecting a first output corresponding to the first and second polarized beam paths; splitting the second shared beam path into a first polarized beam path and a second polarized beam path using a polarized beam splitter; and detecting a second output corresponding to the first and second polarized beam paths.

[0012] In some embodiments, delaying the second shared beam path involves using a delay line having a length that corresponds to a time delay greater than or equal to the time bandwidth of the first and second photons or the jitter of the detector.

[0013] In some embodiments, the delay line provides a delay of 20 to 40 ns. In some embodiments, detecting a first output corresponding to a first polarization beam path includes using a first detector, and detecting a first output corresponding to a second polarization beam path includes using a second detector.

[0014] In some embodiments, detecting a first output corresponding to a first polarization beam path includes using a first detector, and detecting a first output corresponding to a second polarization beam path includes delaying the second polarization beam path and using a first detector. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 shows a configuration 100 for entanglement swapping and Bell state measurement using passive optical elements. [Figure 2] Figure 2 shows a correlation detection configuration 200 for entanglement swapping and bell state measurement according to several embodiments of the technology described herein. [Figure 3] Figure 3 shows a flowchart corresponding to process 300 for detecting a quantum state based on the detection of two photons, according to some embodiments of the technique described herein. [Figure 4] Figure 4 shows an example of a system 400 for detecting correlations between photons using two detectors, according to some embodiments of the technology described herein. [Figure 5] Figure 5 shows an example of a system 500 for detecting correlations between photons using a single detector, according to several embodiments of the technology described herein. [Modes for carrying out the invention]

[0016] The inventors have developed a technology to advance quantum information science by improving the processing efficiency of qubits. Quantum information science will enable unprecedented capabilities for information integration, processing, and distribution. Similar to the development of the classical internet, where current applications were unimaginable in the earliest demonstrations of networks, a "quantum internet" has the potential to enable revolutionary applications such as unconditionally secure private key exchange, distributed quantum computing, enhanced quantum measurement, and verification of fundamental physics.

[0017] Improvements in quantum repeater technology have made it possible to adapt existing communication fiber infrastructure to quantum network processing through the use of optical qubits. However, the inventors recognize and understand that the cost and complexity of the hardware required to detect and process optical qubits impose limitations on the adoption and implementation of quantum network technology. In particular, the use of single-photon detectors can be prohibitively expensive in large-scale implementations. Single-photon detectors are used in conventional techniques to measure entanglement between two qubits. For two qubits, each having two states (e.g., 0 and 1), the superposition of these qubits can be described using four ground states (e.g., Bell states). Determining the ground states is a critical operation in quantum communication. In the case of optical qubits, the ground states can be determined by using four detectors to obtain four polarization measurement results. From the four polarization measurement results, the entanglement between two qubits can be mapped to the ground states.

[0018] Therefore, the inventors have developed a technique to reduce the number of detectors required to perform measurements related to quantum entanglement and quantum communication using optical qubits. The inventors have developed a system in which four polarization measurement results used to determine the ground state can be obtained using two detectors. This halves the hardware requirements for the detectors and substantially reduces the cost and complexity of the system for detecting and processing qubits.

[0019] Therefore, some embodiments provide a system configured to detect correlations between two photons (e.g., entanglement swapping and / or Bell state measurement results), the system comprising an unpolarized beam splitter (e.g., an unpolarized beam splitter cube) configured to receive a first photon (e.g., a qubit) from a first entanglement source at a first input of the unpolarized beam splitter (e.g., the input face of the unpolarized beam splitter cube) and a second photon (e.g., a second qubit) from a second entanglement source at a second input of the unpolarized beam splitter (e.g., a second input face of the unpolarized beam splitter cube orthogonal to the first face), and an unpolarized beam splitter The system comprises a polarizing beam splitter configured to receive a first output of a beam splitter (for example, the first output surface of a non-polarizing beam splitter cube), a polarizing beam splitter configured to receive the first output of a non-polarizing beam splitter at its first input, and to guide light having a first polarization to the first output of the polarizing beam splitter and light having a second polarization to the second output of the polarizing beam splitter, a first photon detector configured to receive an output corresponding to the first polarization, a second photon detector configured to receive an output corresponding to the second polarization, and a delay line configured to receive the second output of the non-polarizing beam splitter and output light to the second input of the polarizing beam splitter.

[0020] In some embodiments, the first and second photon detectors are configured to detect four Bell parameters based on four polarization measurement results, the first two polarization measurement results being detected based on a first output of an unpolarized beam splitter, and the second two polarization measurement results being detected based on a second output of an unpolarized beam splitter.

[0021] In some embodiments, the system further includes an additional detector having respective non-polarizing beam splitters, delay lines, and polarizing beam splitters, the additional detector being configured to detect a correlation between a third photon from a third entanglement source and a fourth photon from a fourth entanglement source.

[0022] Some embodiments provide a system for detecting a correlation between two photons, the system comprising a non-polarizing beam splitter configured to receive a first photon from a first entanglement source at a first input of the non-polarizing beam splitter and a second photon from a second entanglement source at a second input of the non-polarizing beam splitter, a polarizing beam splitter having a first input and a second input, the polarizing beam splitter being configured to receive the first output of the non-polarizing beam splitter at the first input of the polarizing beam splitter, direct light having a first polarization to the first output of the polarizing beam splitter, and direct light having a second polarization to the second output of the polarizing beam splitter, a first delay line configured to receive the second output of the non-polarizing beam splitter and output light to the second input of the polarizing beam splitter, a second delay line configured to receive the second output of the polarizing beam splitter having the second polarization and output the received second output to a common beam path, the common beam path being configured to receive the first output of the polarizing beam splitter and the output of the second delay line, and a photon detector configured to receive photons at different times according to the path between the non-polarizing beam splitter and the photon detector.

[0023] In some embodiments, the first photon detector is configured to detect four Bell parameters based on four polarization measurement results, the four polarization measurement results including a first polarization measurement result detected based on a first output of a non-polarizing beam splitter, a second polarization measurement result detected based on the first output of the non-polarizing beam splitter and a second delay line, a third polarization measurement result detected based on a second output of the non-polarizing beam splitter and a first delay line, and a fourth polarization measurement result detected based on the second output of the non-polarizing beam splitter, the first delay line, and the second delay line.

[0024] Some embodiments provide a method for detecting a quantum state based on the detection of two photons, the method comprising receiving a first photon from a first beam path and a second photon from a second beam path, splitting the first photon beam path and the second photon beam path into a first shared beam path and a second shared beam path, splitting the first shared beam path using a polarization beam splitter into a first polarization beam path and a second polarization beam path having a polarization orthogonal to the first polarization beam path, delaying the second shared beam path, detecting outputs corresponding to the first polarization beam path and the second polarization beam path, splitting the second shared beam path using a polarization beam splitter into the first polarization beam path and the second polarization beam path, and detecting a second output corresponding to the first polarization beam path and the second polarization beam path.

[0025] Some embodiments provide a method for determining the correlation between photons from two different entanglement sources, the method comprising: receiving photons from two entanglement sources, the received photons including a first photon from a first entanglement source and a second photon from a second entanglement source; delaying a portion of the received photons by a set delay time; detecting a first polarization measurement result using a first single-photon detector configured to detect photons having a first polarization; detecting a second polarization measurement result using a second single-photon detector configured to detect photons having a second polarization; detecting a third polarization measurement result using the first single-photon detector after the set delay time; detecting a fourth polarization measurement result using the second single-photon detector after the set delay time; and determining the correlation between the first and second photons based on the first, second, third, and fourth polarization measurement results.

[0026] Figure 1 shows a configuration 100 for entanglement swapping and Bell state measurement using passive optical elements. Using configuration 100, entanglement can be swapped between entanglement source A 102 and entanglement source B 104. As shown in Figure 1, two photons from entanglement sources 102 and 104, respectively, are simultaneously incident on the unpolarized beam splitter 106 of the first element. The two photons exit from one of the two exit ports of the unpolarized beam splitter 106 with equal probability. The photons from each exit port are then led to the respective polarized beam splitters 108 and 110, which then separate the photons into two paths based on polarization. All four outputs of the two polarized beam splitters are connected to individual photon counters 112, 114, 116, and 118.

[0027] The inventors have developed a configuration that significantly reduces the number of elements required for measurement while achieving performance equivalent to that of the configuration shown in Figure 1. Figure 2 shows a correlation detection configuration 200 for entanglement swapping and Bell state measurement according to several embodiments of the technology described herein. The correlation detection configuration 200 includes a non-polarized beam splitter 206, a polarized beam splitter 208, a time delay line 214, a waveplate 216, and single-photon counters 210 and 212.

[0028] As shown in Figure 2, the unpolarized beam splitter 206 receives a first photon from the first entanglement source 202 and a second photon from the second entanglement source 204. The received photons are probabilistically split between the two outputs of the unpolarized beam splitter 206 (for example, reflected to the first output or transmitted to the second output). The first output of the unpolarized beam splitter 206 is configured to direct the photons to the polarized beam splitter 208. The polarized beam splitter 208 directs the photons to one of its two outputs based on their polarization. For example, the polarized beam splitter 208 can direct vertically polarized light to its first output and horizontally polarized light to its second output. To detect photons corresponding to each polarization, single-photon counter 210 is configured to receive light from the first output of the polarization beam splitter 208, and single-photon counter 212 is configured to receive light from the second output of the polarization beam splitter 208.

[0029] The second output of the unpolarized beam splitter 206 is configured to direct photons to a time delay line 214. The time delay line 214 outputs the photons received from the second output of the unpolarized beam splitter 206 to the polarized beam splitter 208. In addition, the time delay line 214 provides an additional optical path length for the photons received from the second output of the unpolarized beam splitter 206. Thus, the additional optical path length corresponds to the time delay in the polarized beam splitter 208 between the arrival of photons from the first output of the unpolarized beam splitter 206 and the arrival of photons from the second output of the unpolarized beam splitter 206.

[0030] In some embodiments, the polarizing beam splitter 208 is a polarizing beam splitter cube. In some embodiments, the polarizing beam splitter 208 is a Gran-Taylor polarizer. In some embodiments, the polarizing beam splitter 208 is a Gran-Laser polarizer. In some embodiments, the polarizing beam splitter 208 is a Gran-Thomson polarizer.

[0031] In some embodiments, the waveplate 216 is located between the output of the time delay line 214 and the polarization beam splitter 208. The waveplate 216 can be used to compensate for any polarization required by the measurement station. In some embodiments, the waveplate may be configured to compensate for phase drift through the delay line. In some embodiments, the waveplate may be configured to eliminate the possibility of obtaining both early and late detection events on the same detector. For example, by rotating the polarization of the light from the time delay line before it passes through the polarization beam splitter 208, the light may be directed to the opposite output of the polarization beam splitter 208. Thus, with respect to some quantum systems, such rotation can significantly reduce the delay length required for the module to avoid detector dead time, which is typically much longer than jitter or the time width of a photon.

[0032] The unpolarized beam splitter 206 may be a 50:50 beam splitter (for example, the received photon is detected with equal probability from both outputs). In some embodiments, the unpolarized beam splitter 206 generates a superposition of the first and second photons.

[0033] In some embodiments, time delay lines can be implemented in any suitable manner so as to provide a longer optical path for photons exiting one of the output ports of the unpolarized beam splitter cube compared to the other output ports. In some embodiments, the minimum delay length of the time delay line is the distance of light propagation in the delay medium, corresponding to the detector dead time and detector jitter. In some embodiments, the minimum delay length of the time delay line is the distance of light propagation in the delay medium, corresponding to the qubit time bandwidth. In some embodiments, the minimum delay length of the time delay line is the distance of light propagation in the delay medium, corresponding to the detector jitter. In some embodiments, the minimum delay length of the time delay line is the distance of light propagation in the delay medium, corresponding to a combination of the detector dead time, the qubit time bandwidth, and / or the detector jitter.

[0034] In some embodiments, the correlation detection configuration 200 may be implemented as a complete fiber-based optical setup. In some embodiments, the correlation configuration 200 may be implemented as a free-space optical setup. For example, at communication wavelengths, low-loss optical fibers (e.g., fiber optic cables) can be used for delay lines. In some embodiments, the correlation configuration 200 may be implemented as a hybrid optical setup including both free-space optical components and fiber-based optical components. The use of fiber optical components or free-space optical components may depend on the wavelength of the system. Any suitable wavelength of light can be used so that light can be transmitted between system components with low loss, and the embodiments of the technology described herein are not limited in this respect.

[0035] The single-photon counter may be any suitable single-photon counter depending on the detection efficiency required for the particular measurement or process. In some embodiments, the single-photon counter is a photomultiplier tube detector. In some embodiments, the single-photon counter is a single-photon avalanche photodiode. In some embodiments, the single-photon counter is a superconducting nanowire single-photon detector. Superconducting nanowire single-photon detectors are used in quantum communication applications requiring the highest detection efficiency. However, in some embodiments, other single-photon counters may be used, and the embodiments of the technology described herein are not limited in this respect.

[0036] Figure 3 shows a flowchart corresponding to process 300 for detecting a quantum state based on the detection of two photons, according to several embodiments of the technique described herein. The two photons are from two separate sources. In some embodiments, the two photons are a first photon from a first entanglement source and a second photon from a second entanglement source. Process 300 can be used with any suitable photons, regardless of linewidth, wavelength, and other characteristics.

[0037] Process 300 begins in operation 302 by receiving a first photon from a first beam path and a second photon from a second beam path, according to some embodiments of the technology described herein. In some embodiments, the first photon is a first qubit from a first entanglement source associated with the first beam path, and the second photon is a second qubit from a second entanglement source associated with the second beam path.

[0038] The first and second photons are received in a non-polarized beam splitter according to some embodiments of the technology described herein. The non-polarized beam splitter may be a non-polarized beam splitter cube. The first and second photons are received simultaneously in the non-polarized beam splitter cube. The first photon may be received at a first input side of the non-polarized beam splitter, and the second photon may be received at a second input side of the non-polarized beam splitter.

[0039] In some embodiments, the non-polarized beam splitter cube is configured to mix a first photon from a first beam path with a second photon from a second beam path to generate a superposition between the state of the first photon and the state of the second photon.

[0040] In some embodiments, the first beam path may be a fiber-based beam path. In some embodiments, the first beam path may be a free-space beam path. In some embodiments, the first beam path may include a mixture of fiber optic components and free-space optic components.

[0041] Similarly, in some embodiments, the second beam path may be a fiber-based beam path. In some embodiments, the second beam path may be a free-space beam path. In some embodiments, the second beam path may include a mixture of fiber optic components and free-space optic components.

[0042] In some embodiments, the first beampath and the second beampath may use the same optical implementation (for example, both may use fiber optic components). In some embodiments, the first beampath and the second beampath may use independent optical implementations (for example, the first beampath may use fiber optic components, and the second beampath may use free-space optical components).

[0043] Next, process 300 proceeds to operation 304 by splitting the first and second photon beam paths into a first shared beam path and a second shared beam path, according to some embodiments of the technology described herein. A beam splitter is used to split the first and second photon beam paths into a first shared beam path and a second shared beam path. A non-polarized beam splitter, receiving the first and second photons at its respective inputs, probabilistically directs each photon to either a first output corresponding to the first shared beam path or a second output corresponding to the second shared beam path. For example, both the first and second photons may be directed to the first output. As another example, both the first and second photons may be directed to the second output. As a third example, the first photon may be directed to the first output and the second photon may be directed to the second output. As a fourth example, the first photon may be directed to a second output, and the second photon may be directed to a first output. In some embodiments, the unpolarized beam splitter is a 50:50 beam splitter, as described herein.

[0044] Next, process 300 proceeds to operation 306 by using a polarizing beam splitter to split the first common beam path into a first polarizing beam path and a second polarizing beam path, according to some embodiments of the technology described herein. The polarizing beam splitter directs the light to one of two outputs depending on the polarization of the light. Light having the first polarization is directed to the first output, and light having the second polarization is directed to the second output, where the second polarization is orthogonal to the first polarization. Thus, the outputs from the polarizing beam cube are two light beam paths with orthogonal polarizations. For example, the first output may be horizontally polarized, and the second output may be vertically polarized, where the horizontal and vertical directions are relative to the reference axis of the polarizer.

[0045] Next, process 300 proceeds to operation 308 by delaying the second shared beam path according to some embodiments of the technology described herein. The delay line is used to delay the light received by the second shared beam path, and this delay is an increase in the path length of the light received by the second shared beam path relative to the path length of the first shared beam path. In some embodiments, the delay line is an optical fiber. For example, an optical fiber of a certain length is configured to add length to the second shared beam path. This added length corresponds to the delay time. In some embodiments, the delay line includes a free-space optical system.

[0046] In some embodiments, the delay time is the duration of the time bandwidth of the received photon (e.g., the duration of the optical pulse in the time domain). In some embodiments, the delay time is the detector jitter. In some embodiments, the delay time is the longer of the time bandwidth of the received photon and the detector jitter. In some embodiments, the delay time is the detector dead time. In some embodiments, the delay time is one or more combinations of the time bandwidth, the detector jitter, and the detector dead time.

[0047] In some embodiments, the delay time is between 20 picoseconds (ps) and 40 ps, ​​between 10 ps and 100 ps, ​​between 1 ps and 500 ps, ​​between 1 ps and 1 ns, between 500 ps and 40 ns, between 1 ns and 100 ns, or between 10 ns and 500 ns.

[0048] Next, process 300 proceeds to operation 310 by detecting outputs corresponding to a first polarization beam path and a second polarization beam path, according to some embodiments of the technology described herein. In some embodiments, a single detector is used to detect outputs corresponding to a first polarization beam path and outputs corresponding to a second polarization beam path. In some embodiments, two detectors are used to detect outputs corresponding to a first polarization beam path and outputs corresponding to a second polarization beam path. A first polarization measurement result is detected by detecting the output corresponding to the first polarization beam path. Similarly, a second polarization measurement result is detected by detecting the output corresponding to a second polarization beam path. In some examples, the measurement result may be that no photons are detected. In some examples, the measurement result may be that one photon is detected. In some embodiments, the measurement may be limited to single-photon detection. In some embodiments, the measurement may detect zero, one, or more photons.

[0049] In a single-detector configuration, the single detector may be configured to receive an output corresponding to a first polarization beam path, and then an output corresponding to a second polarization beam path. Thus, a second delay line can be used to delay the output from the second polarization beam path relative to the path length of the first polarization beam path. In some embodiments, a polarization beam splitter can be used to combine the first polarization beam path and the delayed second beam path before the beam path reaches the detector. In some embodiments, other configurations may be used to combine the first polarization beam path and the delayed second beam path, and the aspects of the techniques described herein are not limited thereto. In some embodiments, the second delay line may be configured to have the same or approximately the same delay time as the first delay line. In some embodiments, the second delay line may be implemented using the techniques described above in relation to the first delay line. For example, the second delay line may be implemented using an optical fiber having a length corresponding to the time bandwidth, detector jitter, detector dead time, or a combination thereof.

[0050] In a two-detector configuration, the first detector is configured to receive the output of the first polarization beam path, and the second detector is configured to receive the output of the second polarization beam path. In some embodiments, the first and second detectors are of the same type. In some embodiments, the first and second detectors are manufactured as a pair of detectors having substantially the same performance.

[0051] In some embodiments, regardless of the number of detectors used, the detectors may be superconducting nanowire single-photon detectors as described herein. In some embodiments, any suitable non-polarization-agnostic single-photon detector may be used.

[0052] Next, process 300 proceeds to operation 312 by using a polarizing beam splitter to split the second shared beam path into a first polarizing beam path and a second polarizing beam path, according to some embodiments of the technology described herein. In some embodiments, the delayed second shared beam path may be recombined with the first shared beam path before the polarizing beam splitter described above in relation to operation 306, so that the second shared beam path can use the same polarizing beam splitter input as the first shared beam path. In some embodiments, the second shared beam path outputs light to the second input face of the polarizing beam splitter described above in relation to operation 306. The second shared beam path is split as described above with reference to the first shared beam path, so that light having the first polarization is directed to the first output and light having the second polarization is directed to the second output, with the second polarization being orthogonal to the first polarization.

[0053] Next, process 300 proceeds to operation 314 by detecting a second output corresponding to a first polarization beam path and a second polarization beam path, according to some embodiments of the technology described herein. The second outputs corresponding to the first polarization beam path and the second polarization beam path may be detected using a single detector or two detectors, as described herein in relation to operation 310. A third polarization measurement result is detected by detecting the output corresponding to the first polarization beam path. Similarly, a fourth polarization measurement result is detected by detecting the output corresponding to the second polarization beam path. In some examples, the measurement result may be that no photons are detected. In some examples, the measurement result may be that one photon is detected. In some embodiments, the measurement may be limited to single-photon detection. In some embodiments, the measurement may detect zero, one, or more photons.

[0054] Following operation 314, process 300 terminates. In the single-detector configuration, an additional operation is included to determine the polarization of each polarization measurement result. Unlike the two-detector configuration, where each detector may correspond to a specific polarization, in the single-photon configuration, the same detector is used to detect each of the four polarization measurement results. Thus, the first detection may be identified as corresponding to the first polarization measurement result, the second detection may be identified as corresponding to the second polarization measurement result, the third detection may be identified as corresponding to the third polarization measurement result, and the fourth detection may correspond to the fourth polarization measurement result. Thus, by knowing the path length and the corresponding delay, the polarization corresponding to each measurement result can be determined based on the fact that the polarization with the shortest path length arrives first and the polarization with the longest path length arrives last.

[0055] In some embodiments, waveplates can be used to adjust the phase of the beam path between optical elements. For example, as described herein, waveplates may be used to change the polarization of the beam path to reduce the possibility of early-arriving / late-arriving photons overlapping on the detector. In some embodiments, waveplates may be used to compensate for phase delays resulting from the optical system used in the beam path.

[0056] Following the completion of process 300, the correlation between the first and second photons may be determined based on the results of the first, second, third, and fourth polarization measurements. In some embodiments, process 300 may be used to perform a BB84 measurement. An example of a BB84 measurement is described in "Eavesdropping Detection in BB84 Quantum Key Distribution Protocols" by C. Lee, I. Sohn, and W. Lee, IEEE Transactions on Network and Service Management, vol. 19, no. 3, pp. 2689-2701 (2022), which is incorporated herein by reference in its entirety. In some embodiments, process 300 may be used to perform an Ekart91 measurement. An example of an Ekart91 measurement is described in "Entanglement-based secure quantum cryptography over 1,120 kilometers" by Yin, J., Li, YH., Liao, SK. et al., Nature 582, 501-505 (2020), which is incorporated herein by reference in its entirety. In some embodiments, process 300 may be used to perform an MDI-QKD measurement. An example of an MKI-QKD measurement is described in "Measurement Device Independent Quantum Key Distribution" by Lo, HK, Curty, M., and Qi, B., Phys. Rev. Lett., vol. 108, no. 13, pp. 120503-120508 (2012). In some embodiments, process 300 may be used for any non-entanglement quantum protocol that relies on two-photon measurements and / or randomized qubit measurements, and the embodiments of the techniques described herein are not limited in this respect.

[0057] Figure 4 shows an example of a system 400 for detecting correlations between photons using two detectors, according to some embodiments of the technology described herein. The system 400 includes a non-polarizing beam splitter 406, a polarizing beam splitter 408, a first detector 410, a second detector 412, a delay line 414, and a waveplate 416.

[0058] The unpolarized beam splitter 406 receives a first photon from a first beam path associated with the entanglement source 402 and a second photon from a second beam path associated with the second entanglement source 404. The unpolarized beam splitter 406 probabilistically divides the first and second beam paths between its two outputs. Thus, the first output from the unpolarized beam splitter 406 is received by the polarized beam splitter 408 on the first input side. The polarized beam splitter 408 directs the received light to either detector 410 or detector 412, depending on the polarization of the received light. For example, horizontally polarized light is sent to detector 412, and vertically polarized light is sent to detector 410.

[0059] Furthermore, a second output from the unpolarized beam splitter 406 is received by a delay line 414. The delay line 414 adds a time delay to the light received from the second output relative to the light sent from the beam path to the detector. In some embodiments, the delay line 414 is an optical fiber that can be coiled to increase the path length within a small device volume, as shown in Figure 4.

[0060] The delay line 414 outputs the delayed light to a second input of the polarizing beam splitter 408. The polarizing beam splitter 408 directs the received light to either detector 410 or detector 412, depending on the polarization of the received light, as described herein. In some embodiments, a polarizer 416 is included between the delay line 414 and the polarizing beam splitter 408. The polarizer 416 may be used to compensate for phase drift caused by the delay line and / or to reduce the possibility of early-arriving / late-arriving photons occurring simultaneously on the same detector, as described herein.

[0061] In some embodiments, system 400 may be a sub-configuration of a larger system for detecting correlations between photons. The system may include two additional detectors for detecting and analyzing each of additional qubit pairs received from separate entanglement sources. For example, multiple correlation detection systems may be included in the correlation detection system, each having a configuration as shown in Figure 4. Thus, additional photons (e.g., additional qubits) may be received from other entanglement sources so as to be processed by the additional detectors. For example, each additional unpolarized beam splitter, delay line, and polarized beam splitter may be included so as to be configured for the additional detectors to detect a correlation between a third photon from a third entanglement source and a fourth photon from a fourth entanglement source.

[0062] Figure 5 shows an example of a system 500 for detecting correlations between photons using a single detector, according to several embodiments of the technology described herein. System 500 includes a non-polarizing beam splitter 506, a first polarizing beam splitter 508, a first detector 510, a first delay line 514, a waveplate 516, a second delay line 520, and a second polarizing beam splitter 526. The non-polarizing beam splitter 506, the first polarizing beam splitter 508, the delay line 514, and the waveplate 516 may be configured similarly to the corresponding components of system 400 shown in Figure 4. The non-polarizing beam splitter 506 receives a first photon from a first beam path associated with an entanglement source 502 and a second photon from a second beam path associated with a second entanglement source 504. The first output of the unpolarized beam splitter 506 is directly received at the first input by the polarized beam splitter 508. The second output of the unpolarized beam splitter 506 is delayed by the delay line 514 before being received at the second input of the polarized beam splitter 508.

[0063] The first output of the polarizing beam splitter 508 is sent to the second delay line 520. Following the delay line 520, the first output of the polarizing beam splitter 508 is coupled to the beam path of the second output of the polarizing beam splitter 508. The first output of the polarizing beam splitter 508 can be coupled to the beam path of the second output of the polarizing beam splitter 508 using the second polarizing beam splitter 526. Thus, the second output of the polarizing beam splitter 508 reaches the detector 510, and then, after a delay introduced by the delay line 520, the first output of the polarizing beam splitter 508 reaches the detector 510.

[0064] In some embodiments, the delay line 520 may include a free-space optical system such as mirrors 522 and 524. Mirrors 522 and 524 are positioned at a 45-degree angle to the beam path of the first output of the polarizing beam splitter 508. Additionally, mirrors 522 and 524 are positioned a certain distance from the polarizing beam splitter 508 and apart from each other, thereby providing an additional path length corresponding to the delay time for the first output of the polarizing beam splitter 508. In some embodiments, additional free-space optical components may be included. In the illustrated embodiments, they are shown as a free-space optical system, but in some embodiments, optical fibers may be used, and / or a combination of a free-space optical system and optical fiber components may be used.

[0065] In some embodiments, delay line 520 delays light by a delay time, which is one or more combinations of time bandwidth, detector jitter, and detector dead time. In some embodiments, delay line 514 delays light for a duration longer than the delay time of delay line 520. Any appropriate delay time can be used so that photons passing through delay line 514 do not reach detector 510 before photons passing through delay line 520 without passing through delay line 514. In some embodiments, delay line 514 may be twice the delay of delay line 520. For example, delay line 520 may correspond to the dead time of detector 510, and delay line 514 may correspond to twice the dead time of detector 510. In some embodiments, other delay line delays may be used, and the embodiments of the technology described herein are not limited thereto.

[0066] Therefore, for photons received by the unpolarized beam splitter 506, there are four time windows in which they can be received by the detector 510: photons transmitted from the first output of the unpolarized beam splitter 506 to the polarized beam splitter 508 and not passing through delay line 514 or delay line 520 after the polarized beam splitter 508; photons transmitted from the first output of the unpolarized beam splitter 506 to the polarized beam splitter 508, not passing through delay line 514, and then passing through delay line 520 after the polarized beam splitter 508; photons transmitted from the second output of the unpolarized beam splitter 506 to the polarized beam splitter 508 through delay line 514 but not passing through delay line 520; and photons transmitted from the second output of the unpolarized beam splitter 506 to the polarized beam splitter 508 through delay line 514 and then passing through delay line 520.

[0067] In some embodiments, system 500 may be a sub-configuration of a larger system for detecting correlations between photons. The system may include additional detectors for detecting and analyzing each of additional qubit pairs received from separate entanglement sources. For example, the correlation detection system may include multiple correlation detection systems, each having a configuration as shown in Figure 5. Thus, additional photons (e.g., additional qubits) may be received from other entanglement sources so as to be processed by the additional detectors. For example, each additional unpolarized beam splitter, delay line, and polarized beam splitter may be included so as to be configured for the additional detectors to detect a correlation between a third photon from a third entanglement source and a fourth photon from a fourth entanglement source.

[0068] While several aspects of at least one embodiment have been described herein, it should be understood that various changes, modifications, and improvements will be readily conceivable to those skilled in the art. Such changes, modifications, and improvements are intended to be part of this disclosure and to be within the spirit and scope of the principles described herein. Accordingly, the foregoing description and drawings are merely examples.

[0069] As used herein and in the claims, the phrase “and / or” should be understood to mean “one or both” of the elements thus combined, i.e., elements that are sometimes present together and sometimes separately. Multiple elements listed using “and / or” should be interpreted in the same manner, i.e., “one or more” of the elements thus combined. Other elements other than those specifically identified by the “and / or” phrase may be present at will, whether related to or unrelated to those specifically identified elements. Thus, as a non-restrictive example, a reference to “A and / or B” when used in conjunction with an open-ended term such as “equipped with” may, in one embodiment, refer to A only (including elements other than B at will), in another embodiment, refer to B only (including elements other than A at will), in yet another embodiment, refer to both A and B (including other elements at will), and so on.

[0070] As used herein and in the claims, the indefinite articles "a" and "an" should be understood to mean "at least one" unless otherwise clearly indicated.

[0071] As used herein and in the claims, the phrase “at least one” should be understood to mean, with respect to a list of one or more elements, at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each element specifically enumerated in the list of elements, nor does it exclude any combination of elements in the list of elements. This definition allows for the optional presence of elements other than those specifically identified in the list of elements to which the phrase “at least one” refers, whether related to or unrelated to those specifically identified elements. Therefore, as a non-restrictive example, “at least one of A and B” (or equivalently, “at least one of A or B” or equivalently, “at least one of A and / or B”) could mean, in one embodiment, at least one which optionally includes two or more A's and no B (and optionally includes elements other than B); in another embodiment, at least one which optionally includes two or more B's and no A (and optionally includes elements other than A); and in yet another embodiment, at least one which optionally includes two or more A's and at least one which optionally includes two or more B's (and optionally includes other elements), and so on.

[0072] The use of ordinal terms such as “first,” “second,” and “third” in a claim to modify a claim element does not, in itself, imply any priority, order, or sequence of one claim element relative to another, or a temporal order in which the actions of the method are performed, but is merely used as a label to distinguish the claim elements, to differentiate one claim element having a certain name from another claim element having the same name (except for the use of ordinal numbers).

[0073] The terms “substantially,” “almost,” and “about” may be used in some embodiments to mean within ±20% of the target value, within ±10% of the target value, within ±5% of the target value, and within ±2% of the target value. The terms “almost” and “about” may include the target value.

[0074] Furthermore, the expressions and terms used herein are for illustrative purposes only and should not be considered limiting. The use of “includes,” “equips,” “has,” “contains,” “accompanies,” and variations thereof herein means that the items listed therein and their equivalents, as well as additional items, are included.

[0075] All definitions defined and used herein should be understood to supersede dictionary definitions, definitions in references to which they are invoked, and / or the ordinary meanings of the terms being defined.

Claims

1. A system for detecting the correlation between two photons, A non-polarizing beam splitter, configured to receive a first photon from a first entanglement source at a first input of the non-polarizing beam splitter, and a second photon from a second entanglement source at a second input of the non-polarizing beam splitter, A polarizing beam splitter having a first input and a second input, wherein the first input of the polarizing beam splitter is configured to receive a first output of a non-polarizing beam splitter, and is configured to guide light having a first polarization to the first output of the polarizing beam splitter and light having a second polarization to the second output of the polarizing beam splitter, A delay line configured to receive the second output of the non-polarized beam splitter and output light to the second input of the polarized beam splitter, A first photon detector configured to receive an output corresponding to the first polarization, A second photon detector configured to receive an output corresponding to the second polarization, and A system that includes these features.

2. The system according to claim 1, wherein the delay line has a length corresponding to a time delay greater than or equal to the time bandwidth of the first and second photons or the jitter of the detector.

3. The system according to claim 2, wherein the delay line provides a delay of 20 to 40 ns.

4. The system according to claim 3, wherein the delay line is an optical fiber cable.

5. The system according to claim 1, wherein the non-polarizing beam splitter is a non-polarizing beam splitter cube.

6. The system according to claim 1, wherein the polarizing beam splitter is a Grand-Taylor prism polarizer.

7. The system according to claim 1, wherein the first detector and the second detector are detectors of the same type.

8. The system according to claim 7, wherein the detector of the same type is a superconducting nanowire single-photon detector.

9. The system according to claim 1, wherein the first photon detector and the second photon detector are configured to detect four Bell parameters based on four polarization measurement results, the first two polarization measurement results are detected based on the first output of the unpolarized beam splitter, and the second two polarization measurement results are detected based on the second output of the unpolarized beam splitter.

10. The system according to claim 1, further comprising an additional detector having a non-polarizing beam splitter, a delay line, and a polarizing beam splitter, wherein the additional detector is configured to detect a correlation between a third photon from a third entanglement source and a fourth photon from a fourth entanglement source.

11. A system for detecting the correlation between two photons, A non-polarizing beam splitter, configured to receive a first photon from a first entanglement source at a first input of the non-polarizing beam splitter, and a second photon from a second entanglement source at a second input of the non-polarizing beam splitter, A polarizing beam splitter having a first input and a second input, wherein the first input of the polarizing beam splitter is configured to receive a first output of a non-polarizing beam splitter, and is configured to guide light having a first polarization to the first output of the polarizing beam splitter and light having a second polarization to the second output of the polarizing beam splitter, A first delay line configured to receive the second output of the non-polarized beam splitter and output light to the second input of the polarized beam splitter, A second delay line configured to receive the second output of the polarization beam splitter having the second polarization and to output the received second output to a shared beam path, wherein the shared beam path is configured to receive the first output of the polarization beam splitter and the output of the second delay line, Photon detector and A system comprising, wherein the photon detector is configured to receive photons at different times depending on the path between the non-polarizing beam splitter and the photon detector.

12. The system according to claim 11, wherein the first delay line has a longer optical path length than the second delay line.

13. The system according to claim 12, wherein the second delay line has an optical path length approximately equal to the time bandwidth of the first and second photons, and the first delay line has an optical path length approximately equal to twice the time bandwidth of the first and second photons.

14. The system according to claim 11, wherein the first photon detector is a superconducting nanowire single-photon detector.

15. The first photon detector is configured to detect four Bell parameters based on four polarization measurement results, and the four polarization measurement results are A first polarization measurement result detected based on the first output of the non-polarized beam splitter, A second polarization measurement result is detected based on the first output and the second delay line of the non-polarized beam splitter, A third polarization measurement result is detected based on the second output and the first delay line of the non-polarized beam splitter, The second output of the non-polarized beam splitter, the first delay line, and the fourth polarization measurement result detected based on the second delay line. The system according to claim 11, including the following:

16. A method for detecting a quantum state based on the detection of two photons, Receiving the first photon from the first beam path and the second photon from the second beam path, Dividing the first beam path and the second beam path into a first shared beam path and a second shared beam path, The first shared beam path is divided into a first polarized beam path and a second polarized beam path having polarization orthogonal to the first polarized beam path using a polarized beam splitter. Delaying the second shared beam path, To detect a first output corresponding to the first polarization beam path and the second polarization beam path, The second shared beam path is split into the first polarized beam path and the second polarized beam path using the polarized beam splitter. To detect a second output corresponding to the first polarization beam path and the second polarization beam path. Methods that include...

17. The method according to claim 16, wherein delaying the second shared beam path includes using a delay line having a length corresponding to a time delay greater than or equal to the time bandwidth of the first and second photons or the jitter of the detector.

18. The method according to claim 17, wherein the delay line provides a delay of 20 to 40 ns.

19. The method according to claim 16, wherein detecting a first output corresponding to the first polarization beam path includes using a first detector, and detecting a first output corresponding to the second polarization beam path includes using a second detector.

20. The method according to claim 16, wherein detecting a first output corresponding to the first polarization beam path includes using a first detector, and detecting a first output corresponding to the second polarization beam path includes delaying the second polarization beam path and using the first detector.

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