Quantum source of telecom entangled photons for multi-user quantum communication

Abstract

The present disclosure relates to a system (100) for generation of entangled photon pairs for quantum communication. The system (100) includes a continuous wave (CW) tunable pump laser (102) to pump a Zinc-indiffused Magnesium-doped Periodically Poled Lithium Niobat (MgO:PPLN) ridge waveguide, an inline variable optical attenuator (VOA) (104) to control 5 the power of the CW tunable pump laser (102), and an input fiber polarization beam splitter (FPBS) (106) with a horizontal polarization (H-pol) arm and a vertical polarization (V-pol) arm, the H-pol arm is automatically rotated to V-pol by a 90 degree fiber twist before a waveguide facet, a first C-L band splitter (108) to separate one or more signal and idler photons into distinct spectral bands, with the signal photons falling within a C-arm and the 0 idler photons falling a L-arm, one or more fiber polarization controllers (110) in both C-arm and L-arm to control polarization of one or more output photons and introduce a desired phase in both C-arm and L-arm and a second C-L band splitter (112) to combine the one or more entangled photon pairs into a single fiber.

Description

QUANTUM SOURCE OF TEUECOM ENTANGEED PHOTONS FOR MULTI-USER QUANTUM COMMUNICATIONTECHNICAL FIELD

[0001] The present disclosure relates, in general, to quantum communication. More particularly, it relates to a system for generation of entangled photon pairs in international telecommunication (ITU)-grid for multi-user quantum key communication.BACKGROUND

[0002] The following description of the related art is intended to provide background information pertaining to the field of the disclosure. This section may include certain aspects of the art that may be related to various features of the present disclosure. However, it should be appreciated that this section be used only to enhance the understanding of the reader with respect to the present disclosure, and not as admission of the prior art.

[0003] The development of quantum communication networks represents a pivotal step towards harnessing the full potential of quantum mechanics in practical applications. At the heart of these netw orks lies the ability to reliably generate and distribute entangled photon pairs, which serve as the fundamental building blocks for secure communication protocols, such as quantum key distribution (QKD). Entangled photon pair sources have emerged as indispensable components in realizing the promise of quantum communication, enabling applications ranging from secure communication to quantum sensing.

[0004] Over the past few decades, significant strides have been made in the field of quantum communication, particularly in the generation and manipulation of entangled photon pairs. Early demonstrations utilized techniques such as spontaneous parametric downconversion (SPDC) and spontaneous four-wave mixing (SFWM) to produce entangled photon pairs. While these techniques have been successful in generating entanglement between two parties, their scalability for multi-user communication remains limited.

[0005] Various approaches have been explored to generate entangled photon pairs, including spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM) processes. While these methods have demonstrated success in producing entangled photons, their applicability to multi-user scenarios has been limited. Previous attempts at creating multi-user entangled photon pair sources have yielded promising results, with some sources achieving entanglement in multiple channel pairs. However, challenges remain in terms of scalability, spectral correlation, and ease of deployment.

[0006] There is, therefore, a need to provide an optimum solution that can obviate the above-mentioned limitations and provide the generation of entangled photon pairs for different user pairs simultaneously for quantum communication applications.OBJECTS OF THE PRESENT DISCLOSURE

[0007] An object of the present disclosure is to provide an effective that can obviate the above-mentioned limitations and provide generation of entangled photon pairs for different user pairs simultaneously for quantum communication applications.

[0008] An object of the present disclosure is to provide a system to implement a fully fiber-integrated design to facilitate easy deployment within existing optical networks, ensuring compatibility and seamless integration with current infrastructure.

[0009] An object of the present disclosure is to provide a system that the requirements for quantum communication applications, such as quantum key distribution (QKD), entanglement swapping, and quantum sensing, by providing reliable and high-quality entangled photon pairs.

[0010] An object of the present disclosure is to provide a system that validates the performance and functionality of the sy stem through experimental characterization, including the assessment of entanglement parameters.SUMMARY

[0011] Aspects of the present disclosure relate to quantum communication. More particularly, it relates to a system for generation of entangled photon pairs in international telecommunication (ITU)-grid for multi-user quantum key communication.

[0012] According to an aspect, the present disclosure pertains to a system to generate multiple entangled photon pairs for quantum communication. The system includes a continuous wave (CW) tunable pump laser configured to pump a Zinc -indiffused Magnesium-doped Periodically Poled Lithium Niobate (MgO:PPLN) ridge waveguide, an inline variable optical attenuator (VOA) configured to control the power of the CW tunable pump laser and an input fiber polarization beam splitter (FPBS) with a horizontal polarization (H-pol) arm and a vertical polarization (V-pol) arm, the H-pol arm is automatically rotated to V-pol by a 90 degree fiber twist before a waveguide facet. Further, the system includes a first C-L band splitter configured to separate signal and idler photons into distinct spectral bands, with the signal photons falling within a C-arm and the idler photons falling in L-arm, one fiber polarization controllers in both C-arm and L-arm to control polarization of outputphotons and introduce a desired phase in both C-arm and L-arm and a second C-L band splitter configured to combine the entangled photon pairs into a single fiber.

[0013] In an aspect, the MgO:PPLN ridge waveguide is placed in a Sagnac configuration.

[0014] In an aspect, the system further includes an inline long-pass filter (LPF) positioned after the MgO:PPLN ridge waveguide to reject residual photons from the CW tunable pump laser.

[0015] In an aspect, the CW tunable pump laser operates at a wavelength of 775 nm.

[0016] In an aspect, the first C-L band splitter is configured to separate the signal photons falling within the C-arm in the range of approximately 1520-1550 nm and to separate the idler photons falling within the L-arm in the range of approximately 1550-1580 nm.

[0017] In an aspect, the system further includes an arrayed waveguide grating (AWG) configured to route or demultiplex the one or more entangled photons into one or more International Telecommunication Unit (ITU) channels with a channel separation of 100 GHz.

[0018] In an aspect, the system further includes one or more single-photon avalanche detectors configured to detect one or more coincidence events between the one or more entangled photons.

[0019] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with accompanying drawing figures in which like numerals represent like components.BRIEF DESCIRPTION OF DRAWINGS

[0020] The accompanying drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0021] FIG. 1 illustrates an exemplary block diagram of the proposed system to generate one or more entangled photon pairs for quantum communication, in accordance with an embodiment of the present disclosure.

[0022] FIGs. 2(a) to 2(c) illustrate: (a) an exemplary diagram of the experimental setup for polarization-entangled photon pair generation and distribution to multi-users for secure key generation and communication at telecom wavelengths, (b) schematic diagram for simultaneous multi-user communication through the source, and (c) experimental setup forlong-distance transmission of entangled photon pair in ITU wavelength channels, in accordance with an embodiment of the present disclosure.

[0023] FIG. 3 illustrates a graphical representation of the channel pair correlation of the arrayed waveguide grating (AWG), in accordance with an embodiment of the present disclosure.

[0024] FIG. 4 illustrates a graphical representation of the photon pair spectrum centered around 1550 nm, in accordance with an embodiment of the present disclosure.

[0025] FIGs. 5(a) to 5(d) illustrate graphical representations of (a) two-photon interference for a |<b-) Bell state, (b) two-photon interference for a |<b+) Bell state, (c) real and imaginary parts of a reconstructed density matrix obtained via quantum state tomography for a |<b“) Bell state, (d) real and imaginary parts of a reconstructed density matrix obtained via quantum state tomography for a | <b+) Bell state, in accordance with an embodiment of the present disclosure.

[0026] FIGs. 6(a) to 6(c) illustrate graphical representations of the raw fidelity, concurrence, and S-parameter respectively for a |<b-) and |<b+) Bell state in 14-different ITU channel pairs, in accordance with an embodiment of the present disclosure.

[0027] FIGs. 7(a) & 7(b) illustrate the graphical representations of (a) the real and imaginary parts of the reconstructed density matrix of a |<b-) Bell state without using any fiber spool and (b) the density matrix of a |<b-) Bell state after using a fiber spool, in accordance with an embodiment of the present disclosure.

[0028] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following from the following detailed description of preferred embodiments, along with accompanying drawing figures in which like numerals represent like components.DETAILED DESCRIPTION

[0029] Embodiments explained herein relates, in general, to quantum communication. More particularly, it relates to a system for generation of entangled photon pairs in international telecommunication (ITU)-grid for multi-user quantum key communication.

[0030] FIG. 1 illustrates an exemplary block diagram of the proposed system to generate multiple entangled photon pairs for quantum communication, in accordance with an embodiment of the present disclosure.

[0031] Referring to FIG. 1, in an embodiment, a system 100 to generate multiple entangled photon pairs for quantum communication is disclosed. The system 100 can includea continuous wave (CW) tunable pump laser 102 configured to pump a Zinc -indiffused Magnesium-doped Periodically Poled Lithium Niobate (MgO:PPLN) ridge waveguide, an inline variable optical attenuator (VOA) 104 configured to control the power of the CW tunable pump laser 102 and an input fiber polarization beam splitter (FPBS) 106 with a horizontal polarization (H-pol) arm and a vertical polarization (V-pol) arm, the H-pol arm is automatically rotated to V-pol by a 90 degree fiber twist before a waveguide facet. Further, the system 100 can include a first C-L band splitter 108 configured to separate one or more signal and idler photons into distinct spectral bands, with the signal photons falling within a C-arm and the idler photons falling a L-arm, one or more fiber polarization controllers 110 in both C-arm and L-arm to control polarization of one or more output photons and introduce a desired phase in both C-arm and L-arm and a second C-L band splitter 112 configured to combine the one or more entangled photon pairs into a single fiber.

[0032] In an example, a continuous wave (CW) tunable pump laser is a type of laser that emits a continuous beam of light at a single wavelength (or very narrow range of wavelengths) and offers the flexibility of tuning that wavelength across a certain range. An inline variable optical attenuator (VOA) controls and adjusts the power level of the signal without distorting its waveform or causing signal degradation. The C-L band splitter is a device used in telecommunications and signal processing to separate or combine signals in the C and L bands of the telecom band. Fiber polarization controllers are devices designed to manipulate and control the polarization state of light within optical fibers.

[0033] In an embodiment, the MgO:PPLN ridge waveguide can be placed in a Sagnac configuration. In an example, at its core, the Sagnac configuration involves sending a beam of light around a closed path in opposite directions. The crucial aspect here is that the light travels along a loop in two opposing directions, allowing for interference upon its return. This creates an interference pattern that can be analyzed to derive valuable information.

[0034] In an embodiment, the system 100 can further include an inline long -pass filter (LPF) positioned after the MgO:PPLN ridge waveguide to reject residual photons from the CW tunable pump laser 102. In an example, an inline long-pass filter attenuates or blocks frequencies below a specified cutoff frequency while allowing higher frequencies to pass through relatively unaltered.

[0035] In an embodiment, the CW tunable pump laser 102 can operate at a wavelength of 775 nm.

[0036] In an embodiment, the first C-L band splitter 108 can be configured to separate the signal photons falling within the C-arm in the range of approximately 1520-1550nm and the idler photons falling within the L-arm in the range of approximately 1550-1580 nm.

[0037] In an embodiment, the system 100 can further include an arrayed waveguide grating (AWG) configured to route or demultiplex the one or more entangled photons into one or more International Telecommunication Unit (ITU) channels with a channel separation of 100 GHz. In an example, AWG serves as a multiplexer and demultiplexer, allowing multiple optical signals of different wavelengths to be combined and separated efficiently.

[0038] In an embodiment, the system 100 can further include one or more singlephoton avalanche detectors configured to detect one or more coincidence events between the one or more entangled photons. In an example, single-photon avalanche detectors SPADs are semiconductor devices designed to detect single photons with high efficiency and low noise. They operate based on a principle known as avalanche photodiode (APD) breakdown.

[0039] FIGs. 2(a) to 2(c) illustrate: (a) an exemplary diagram of the experimental setup for polarization-entangled photon pair generation and distribution to multi-users for secure key generation and communication at telecom wavelengths, (b) schematic diagram for simultaneous multi-user communication through the source, and (c) experimental setup for long-distance transmission of entangled photon pair in ITU wavelength channels, in accordance with an embodiment of the present disclosure.

[0040] Referring to FIGs. 2(a) to 2(c), in an embodiment, a continuous wave (CW) tunable pump laser at 775 nm (Toptica Photonics AG), whose power can be controlled through an inline variable optical attenuator (VOA), can be used to pump a 4-cm long Zinc- indiffused MgO:PPUN ridge waveguide (Covesion Utd.) placed in the Sagnac configuration. A horizontal polarization (H-pol) arm of an input fiber polarization beam splitter (FPBS) can be automatically rotated to vertical polarization (V-pol) by a 90-degree fiber twist before the waveguide facet, which can be type-0 phase-matched for degenerate down-conversion i.e. 775 nm (pump, V-pol)1550 nm (signal, V-pol) + 1550 nm (idler, V-pol). The two counter-propagating input pump waves can be always V-pol within the waveguide and thus generate V-pol signal and idler photons. The signal and idler can be generated in the anticlockwise direction remain V-pol, whereas those generated in the clockwise direction can be rotated to H-pol before recombining at the fiber polarization beam splitter (FPBS) and resulting in a symmetric polarization-entangled state due to loss of "which-path" information. The residual pump in the output of WDM can be rejected using an inline long -pass filter (UPF) with a cut-off wavelength of 1518 nm and insertion loss of < 1 dB. The signal (shorter- wavelength) and idler (longer-wavelength) photons can be separated into C- (-1520-1550nm) and L- (-1550-1580 nm) arms, respectively, of a telecom C-L band splitter. A fiber polarization controller can be used in both C- and L-arm to control the output polarization and to introduce a desired phase in the two paths. The generated entangled photon pairs can then be combined into a single fiber through another C-L band splitter / combiner. Eventually, the entangled photons can be routed / demultiplexed in various ITU channels using an AWG with a channel separation of 100 GHz. The quantum state fidelity, two-photon interference, and violation of CHSH-Bell's inequality can then be characterized in 14 channel pairs using a set of quarter-wave plates (QWP), half-wave plates (HWP), and an fiber polarization beam splitter (FPBS) in C- and L-arms.

[0041] The experimental setup can be designed in such a manner that once the source is switch on, it can generate 14 independent polarization-entangled states, and the entanglement in all the channel pairs can be characterized simultaneously in a single measurement. Here, the entanglement in all the channel pairs can be characterized one after another in a single measurement by using a pair of single -photon avalanche photodetectors (SPAD) (ID Qube NIR), each with a detection efficiency of 20%, dead time of 5 ps, and dark counts < 700 cps. Rather than optimizing the setup for optimal fidelity in each channel pair, the setup can be optimized once, and the measurement can be carried out in all channel pairs (sequentially) to see the realistic application of the source. The coincidence events between the two detectors can be recorded in a time-tagger unit (PicoQuant, PicoHarp 300) with a time-bin-width of 512 ps. The AWG used in the experiment can be a passive optical component with a total of 32 channels from 1537.407 nm (C50) to 1562.23 nm (C19) in the ITU grid. The channel spacing can be 100 GHz, and insertion loss in each channel can be <3 dB. The full width at half-maximum bandwidth (3 dB bandwidth) for each channel pair can be slightly non-uniform i.e. it can vary from 0.21 nm to 0.35 nm, and polarization-dependent loss can be < 0.2 dB for each channel pair. To characterize the channel pair correlation, first can be performed the single pass SPDC in Zinc indiffused MgO:PPEN ridge waveguide and can measure the coincidences between different channels of AWG. The experimental setup can be placed as following: A CW pump laser, VOA, polarization controller, type-0 MgO:PPEN waveguide, WDM at 775 / 1550 nm, EPF, AWG, SPAD, and coincidence counter. The pump laser can be at - 774.89 nm, and the waveguide can be operated at a temperature of 34.33°C for the down-conversion process.

[0042] FIG. 3 illustrates a graphical representation of the channel pair correlation of the arrayed waveguide grating (AWG), in accordance with an embodiment of the present disclosure.

[0043] Referring to FIG. 3, in an embodiment, a graphical representation of the channel pair correlation of the arrayed waveguide grating (AWG) is disclosed. The channel pair correlation of the arrayed waveguide grating (AWG) at waveguide temperature 34.33 °C, pump wavelength 774.89 nm and pump power ~ 39 pW can be shown. The x-axis can show the ITU channels C35 (1549.318 nm) to C50 (1537.407 nm), and the y-axis can show ITU channels C34 (1550.117 nm) to C19 (1562.23 nm). The color bar can indicate the coincidence counts / 20s. FIG. 3 shows the measured coincidences between different channels i.e. channel pair correlations. It can be comprehensible that channel pairs are highly spectrally correlated; for example, channel number 35 (C35) can show a coincidence only with channel 34 (C34), and channel 36 (C36) can show a coincidence with channel 33 (C33). Negligible crosstalk can be shown throughout the sixteen channel pairs, thus all the channel pairs can be used to route the entangled photon pairs for multi-user quantum key distribution and communication.

[0044] FIG. 4 illustrates a graphical representation of the photon pair spectrum centered around 1550 nm, in accordance with an embodiment of the present disclosure.

[0045] Referring to FIG. 4, in an embodiment, a graphical representation of the photon pair spectrum centered around 1550 nm is disclosed. The pink dots can represent the experimental values, and the dotted blue line can be the theoretical sine2. The colored bars can show the channels of a 100 GHz AWG in the ITU grid. The same color bars around 1550- nm can be the channel pairs that include an entangled photon pair. FIG.4 shows the photon pair emission spectrum / bandwidth of the source and the channel pair capacity that can be utilized for quantum communication. The emissions can degenerate around 1550-nm and the colored bars can be the channel wavelengths of AWG. The red, green, and blue bars on the signal side can correspond to channels 37 (C37 = 1547.721 nm), 38 (C38 = 1546.921 nm), and 39 (C39 = 1546.125 nm), respectively. The grey color bar can show channel number 50 (C50 = 1537.407 nm). On the idler side, the color bars in the same sequence can correspond to channel numbers 32 (C32 = 1551.724 nm), 31 (C31 = 1552.538 nm), 30 (C30 = 1553.322 nm), and 19 (C19 = 1562.23 nm) respectively. The same color bars can indicate entangled pairs of signal and idler photons e.g. photons of channel C37 are entangled with C32. The entanglement can be characterized between these 14 channel pairs of the AWG that can be restricted to the highly efficient region of the spectrum. FIG. 4 can indicate that more channel pairs can be exploited for quantum communication / key distribution since a large portion of the emission spectrum is still unoccupied.

[0046] FIGs. 5(a) to 5(d) illustrate graphical representations of (a) two-photon interference for a IO-) Bell state, (b) two-photon interference for a |<b+) Bell state, (c) real and imaginary parts of a reconstructed density matrix obtained via quantum state tomography for a |<b“) Bell state, (d) real and imaginary parts of a reconstructed density matrix obtained via quantum state tomography for a | <b+) Bell state, in accordance with an embodiment of the present disclosure.

[0047] Referring to, FIGs. 5(a) to 5(d), in an embodiment, FIG. 5(a) represents the two-photon interference for a |<b-) Bell state and FIG. 5(b) represents the two-photon interference for a |<b+) Bell state in ITU channel pair C38 (1546.921 nm)-C31 (1552.538 nm). The dots and the dashed lines show the experimental data and sinusoidal fit, respectively. FIG. 5(c) shows the real and imaginary parts of a reconstructed density matrix obtained via quantum state tomography for a |<b-) Bell state and FIG. 5(d) shows the real and imaginary parts of a reconstructed density matrix obtained via quantum state tomography for a |<b+) Bell state in ITU channel pair C38-C31. Raw fidelities for the Bell states |<b-) and |+) are ~ 94% and ~ 95%, respectively.

[0048] Further, the generation of tunable Bell states can be done through pumping of the waveguide (at T = 34.33 °C) placed in the Sagnac configuration through an fiber polarization beam splitter (FPBS) to generate the polarization-entangled state. The probability amplitudes of photon pair generation from the clockwise and anti-clockwise directions can add up after the input fiber polarization beam splitter (FPBS) in the same mode to generate a symmetric polarization entangled state Themaximally entangled Bell-state can be accomplished by precisely controlling the input pump polarization and setting an equal path length for the polarization maintaining fibers on both sides of the waveguide. First the waveguide input pump (A = 774.89-nm) power can be attenuated to ~ 50 pW using a VOA and then an input fiber polarization controller can be used to adjust the pump polarization for the |<b+) Bell-state generation. The residual pump can be filtered using an inline long -pass filter (transmission wavelength > 1518 nm), and the photon emission spectrum can be split into two halves at 1550 nm using a commercial C-U band splitter. A set of manual fiber polarization controllers can be controlled to compensate for the polarization rotation in the two arms (C and U) and to introduce a desired phase difference in the signal and idler photons. A set of quantum state analyzer tools (QWPs, HWPs, FPBSs) can be used in the two paths before combining the signal and idler photons through another C-U band splitter. Finally, the photon pairs can be demultiplexed intodifferent ITU channels using a commercially available AWG. This can allows characterization the entanglement in all channel pairs at once without altering the setup. Here, only 14 channel pairs (from C37-C32 to C50-C19) can be selected for entanglement characterization, leaving out C35-C34 and C36-C33 due to the C-L band splitter's substantial uneven insertion loss at those wavelengths as a result of the device's transmission spectrum. The experimental setup can also generate a — |U)| )) Bell state. Thiscan be achieved through phase-optimization of the C (signal) and L (idler) arms with the help of two fiber polarization controllers placed at the end of the first C-L band splitter. Here, the source for both the entangled states can be characterized.

[0049] The source for the maximally entangled two-qubit Bell state generation can be optimized in channel pair C38-C31 and can investigate the entanglement corresponding to both the generated states. FIGs. 5(a) and 5(b) shows the two-photon interference in channel pair C38-C31 for a |<b-) and |<b+) Bell state, respectively. The idler photons traveling in the L-arm can be projected into horizontal / vertical (H / V) and diagonal / anti-diagonal (D / A) basis using a HWP and an fiber polarization beam splitter (FPBS); the polarization of signal photons traveling in the C-arm can be continuously varied by rotating the corresponding HWP. A coincidence can be recorded between the two photons during this process. It can be noticed that for a |<b-) Bell state, the maximum coincidence can occur when the idler photon can be projected onto D-polarization and the signal can be A-polarized while for a |<b+) Bell state, the maximum coincidence can occur when the signal is also D-polarized. The behavior of interference for a |<b-) and |<b+) Bell state can be similar in H-V basis and can be opposite in D-A basis. In FIGs. 5(a) and 5(b), the dots can stand for experimental values, and the dashed lines can be the sinusoidal fit. The fringe visibilities can be 95.98%, 88.23%, 96.91%, and 89.38% for H (0°), D (22.5°), V (45°), and A (67.5°) polarization projections, respectively in |_) Bell state. For a |<b+) Bell state the corresponding visibilities can be 86.71%, 94.07%, 97.71%, and 90.01%. FIGs. 5(C) and 5(D) show the density matrices for a |<b“) and |<b+) Bell states respectively, in channel pair C38-C31. The real and imaginary part of the density matrix can be calculated from the experimental coincidences observed in the quantum state tomography (QST) of the generated Bell state. The raw fidelities for a |<b-) and |<b+) Bell states can be ~ 94% and ~ 95%, respectively. This show that the source can generate both the Bell states with high fidelity, and the output state can be tuned from |_) to |<b+) or vice-versa with the help of a fiber-based polarization controller connected in the C- and L-arm of first C-L band splitter.

[0050] FIGs. 6(a) to 6(c) illustrate graphical representations of the raw fidelity, concurrence, and S-parameter respectively for a IO-) and |<b+) Bell state in 14-different ITU channel pairs, in accordance with an embodiment of the present disclosure.

[0051] Referring to FIGs. 6(a) to 6(c), in an embodiment, graphical representations of the raw fidelity, concurrence, and S-parameter respectively for a |<b-) and |<b+) Bell state in 14-different ITU channel pairs is disclosed. Each channel pair can contain a polarization entangled photon pair. For the scaling of a WDM-based quantum communication network, the source can have the capacity to employ a large number of users. This can reduce the total number of sources required to build such a network, and also the financial cost of infrastructure. The user hardware needed for simultaneous quantum communication between multi-users can be reduced when a large number of wavelength channels of the source are exploited. As with a large number of available channel pairs, more users can be fully connected such that each user shares an entangled photon with the other. In this regard, the capacity of the source and the number of channel pairs that can be used for secure communication, key distribution, and entanglement distribution to remote users can be evaluated. Here, the violation of CHSH inequality, concurrence, and entanglement fidelity can be measured by performing the QST in 14 channel pairs. FIGs. 6(a), 6(b), and 6(c) show the fidelity of the quantum state, concurrence, and S-parameter, respectively for |_) and |<b+) Bell states in 14 ITU channel pairs (C37-C32 to C50-C19). The raw fidelity can be > 90% in all channel pairs for IO-) Bell state, and the maximum fidelity can be ~ 94% in channel pair C38-C31 and C39-C30. For a |<b+) Bell state, the raw fidelity in the first 12 channel pairs can be > 89%, and the maximum fidelity can be- 95% in channel pair C38- C31. The raw concurrence can be > 0.8 in all channel pairs for a |<b-) Bell state, and the maximum value can be - 0.88 in channel pairs C38-C31 and C39-C30. For a | <b+) Bell state, the raw concurrence can be > 0.8 in the first 12 channel pairs, and the maximum concurrence can be - 0.91 in channel pair C38-C31. It can be apparent from FIG. 5(c) that each channel pair can carry an entangled photon pair since each channel pair can violate the CHSH-Bell inequality e.g. the min value of S-parameter can be - 2.565 ± 0.037 and max value can be - 2.682 ± 0.03 for a |<b+) Bell state and for a |<b-) state the min value can be - 2.652 ± 0.033 and the max value can be - 2.753 ± 0.029. This can suggest that all these channel pairs can be utilized in a WDM-based quantum communication network. As shown in FIG. 2(b), 14 user pairs can access the source simultaneously for communication / secret key distribution if a two-user communication scheme is deployed. Alice 1 can communicate with Bob 1, Alice 2can communicate with Bob 2, and so on since each user pair owns a unique bipartite entangled state independent of the other user pair. Alice 1 cannot communicate with any other user except Bob 1 since they do not share any entangled photons. However, this could be possible if optical switches can be used to route the photons of Bobl to any other user with whom Alice 1 wants to communicate or the whole channels can be further multiplexed to form a complete network where each user shares an entangled photon with the rest of the users. The latter scheme can allow any user to communicate with any other user.

[0052] FIGs. 7(a) & 7(b) illustrate the graphical representations of (a) the real and imaginary parts of the reconstructed density matrix of a |<b-) Bell state without using any fiber spool and (b) the density matrix of a |<b-) Bell state after using a fiber spool, in accordance with an embodiment of the present disclosure.

[0053] Referring to, FIGs. 7(a) & 7(b), in an embodiment, FIG. 7(a) represents the real and imaginary parts of the reconstructed density matrix of a |<b-) Bell state without using any fiber spool i.e. just after the AWG. FIG. 7(b) shows the density matrix of a |<b-) Bell state after using a fiber spool of 10 Km in wavelength channel pair C38-C31. The concurrences can be ~ 0.902 and ~ 0.94, and the fidelities can be approximately 95.09% and 96.87%, respectively. The source can be highly efficient to transfer the entangled photons to remote parties for quantum communication and key distribution. In a realistic scenario, the users of a quantum network can typically be located dozens of kilometers apart; therefore, the source must be able to send entangled photons to these far-off users. This can require the multi-user source to have high-fidelity entanglement, high stability, and compactness. The present disclosure can assess the source in this context for long-distance transmission. The decrease in coincidence rate can be monitored and the effect on fidelity when these entangled photon pairs are sent over a long distance. The entanglement can be quantified before and after the fiber spool in channel pair C38-C31. The QST in wavelength channels C38 and C31 can be conducted, and next, two fiber spools, each 500 m long can be put, in channels C38 and C31 and re-conduct the QST. Finally, two fiber spools can be put, each 10-Km long, in channels C38 and C31 and can perform the QST measurement. It can be noticed that there can be a slight drop in the coincidence rate after transferring the photons to 0.5 Km but a significant drop when the photons can be sent to 10 Km. The fidelity can remain almost equal even after sending the photons to a 10 Km distance. This can be feasible due to the low propagation loss of single-mode fibers at telecom wavelength. The coincidence rates for remote users can be increased by simply increasing the input pump power. The reconstructeddensity matrix of the IO-) Bell state, before and after using the fiber spool in channel pair C38-C31, can be shown in FIGs. 7(a) and 7(b), respectively. The raw state fidelity / concurrence before using the fiber spool can be ~ 95.09% / - 0.902 and after using a 0.5 Km (i.e. 500 m) spool can be - 94.76% / ~0.896. The corresponding value can become - 96.87% / - 0.94 after using a 10 Km fiber spool in channel pair C38-C31. The above results can show the successful transfer of polarization-entangled photons to a 10 Km distance. This can indicate that the entangled photons can be transferred to remote users with very low loss.

[0054] It will be apparent to those skilled in the art that the method and system of the disclosure may be provided using some or all of the mentioned features and components without departing from the scope of the present disclosure. While various embodiments of the present disclosure have been illustrated and described herein, it will be clear that the disclosure is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the disclosure, as described in the claims.ADVANTAGES OF THE PRESENT DISCLOSURE

[0055] The present disclosure provides a system utilizing continuous wave (CW) tunable pump laser that allows for precise control over the generation process, ensuring optimal efficiency and performance of the entangled photon pairs.

[0056] The present disclosure provides a system with inclusion of an inline variable optical attenuator (VOA) that enables the fine-tuning of the pump laser power, providing flexibility in adjusting the photon generation parameters to suit specific requirements.

[0057] The present disclosure provides a system utilizing an input fiber polarization beam splitter (FPBS) with automatic polarization rotation to ensure efficient polarization alignment before entering the waveguide facet, enhancing the generation efficiency of entangled photon pairs.

[0058] The present disclosure provides a system efficiently separating signal and idler photons into distinct spectral bands, allowing for precise manipulation and control of the generated photons in each arm.

[0059] The present disclosure provides a system with inclusion of fiber polarization controllers in both the C-arm and L-arm enables precise control over the polarization of output photons, facilitating the introduction of desired polarization states as needed.

[0060] Hie present disclosure provides a system that efficiently combines the generated entangled photon pairs into a single fiber, ensuring efficient collection and transmission of the entangled photons for further processing or utilization.

Claims

We Claim:

1. A system (100) to generate one or more entangled photon pairs for quantum communication, said system (100) comprising: a continuous wave (CW) tunable pump laser (102) configured to pump a Zinc-indiffused Magnesium-doped Periodically Poled Lithium Niobate (MgO:PPLN) ridge waveguide; an inline variable optical attenuator (VOA) (104) configured to control the power of the CW tunable pump laser (102); an input fiber polarization beam splitter (FPBS) (106) with a horizontal polarization (H- pol) arm and a vertical polarization (V-pol) arm, the H-pol arm is automatically rotated to V-pol through a 90 degree fiber twist before a waveguide facet; a first C-L band splitter (108) configured to separate one or more signal and idler photons into distinct spectral bands, with the signal photons falling within a C-arm and the idler photons falling within a L-arm; one or more fiber polarization controllers (110) in both C-arm and L-arm to control polarization of one or more output photons and introduce a desired phase in both C-arm and L- arm; and a second C-L band splitter (112) configured to combine the one or more entangled photon pairs into a single fiber, wherein the MgO:PPLN ridge waveguide is placed in a Sagnac configuration, wherein the system (100) comprises an arrayed waveguide grating (AWG) configured to route or demultiplex the one or more entangled photons into one or more International Telecommunication Unit (ITU) channels with a channel separation of 100 GHz.

2. The system (100) as claimed in claim 1, wherein the system (100) comprises an inline- long pass filter (LPF) positioned after the MgO:PPLN ridge waveguide to reject residual photons from the CW tunable pump laser (102).

3. The system (100) as claimed in claim 1, wherein the CW tunable pump laser (102) operates at a wavelength of 774.89 nm.

4. The system (100) as claimed in claim 1, wherein the first C-L band splitter (108) is configured to separate the signal photons falling within the C-arm in the range of approximately 1520-1550 nm.

5. The system (100) as claimed in claim 1, wherein the first C-L band splitter (108) is configured to separate the idler photons falling within the L-arm in the range of approximately 1550-1580 nm.

6. The system (100) as claimed in claim 1, wherein the system (100) comprises one or more single-photon avalanche detectors configured to detect one or more coincidence events between the one or more entangled photons.

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