Homodyne measurement system

Abstract

A homodyne measurement system (10) according to the present disclosure comprises a two-input four-output optical multiplexer (112), a first homodyne measurement device (116), and a second homodyne measurement device (118). The two-input four-output optical multiplexer causes light under test and local oscillator light to interfere with each other, and forms four branch outputs. The first homodyne measurement device performs a first homodyne measurement on two of the four branch outputs of the two-input four-output optical multiplexer. The second homodyne measurement device performs, on the remaining two branch outputs of the two-input four-output optical multiplexer, a second homodyne measurement slower than the first homodyne measurement, and generates a phase control signal to be used for controlling phase synchronization between the light under test and the local oscillator light.

Description

Homodyne Measurement System

[0001] The present disclosure relates to a homodyne measurement system.

[0002] Towards the realization of quantum information processing, research has been conducted to generate squeezed states using various structures or materials. For example, homodyne measurement (or homodyne detection) can be used to measure the amplitude and phase of squeezed light. According to homodyne measurement, the quadrature amplitude and phase of light can be measured with high sensitivity using a relatively simple measurement system.

[0003] In homodyne measurement, for example, squeezed light as the measured light and local oscillator (LO) light are caused to interfere, split into two branches, each branched light is detected by an individual photodetector (PD), and amplitude and phase information of the measured light is obtained from the difference between the electrical detection signals.

[0004] In existing homodyne measurement, there is a trade-off between measurement efficiency and speed (or bandwidth). However, as described in Patent Document 1, for example, using an optical parametric amplifier (OPA) as a preamplifier makes it possible to achieve both improvement in measurement efficiency and improvement in measurement speed. According to this technique, for example, gigabithertz (GHz)-class high-speed homodyne measurement, which has been used in optical communication, can be applied to quantum information processing.

[0005] Japanese Patent No. 7460931

[0006] In existing homodyne measurement techniques, a signal used for phase control to establish phase synchronization between measured light and LO light (phase control signal) is separated from the electrical signal after photoelectric conversion by a balanced PD. However, in homodyne measurement of high-speed measured light such as GHz-class light using an OPA, it is difficult to separate the phase control signal from the electrical signal output from the balanced PD.

[0007] In other words, in a configuration using an electrical separator that electrically separates the phase control signal from the output of the balanced PD, the speeding up of phase synchronization between the measured light and the LO light may be restricted, and as a result, the speeding up of the entire homodyne measurement system may be restricted.

[0008] Therefore, one exemplary objective of this disclosure is to speed up homodyne measurement systems by eliminating the use of electrical separators, which are difficult to speed up.

[0009] Therefore, a homodyne measurement system according to one aspect of the present disclosure comprises: a two-input, four-output optical multiplexer that interferes with the light to be measured and the local oscillator light to split them into four; a first homodyne measuring instrument that performs a first homodyne measurement on two of the four branch outputs of the two-input, four-output optical multiplexer; and a second homodyne measuring instrument that performs a second homodyne measurement, which is slower than the first homodyne measurement, on the remaining two of the four branch outputs, and outputs a phase control signal used to control the phase synchronization between the light to be measured and the local oscillator light.

[0010] This figure shows an exemplary configuration of a homodyne measurement system. This figure shows an exemplary configuration of a homodyne measurement system according to one embodiment. (A) is a diagram showing a partial configuration of Figure 1, and (B) is a diagram showing a partial configuration of Figure 2. (A) and (B) are diagrams showing example configurations of a 2x4 optical multiplexer in Figure 2. (A) to (C) are diagrams showing example configurations of an optical path length adjustment mechanism in Figure 2. This figure shows an example configuration of an optical phase shifter in Figure 2. This figure shows an example configuration for variably controlling the branching ratio of a 2x4 optical multiplexer in Figure 2.

[0011] Embodiments will be described in detail below with reference to the drawings. However, the accompanying drawings and the following description are provided for the benefit of those skilled in the art to fully understand this disclosure and are not intended to limit the subject matter described in the claims. Furthermore, unnecessary details may be omitted. For example, detailed descriptions of already well-known matters or redundant descriptions of substantially identical configurations may be omitted.

[0012] Furthermore, in the drawings, identical or corresponding elements are appropriately denoted by the same reference numeral. The drawings are schematic, and the dimensional relationships or ratios of each element may differ from reality. There may also be differences in dimensional relationships or ratios between drawings. When numerical values ​​are given in the following explanation, these values ​​are merely examples, and other values ​​may be used additionally or as substitutes.

[0013] <Overview> Before describing the embodiments, an exemplary configuration of a homodyne measurement system in which a phase control signal for phase synchronization is separated by an electrical separator will be described as a comparative example of the embodiments, with reference to Figure 1.

[0014] The homodyne measurement system 1000 illustrated in Figure 1 includes, for example, an optical phase shifter 1002, an optical multiplexer 1004, an optical parametric amplifier (OPA) 1006, an optical demultiplexer 1008, an optical demultiplexer 1010, a 2-input 2-output (2x2) optical multiplexer 1012, a balanced homodyne measuring instrument 1014, an electrical separator 1016, a photodetector (PD) 1011, h0, an optical phase shifter 1022, and h4.

[0015] The homodyne measurement system 1000 receives both the light to be measured, such as squeezed light, and the LO light as input. The light to be measured is phase-controlled in the optical phase shifter 1002 for optical parametric amplification (phase-sensitive amplification) by the OPA 1006, and then combined with the excitation light in the optical multiplexer 1004 before being input to the OPA 1006.

[0016] OPA 1006 parametrically amplifies the light to be measured using excitation light that is phase-locked to the light to be measured. The light amplified by OPA 1006 is input to optical demultiplexer 1008 and demultiplexed into the light to be measured and the excitation light. The excitation light is not needed for homodyne measurement and may be discarded (or blocked).

[0017] The light to be measured, delimited by the optical demultiplexer 1008, is input to the optical demultiplexer 1010, where a portion (for example, about 1% to 10% of the optical power) is branched to the PD 1011, and the remainder is input to one of the two input ports of the 2x2 optical multiplexer 1012.

[0018] The light intensity signal detected by PD 1011 is supplied to controller 1020 for phase control of the optical phase shifter 1002. Based on the output of PD 1011, controller 1020 controls the phase of the light under test by the optical phase shifter 1002, for example, so that phase synchronization between the light under test and the excitation light is established or maintained.

[0019] One of the two input ports of the 2x2 optical multiplexer 1012 receives LO light, which has undergone phase control for phase synchronization with the light under test in the optical phase shifter 1022. In the 2x2 optical multiplexer 1012, the light under test and the LO light, which are phase-synchronized with each other, mix (interfere), and the interfered light is output from each of the two output ports of the 2x2 optical multiplexer 1012 to the balanced homodyne measuring instrument 1014.

[0020] The balanced homodyne detector 1014 includes PDs 1141 and 1142 corresponding to each of the two interference light streams, and a differencer 1143 that outputs the difference between the detected values ​​(electrical signals) from these PDs 1141 and 1142 as an error signal. The difference signal is input to the electrical separator 1016 as the measurement result from the balanced homodyne detector 1014.

[0021] The electrical separator 1016 electrically separates the electrical signal resulting from the measurement into the signal component of the light under test (e.g., high-frequency component) and the signal component for phase control of the LO light (e.g., low-frequency component). The separated signal component of the light under test is input to a measuring instrument 1030, such as an oscilloscope, to monitor (or observe) the state of the light under test.

[0022] Meanwhile, the signal component for phase control (error signal) separated in the electrical separator 1016 is supplied to the controller 1024, and the controller 1024 establishes phase synchronization between the LO light and the light under measurement by controlling the optical phase shifter 1022 of the LO light based on the signal component.

[0023] In this context, an electrical separator 1016, as used in previous quantum optics experiments, can separate and output, for example, the signal component of the light under measurement at around 100 MHz from the signal component for phase control (synchronization) at around 1 to several MHz or less.

[0024] However, if the application of OPA 1006 increases the signal component of the light being measured to a speed of GHz class, it becomes difficult for the electrical separator 1016 to separate the GHz class signal component being measured from the phase control (synchronization) signal component of approximately 1 to several MHz or less.

[0025] Currently, it is difficult to manufacture a homodyne analyzer capable of separating and outputting the GHz-class signal component under test from the phase control signal component of approximately 1 to several MHz or lower, and no commercially available high-speed (or wideband) homodyne analyzers have such a function.

[0026] While it is possible to separate the signal under measurement from the phase control signal by using a power splitter or the like to branch the output electrical signal of the balanced homodyne measuring instrument 1014, electrical circuits or components that can handle GHz-class speeds are extremely expensive. For example, a high-speed electrical separator 1016 operating at 100 GHz is very expensive, costing several hundred thousand yen per unit.

[0027] Furthermore, even with such an expensive electrical separator 1016, it is difficult to achieve impedance matching with the slow electrical circuit for phase control of the LO light. This raises technical concerns that reflected high-frequency signals may be mixed into measuring instruments such as oscilloscopes 1030.

[0028] Thus, even if it were possible to apply a GHz-class high-speed measuring instrument to the balanced homodyne measuring instrument 1014, its application may be limited by the limitations of the signal separation performance of the electrical separator 1016. As a result, the overall measurement speed performance of the homodyne measuring system 1000 may be limited.

[0029] Therefore, for example, by performing the separation for obtaining the phase control signal at the optical stage rather than the electrical stage, the expensive electrical separator 1016 is not required, and the reflection of electrical high-frequency signals is not a problem.

[0030] Therefore, the following describes an exemplary embodiment of a homodyne measurement system in which separation for obtaining a phase control signal is performed at the optical stage rather than the electrical stage.

[0031] <One Embodiment> Figure 2 shows an exemplary configuration of a homodyne measurement system 10 according to one embodiment. The homodyne measurement system 10 illustrated in Figure 2 differs from the configuration illustrated in Figure 1 in that, instead of the 2x2 optical multiplexer 1012, balanced homodyne measuring instrument 1014, and electrical separator 1016, it is equipped with a 2-input 4-output (2x4) optical multiplexer, an optical path length adjustment mechanism 114, and balanced homodyne measuring instruments 116 and 118. Note that the optical path length adjustment mechanism 114 may be optional.

[0032] The optical phase shifter 102, optical multiplexer 104, OPA 106, optical demultiplexer 108, 110, photodetector (PD) 111, controller 120, optical phase shifter 122, and controller 124 illustrated in Figure 2 have functions equivalent to or similar to the optical phase shifter 1002, optical multiplexer 1004, OPA 1006, optical demultiplexer 1008, 1010, PD 1011, controller 1020, optical phase shifter 1022, and controller 1024 illustrated in Figure 1, respectively.

[0033] Figures 3(A) and 3(B) are diagrams that show partial excerpts of the configurations of Figure 1 and Figure 2, focusing on the differences mentioned above. Figure 3(A) corresponds to a partial configuration of Figure 1, and Figure 3(B) corresponds to a partial configuration of Figure 2.

[0034] As shown in Figures 2 and 3(B), the homodyne measurement system 10 of this embodiment (hereinafter sometimes abbreviated as "measurement system 10") does not require the electrical separator 1016 shown in Figure 1.

[0035] In Figures 2 and 3(B), the 2x4 optical multiplexer 112 receives the light under test, which has been optically parametrically amplified by the OPA 106, as input to one of its two input ports, and the LO light as input to the other of its two input ports. The 2x4 optical multiplexer 112 interferes with the input light under test and the LO light and then branches (or distributes) them to four output ports.

[0036] Of the four output ports #1 to #4, for example, two output ports #2 and #3 are connected to the two input ports of the first balanced homodyne analyzer 116, and the remaining two output ports #1 and #4 are connected to the two input ports of the second balanced homodyne analyzer 118.

[0037] The first balanced homodyne detector 116 has a configuration that allows for GHz-class measurement speeds, for example, to perform measurements on light to be measured that has been optically parametrically amplified by the OPA 106.

[0038] For example, the first balanced homodyne detector 116 includes balanced PDs 1161 and 1162 that correspond to GHz-class measurement speeds, and a differencer 1163 that outputs the difference between each detected value from each PD 1161 and 1162 as an error signal. The difference signal is input to a measuring instrument 130, such as an oscilloscope, to monitor (or observe) the state of the light being measured.

[0039] On the other hand, the second balanced homodyne measuring instrument 118 acquires a signal (phase control signal) for controlling the optical phase shifter 122 for LO light by balanced homodyne measurement, and therefore has a configuration that can handle a measurement speed slower than GHz class (for example, a measurement speed of MHz class).

[0040] For example, the second balanced homodyne detector 118 includes balanced PDs 1181 and 1182, and a differencer 1183 that outputs the difference between the detected values ​​from each PD 1181 and 1182 as an error signal. The balanced PDs 1181 and 1182 are capable of highly efficient optical detection, although at a lower speed than GHz class. The difference signal is output to the controller 124 as a phase control signal for the LO light.

[0041] The second balanced homodyne detector 118 generates a phase control signal from the two branched output lights of the 2x4 optical multiplexer 112, so the electrical separator 1016 is not required, as shown in the configurations in Figures 1 and 3(A).

[0042] In other words, the second balanced homodyne detector 118 generates a phase control signal from interference light that is partially branched (or separated) by the 2×4 optical multiplexer 112 at the optical stage before photoelectric conversion, so there is no need to separate the phase control signal from an electrical signal.

[0043] In the following description, for convenience, the first balanced homodyne detector 116 and the second balanced homodyne detector 118 may be abbreviated as "high-speed homodyne detector 116" and "low-speed homodyne detector 118", or simply "homodyne detector 116" and "homodyne detector 118", respectively. The low-speed homodyne detector 118 is less expensive than the high-speed homodyne detector 116, and for example, may cost on the order of tens of thousands of yen or less per unit.

[0044] Here, since the measured light after being optically parametric amplified by the OPA 106 is resistant to a certain degree of optical loss, even if a part of the output optical power of the OPA 106, for example, about several percent to more than ten percent of the power, is separated, there is little influence on the homodyne measurement of the measured light.

[0045] Therefore, in order to obtain a phase control signal, it is permissible to separate part of the power of the measured light as it is without photoelectric conversion in the 2×4 optical multiplexer 112 and supply it to the low-speed homodyne detector 118.

[0046] In this respect, the input optical power to the low-speed homodyne detector 118 may be lower than the input optical power to the high-speed homodyne detector 116. However, it is preferable that the optical power input to the two input ports of each of the homodyne detectors 116 and 118 is balanced, that is, equally distributed, for balanced homodyne measurement.

[0047] As a non-limiting example, the 2×4 optical multiplexer 112 equally divides 10% of the power of the measured light input from the optical demultiplexer 110 into two 5% portions, outputs the 5% portions to the low-speed homodyne detector 118, and equally divides the remaining 90% into two 45% portions, outputs the 45% portions to the high-speed homodyne detector 116.

[0048] That is, in this case, the branching ratio of output optical power at output ports #1 to #4 of the 2×4 optical multiplexer 112 is (#1:#2:#3:#4) = (5%:45%:45%:5%). However, other branching ratios are not excluded; for example, a branching ratio where the output optical power is equally distributed among each of the output ports #1 to #4, such as (#1:#2:#3:#4) = (25%:25%:25%:25%), is also allowed.

[0049] Note that the optical power of the two systems balanced and input to each of the homodyne detectors 116 and 118 does not need to be completely equally distributed (1:1). Ratios that deviate slightly from the 1:1 equal distribution are allowed within a range that enables balanced homodyne measurement.

[0050] The optical path length adjusting mechanism 114 may be provided on at least one of the two output ports #2 and #3 to which the high-speed homodyne detector 116 is connected, among the four output ports #1 to #4 of the 2×4 optical multiplexer 112.

[0051] For example, when observing a signal with a frequency up to about 100 GHz or lower, the optical path length adjusting mechanism 114 can match the optical path length difference of the two systems between the 2×4 optical multiplexer 112 and the high-speed homodyne detector 116 with an accuracy of, for example, micrometer (μm) order or lower. Therefore, a decrease in measurement accuracy caused by the optical path length difference between the 2×4 optical multiplexer 112 and the high-speed homodyne detector 116 can be prevented or suppressed.

[0052] Hereinafter, the overall operation of the homodyne measurement system 10 according to the present embodiment will be described. The homodyne measurement system 10 receives measured light such as squeezed light and LO light. After the measured light undergoes phase control for optical parametric amplification (phase-sensitive amplification) by the OPA 106 in the optical phase shifter 102, it is multiplexed with pump light in the optical multiplexer 104 and input to the OPA 106.

[0053] OPA 106, for example, parametrically amplifies the light to be measured using excitation light that is phase-locked to the light to be measured. The light amplified by OPA 106 is input to optical demultiplexer 108 and demultiplexed into the light to be measured and the excitation light. The excitation light is not needed for homodyne measurement and may be discarded (or blocked).

[0054] The light to be measured, delimited by the optical demultiplexer 108, is input to the optical demultiplexer 110, where a portion (for example, about 1% to 10% of the optical power) is branched to the PD 111, and the remainder is input to one of the two input ports of the 2x4 optical multiplexer 112.

[0055] The detected value (electrical signal) of the light intensity detected by PD111 is supplied to controller 120. Based on the detected value of PD111, controller 120 controls the phase of the light to be measured by the optical phase shifter 102, for example, so that phase synchronization between the light to be measured and the excitation light is established or maintained.

[0056] One of the two input ports of the 2x4 optical multiplexer 112 receives LO light, which has undergone phase control for phase synchronization with the light under test in the optical phase shifter 122. In the 2x4 optical multiplexer 112, the light under test and the LO light, which are phase-synchronized with each other, are mixed (interfere) and branched to four output ports #1 to #4. The branching ratio is as previously described.

[0057] Power-balanced interference light from two of the four output ports #1 to #4, output ports #2 and #3, is input to a high-speed homodyne analyzer 116 for measuring the light under test. Power-balanced interference light from the remaining two of the four output ports #1 to #4, output ports #1 and #4, is input to a low-speed homodyne analyzer 118 for obtaining a phase control signal.

[0058] The high-speed homodyne detector 116 detects the light intensity values ​​of each of the two input light streams from the 2x4 optical multiplexer 112 using balanced PDs 1161 and 1162, and generates a difference signal of the respective detected values ​​(electrical signals) using a differencer 1163. The difference signal is output to a measuring instrument 130, such as an oscilloscope, and the state of the light being measured is monitored (or observed) in the measuring instrument 130.

[0059] Meanwhile, the low-speed homodyne detector 118 detects the light intensity values ​​of each of the two input light streams from the 2x4 optical multiplexer 112 using balanced PDs 1181 and 1182, and the difference signal of the respective detected values ​​(electrical signals) is obtained by the differencer 1183. The difference signal is input to the controller 124 as an error signal for phase control.

[0060] The controller 124 controls the phase shifter 122 to control the phase of the LO light, based on the error signal from the low-speed homodyne measuring instrument 118, so that synchronization between the light under measurement and the LO light is established (or maintained).

[0061] As described above, according to the measurement system 10 of this embodiment, a phase control signal is obtained by splitting a portion of the interference light between the light to be measured and the LO light at the optical stage and inputting it to the low-speed homodyne measuring instrument 118. Therefore, it is not necessary to separate the phase control signal from the electrical signal. Accordingly, the use of the electrical separator 1016, which is difficult to speed up as shown in Figures 1 and 3(A), can be eliminated, and the overall measurement speed of the measurement system 10 can be improved.

[0062] <About OPA106> In the configuration illustrated in Figure 2, OPA106 has, for example, a lithium niobate (LiNbO3) waveguide (PPLN waveguide) having a periodic polarization reversal structure for optical parametric amplification.

[0063] LiNbO3 is just one example of a nonlinear optical material. Other materials that may be used as waveguide materials for periodic polarization reversal structures include KNbO3, LiTaO3(LT), LiNb(x)Ta(1-x)O3 (0≦x≦1), or KTiOPO4, or materials containing at least one additive selected from the group consisting of Mg, Zn, Sc, and In.

[0064] The OPA106 has the characteristic of amplifying the light to be measured by the excitation light when the phases of the input light to be measured and the excitation light coincide, and attenuating the light to be measured when the phases of the two are in an orthogonal phase relationship with a 90-degree phase difference.

[0065] By utilizing this characteristic to match the phase between the light under test and the excitation light, the generation of spontaneous emission light that is orthogonal in phase to the light under test is suppressed. Furthermore, even for components with a common phase relationship, the generation of excessive spontaneous emission light that exceeds the noise of the light under test is suppressed. Therefore, OPA106 can amplify the light under test (low-noise amplification) without degrading the signal-to-noise ratio.

[0066] To establish or maintain phase synchronization between the light under test and the excitation light, as described above, the controller 102 illustrated in Figure 2 controls the phase shifter 102 to control the phase of the light under test. For this phase synchronization control, the output optical power of the OPA 106 is monitored by the optical demultiplexer 110 and PD 111.

[0067] The controller 120 controls the optical phase shifter 102 for the light under test based on the detected value of the PD 111, so that phase synchronization between the light under test and the excitation light is established or maintained. Note that the phase synchronization between the light under test and the excitation light may be controlled by phase control of the excitation light.

[0068] <About the 2x4 Optical Multiplexer 112> The structure of the 2x4 optical multiplexer 112, which splits (distributes) the light under measurement and the LO light into four outputs, may be, for example, a 2-input, 4-output (2x4) multimode interferometer (MMI) using a waveguide structure, as shown in Figure 4(A), or it may be a structure in which one or more of the following are connected in multiple stages: a beam splitter (BS), a 2x2 directional coupler (DC), and a 2x2 MMI, as shown in Figure 4(B).

[0069] The structure illustrated in Figure 4(B) may be a structure in which optical fiber type BS, DC and / or MMI are spatially coupled to each other, or a structure in which waveguide structure BS, DC and / or MMI are integrated into a planar optical circuit (PLC). When integrated into a PLC, the 2x4 optical multiplexer 112 may have a symmetrical structure in the vertical direction of the paper in Figure 4(B) in order to balance the output optical power between two of the four output ports.

[0070] <About the Optical Path Length Adjustment Mechanism 114> As described above, the optical path length adjustment mechanism 114 illustrated in Figures 2 and 3(B) is provided at least one of the two input ports of the high-speed homodyne measuring instrument 116 in order to adjust the two optical path lengths between the 2x4 optical multiplexer 112 and the high-speed homodyne measuring instrument 116.

[0071] The optical path length adjustment mechanism 114 is not particularly limited in its structure, as long as it has a structure that can vary the optical path length. As an unrestricted example, the optical path length adjustment mechanism 114 may have any of the structures shown in Figures 5(A) to 5(C), either individually or in combination.

[0072] The structure illustrated in Figure 5(A) is a structure that uses an optical element 501, such as a movable mirror, in free space, and varies the optical path length by shifting the position or orientation of the optical element 501.

[0073] The structure illustrated in Figure 5(B) is a fiber stretcher in which an optical fiber 505 is wound around a piezoelectric element 503, such as a piezo element, and the optical path length is varied by physically changing (stretching or unstretching) the length of the optical fiber 505.

[0074] Furthermore, the adjustment (or setting) of the optical path length by the optical path length adjustment mechanism 114 can be performed manually, for example, by connecting the output of the high-speed homodyne detector 116 to a measuring instrument capable of measuring phase shift, such as a network analyzer, and minimizing the observed phase shift. However, this does not preclude automatic adjustment based on the output of the high-speed homodyne detector 116.

[0075] The structure illustrated in Figure 5(C) is a structure in which two output waveguides 509 between a 2x4 optical multiplexer 112 and a high-speed homodyne detector 116 are formed on an optical chip 511, and the refractive index or waveguide length of the output waveguide 509 is varied by arranging an electro-optic (EO) element or thermo-optic (TO) element indicated by reference numeral 513 near at least one of the output waveguides 509.

[0076] The EO element changes the refractive index of the output waveguide 509 by utilizing the EO effect, for example, which occurs when an electric field is applied to electrodes formed along the output waveguide 509. The TO element changes the refractive index of the output waveguide 509 by utilizing the thermo-optic effect, for example, which occurs when an electric current is passed through electrodes formed on the output waveguide 509.

[0077] <Regarding the optical phase shifters 102 and 122> For example, a phase modulator using the Pockels effect may be applied to the optical phase shifters 102 and 122, or a fiber stretcher using a piezoelectric element may be applied.

[0078] Furthermore, as illustrated in Figure 6, when the optical phase shifter 102 and the OPA 106 are integrated into the same optical chip 601, an EO element or TO element, indicated by reference numeral 603, is provided near the waveguide 602 located upstream of the OPA 106. By varying the refractive index of the waveguide 602 using the EO element or TO element, a structure (optical phase shifter 102) is obtained that varies the phase of light propagating through the waveguide 602.

[0079] <Regarding other optical systems> In the homodyne measurement system 10 illustrated in Figure 2, the optical system, excluding the electrical circuits including controllers 120 and 124, may be implemented using, for example, fiber-coupled optical elements, or it may be integrated and implemented on the same optical chip using waveguide-type optical elements.

[0080] <Regarding variable branching ratio control of the 2x4 optical multiplexer 112> The branching ratio of the 2x4 optical multiplexer 112 described above can be variable. For example, in the configuration illustrated in Figure 4(B), DC can be used to change the branching ratio by voltage control.

[0081] When using a 2x4 optical multiplexer 112 with a variable branching ratio, for example, as shown in Figure 7, the deviation of the 2x4 optical multiplexer 112 from the intended branching ratio can be dynamically compensated for by the controller 702 based on the measurement results of the low-speed homodyne meter 118. In other words, the measurement results of the low-speed homodyne meter 116 can be used for two types of control: the phase of the LO light and the branching ratio of the 2x4 optical multiplexer 112. Since the deviation of the branching ratio is an extremely slow change compared to the phase change of the LO light, for example, when a signal with a speed of several tens of kHz to several MHz is used for controlling the phase of the LO light, a DC signal to several Hz can be used for the control signal of the branching ratio.

[0082] For example, if the excitation light wavelength deviates from the intended wavelength, and / or if the branching ratio deviates from the intended branching ratio due to aging, it is possible to automatically fine-tune the branching ratio to compensate. Therefore, the accuracy of balanced homodyne measurements can be stably ensured.

[0083] <Supplement> In this disclosure, the term "measurement" may be replaced with other terms such as "detection," "measurement," "observation," or "monitor" as appropriate to the context in which it is used. Also, the term "…instrument" may be replaced with "…circuit," "…part," or "…means."

[0084] Furthermore, in this disclosure, the terms “connection” or “joining,” when used, should be understood to mean any direct or indirect “connection” or “joining” between two or more elements. For example, the terms should be understood to include indirect “connection” or “joining” between two elements that are mutually “connected” or “joined” by one or more intermediate elements.

[0085] Furthermore, any reference in this disclosure to elements designated as "first..." or "second..." does not limit the quantity or order of those elements. These designations are merely a convenient way of distinguishing between two or more elements. For example, references to first and second elements do not imply that only two elements may be adopted, nor do they imply that the first element must take precedence over the second element in any physical quantity.

[0086] Furthermore, in this disclosure, unless otherwise specified, the phrase "based on..." does not mean "based solely on...", but includes both "based solely on..." and "based at least on...".

[0087] While the Disclosure has been described in detail above, it will be apparent to those skilled in the art that the purpose and scope of this Disclosure are not limited to what has been described herein. This Disclosure can be implemented in modified and altered forms without exceeding the purpose and scope of this Disclosure as defined by the claims. Therefore, the descriptions in this Disclosure are for illustrative purposes only and are not intended to be restrictive in any way to the purpose and scope of this Disclosure.

[0088] This disclosure is useful, for example, in quantum information technology.

[0089] 10 Homodyne Measurement System 102, 122 Optical Phase Shifter 104 Optical Multiplexer 106 Optical Parametric Amplifier (OPA) 108, 110 Optical Demultiplexer 111 Photodetector (PD) 112 2-Input 4-Output (2x4) Optical Multiplexer 114 Optical Path Length Adjustment Mechanism 116, 118 Balanced Homodyne Measuring Instrument 120, 124, 702 Controller 130 Measuring Instrument 501 Optical Element 503 Piezoelectric Element 505 Optical Fiber 507 Distributor 509 Output Waveguide 511, 601 Optical Chip 513, 603 Electro-Optical (EO) Element or Thermo-Optical (TO) Element 1161, 1162 Balanced PD 1181, 1182 Balanced PD 1163, 1183 Differencer

Claims

1. A homodyne measurement system comprising: a two-input, four-output optical multiplexer that interferes with the light under test and the local oscillator light to split them into four; a first homodyne measuring instrument that performs a first homodyne measurement on two of the four branch outputs of the two-input, four-output optical multiplexer; and a second homodyne measuring instrument that performs a second homodyne measurement on the remaining two of the four branch outputs, which is slower than the first homodyne measurement, and generates a phase control signal used to control the phase synchronization between the light under test and the local oscillator light.

2. The homodyne measurement system according to claim 1, further comprising: an optical parametric amplifier that optically parametrically amplifies the light to be measured input to the two-input four-output optical multiplexer with excitation light; a first optical phase shifter that controls the phase of the local oscillator light; and a controller that controls the first optical phase shifter based on the phase control signal to establish phase synchronization between the light to be measured and the local oscillator light.

3. The homodyne measurement system according to claim 1, wherein the two-input, four-output optical multiplexer has a branching ratio that balances the output optical power between the two branch outputs to the first homodyne measuring instrument and between the two branch outputs to the second homodyne measuring instrument.

4. The homodyne measurement system according to claim 3, wherein the output optical power of the two branch outputs to the second homodyne detector is lower than the output optical power of the two branch outputs to the first homodyne detector.

5. The homodyne measurement system according to claim 1, wherein the two-input four-output optical multiplexer is composed of a two-input four-output multimode interferometer, or is composed of one or more of the following connected in multiple stages: a beam splitter, a two-input two-output directional coupler, and a two-input two-output multimode interferometer.

6. The homodyne measurement system according to claim 1, further comprising an optical path length adjustment mechanism for adjusting the optical path length of at least one of the two branch outputs to the first homodyne measuring instrument.

7. The homodyne measurement system according to claim 2, wherein the optical parametric amplifier has a lithium niobate waveguide with a periodic polarization reversal structure.

8. The homodyne measurement system according to claim 2, further comprising a second optical phase shifter for controlling the phase of the light to be measured input to the optical parametric amplifier, wherein the optical parametric amplifier and the second optical phase shifter are integrated into the same planar light wave circuit.

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