Self-balancing homodyne detectors and methods thereof

Abstract

Disclosed herein is a homodyne detector (200) comprising a first beam splitter (202) and a second beam splitter (204) optically coupled with each other via at least an optical modulator (210). The first beam splitter (202) is configured to receive an optical signal and a reference optical signal as input and generate a first optical beam and a second optical beam based on the optical signal and the reference optical signal. The second beam splitter (204) is configured to receive at least the first optical beam through the optical modulator (210). The homodyne detector (200) comprises a control circuit (212) coupled to an output end of an amplifier (110) of the homodyne detector (200) and the optical modulator (210). The control circuit (212) is configured to balance one or more imbalances in the homodyne detector (200) by controlling the first optical beam passing through the optical modulator (210) based on an output signal of the amplifier (110).

Description

[0001] SELF-BALANCING HOMODYNE DETECTORS AND METHODS THEREOF TECHNICAL FIELD

[0001] The present disclosure relates to homodyne measurements in quantum optics. More particularly, the present disclosure relates to self-balancing homodyne detectors and methods thereof. BACKGROUND OF THE INVENTION

[0002] The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

[0003] Homodyne measurements are integral to quantum optics and quantum information science, facilitating measurement of quadrature amplitudes of various quantum states of light. The various quantum states of light may include, coherent states, Fock states, Squeezed states and Entangled states. A device for the homodyne measurements is known as a homodyne detector. The homodyne detector comprises a beam splitter that receives an optical signal for which the quantum state needs to be measured and a reference optical signal from a local oscillator as input. The homodyne detector interferes the quantum state of light with the reference optical signal. The reference optical signal comprises known amplitude and phase. By adjusting a relative phase between the quantum state and the local oscillator, different quadrature amplitudes can be selectively measured.

[0004] The process of homodyne measurement involves combining the optical signal and the reference optical signal using the beamsplitter. The beam splitter may interfere the optical signal and the reference optical signal to generate two output optical beams. The two output optical beams are then measured using two photodetectors to generate electrical current signals proportional to the two optical beams. Further, a subtractor may evaluate a difference between the two electrical current signals representing the quantum states of the optical signal. Depending on chosen relative phase of the reference optical signal, the measurement provides information about amplitude of one specific quadrature of quantum state while degrading information about conjugate quadrature. By varying the relative phase, a comprehensive characterization of the quantum state in terms of its quadrature amplitudes can be obtained.

[0005] Conventionally, homodyne detectors rely on perfectly balanced pairs of photodiodes and a perfectly balanced beam splitter for accurate measurements. However, in practice, achieving the perfect balance is challenging, leading to issues such as a Direct Current (DC) shift and larger classical noise. In critical applications such as quantum random number generation and quantum state tomography, the classical noise may result in inaccurate results.

[0006] Hence, there exists a need for methods and systems for efficiently balancing the homodyne detector for accurate measurements.

[0007] The present disclosure is directed to overcome one or more limitations stated above or any other limitations associated with the prior art. SUMMARY

[0008] In view of aforementioned challenges, the present disclosure provides techniques to self-balance a homodyne detector, even when the photodiodes and / or the beam splitter are not perfectly balanced. This present disclosure aims to enhance the accuracy and reliability of homodyne measurements, particularly in sensitive quantum applications, by mitigating the effects of imbalance and DC shift.

[0009] Disclosed herein is a homodyne detector comprising a first beam splitter and a second beam splitter optically coupled with each other via at least an optical modulator. The first beam splitter is configured to receive an optical signal and a reference optical signal as input and generate a first optical beam and a second optical beam based on the optical signal and the reference optical signal. The second beam splitter is configured to receive at least the first optical beam through the optical modulator. The homodyne detector comprises a control circuit coupled to an output end of an amplifier of the homodyne detector and the optical modulator. The control circuit is configured to balance one or more imbalances in the homodyne detector by controlling the first optical beam passing through the optical modulator based on an output signal of the amplifier.

[0010] Also disclosed herein is a homodyne detector comprising a beam splitter configured to receive an input optical signal and a reference optical signal as input and generate a first optical beam and a second optical beam based on the input optical signal and the reference optical signal. The homodyne detector comprises a first optical attenuator configured to attenuate the first optical beam and a second optical attenuator configured to attenuate the second optical beam. The homodyne detector comprises a control circuit coupled to an output end of an amplifier of the homodyne detector, the first optical attenuator and the second optical attenuator. The control circuit is configured to balance one or more imbalances in the homodyne detector by controlling attenuation of the first optical beam and the second optical beam based on an output signal of the amplifier.

[0011] Further disclosed herein is a method for homodyne detection performed by a homodyne detector. The method comprises receiving, at a first beam splitter of the homodyne detector, an optical signal and a reference optical signal for homodyne detection and interfering, by the first beam splitter, the optical signal and the reference optical signal to generate a first optical beam and a second optical beam. The method comprises receiving, at a second beam splitter of the homodyne detector, at least the first optical beam through an optical modulator and the second optical beam. The second beam splitter is optically coupled with the first beam splitter via at least the optical modulator. The method comprises balancing, by a control circuit of the homodyne detector, one or more imbalances in the homodyne detector by controlling the first optical beam passing through the optical modulator based on an output signal of an amplifier of the homodyne detector. The control circuit coupled to an output end of the amplifier and the optical modulator.

[0012] Furthermore, disclosed herein is a method for homodyne detection performed by a homodyne detector. The method comprises receiving, at a beam splitter of the homodyne detector, an optical signal and a reference optical signal as input. The method comprises interfering, by the beam splitter, the optical signal and the reference optical signal to generate a first optical beam and a second optical beam. The method further comprises balancing, by a control circuit of the homodyne detector, one or more imbalances in the homodyne detector by controlling attenuation of the first optical beam and the second optical beam based on an output signal of an amplifier of the homodyne detector. The first optical beam is attenuated by a first optical attenuator and the second optical beam is attenuated by a second optical attenuator. The first optical attenuator and the second optical attenuator are coupled with the control circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate exemplary embodiments and, together with the description, serve to explain the disclosed principles. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of device or system and / or methods in accordance with embodiments of the present subject matter are now described, by way of example only, and with reference to the accompanying figures, in which:

[0014] FIG.1 illustrates a conventional homodyne detector;

[0015] FIG.2 illustrates an exemplary homodyne detector to balance one or more imbalances in the homodyne detector, in accordance with an embodiment of the present disclosure;

[0016] FIG. 3 illustrates another exemplary homodyne detector to balance one or more imbalances in the homodyne detector, in accordance with another embodiment of the present disclosure.

[0017] FIG.4 is a flowchart of an exemplary method to self-balance the homodyne detector, in accordance with an embodiment of the present disclosure; and

[0018] FIG. 5 is a flowchart of another method to self-balance the homodyne detector, in accordance with an embodiment of the present disclosure.

[0019] The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein. DETAILED DESCRIPTION

[0020] Exemplary In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

[0021] While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.

[0022] The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a device or system or apparatus proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the device or system or apparatus.

[0023] The present disclosure relates to systems and methods for a self-balancing homodyne detector to balance one or more imbalances caused due to inefficiencies in one or more components of the conventional homodyne detector. The present disclosure provides various techniques for the self-balancing the homodyne detector. In one embodiment, a first homodyne detector includes a first beam splitter, a second beam splitter, an optical modulator, and a control circuit for self-balancing. The first beam splitter receives an optical signal and a reference optical signal and interferes the optical signal and the reference optical signal. The optical signal is defined as the signal for which homodyne measurement is to be made. The first beam splitter generates two interfered signals, wherein at least one interfered signal is passed through the optical modulator.

[0024] The optical modulator is a phase modulator, whose phase is controlled by the control circuit. The control circuit is connected to an output end of an amplifier and a phase control input of the optical modulator. The control circuit converts the amplifier output into a DC signal that may be representative of the one or more imbalances in the homodyne detector. Further, the control circuit generates a control signal to control the optical modulator to modulate a phase of the optical beam of the at least one interfered signal. The control circuit controls the phase of the at least one interfered signal so that the DC signal is mitigated indicating balancing the one or more imbalances. Thus, the first homodyne detector is capable of balancing the one or more imbalances, thereby providing enhanced homodyne measurement. The first homodyne detector provides a simple apparatus for self-balancing the first homodyne detector and also provides a simple mechanism of achieving the balance by only controlling the optical modulator.

[0025] In another embodiment, a second homodyne detector comprises two variable optical attenuators and a control circuit in addition to the one or more components of the conventional homodyne detector. The two optical attenuators are placed at optical paths of the interfered beams generated by the beam splitter of the conventional homodyne detector. The control circuit is connected to the output end of the amplifier and the two variable optical attenuators. The control circuit controls the two variable optical attenuators so that the two variable optical attenuators dynamically attenuate the two interfered beams respectively to reduce the one or more imbalances in the homodyne detector. Thus, the second homodyne detector provides a compact architecture of homodyne detector by adding few components to the conventional homodyne detector while mitigating the one or more imbalances in the second homodyne detector.

[0026] FIG.1 illustrates a conventional homodyne detector.

[0027] As shown in Fig.1, the conventional homodyne detector 100 comprises a beam splitter 102, a first photo detector 104, a second photo detector 106, a difference module 108 and an amplifier 110. The conventional homodyne detector 100 may be configured to perform homodyne measurement of an optical signal 112 and to detect a quantum state of the optical signal 112. In operation, the beam splitter 102 may receive the optical signal 112 and a reference optical signal 114 as input. The reference optical signal 114 may be of same wavelength as that of the optical signal 112, and the reference optical signal 114 and the optical signal 112 may be generated from the same source.

[0028] As shown in FIG.1, the beam splitter 102 may receive one or more input optical signals at two input ports and may be coupled to the first photo detector 104 and the second photo detector 106 at two output ports of the beam splitter 102. The difference module 108 may be coupled with output ends of the first photo detector 104 and the second photo detector 106. The amplifier 110 may be coupled with an output port of the difference module 108.

[0029] In operation, the beam splitter 102 may perform interference of the optical signal 112 and the reference optical signal 114 to generate two output optical signals 116 and 118 as illustrated in FIG.1. The two output optical signals may be generated using transmission and reflection properties of the beam splitter 102. The beam splitter 102 may be associated with a transmission coefficient (^^) and a reflection coefficient (^^). The transmission coefficient (^^) may be defined as a ratio of an amount of optical signal that has been transmitted to an amount of incident optical signal. The reflection coefficient (^^ may be defined as a ratio of an amount of optical signal that has been reflected to an amount of incident optical signal. Each of the two output optical signals may include a combination of a reflected signal and a transmitted signal. For example, the output optical signal 116 may be a combination of a transmitted component of the optical signal 112 and a reflected component of the reference optical signal 114. Alternatively, the output optical signal 118 may be a combination of a transmitted component of the reference optical signal 114 and a reflected component of the optical signal 112.

[0030] The two output optical signals may be incident on the first photo detector 104 and the second photo detector 106 respectively and may be converted to current signals proportional to intensities of the two output optical signals respectively. Each photo detector 104, 106 may be associated with an efficiency for converting an optical signal into a current signal. The efficiency may be defined as a ratio of a number of electrons generated to a number of incident photons on the photo detector. The higher the efficiency of the photo detector, the better is the proportionality of the current signal with the optical signal. Further, the difference module 108 may generate a difference signal based on the electrical signals generated by the first and the second photo detectors. The difference signal may be defined as a difference between the electrical signals generated by the first and the second photo detectors. The difference signal may represent one or more characteristics of the optical signal 112. For example, the difference signal may be proportional to a quadrature component of the optical signal 112.

[0031] An optical signal, as an electromagnetic wave, may be described as a combination of two components including a sine wave and a cosine wave. The two components may be referred to as in-phase quadrature and out-of-phase quadrature or alternatively X-quadrature and P- quadrature components. The X-quadrature Component may be defined as an amplitude of the optical signal that is synchronized with a reference phase of the reference optical signal. The P-quadrature Component may be defined as an amplitude of the light wave that is 90 degrees (π / 2 radians) out of phase with the reference optical signal. The difference signal may be proportional to the quadrature component of the optical signal.

[0032] Further, the amplifier 110 may amplify and convert the difference signal generated by the difference module 108 to a voltage signal, also referred to here in as the output signal 120. For example, the amplifier 110 may be a transimpedance amplifier. Further, the quantum states of the optical signal 112 may be evaluated from the output signal 120.

[0033] One or more problems envisaged due to the conventional homodyne detector 100 have been discussed herein below.

[0034] Typically, homodyne measurement relies on perfectly balanced pair of photodiodes to perform the measurement. For example, the first photo detector 104 and the second photo detector 106 need to have same amount of efficiencies to perform the measurement. However, in practice it is extremely difficult to achieve the perfect balance, which may result in a DC shift and consequently larger errors in the measurement. Particularly, in some applications such as quantum random number generation, in quantum quadrature measurements and in Fock state measurements, the DC shift may cause incorrect results. Furthermore, an imperfect beam splitter may also result in the DC shift. The imperfect beam splitter may be caused due to asymmetric values of transmission coefficient and reflection coefficient. The above problems envisaged in the homodyne measurement have been explained in greater detail herein after below.

[0035] At an initial stage, the quantum state of the optical signal 112 may be indicated by equation (1). ^^ Wherein |^^^^^is the quantum state of the optical signal 112, ^^^^represents expansion coefficients |^^^ is an nthFock state ^^^^is an annihilation operator for a single photon of the optical signal 112 and is a creation operator for a single photon of the optical signal

[0036] Fock state may be defined as a quantum state including a well-defined number of photons, that may be used to determine discrete quantum properties of light. The annihilation operator^^^^may be used to lower a number of photons in the quantum state by one. Alternatively, the creation operator may be used to increase the number of photons in quantum state by one.

[0037] A quantum state of the reference optical signal 114 may be an optical coherent state indicated by equation (2). Wherein ^^is complex number such that|^^|2represents a mean value of a number of photons in the local oscillator, and ^^^^is an annihilation operator of a local oscillator mode.

[0038] A joint state of the signal mode and the local oscillator mode may be indicated by equation (3). |Ψ^ = |^^^^|^^^-----------(3)

[0039] The beam splitter 102 may perform an interference of the optical signal 112 and the reference optical signal 114. The interference may be represented using an interference matrix (B) indicated by equation (4). Wherein ^^ is the transmission coefficient and ^^ is the reflection coefficient.

[0040] The ^^ and ^^ are complex numbers such that |^^|2 + |^^|2 = 1. The beam splitter 102generates two output optical signals, ^^^^and ^^^^, 116 and 118, along two output optical paths, represented by the equations (5a) and (5b). ^^^^ = ^^^^^^ + ^^^^^^------(5a)^^^^ = −^^∗^^^^ + ^^∗^^^^--------(5b)

[0041] To perform the homodyne measurement, a difference between intensities of the two output optical signals 116 and 118 represented by equation (6) may be evaluated. Wherein and cc represents complex conjugate.

[0042] Since the Local Oscillator (LO) mode is an optical coherent mode represented by ^^^, ^^^^,^^^^and ^^† ^^may be determined using the equations 7(a), 7(b) and 7(c) respectively. ^^^^ = |^^|2------ 7(a) ^^^^^^ = ^^-------7(b)

[0043] Furthermore, as the optical signal 112 is generally much weaker than the reference optical signal 114, ns may be discarded in comparison to nl. Accordingly, ^^^^^^^^^^may be represented by equation (8). Wherein 2√2^^∗^^^^ = ^^^^−^^^^ for real values of t and ^^. The values ‘t’ and ^^ may be defined asa magnitude and a phase of the number 2√2^^∗^^^^. Thus, from the equation (8), the difference in intensity (^^^^^^^^^^) is proportional to an expectation value of a quadrature operator along an angle ^^ (^^(^^)) with a DC shift represented by the first component of the equation (8). The quadrature operator may be required for analyzing and manipulating the quantum states of light. The quadrature operator may be indicated by the equation (9).

[0044] The quantum states of the optical signal 112 may be obtained by modifying the valueof ^^ in the equation (9). When ^^ = 0, the X-quadrature component may be measured, whereaswhen the ^^ =^^ 2, the P-quadrature component may be measured.

[0045] The equations (1)-(9) may hold good in ideal conditions when the two photo detectors 104 and 106 are perfectly balanced and when the beam splitter 102 is lossy or asymmetric. However, when the two photodetectors 104 and 106 are imperfect, the equation (6) may includethe efficiencies (^^1, ^^2) of the two photo detectors 104 and 106 be represented by equation (10).

[0046] Thus, the DC shift component may be worsened when the two photo detectors 104 and 106 are not equal.

[0047] Present disclosure provides systems and methods to solve one or more problems discussed above.

[0048] FIG. 2 illustrates a homodyne detector for balancing one or more imbalances, in accordance with a first embodiment of the present disclosure.

[0049] With reference to FIG. 2, there is shown a homodyne detector 200, also referred to herein as an apparatus 200. The apparatus 200 may comprise a first beam splitter 202 and a second beam splitter 204 coupled through a plurality of optical mirrors 206, 208 and an optical modulator 210. The apparatus 200 may also comprise the first photodetector 104, the second photo detector 106, the difference module 108 and the amplifier 110 similar to the conventional homodyne detector 100. The apparatus 200 may further comprise a control circuit 212 whose input end is coupled with the amplifier 110 and output end is coupled with the optical modulator 210. The control circuit 212 may comprise a mean value detector 214 and a control signal generator 216.

[0050] The first beam splitter 202 is configured to receive the optical signal 112 and the reference optical signal 114 as input signals and may perform interference of the input signals. In operation, the first beam splitter 202 may be similar to that of the beam splitter 102 of the conventional homodyne detector 100 of FIG.1. Further, the first beam splitter 202 may split the interfered optical signals into a first optical beam 218 and a second optical beam 220.

[0051] The plurality of optical mirrors 206, 208 are configured to guide each of the first optical beam 218 and the second optical beam 220 towards the second beam splitter 204, as shown in FIG. 2. The plurality of optical mirrors 206 and 208 may be configured with one or more reflection mechanisms to reflect the optical beams incident on the plurality of optical mirrors 206 and 208. In an embodiment, one of the first optical beam 218 and the second optical beam 220 are passed through the optical modulator 210 before guiding to the second beam splitter 204, as shown in FIG.2.

[0052] The optical modulator 210 may be configured to modulate one or more optical characteristics of the first optical beam 218 reflected from the first optical mirror 206. In one embodiment, the optical modulator 210 may be an optical phase modulator that is configured to modulate a phase of the first optical beam 218. Location of the optical modulator 210 has been indicated between the first optical mirror 206 and the second beam splitter 204 only for illustrative purpose and cannot be construed as limiting. In other embodiments, the optical modulator 210 may be deployed at any other location between the first beam splitter 202 and the second beam splitter 204, though not shown in FIG.2. For example, the optical modulator 210 may be deployed prior to the second optical mirror 208 to modulate the second optical beam 220.

[0053] In operation, the optical modulator 210 may be controlled by the control circuit 212, particularly the control signal generator 216. The optical modulator 210 may receive a control signal 222 from the control signal generator 216 that may enable in mitigating the one or more imbalances in the apparatus 200. The optical modulator 210 may modulate the one or more optical characteristics of the first optical beam 218 based on the control signal 222. For example, the optical modulator 210 may modify a phase of the first optical beam 218 proportional to an amplitude of the control signal 222.

[0054] The second beam splitter 204 is configured to perform interference of the modulated first optical beam 218’ received through a first optical path and the second optical beam 220 received through a second optical path. The optical path through which the first optical beam 218 traverses through the first optical mirror 206 and the optical modulator 210 may be defined as the first optical path. Alternatively, the optical path through which the second optical beam 220 traverses through the second optical mirror 208 may be defined as the second optical path. In other embodiments where the optical modulator 210 is deployed in the second optical path, the second beam splitter 204 may receive modulated second optical beam and the first optical beam.

[0055] Further, the second beam splitter 204 may perform interference of the modulated first optical beam 218’ and the second optical beam 220 and may generate two output optical beams. The two output optical beams of the second beam splitter 204 may be referred to herein as a third optical beam 224 and a fourth optical beam 226. Each of the third optical beam 224 and the fourth optical beam 226 are passed through respective photo detectors 104 and 106, as shown in FIG.2. The two photo detectors 104 and 106 may convert the third optical beam 224 and the fourth optical beam 226 into a first current signal and a second current signal respectively.

[0056] The difference module 108 may be configured to generate a difference current signal 228 based on a difference between the first current signal and the second current signal. Further, the difference module 108 may provide the difference current signal 228, also referred to here in as a difference signal 228, to the amplifier 110 as an input. The amplifier 110 may be configured to convert the difference current signal 228 into the output voltage signal 116.

[0057] In operation, the control circuit 212 may be configured to balance the one or more imbalances when the optical signal 112 has been disabled or when the optical signal 112 has a zero power. In one embodiment, the control circuit 212 may balance the one or more imbalances in the apparatus 200 before the optical signal 112 is provided to the apparatus 200 as part of a calibration phase. Upon calibrating the apparatus 200, the apparatus 200 may deactivate the control circuit 212 and may receive the optical signal 112 to initiate the homodyne measurement. In another embodiment, when a power of the optical signal 112 is of zero value, the control circuit 212 may be active in real-time and may continuously balance the one or more imbalances in the apparatus 200 even when receiving the optical signal 112. Thus, the control circuit 212 may be configured to dynamically remain active or passive based on the characteristics of the optical signal 112 and thereby balance the apparatus 200 efficiently.

[0058] The mean value detector 214 may be configured to detect a mean value of the output signal 120 that may represent the one or more imbalances within the apparatus 200. In one embodiment, the mean value detector 214 may comprise a Low Pass Filter (LPF) to filter higher frequencies of the output signal 120 and to pass only the lower frequencies to output. The lower frequencies may include, but not limited to, a Direct Current (DC) value of the output signal 120. For example, the DC value may be the first component of the equations (8) and / or (10). The mean value of the output signal 120 may be the DC value.

[0059] The control signal generator 216 may be configured to receive the mean value of the output signal 120 and may generate the control signal 222 to control the optical modulator 210. In one embodiment, the control signal generator 216 may comprise a Digital to Analog Converter (DAC) that may generate an analog signal to control the optical modulator 210. The control signal 222 may be proportional to the mean value and may be configured to adjust one or more characteristics of one of the first optical beam 218 and the second optical beam 220 until the mean value is reduced to zero. Thus, the control signal generator 216 may continuously generate the control signal 222 until the mean value or the one or more imbalances in the apparatus 200 are reduced to zero. Thus, the apparatus 200 may operate as a self- balancing homodyne detector by self-balancing the one or more imbalances arising from imperfections in various components of the apparatus 200. Further, the apparatus 200 also enhances the functionality of homodyne detector systems by optimizing the homodyne measurement of optical signals and improving the overall performance of optical communication systems. Furthermore, as the apparatus 200 compensates the effects of the one or more imbalances in the optical domain, using the optical modulator 210 in contrast to the electronic domain, the self-balancing is more efficient.

[0060] FIG. 3 illustrates a homodyne detector for balancing one or more imbalances, in accordance with a second embodiment of the present disclosure.

[0061] With reference to FIG. 3, there is shown a homodyne detector 300, also referred to herein as an apparatus 300. The apparatus 300 may comprise a first optical attenuator 302, a second optical attenuator 304 and the control circuit 306. The control circuit 306 may comprise the mean value detector 214 and a control signal generator 308. Further, the apparatus 300 may also comprise the beam splitter 102, the first photo detector 104, the second photo detector 106, the difference module 108 and the amplifier 110 similar to that of the conventional homodyne detector of FIG.1.

[0062] The first optical attenuator 302 and the second optical attenuator 304 may be coupled with the two output ends of the beam splitter 102 and the first photo detector 104 and the second photo detector 106 respectively. The control circuit 306 may be coupled with the output end of the amplifier 110 at an input end and with the optical attenuators 302 and 304 at the output ends.

[0063] The first optical attenuator 302 and the second optical attenuator 304 may comprise variable optical attenuators and may be configured to attenuate the first and second optical signals 116 and 118 respectively. The variable optical attenuator may be defined as an optical attenuator that may dynamically attenuate an optical beam / signal passing through the optical attenuator based on a control signal. The first optical attenuator 302 may be similar to the second optical attenuator 304 in configuration. Accordingly, each optical attenuator 302 and 304 may attenuate the first and second optical signals 116 and 118 respectively by a base attenuation value (^^) in the absence of control signals. In operation, each of the first and second optical attenuators 302 and 304 may be configured to attenuate the optical signals 116 and 118 based on a plurality of control signals 310 and 312 respectively to self-balance the homodyne detector 300, which may be explained herein below in greater detail.

[0064] The optical attenuators 302 and 304 may generate two attenuated optical signals 314 and 316 that may be passed through the first and the second photo detectors 104 and 106 to generate the two current signals. Further, the difference module 108 may generate the difference signal 318 based on a difference of the two current signals. The amplifier 110 may generate the output signal 120. When one or more components of the apparatus 300 have one or more imbalances, the output signal 120 may comprise a DC value representing the one or more imbalances.

[0065] In operation, the control circuit 306 may be configured to balance the one or more imbalances when the optical signal 112 has been disabled or when the optical signal 112 has a zero power. In one embodiment, the control circuit 306 may balance the one or more imbalances in the apparatus 300 before the optical signal 112 is provided to the apparatus 300 as part of a calibration phase. Upon calibrating the apparatus 300, the apparatus 300 may deactivate the control circuit 306 and may receive the optical signal 112 to initiate the homodyne measurement. In another embodiment, when the optical signal 112 is of zero mean, the control circuit 306 may be active in real-time and may continuously balance the one or more imbalances in the apparatus 300 even when receiving the optical signal 112. Thus, the control circuit 306 may be configured to dynamically remain active or passive based on the characteristics of the optical signal 112 and thereby balance the apparatus 300 efficiently.

[0066] The mean value detector 214 may detect the mean value or the DC value of the output signal 120. Further, the control signal generator 308 may generate the plurality of control signals 310 and 312 at a first output end and a second output end of the control signal generator 308. The plurality of control signals 310 and 312 may be further used to control the optical attenuators 302 and 304 to attenuate the first and the second optical signals 116 and 118 so as to reduce the one or more imbalances in the apparatus 300. In one embodiment, the control signal generator 308 may comprise a differential amplifier. A first control signal 310 may be represented as ‘D’ whereas a second control signal 312 may be represented as ‘-D’ which is a negative value of the control signal 310.

[0067] In operation, the optical attenuators 302 and 304 may attenuate the optical signals 116 and 118 respectively based on attenuation value (^^) and respective plurality of control signals 310 and 312. Thus, the first output signal 116 may be attenuated by the first optical attenuatorby factor (^^ − ^^), whereas the second optical signal 118 may be attenuated by a factor of(^^ + ^^). The attenuation of the optical signals 116 and 118 based on the control signals 314and 316 may reduce the one or more imbalances in the apparatus 300, thereby reducing the mean value of the output signal 120. Further, the first and the second photo detectors 104 and 106 may convert the attenuated optical signals 314 and 316 into first and second current signals respectively. The difference module 108 may generate the difference signal 318 based on the difference between the first and the second current signals. The amplifier 110 generates the output signal 120 based on the difference signal 318, which may be again used by the control circuit 3036 to generate updated control signals 310 and 312 based on the reduced mean value.

[0068] Thus, the control circuit 306 acts as a feedback loop that continuously controls the attenuation of the optical signals 116 and 118 until the mean value of the output signal 120 reduces to zero, thereby mitigating the one or more imbalances in the apparatus 300.

[0069] In an example, the efficiencies of the first photo detector 104 and the second photo detector 106 may be different and represented by ^^1and ^^2. In this example, the output signals 314 and 316 of the beam splitter 102 may be equal when the optical signal is of zero amplitude or a zero power. Accordingly, the output signals of the photo detectors 204 and 206 may be represented by equation (11).

[0070] Accordingly, the first control signal 310 (D) may be generated based on a sum of efficiencies and a difference between the efficiencies of the first photo detector 104 and the second photo detector 106 and represented by equation (12). The second control signal 312 may be represented as a negative value of ‘D’, which is ‘-D’. Thus, the apparatus 300 may be capable of balancing any imbalances arising due to imperfect efficiencies in the photo detectors 104 and 106.

[0071] Thus, the apparatuses 200 and 300 provide for self-balancing homodyne detector that are capable of balancing any types of imbalances arising due to abnormal characteristics of the components such as imperfectly balanced beam splitter and inequivalent photo detectors 204 and 206. Further, the apparatuses 200 and 300 also provide for a compact design for a self- balanced homodyne detector. By continuously monitoring and adjusting the phase modulation based on the output signal 120, the apparatus 200 ensures optimal performance without a need for manual intervention. The apparatuses 200 and 300 also reduce maintenance requirements for the homodyne detector system, by eliminating a need for manual adjustments to maintain balance. Thus, the apparatuses 200 and 300 also provide cost-effective solution and decreased downtime associated with system calibration and tuning. Furthermore, as the apparatus 300 compensates the effects of the one or more imbalances in the optical domain, using the optical attenuators 302 and 304 in contrast to the electronic domain, the self-balancing is more efficient.

[0072] The apparatuses 200 and 300 also optimize the operation of the homodyne detector, leading to increased efficiency in detecting the quantum states of the optical signal 112. Such improvement in the efficiency may result in better overall performance and higher throughput in optical communication systems.

[0073] FIG.4 an exemplary flowchart 400 of a method performed by the homodyne detector 200 of FIG.2 (which may also be referred to as a “first homodyne detector”) to balance one or more imbalances in the homodyne detector, in accordance with an embodiment of the present disclosure.

[0074] At block 402, the homodyne detector 200 may receive, at the first beam splitter 202 of the homodyne detector 200, the optical signal 112 and the reference optical signal 114 for homodyne detection.

[0075] At block 404, the homodyne detector 200 may interfere, by the first beam splitter 202, the optical signal 112 and the reference optical signal 114 to generate the first optical beam 218 and the second optical beam 220.

[0076] At block 406, the homodyne detector 200 may receive, at the second beam splitter 204 of the homodyne detector 200, at least the first optical beam 218 through the optical modulator 210 and the second optical beam 220. The second beam splitter 204 is optically coupled with the first beam splitter 202 via at least the optical modulator 210.

[0077] At block 408, the homodyne detector 200 may balance, using the control circuit 212 of the homodyne detector 200, one or more imbalances in the homodyne detector 200 by controlling the first optical beam 218 passing through the optical modulator 210 based on the output signal 120 of the amplifier 110 of the homodyne detector 200. The control circuit 212 coupled to the output end of the amplifier 110 and the optical modulator 210.

[0078] FIG.5 an exemplary flowchart 500 of a method performed by the homodyne detector 300 of FIG.3 (which may also be referred to as a “second homodyne detector”) to balance one or more imbalances in the homodyne detector, in accordance with an embodiment of the present disclosure.

[0079] At block 502, the homodyne detector 300 may receive, at the beam splitter 102 of the homodyne detector 300, the optical signal 112 and the reference optical signal 114 as input.

[0080] At block 504, the homodyne detector 300 may interfere, with the beam splitter 102, the optical signal 112 and the reference optical signal 114 to generate the first optical signal 116 and the second optical signal 118.

[0081] At block 506, the homodyne detector 300 may balance, using the control circuit 306 of the homodyne detector 300, one or more imbalances in the homodyne detector 300 by controlling attenuation of the first optical signal 116 and the second optical signal 118 based on the output signal 120 of the amplifier 110 of the homodyne detector 300.

[0082] The order in which the flowcharts 400 and 500 is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the flowcharts 400 and 500 or alternate methods. Additionally, individual blocks may be deleted from the flowchart 400 and 500 without departing from the scope of the subject matter described herein. Furthermore, the method can be implemented in any suitable hardware, software, firmware, or combination thereof.

[0083] The illustrated steps are set out to explain the exemplary embodiments shown, and it should be anticipated that ongoing technological development will change the manner in which particular functions are performed. These examples are presented herein for purposes of illustration, and not limitation. Further, the boundaries of the functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternative boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope of the disclosed embodiments. Also, the words "comprising," "having," "containing," and "including," and other similar forms are intended to be equivalent in meaning and be open ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items or meant to be limited to only the listed item or items. It must also be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise.

[0084] Finally, the language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. Accordingly, the disclosure of the embodiments of the disclosure is intended to be illustrative, but not limiting, of the scope of the disclosure.

[0085] With respect to the use of substantially any plural and / or singular terms herein, those having skill in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. The various singular / plural permutations may be expressly set forth herein for sake of clarity.

Claims

We claim:

1. A homodyne detector (200), comprising: a first beam splitter (202) and a second beam splitter (204) optically coupled with each other via at least an optical modulator (210); wherein the first beam splitter (202) is configured to receive an optical signal and a reference optical signal as input and generate a first optical beam and a second optical beam based on the optical signal and the reference optical signal; wherein the second beam splitter (204) is configured to receive at least the first optical beam through the optical modulator (210); and a control circuit (212) coupled to an output end of an amplifier (110) of the homodyne detector (200) and the optical modulator (210), wherein the control circuit (212) is configured to balance one or more imbalances in the homodyne detector (200) by controlling the first optical beam passing through the optical modulator (210) based on an output signal of the amplifier (110).

2. The homodyne detector (200) of claim 1, wherein the control circuit (212) comprises: a mean value detector (214) connected at the output end of the amplifier (110) and configured to determine a mean value of the output signal when a power of the optical signal is set to zero or when the optical signal is disabled, wherein the mean value indicates the one or more imbalances in the homodyne detector (200); and a control signal generator (216) connected between the mean value detector (214) (214) and the optical modulator (210) and configured to: generate a control signal for the optical modulator (210) based on the mean value; and control the optical modulator (210) based on the control signal until the mean value is zero, wherein the control signal enables the optical modulator (210) to modulate the phase of the first optical beam to balance the one or more imbalances in the homodyne detector (200).

3. The homodyne detector (200) of claim 1, wherein the optical modulator (210) is an optical phase modulator coupled with the control circuit (212) and is configured tomodulate a phase of the first optical beam based on a control signal received from the control circuit (212).

4. The homodyne detector (200) of claim 1, further comprises: a first photo detector (104) coupled with the second beam splitter (204) and is configured to convert a third optical beam into a first current signal; a second photo detector (106) coupled with the second beam splitter (204) and is configured to convert a fourth optical beam into a second current signal; a difference module (108) coupled with the first photo detector (104) and the second photo detector (106) and configured to generate a difference signal indicating a difference between the first current signal and the second current signal; and the amplifier (110) coupled with the difference module (108) configured to convert the difference signal into the output signal, wherein the one or more imbalances are associated with one or more of the first photo detector (104), the second photo detector (106), the first beam splitter (202) and the second beam splitter (204).

5. A homodyne detector (300), comprising: a beam splitter (102) configured to receive an optical signal and a reference optical signal as input and generate a first optical beam and a second optical beam based on the optical signal and the reference optical signal; a first optical attenuator (302) configured to attenuate the first optical beam; a second optical attenuator (304) configured to attenuate the second optical beam; and a control circuit (306) coupled to an output end of an amplifier (110) of the homodyne detector (300), the first optical attenuator (302) and the second optical attenuator (304), wherein the control circuit (306) is configured to balance one or more imbalances in the homodyne detector (300) by controlling attenuation of the first optical beam and the second optical beam based on an output signal of the amplifier (110).

6. The homodyne detector (300) of claim 5, wherein the first optical attenuator (302) and the second optical attenuator (304) are variable optical attenuators.

7. The homodyne detector (300) of claim 5, wherein the control circuit (306) comprises: a mean value detector (214) coupled at the output end of the amplifier (110) and is configured to determine a mean value of the output signal when a power of the optical signal is set to zero or when the optical signal is disabled, wherein the mean value indicates one or more imbalances in the homodyne detector (300); and a control signal generator (308) connected to the mean value detector (214) at an input end, the first optical attenuator (302) at a first output end and the second optical attenuator (304) at a second output end and is configured to: generate a first control output at the first output end and a second control output at the second output end based on the mean value; and control the first optical attenuator (302) using the first control output, and the second optical attenuator (304) based on the second control output, until the mean value of the output signal is zero, wherein the second control output is a negative value of the first control output.

8. The homodyne detector (300) of claim 5, wherein to attenuate the first optical beam, the first optical attenuator (302) is configured to: receive a first control output from the control circuit (306); and attenuate the first optical beam with an attenuation value based on a base attenuation value, and the first control output.

9. The homodyne detector (300) of claim 5, wherein to attenuate the second optical beam, the second optical attenuator (304) is configured to: receive a second control output from the control circuit (306); and attenuate the first optical beam with an attenuation value based on a base attenuation value, and the second control output.

10. The homodyne detector (300) of claim 5, further comprising:the first photo detector (104) coupled with the beam splitter (102) and configured to convert attenuated first optical beam into a first current signal; the second photo detector (106) coupled with the beam splitter (102) and configured to convert attenuated second optical beam into a second current signal; a difference module (108) coupled with the first photo detector (104) and the second photo detector (106) and configured to generate a difference signal indicating a difference between the first current signal and the second current signal; and the amplifier (110) coupled with the difference module (108) configured to convert the difference signal into the output signal, wherein the one or more imbalances are associated with one or more of the first photo detector (104), the second photo detector (106) and the beam splitter (102).

11. A method for homodyne detection performed by a homodyne detector (200), the method comprising: receiving, at a first beam splitter (202) of the homodyne detector (200), an optical signal and a reference optical signal for homodyne detection; interfering, by the first beam splitter (202), the optical signal and the reference optical signal to generate a first optical beam and a second optical beam; receiving, at a second beam splitter (204) of the homodyne detector (200), at least the first optical beam through an optical modulator (210) and the second optical beam, wherein the second beam splitter (204) is optically coupled with the first beam splitter (202) via at least the optical modulator (210); and balancing, by a control circuit (212) of the homodyne detector (200), one or more imbalances in the homodyne detector (200) by controlling the first optical beam passing through the optical modulator (210) based on an output signal of an amplifier (110) of the homodyne detector (200), wherein the control circuit (212) coupled to an output end of the amplifier (110) and the optical modulator (210).

12. The method of claim 11, wherein balancing the one or more imbalances comprises: determining a mean value of the output signal when a power of the optical signal is set to zero or when the optical signal is disabled, wherein the mean value indicates the one or more imbalances in the homodyne detector (200);generating a control signal for the optical modulator (210) based on the mean value; and controlling the optical modulator (210) based on the control signal until the mean value is zero.

13. The method of claim 11, further comprising: modulating, by the optical modulator (210), the first optical beam based on a control signal received from the control circuit (212), wherein the optical modulator (210) is an optical phase modulator coupled with the control circuit (212).

14. The method of claim 11, further comprising: converting, by a first photo detector (104), a third optical beam into a first current signal, wherein the first photo detector (104) is coupled with the second beam splitter (204); converting, by a second photo detector (106), a fourth optical beam into a second current signal, wherein the second photo detector (106) is coupled with the second beam splitter (204); generating, by a difference module (108), a difference signal indicating a difference between the first current signal and the second current signal, wherein the difference module (108) is coupled with the first photo detector (104) and the second photo detector (106); and generating, by an amplifier (110), the output signal based on the difference signal, wherein the amplifier (110) is coupled with the difference module (108), wherein the one or more imbalances are associated with one or more of the first photo detector (104), the second photo detector (106), the first beam splitter (202) and the second beam splitter (204).

15. A method for homodyne detection performed by a homodyne detector (300), the method comprising: receiving, at a beam splitter (102) of the homodyne detector (300), an optical signal and a reference optical signal as input;interfering, by the beam splitter (102), the optical signal and the reference optical signal to generate a first optical beam and a second optical beam; and balancing, by a control circuit (306) of the homodyne detector (300), one or more imbalances in the homodyne detector by controlling attenuation of the first optical beam and the second optical beam based on an output signal of an amplifier (110) of the homodyne detector, wherein the first optical beam is attenuated by a first optical attenuator (302) and the second optical beam is attenuated by a second optical attenuator (304), wherein the first optical attenuator (302) and the second optical attenuator (304) are coupled with the control circuit (306).

16. The method of claim 15, wherein the first optical attenuator (302) and the second optical attenuator (304) are variable optical attenuators.

17. The method of claim 15, wherein balancing the homodyne detector (300) comprises: determining a mean value of the output signal when a power of the optical signal is set to zero or when the optical signal is disabled, wherein the mean value indicates one or more imbalances in the homodyne detector (300); generating a first control output and a second control output based on the mean value; controlling the first optical attenuator (302) using the first control output until the mean value is zero; and controlling the second optical attenuator (304) using the second control output until the mean value is zero, wherein the second control output is a negative value of the first control output.

18. The method of claim 15, wherein attenuating the first optical beam comprises: receiving a first control output from the control circuit (306); and attenuating the first optical beam with an attenuation value based on a base attenuation value, and the first control output.

19. The method of claim 15, wherein attenuating the second optical beam comprises:receiving a second control output from the control circuit (306); and attenuating the second optical beam with an attenuation value based on a base attenuation value, and the second control output.

20. The method of claim 15, further comprising: converting, by the first photo detector (104) of the homodyne detector (300), attenuated first optical beam into a first current signal; converting, by the second photo detector (106) of the homodyne detector (300), attenuated second optical beam into a second current signal, wherein the first photo detector (104) and the second photo detector (106) are coupled with the beam splitter (102); generating, by a difference module (108) of the homodyne detector (300), a difference signal indicating a difference between the first current signal and the second current signal; and converting, by an amplifier (110), the difference signal into the output signal, wherein the one or more imbalances are associated with one or more of the first photo detector (104), the second photo detector (106), and the beam splitter (102).

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