Using entangled photons to directly measure unbalanced optical paths

Abstract

The present invention is entitled "Direct Measurement of Unbalanced Optical Paths Using Entangled Photons". The invention provides a system and method for directly measuring unbalanced optical paths using entangled photons. The system includes a light source for generating a pair of simultaneously generated photons. The system also includes a first transmitter / receiver and a second transmitter / receiver that emit a first photon and a second photon from the pair of simultaneously generated photons toward a first remote reflector and a second remote reflector, and receive reflected first and second photons along a first optical path and a second optical path. Additionally, the system includes a mode combiner for combining the reflected first and second photons into a first output port and a second output port. Furthermore, the system includes a photodetector for detecting photons from the first output port and the second output port. Additionally, the system includes a processor that measures a time delay difference between the first optical path and the second optical path based on the time difference of arrival of signals from the photodetector.

Description

[0001] Statement regarding federally sponsored research or development

[0002] This invention was carried out under government funding in accordance with 15-C-0285. The government owns certain rights to this invention. Background Technology

[0003] Satellites orbiting the Earth can communicate with each other to synchronize their clocks. Typically, for precise clock synchronization, satellites use interferometry to establish their relative positions. Satellites generally use the relative positions of satellites and atomic clocks to provide accurate time and frequency standards. Atomic clocks are used for international time allocation, for controlling the frequencies of broadcast signals, and in Global Navigation Satellite Systems (GNSS) such as GPS, GLONASS, BeiDou, or Galileo. In some satellite constellations, the relative positions of satellites may not be known with the precision required to synchronize atomic clocks to the desired level. Summary of the Invention

[0004] This invention provides a system and method for directly measuring an unbalanced optical path using entangled photons. The system includes a light source for generating a pair of simultaneously generated photons. The system also includes a first transmitter / receiver that emits a first photon from the pair toward a first remote reflector and receives a first photon reflected by the first remote reflector, the first photon traveling along a first optical path. Additionally, the system includes a second transmitter / receiver that emits a second photon from the pair toward a second remote reflector and receives a second photon reflected by the second remote reflector, the second photon traveling along a second optical path. Furthermore, the system includes a mode combiner for combining the reflected first photon and the reflected second photon into a first output port and a second output port. Moreover, the system includes a plurality of photodetectors that detect photons from the first output port and photons from the second output port. Finally, the system includes a processor executing executable code that causes the processor to measure a time delay difference between the first and second optical paths based on the arrival time differences of signals from the plurality of photodetectors. Attached Figure Description

[0005] It should be understood that the accompanying drawings only illustrate some embodiments and should not be considered as limiting the scope. Exemplary embodiments will be described with additional features and details in the drawings, in which:

[0006] Figure 1 This is a schematic diagram illustrating an exemplary interferometer according to one aspect of this disclosure;

[0007] Figure 2 This is a block diagram illustrating an exemplary system for dynamic optical interferometer locking using entangled photons according to one aspect of this disclosure;

[0008] Figure 3 This is a schematic diagram illustrating different paths in a chip-level device according to one aspect of this disclosure;

[0009] Figure 4 This is a schematic diagram illustrating different optical paths for receiving photon pairs according to one aspect of this disclosure;

[0010] Figure 5 This is a schematic diagram illustrating a method for adjusting an optical path with a large delay according to one aspect of this disclosure;

[0011] Figure 6 This is a block diagram illustrating a network of a light source and a remote reflector according to one aspect of this disclosure; and

[0012] Figure 7 This is a flowchart of dynamic optical interferometer locking using entangled photons according to one aspect of this disclosure.

[0013] As is customary, the various features described are not necessarily drawn to scale, but rather to emphasize specific features relevant to the exemplary implementation. Detailed Implementation

[0014] In the following detailed description, reference is made to the accompanying drawings, which form a part of the description, and specific exemplary embodiments are illustrated therein. However, it should be understood that other embodiments may be utilized, and logical, mechanical, and electrical changes may be made.

[0015] This paper describes a system and method for directly measuring unbalanced optical paths using entangled photons. In some implementations, the system generates two “twin” photons that are “born” at nearly the same time. The twin photons can be used to achieve real-time locking of the lengths of two optical paths between a central source and two remote locations (e.g., between a source satellite and two remotely positioned satellites). The source acquires the lock by projecting the twin photons toward a reflector at a remote location. The source can receive the reflected photons and recombine them within a Hong-Ou-Mandel (HOM) interferometer. A computing device connected to the interferometer can directly measure the arrival time difference of the twin photons due to the unbalanced optical path. The source can use the direct measurement to adjust the optical path to facilitate its balance.

[0016] In some systems, two photodetectors can simultaneously receive twin photons and provide electrical signals to a timing module for comparing their arrival times. If two photons are received simultaneously on the photodetectors, the timing module can record the disappearance of the coincidence detection rate for one photon on each photodetector. Alternatively, if twin photons are received on the photodetectors at different times, the timing module can observe a measurable coincidence detection rate of the twin photons. In some systems, computing devices can identify the disappearance of the coincidence detection rate to synchronize the optical path link between the source and the two remote locations.

[0017] Typically, the delay between the remote and source locations is largely unknown, and the differences in distances between different satellites can also be largely unknown. In these cases, identifying the dip caused by the simultaneous arrival of twin photons can be particularly challenging. For example, the dip width can be sub-picosecond, and the unknown ranging distance can be relatively large (>10⁻¹⁰⁰ m). The system can interpret large-scale delays by monitoring a continuously acquired, time-stamped photon detection stream and directly measuring the large path difference (potentially greater than hundreds of meters) by analyzing the time-delay histograms of the two detectors in the interferometer.

[0018] In some implementations, after measuring the path difference in flight time, the system can add a delay to the path with the shorter flight time. Therefore, the system can identify the difference in photon paths without utilizing quantum interference of twin photons. Additionally, the system can identify the path difference without precisely measuring the individual flight time of the photons. If the flight times are still unbalanced within the desired accuracy after adding the delay, other systems may be able to eventually adjust the difference in flight time. Furthermore, the system can monitor the path length and adjust as needed to maintain synchronization of different clocks.

[0019] Figure 1 This is a schematic diagram illustrating a system 100 for a Hong-Ou-Mandel (HOM) interferometer. The system described herein can be combined with a HOM interferometer. As used herein, a HOM interferometer is a device that uses the HOM effect to measure the properties of two received photons. System 100 can generate a pump photon 101. System 100 can separate the pump photon 101 into two twin photons 103 (referred to herein separately as photons 103-A and 103-B). For example, the pump photon 101 can be generated by a laser source that produces photons with a wavelength of 405 nm or other potential wavelengths.

[0020] In some embodiments, pump photons 101 are split into twin photons 103, which are guided through optical structures for recombination. For example, pump photons 101 are split into twin photons 103-a and 103-b by optical structure 105. Twin photons 103-a and 103-b may each have twice the wavelength of pump photon 101 (i.e., where the pump photon may have a wavelength of 405 nm, and twin photons 103-a and 103-b may each have a wavelength of 810 nm). Additionally, system 100 may include guiding optics 107 that guide twin photons 103-a and 103-b to detector 109 for reception of twin photons 103-a and 103-b. For example, detector 109-a can receive and detect twin photon 103-a, and detector 109-b can receive and detect twin photon 103-b.

[0021] In some implementations, when detector 109 receives the associated twin photon 103, detector 109 may provide a signal to timing module 111, which compares the arrival times of electrical signals to determine the degree of overlap. Timing module 111 may attempt to identify a delay in one or both of the optical paths that can be inserted, such that the overlap rate of the signals provided by the photodetector decreases toward zero. For example, when the photodetector detects the twin photon 103 at substantially the same time, the overlap rate of the signals provided by the photodetector may decrease toward zero. This decrease toward zero is referred to as the HOM dip shown in trajectory diagram 113. The dip occurs when the two twin photons 103 are substantially identical in all characteristics and recombine at separator 110. The HOM dip disappears when the photons become distinguishable.

[0022] System 100 and other systems described herein can be used within a larger system to synchronize atomic clocks. For example, HOM interferometry can provide a femtosecond-precision time base reliably distributed across LEO or MEO satellite constellations. One advantage of the systems and apparatus disclosed herein is the use of a phase-synchronized receiver array. Phase-synchronized receivers distributed over a large area can improve the resolution of satellite-acquired images by forming a large aperture from a dispersed set of smaller apertures, thereby breaking the diffraction limit of individual smaller apertures. Ultra-high resolution imaging can support a variety of tasks, from sparse aperture imaging to geolocation to ground moving target indication. Additional advantages of this system include real-time computation of interferometry. For example, the returned signals can be synchronized during acquisition and a better signal-to-noise ratio can be achieved on the image than after collection and averaging. Further advantages can include active beamforming, where the power of the pulses can be concentrated into a smaller area than that achievable with the angular resolution of any single satellite antenna. Active beamforming not only improves the signal-to-noise ratio of the returned signals but also reduces signal spillover and target dwell time, thus increasing the stealth of the implementation process. In addition, receiver arrays are more resilient to single-point attacks and can be configured to be optimized for different specific mission profiles.

[0023] As mentioned above, the identification of HOM (Homo Mirror) depressions and the synchronization of different satellites using HOM interferometry have many different applications. However, these applications rely on balancing the different optical paths of the interferometer with sub-millimeter precision. Balancing the different optical paths of remotely positioned satellites to a high level of precision can be difficult, especially when the paths differ significantly. The system and method described herein adjust the optical paths between different satellites (or other remotely positioned devices) to facilitate optical path balancing.

[0024] Figure 2 This is a block diagram of the balancing system 200. The balancing system 200 can use various components to balance the different optical paths between the light source and receiver 206 and the individual remote reflectors 207 and 209. Additionally, the balancing system 200 can be part of a balancing network that enables time synchronization between multiple sources. The balancing network is described in more detail below. The light source and receiver 206 can emit a first twin photon toward the remote reflector 207 and a second twin photon toward the remote reflector 209. In some networks, the light source and receiver 206 can be a satellite, and the remote reflectors 207 and 209 can be different remotely positioned satellites. Alternatively, the light source and receiver 206 can be a device capable of emitting twin photons, and the remote reflectors 207 and 209 can be individual devices capable of receiving twin photons and reflecting them toward the light source and receiver 206.

[0025] In some embodiments, the light source and receiver 206 may include a photon source 201. The photon source 201 may be a device that generates photons and separates them into two twin photons. The photon source 201 may be a laser generating device or other device capable of generating twin photons. For example, the photon source 201 may use spontaneous parametric downconversion (SPDC) to generate correlated or entangled photon pairs. When the photon source 201 generates twin photons, it may provide a first twin photon to a first transmitter / receiver 203 and a second twin photon to a second transmitter / receiver 205. The first transmitter / receiver 203 and the second transmitter / receiver 205 may be devices that emit and receive photons. For example, the transmitter / receiver may be a combination of optical devices that can focus and guide photons toward a corresponding remote reflector and also receive photons reflected from the corresponding remote reflector. For example, the first transmitter / receiver 203 may emit photons toward a remote reflector 207. Additionally, when the remote reflector 207 reflects photons toward the light source and receiver 206, the first emitter / receiver 203 can receive the reflected photons. Similarly, the second emitter / receiver 205 can emit photons toward the remote reflector 209. Furthermore, when the remote reflector 209 reflects photons toward the light source and receiver 206, the second emitter / receiver 205 can receive the reflected photons.

[0026] In some implementations, when the first transmitter / receiver 203 receives a first twin photon reflected from the remote reflector 207 and the second transmitter / receiver 205 receives a second twin photon reflected from the remote reflector 209, the first transmitter / receiver 203 and the second transmitter / receiver 205 may provide the received twin photons to the mode combiner 211. In some specific implementations, the first transmitter / receiver 203 and the second transmitter / receiver 205 may act as multiple devices, wherein photons are emitted by a first type of device and received by a second type of device. Alternatively, the first transmitter / receiver 203 and the second transmitter / receiver 205 may include a device (such as a circulator) that receives a corresponding photon for emission at a first port, emits photons toward and receives photons from the remote reflector at a second port, and provides the received photons to the mode combiner 211 at a third port.

[0027] In some implementations, the mode combiner 211 may include multiple outputs directed to photodetectors 221 and 223. The photodetectors convert received light into electrical signals and provide these signals to the timing module 213. The timing module 213 (and...) Figure 1 (Similar to timing module 111 in the image) can combine the received electrical signals and perform interferometry on the received electrical signals. Then timing module 213 can provide data indicating the coincidence result of the electrical signals arriving at the timer module 213 in the light source and receiver 206.

[0028] In another embodiment, timing module 213 may be a device capable of acquiring timing information from interferometric data. As described herein, timing information may refer to information describing the time difference from when the first twin photon is incident on the first photodetector 221 and the second twin photon is incident on the second photodetector 223. Additionally, timing information may refer to information relating to the overlap rate of the twin photons on the respective photodetectors. Timing module 213 may provide the acquired timing information to processor 215. In some embodiments, timing module 213 may be a function executed on processor 215. In other embodiments, timing module 213 may be implemented on a computing device separate from processor 215.

[0029] In some implementations, processor 215 may use timing information from timing module 213 to identify a delay based on the difference in the incident times of the twin photons. Using the identified delay, processor 215 may determine adjustments to be made to the path length between photon source 201 and one or both of the first transmitter / receiver 203 and the second transmitter / receiver 205, such that the path length between photon source 201 and remote reflector 207 and the path length between photon source 201 and remote reflector 209 are substantially equal. As used herein, the different optical paths are substantially equal as the coincidence rate of twin photon detection at the first photodetector 221 and the second photodetector 223 decreases toward zero, as described above with respect to the HOM recess.

[0030] In some implementations, the difference in path length between photon source 201 and the corresponding remote reflectors 207 and 209 can be substantially larger (>10m-100m) than the maximum difference in path length between these paths that are substantially equal in width in the HOM (such as in the sub-millimeter range). Therefore, to achieve precise equalization of path lengths over a wide range of initial path length differences, processor 215 can adjust the path lengths to facilitate fine-tuning. Specifically, processor 215 can determine the adjustment of the actuator in control regulator 219. Regulator 219 can be a device capable of making large changes to different optical paths to facilitate small changes to different optical paths. When processor 215 determines that the difference between optical path lengths is substantially large, processor 215 can control regulator 219 to change the optical paths such that the difference between optical path lengths is within a fine-tuning range.

[0031] Furthermore, once the processor 215 has controlled the regulator 219 to adjust the difference between optical path lengths within a fine adjustment range, the processor 215 can monitor the coincidence rate observed by the mode combiner 211 to monitor and adjust the different optical path lengths, taking into account the movement of the remote reflectors 207 and 209 relative to each other and relative to the light source and receiver 206. For example, if the coincidence rate observed by the mode combiner 211 moves out of the fine adjustment range, the processor 215 can guide the regulator 219 to adjust one or more optical path lengths so that the path length difference is within the fine adjustment range.

[0032] The processor 215 and / or other computing devices used in the light source and receiver 206 or other systems and methods described herein can be implemented using software, firmware, hardware, or appropriate combinations thereof. The processor 215 and other computing devices may be supplemented or incorporated therein by specially designed application-specific integrated circuits (ASICs) or field-programmable gate arrays (FPGAs). The processor 215 and other computing devices may also include or run with software programs, firmware, or other computer-readable instructions to perform various processing tasks, computational, and control functions used in the methods and systems described herein.

[0033] The methods described herein can be implemented using computer-executable instructions (such as program modules or components) that are executed by at least one processor (such as processor 215). Typically, program modules include routines, programs, objects, data components, data structures, algorithms, etc., that perform specific tasks or implement specific abstract data types.

[0034] Various procedural tasks, calculations, and generation instructions used in performing the operations described herein with respect to other data may be implemented in software, firmware, or other computer-readable instructions. These instructions are typically stored on a suitable computer program product, which includes a computer-readable medium for storing computer-readable instructions or data structures. Such a computer-readable medium may be a usable medium accessible by a general-purpose or special-purpose computer, processor, or other programmable logic device.

[0035] Suitable computer-readable storage media may include, for example, non-volatile memory devices, including semiconductor memory devices such as random access memory (RAM), read-only memory (ROM), electrically erasable programmable ROM (EEPROM), or flash memory devices; disks such as internal hard disks and removable disks; optical disk storage devices such as optical discs (CDs), digital versatile optical discs (DVDs), Blu-ray discs; or any other medium that may be used to carry or store desired program code in the form of computer-executable instructions or data structures.

[0036] As described above, the light source and receiver 206 may be able to adjust the different path lengths between the photon source 201 and the different remote reflectors 207 and 209 associated with the twin photons generated by the photon source, such that the different optical path lengths are substantially equal enough to allow for fine-tuning. Additionally, the light source and receiver 206 can adjust the different path lengths to account for movement of the remote reflectors 207 and 209 relative to the photon source 201. Therefore, the light source and receiver 206 can be used to synchronize the operation of different satellites associated with the photon source 201 and the remote reflectors 207 and 209 with high precision.

[0037] Figure 3 Different paths of photons passing through chip-level device 300 are illustrated, each path capable of generating photons, separating photons into twin photons, providing twin photons to a remote reflector, receiving photons from the remote reflector, and providing the received photons to an interferometer for performing HOM interferometry. Therefore, chip-level device 300 is illustrated in one embodiment, which includes the above-described... Figure 2 Certain components of the light source and receiver 206 described herein. Specifically, photon source 201 and Figure 3 The other components shown facilitate the transmission and reception of photons to remote reflectors 207 and 209, the delivery of received photons to mode combiner 211, the delivery of output ports from mode combiner 211 to photodetectors 321 and 323, and the delivery of electrical signals to timing module 311. Although these components are shown as part of a chip-level device, they can also be implemented using separate components coupled to each other with an optical transmission medium.

[0038] As shown in the figure Figure 3 The source path 310 and interferometer path 312 through the chip-level device 300 are shown. In the source path 310, photons are generated and separated into twin photons 303 and 305 for use by different remote reflectors. In the interferometer path 312, twin photons 303 and 305 are received from the remote reflectors and supplied to the timing module 311.

[0039] In some implementations, the chip-scale device 300 utilizes the nonlinear optical effect of degenerate spontaneous parametric down-conversion (dSPDC), where pump photons 308 split into two “twin” photons 306A and 306B that are “born” at nearly the same time (e.g., within <100 femtoseconds of each other). This simultaneity, enforced by quantum mechanics, can be used to synchronize separate atomic clocks. To synchronize separate atomic clocks (i.e., when different atomic clocks are located on different satellites or associated with other types of remote reflectors), synchronization is achieved by projecting twin photons 303 and 305 from the chip-scale device 300, reflecting some of photons 303 and 305 from each of the remote reflectors, and providing them with recombination in a Hong-Ou-Mandel (HOM) interferometer, where a purely quantum mechanical interference “dig” of the coincidence rate is observed when the paths are substantially equal, as described above relative to... Figures 1-2 The arrival times of some entangled photons from each satellite are correlated over a classical channel, thereby enabling clock synchronization with high precision (i.e., potentially femtosecond precision).

[0040] In some implementations, the chip-level device 300 is a chip-level photonic integrated circuit that generates and interferes with time-entangled photons. The chip-level device 300 may include optical functions and components on a hybrid optical waveguide platform that combines the nonlinear characteristics of a ppKTP waveguide (or other waveguides made of materials with similar properties) with the high confinement and filtering capabilities of a silicon nitride waveguide.

[0041] In another embodiment, the chip-scale device 300 can generate pump photons 308. From the pump photons 308, the chip-scale device 300 can generate high throughput time-energy entangled twin photons via type II degenerate spontaneous parametric down-conversion (dSPDC) pumped by a continuous wave (CW) in a waveguide providing the photons (such as a ppKTP waveguide or a waveguide made of a material with similar functionality). The twin photon state (also known as a two-photon state or "2002" state) can have robust time-spectral entanglement and is an ideal quantum system for synchronizing time between clocks on remote satellites because it is not prone to decoherence due to absorption or loss, and the transmission channel (free space) is naturally free of dispersion.

[0042] In an additional embodiment, the chip-level device 300 may provide a hybrid waveguide platform that combines the nonlinear optical capabilities of a photonic waveguide with the tight guiding and filtering capabilities of a photonic component waveguide. This combination allows for miniaturization, efficiency, and robustness, while increasing the available throughput of twin photons 306A and 306B (emitted from the chip-level device as photons 303 and 305).

[0043] In some implementations, twin photons 306A and 306B are formed via dSPDC in a waveguide providing photons in source path 301. Each twin photon in twin photons 306A and 306B may occupy a different waveguide mode (transversely electric (TE) or transversely magnetic (TM)). A vertical coupler (VC) region adiabatically extracts the twin photons from the waveguide providing photons and places them into a photonic component waveguide patterned on top of the waveguide providing photons. Additionally, the TM and TE photons may be separated by two diffractive waveguide mode separators (MS). The TE photon then passes through a bandpass filter (BPF) to repel background photons, passes through a second MS, and subsequently exits the chip as twin photon 305. Furthermore, the initial TM photon is converted to TE mode by a diffractive mode converter (MC), which also reverses its propagation direction. This (now TE-polarized) photon passes through the BPF and exits the chip as twin photon 303. The various functions performed on the chip can be performed by photonic component waveguides (in some embodiments, made of silicon nitride or other similar materials), wherein the waveguide structure is patterned in a film deposited on top of a substrate containing the waveguides that provide photons.

[0044] In an additional embodiment, twin photons 303 and 305 can be reflected from a remote reflector in interferometer path 316. Twin photons 303 and 305 can be recoupled into a photonic component waveguide on the chip-scale device 300 for delivery to the HOM interferometer. (In some specific embodiments, these photons may also have their polarization rotated 90 degrees by a conventional waveplate). Although twin photons 303 and 305 re-enter the same waveguide from which they were previously emitted, they are coupled into an orthogonal waveguide mode (i.e., TM) because their polarization has now been rotated. Each of the twin photons 303 and 305 can then interact with a MS that reverses their respective propagation directions within the waveguide, causing twin photons 303 and 305 to propagate through the reflection port of the respective MS toward a 50 / 50 waveguide coupler, where the overlapping twin photons can recombine. The photons that fail to recombine can then exit the chip-level device 300 for detection by photodetectors 321 and 323 and for subsequent interferometry to be performed by timing module 311. The chip-level device 300 and the components contained therein are described in more detail in the following applications: U.S. Non-Provisional Application Serial No. 16 / 803,841, filed February 27, 2020, entitled “Integrated Photonic Source and Detector of Entangled Photons”; U.S. Non-Provisional Application Serial No. 16 / 803,820, filed February 27, 2020, entitled “Integrated Photonic Mode Splitter and Converter”; and U.S. Non-Provisional Application Serial No. 16 / 803,831, filed February 27, 2020, all of which are incorporated herein by reference.

[0045] Figure 4This is a schematic diagram illustrating various combinations 440 of twin photons 403 and 405 that can be detected by photodetectors 421 and 423. Some systems may include a mode combiner 433 that directs the twin photons 403 and 405 to one of the two photodetectors 421 and 423, such as those detected as part of an interferometer. In some embodiments, the first twin photon 403 may be described as a signal photon 403, and the second twin photon 405 may be described as an idler photon 405. As described above, the signal photon 403 may be projected along a first optical path, and the idler photon 405 may be projected along a second optical path. The first optical path may direct the signal photon 403 toward a first remote reflector (such as a satellite equipped with retroreflective optics), and the second optical path may direct the idler photon 405 toward a second remote reflector. Both the signal photon 403 and the idler photon 405 are reflected backward toward a source and coupled to the mode combiner 433 (such as those described above). Figure 2 The mode combiner 433 is part of a 50 / 50 mode combiner (a portion of the light source and receiver 206). As described above, the mode combiner 433 may have two output ports directed to two photodetectors 421 and 423. In some specific implementations, photodetectors 421 and 423 may be single-photon avalanche photodetectors; however, other types of photodetectors may be used. Photodetectors 421 and 423 may be provided by a time difference counter (such as one that can be used as a time difference counter). Figure 2 The electrical signal was found in part of the timing module 213.

[0046] Sometimes, the first optical path of the signal photon 403 may be shorter than the second optical path of the idle photon 405. Therefore, the flight time of the signal photon 403 may be shorter than the flight time of the idle photon 405. At other times, the second optical path of the idle photon 405 may be shorter than the first optical path of the signal photon 403. Therefore, the flight time of the idle photon 405 may be shorter than the flight time of the signal photon 403. When the flight time is shorter, the associated photon may be incident on one of photodetectors 421 and 423 before another photon is incident on one of photodetectors 421 and 423. For example, when the flight time of the signal photon 403 is shorter than the flight time of the idle photon 405, the signal photon 403 will be incident on one of photodetectors 421 and 423 before the idle photon is incident on one of them. Conversely, when the flight time of the idle photon 405 is shorter than the flight time of the signal photon 403, the idle photon 405 will be incident on one of the photodetectors 421 and 423 before the signal photon 403.

[0047] When signal photon 403 is incident on one of photodetectors 421 and 423, signal photon 403 may be randomly incident on one of photodetectors 421 and 423. Similarly, when idle photon 405 is incident on one of photodetectors 421 and 423, idle photon 405 may be randomly incident on one of photodetectors 421 and 423. Therefore, the associated signal photon 403 and idle photon 405 may be incident on photodetectors 421 and 423 in one of combinations 440-a–440-d. Since signal photon 403 and idle photon 405 are each randomly incident on photodetectors 421 and 423, each combination in combinations 440-a–440-d may have a substantially 25% chance of occurrence.

[0048] In combination 440-a, both signal photon 403 and idle photon 405 pass through mode combiner 433 and are incident on different photodetectors 421 and 423. Specifically, signal photon 403 can pass through mode combiner 433 and be incident on second photodetector 423 at the second photodetector detection time. Idle photon 405 can pass through mode combiner 433 and be incident on first photodetector 421 at the first photodetector detection time. Photodetectors 421 and 423 can provide their signals to a timing module, wherein these signals contain information describing the detection times of the first and second photodetectors. The timing module can calculate a time difference equal to the first photodetector detection time minus the second photodetector time.

[0049] In combination 440-b, both signal photon 403 and idle photon 405 can be reflected by mode combiner 433 and incident on different photodetectors 421 and 423. Specifically, signal photon 403 can be reflected by mode combiner 433 and incident on first photodetector 421 at the first photodetector detection time. Idle photon 405 can be reflected by mode combiner 433 and incident on second photodetector 423 at the second photodetector detection time. Photodetectors 421 and 423 can provide their signals to a timing module, wherein these signals contain information describing the detection times of the first and second photodetectors. The timing module can calculate a time difference equal to the first photodetector detection time minus the second photodetector time.

[0050] In combinations 440-c and 440-d, signal photon 403 and idle photon 405 may be incident on the same photodetector. For example, in combination 440-c, signal photon 403 may pass through mode combiner 433 and idle photon 405 may be reflected by mode combiner 433. Therefore, both signal photon 403 and idle photon 405 are incident on second photodetector 423. Alternatively, in combination 440-d, signal photon 403 may be reflected by mode combiner 433 and idle photon 405 may pass through mode combiner 433. Therefore, both signal photon 403 and idle photon 405 are incident on first photodetector 423. When signal photon 403 and idle photon 405 are incident on the same photodetector, the timing module does not receive separate signals from the first photodetector 421 and the second photodetector 423 from which relative timing information is inferred.

[0051] As described above, when signal photon 403 and idle photon 405 arrive at different times, combination 440-a causes the timing module to measure the arrival time of idle photon 405 minus the arrival time of signal photon 403. Additionally, combination 440-b causes the timing module to measure the arrival time of signal photon 403 minus the arrival time of idle photon 405. The timing module may not need to obtain information from combinations 440-c and 440-d. Therefore, the measurements from the timing module can produce two clusters of results that are substantially equal in size but opposite in sign, wherein the magnitude of these measurements is proportional to the difference in optical length between these paths.

[0052] Figure 5 This is a block diagram of a system 500 for adjusting path length based on measurements from a timing module. As described above, the timing module can generate two clusters of results for measurements of the time difference between twin photons measured at different photodetectors. The timing module can provide the results of the measurements to a processor, which can perform statistical analysis of the recorded time differences. Figure 550 is a histogram of the time differences measured by the timing module. Figure 550 shows two clusters of measurements that are substantially symmetric about the simultaneous arrival time of the twin photons. The processor can perform bimodal fitting or other mathematical processes to extract the time delay from the histogram. The processor can use the extracted information to implement one or more adjustments to remove the time delay between the different optical paths of the twin photons, such that the histogram of the time difference results measured by the timing module has a single cluster at the simultaneous arrival time of the twin photons, as shown in Figure 551.

[0053] In some embodiments, to achieve modulation, system 500 includes a photon source 501 that provides twin photons to remote reflectors 507 and 509. Specifically, photon source 501 may provide a first photon to remote reflector 507 and a second photon to remote reflector 509. As shown, an unknown path length difference 520 may exist on the optical path of the first photon between photon source 501 and remote reflector 507. Due to the path length difference 520, a timing module on the photon source can measure the difference in the time when the first and second photons are incident on the photodetector on the photon source. Therefore, the processor can observe two clusters of measurements that are substantially symmetric about the time of simultaneous arrival. The processor can then extract a time delay from the measurements provided by the timing module. For example, the width between the clusters of measurements may be equal to twice the delay derived from the path length difference 520. The processor can then add a delay 519 to the shorter of the optical paths, such that the different optical paths between photon source 501 and remote reflectors 507 and 509 are substantially equal. In some implementations, the paths are substantially equal when the path length difference after the added delay 519 is within the fine adjustment range of the adjustment performed by regulator 219.

[0054] In some implementations, the processor may be able to calculate the magnitude of the delay from the information provided by the timing module, but not the sign of the delay. To determine the sign of the delay (which optical path should be delayed by inserting the added delay 519), the processor may add an additional delay to one of the two optical paths between the photon source 501 and the two remote reflectors 507 and 509. After adding the additional delay 519 to one of these paths, the processor may again determine whether the timing module measures the difference in incident times. If the measurements provided by the timing module produce two clusters of measurements separated by twice the initial delay present in the optical path before the addition of delay 519, the processor may determine that delay 519 has been added to the wrong optical path. Therefore, the processor may then remove delay 519 and insert delay 519 into another optical path such that the histogram of the time difference measured by the timing module has a single cluster at the simultaneous arrival times of the twin photons, as shown in Figure 551. Alternatively, if the measurements provided by the timing module produce a single cluster of measurements at the simultaneous arrival times, the processor may determine that delay 519 has been added to the correct optical path.

[0055] In another embodiment, delay 519 can be added as a series of discrete optical path lengths. For example, system 500 may be able to insert a large delay 519 into one or both of these optical paths. Delay 519 can be inserted using discrete optical delay elements of various lengths. For example, delay element 519 can be any combination of optical fiber, mirror, or other optical device. Additionally, delay element 519 can include insertable fiber optic segments with lengths such as 100m, 10m, 1m, 1cm, etc. Furthermore, delay element 519 can include adjustable mirrors that allow light to be reflected between mirrors a desired number of times to delay light propagation therebetween, as determined by the angles of the mirrors relative to each other, which are controlled by the processor. Other delay elements can be used to control the length of delay 519, such as piezoelectric transducers, adjustable prisms, etc. Therefore, by using various delay lengths, the processor can remove path length differences between multiple optical paths, allowing these path length differences to be within a finely adjustable range.

[0056] In some implementations, as the photon source 501 moves relative to one or both of the remote reflectors 507 and 509, the processor can adjust the delay 519 to maintain substantially equal optical path lengths. For example, if the processor determines that there is more than one cluster of measurements, the processor can adjust the length of the delay 519. Alternatively, the processor can adjust the delay 519 when it determines that the difference in optical path lengths is greater than a fine adjustment range.

[0057] Figure 6 This is a block diagram of a network 600 of optical devices that implement the systems and methods described above to synchronize multiple systems in the network 600. As shown, the network 600 may include a first light source 601. In some embodiments, the first light source 601 acts as a light source for initializing the synchronization of various nodes in the network 600. Thus, the first light source 601 serves as a node in the network that uniquely acts as a source of entangled photon pairs and not uniquely acts as a destination of reflected photons. In other embodiments, the first light source 601 simultaneously acts as both a source of entangled photon pairs and a destination of reflected photon pairs. Thus, the first light source 601 acts as a multifunctional node 601, which simultaneously acts as both a destination and a source of entangled photons. Additionally, the network 600 may include destination nodes 607 and 609 that function in a manner similar to the remote reflectors described above. Furthermore, the network 600 may include multifunctional nodes 608-a–608-d (generally and collectively referred to as multifunctional node 608) that simultaneously act as both destinations and sources of entangled photon pairs.

[0058] In some embodiments, to initiate synchronization, a first light source 601, serving as a first multi-function node 601, synchronizes optical path 610-a between the first multi-function node 601 and the endpoint node 609 with optical path 610-b between the first multi-function node 601 and multi-function node 608-a. The first multi-function node 601 can use a combination of the coarse and fine adjustments described above to synchronize optical paths 610-a and 610-b. When optical paths 610-a and 610-b are synchronized, multi-function node 608-a can act as a light source, using multi-function node 608-b and the first multi-function node 601 as reflectors to substantially synchronize optical path 610-c with optical path 610-b, as described above. Similarly, multi-function node 608-b synchronizes optical path 610-d with optical path 610-c, multi-function node 608-c synchronizes optical path 610-e with optical path 610-d, and multi-function node 608-d synchronizes optical path 610-f with optical path 610-e.

[0059] Figure 7 This is a flowchart of a method 700 for directly measuring the difference in optical path length between a light source and a remote reflector. For example, method 700 is performed at 701, where multiple pairs of simultaneously generated photons are generated. Additionally, method 700 is performed at 703, where a first photon from a pair of photons is emitted along a first optical path toward a first remote reflector, and a second photon from that pair is emitted along a second optical path toward a second remote reflector. Furthermore, method 700 is performed at 705, where the first and second photons are received. Finally, method 700 is performed at 707, where the received first and second photons are combined in a mode combiner having a first output port and a second output port.

[0060] In another embodiment, method 700 is performed at 709, wherein a first output from a first output port is detected at a first photodetector, and a second output from a second output port is detected at a second photodetector. Additionally, method 700 is performed at 711, wherein the optical path length difference between the first and second optical paths is measured based on the time difference between the detection of the first and second outputs. Furthermore, method 700 may be performed at 713, wherein the length of one or more of the first and second optical paths may be adjusted to reduce the optical path length difference.

[0061] Exemplary Implementation

[0062] Example 1 includes a system comprising: a light source for generating a pair of simultaneously generated photons; a first transmitter / receiver for emitting a first photon from the pair toward a first remote reflector and receiving the reflected first photon along a first optical path; a second transmitter / receiver for emitting a second photon from the pair toward a second remote reflector and receiving the reflected second photon along a second optical path; a mode combiner for combining the reflected first photon and the reflected second photon into a first output port and a second output port; a plurality of photodetectors for detecting photons from the first output port and photons from the second output port; and a processor for executing executable code that causes the processor to measure a time delay difference between the first optical path and the second optical path based on the time difference of arrival of signals from the plurality of photodetectors.

[0063] Example 2 includes the system according to Example 1, wherein the executable code instructs the processor to send a command to the actuator, the command causing the actuator to adjust the length of one or more of the first optical path and the second optical path to reduce the time delay difference.

[0064] Example 3 includes the system according to Example 2, wherein the time delay difference is reduced such that the difference is within a fine adjustment range.

[0065] Example 4 includes the system according to any one of Examples 2-3, wherein the adjustment of the length is substantially equal to the length associated with the measured time delay difference.

[0066] Example 5 includes a system according to any one of Examples 2-4, wherein the executable code instructs the processor to: command the actuator to insert a length of optical transmission medium into the first optical path, wherein the length of the optical transmission medium is substantially equal to a length associated with a measured delay difference; after the length of the optical transmission medium is inserted into the first optical path, measure a changed delay difference between the first optical path and the second optical path; determine whether the changed delay difference is greater than the measured delay difference; and when the changed delay difference is greater than the measured delay difference, command the actuator to remove the length of optical transmission medium from the first optical path and insert the length of optical transmission medium into the second optical path.

[0067] Example 6 includes the system according to Example 5, wherein the optical transmission medium includes at least one of the following: an optical fiber of a certain length; a plurality of mirrors; and a mechanical delay stage.

[0068] Example 7 includes a system according to any one of Examples 5-6, wherein the optical transmission medium comprises a combination of optical transmission media of different combinable lengths.

[0069] Example 8 includes a system according to any one of Examples 1-7, wherein the processor measures the time delay difference by: constructing a histogram of the time difference between when a photon in the pair of photons is incident on a first photodetector among the plurality of photodetectors and when the photon is incident on a second photodetector among the plurality of photodetectors; and measuring the time delay as the time difference between a first peak and a second peak in the histogram.

[0070] Example 9 includes the system according to Example 8, wherein bimodal fitting is used to calculate the time difference between the first peak and the second peak.

[0071] Example 10 includes a method comprising: generating multiple pairs of simultaneously generated photons; emitting a first photon from one pair of photons in the multiple pairs of simultaneously generated photons toward a first remote reflector, and emitting a second photon from the pair of photons toward a second remote reflector; receiving the first photon and the second photon, wherein the first photon and the second photon travel along a first optical path and a second optical path, respectively; combining the received first photon and the received second photon in a mode combiner having a first output port and a second output port; detecting a first output from the first output port at a first photodetector, and detecting a second output from the second output port at a second photodetector; and measuring an optical path length difference between the first optical path and the second optical path based on a time difference between the detections of the first output and the second output by the first photodetector and the second photodetector.

[0072] Example 11 includes the method according to Example 10, and further includes adjusting the length of one or more of the first optical path and the second optical path to reduce the optical path length difference.

[0073] Example 12 includes the method according to Example 11, wherein adjusting the length includes reducing the optical path length difference to a fine adjustment range.

[0074] Example 13 includes the method according to any one of Examples 11-12, wherein adjusting the length includes reducing the optical path length difference by a discrete distance, the discrete distance being close to the measured optical path length difference.

[0075] Example 14 includes the method according to any one of Examples 11-13, wherein adjusting the length further includes: inserting a certain length of optical transmission medium into the first optical path; after the certain length of optical transmission medium is inserted into the first optical path, measuring a changed optical path length difference between the first optical path and the second optical path; determining whether the changed optical path length difference is greater than the measured optical path length difference; and when the changed optical path length difference is greater than the measured optical path length difference, removing the certain length of optical transmission medium from the first optical path and inserting the certain length of optical transmission medium into the second optical path.

[0076] Example 15 includes the method according to Example 14, wherein the optical transmission medium includes at least one of: an optical fiber of a certain length; a plurality of mirrors; and a mechanical delay stage.

[0077] Example 16 includes the method according to any one of Examples 14-15, wherein the optical transmission medium comprises a combination of optical transmission media of different combinable lengths.

[0078] Example 17 includes the method according to any one of Examples 10-16, further comprising: constructing a histogram of the time difference between the detection of the first output and the second output; measuring the time difference between a first peak and a second peak in the histogram; and identifying a length associated with the time difference.

[0079] Example 18 includes the method according to Example 17, wherein measuring the time difference between the first peak and the second peak includes using bimodal fitting.

[0080] Example 19 includes a system comprising: a light source for generating multiple pairs of simultaneously generated photons; a first transmitter / receiver that emits a first photon from the multiple pairs of simultaneously generated photons toward a first remote reflector and receives the reflected first photon along a first optical path; a second transmitter / receiver that emits a second photon from the multiple pairs of simultaneously generated photons toward a second remote reflector and receives the reflected second photon along a second optical path; a mode combiner that combines the reflected first photon from the first optical path and the reflected second photon from the second optical path; and a plurality of photodetectors that detect photons from the first optical path. A first output from a first output port of the mode combiner and a second output from a second output port of the mode combiner; an actuator for controlling the lengths of the first optical path and the second optical path; and a processor that executes executable code that causes the processor to: measure the optical path length difference between the first optical path and the second optical path based on the time difference between associated photons in the plurality of simultaneously generated photons incident on different photodetectors of the plurality of photodetectors; and send a command to the actuator that causes the actuator to adjust the length of one or more of the first optical path and the second optical path to reduce the optical path length difference.

[0081] Example 20 includes the system according to Example 19, wherein the processor measures the length difference by: constructing a histogram of the time difference between when a photon from one of the plurality of simultaneously generated photons is incident on a first photodetector of the plurality of photodetectors and when the associated photon is incident on a second photodetector of the plurality of photodetectors; measuring the time difference between a first peak and a second peak in the histogram; and identifying the length associated with the time difference.

[0082] Although specific embodiments have been illustrated and described herein, those skilled in the art will recognize that any arrangement calculated to achieve the same purpose may replace the specific embodiments shown. Therefore, it is apparent that the invention is limited only by the claims and their equivalents.

Claims

1. A system for adjusting the optical path between different photons, the system comprising: Light source (206), the light source being used to generate a pair of photons produced simultaneously; A first transmitter / receiver (203) emits a first photon of the pair of photons toward a first remote reflector (207) and receives the first photon reflected by the first remote reflector (207), the first photon traveling along a first optical path; A second transmitter / receiver (205) emits a second photon of the pair of photons toward a second remote reflector (209) and receives the second photon reflected by the second remote reflector (209), the second photon traveling along a second optical path; A mode combiner (211) is used to combine a reflected first photon and a reflected second photon into a first output port and a second output port; Multiple photodetectors (221, 223) detect photons from the first output port and photons from the second output port; and Processor (215), the processor executing executable code, the executable code causing the processor (215) to: The time delay difference between the first optical path and the second optical path is measured based on the arrival time difference of signals from the plurality of photodetectors (221, 223). A command is sent to the actuator, the command causing the actuator to insert a certain length of optical transmission medium into the first optical path, wherein the length of the optical transmission medium is substantially equal to the length associated with the measured time delay difference; The executable code therein instructs the processor (215): A command is sent to the actuator, the command causing the actuator to adjust the length of one or more of the first optical path and the second optical path to reduce the time delay difference; After the optical transmission medium of a certain length is inserted into the first optical path, the time delay difference between the changes in the first optical path and the second optical path is measured. Determine whether the changed delay difference is greater than the measured delay difference; as well as When the changed time delay difference is greater than the measured time delay difference, the actuator is commanded to remove the optical transmission medium of a certain length from the first optical path and insert the optical transmission medium of a certain length into the second optical path.

2. A method for adjusting the optical path between different photons, the method comprising: Generate multiple pairs of photons produced simultaneously; The first photon of one of the multiple pairs of photons generated simultaneously is emitted toward the first remote reflector (207), and the second photon of the pair of photons is emitted toward the second remote reflector (209); Receive the first photon and the second photon, wherein the first photon and the second photon travel along a first optical path and a second optical path, respectively; The received first photon and the received second photon are combined in a mode combiner (211) having a first output port and a second output port; A first output from the first output port is detected at a first photodetector (221), and a second output from the second output port is detected at a second photodetector (223); The optical path length difference between the first optical path and the second optical path is measured based on the time difference between the detections of the first output and the second output by the first photodetector (221) and the second photodetector (223). A command is sent to the actuator, the command causing the actuator to insert a certain length of optical transmission medium into the first optical path, wherein the length of the optical transmission medium is substantially equal to the length associated with the measured time delay difference; A command is sent to the actuator, the command causing the actuator to adjust the length of one or more of the first optical path and the second optical path to reduce the time delay difference; After the optical transmission medium of a certain length is inserted into the first optical path, the time delay difference between the changes in the first optical path and the second optical path is measured. Determine whether the changed delay difference is greater than the measured delay difference; as well as When the changed time delay difference is greater than the measured time delay difference, the actuator is commanded to remove the optical transmission medium of a certain length from the first optical path and insert the optical transmission medium of a certain length into the second optical path.

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