WO2022123594A1

WO2022123594A1 - System and method for plug-and-play differential phase encoded measurement-device-independent quantum key distribution

Info

Publication number: WO2022123594A1
Application number: PCT/IN2021/051137
Authority: WO
Inventors: Shashank Kumar RANU; Prabha MANDAYAM; Dr. Anil PRABHAKAR
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2020-12-07
Filing date: 2021-12-06
Publication date: 2022-06-16

Abstract

Disclosed herein is a plug-and-play differential phase-encoded measurement device independent (DPS MDI) quantum key distribution (QKD) system that uses single photons in a linear superposition of three orthogonal time-bin states. The system shown in FIG. 1 comprises a transmitter (Tx), a pair of receiver sub-systems (Rx1, Rx2) and a measurement unit. The method comprises the steps of: generating a high-powered optical signal, polarizing the generated signal to 45o and obtaining pulsed signal, obtaining three time-bins from the pulsed signal, splitting the pulsed signal into horizontally polarized and vertically polarized signals, transmitting the horizontally polarized signal to the first receiver sub-system (Rx1) and reflecting the vertically polarized signal to transmitter (Tx), horizontally polarising the vertically polarized signal and transmitting it to second receiver sub-system (Rx2). The receiver sub-systems (Rx1, Rx2) convert horizontally polarized signals into phase encoded key. The keys from receiver sub-systems are transmitted to measurement unit for photon detection.

Description

SYSTEM AND METHOD FOR PLUG-AND-PLAY DIFFERENTIAL PHASE ENCODED MEASUREMENT-DEVICE-INDEPENDENT QUANTUM KEY DISTRIBUTION

FIELD OF THE INVENTION:

[0001 ] The present invention relates to differential phase-encoded measurement (DPS) quantum key distribution (QKD) systems. More particularly, the present invention relates to a plug-and-play differential phase-encoded measurement device independent (DPS MDI) quantum key distribution (QKD) system which uses single photons in a linear superposition of three orthogonal time-bin states.

BACKGROUND:

[0002] In recent days, the process of sharing information between two entities without revealing any information to an eavesdropper is a challenging task. Also, most of the communication takes place over a long distance and is cloud-based. This is making secure data transmission a vital aspect of modern communication systems. There are different methods which are used to transmit data between two communicating parties in a secure way. Quantum Key Distribution (QKD) is one such preferred method in recent days for secure communication.

[0003] The QKD system consists of a sender (Alice) transmitting optical signals at single-photon levels to a receiver (Bob). The photons are phase modulated at the Alice and then transmitted to Bob. Differential Phase Shift Quantum Key Distribution (DPS-QKD) is one of the most commonly used protocol in QKD which shifts the keys generated through photon transmission in relative phase signals. However, various improvements are made to the QKD system for providing a secure transmission. There are many prior art relating to QKD systems for generating key and sharing information in a safe way, there are still sidechannel attacks on measurement unit. The side-channel attacks on measurement is overcome by using Measurement-device-independent QKD (MDI-QKD). This MDI arrangement is also combined with DPS QKD systems.

[0004] In MDI implementations, active stabilization systems are typically used to ensure indistinguishability of photons from transmitter to receiver, mainly in three dimensions: spectrum, polarization, and timing. These stabilization systems increase the implementation complexity of QKD experiments. Hence, there is a need to reduce the experimental complexity arising because of reference frame alignment between transmitter and receiver.

OBJECTS OF THE INVENTION:

[0005] The primary object of the present invention is to provide a DPS MDI QKD system that employs plug-and-play architecture to reduce the experimental complexity arising due to reference frame alignment between transmitter and receiver.

SUMMARY:

[0006] The present invention provides a plug-and-play differential phase-encoded measurement device independent (DPS MDI) quantum key distribution (QKD) system that uses single photons in a linear superposition of three orthogonal time-bin states. The plug- and-play architecture implemented in the proposed system eliminates the requirement of optical phase-locked-loop (OPLL) which is a typical subsystem used in MDI implementations. The proposed system guarantees indistinguishability between transmitter and receiver photons in terms of frequency, time and polarization degree of freedoms. The key information is encoded as phase difference between two time-bins which makes protocol robust against random phase fluctuations.

[0007] According to the present invention, the system comprises a transmitter, a pair of two receiver sub-systems and a measurement unit. The transmitter comprises a laser source, a pair of polarization controllers, an intensity modulator, a delay line interferometer, plurality of polarization beam splitters and a half-wave plate. Each receiver sub-system comprises a beam splitter, an intensity modulator, a phase modulator, a phase randomizer, a photo-detector and a faraday mirror. The system also comprises a measurement unit for detecting the signal from the two receiver sub-systems. The measurement unit comprises a beam splitter, two acousto-optic deflectors and four single-photon detectors.

[0008] In accordance with present invention, the method for securely transmitting key using a plug-and-play DPS MDI QKD system comprises the steps of: generating a high- powered optical signal using a laser source in the transmitter, transmitting the generated signal through a pair of polarization controllers for polarizing the signal to 45°, passing the signal through an intensity modulator for obtaining a pulsed signal, passing the pulsed signal through a delay line interferometer for obtaining three time-bins, transmitting the signal from the delay line interferometer to a polarization beam splitter for splitting into horizontally polarized and vertically polarized signals, passing the horizontally polarized signal from the polarization beam splitter to another polarization beam splitter and reflecting the vertically polarized signal to the polarization beam splitter. The horizontally polarized signal is transmitted from the polarization beam splitter to a receiver sub-system through an optical channel and converted into a phase encoded key. The vertically reflected signal is again horizontally polarised using half-wave plate and transmitted to another receiver subsystem through an optical channel. This signal is also converted into a phase encoded key in the receiver sub-system. The phase encoded keys from both the receiver sub-systems are transmitted to measurement unit for photon detection.

[0009] The proposed system has obtained secure channel length for a weak coherent source (WCS) based implementation using the decoy-state analysis. In this system, the transmitter encodes the key information as phase difference between time-bins. As these time bins are few nanoseconds apart, the quantum channel affects them in a similar way, thereby cancelling the effects of phase fluctuations. The receiver sub-system also does a similar encoding, and hence gets the same advantage. In other MDI protocols that use only a pulse-train without any spatial or temporal superposition, the transmitter encodes key information as phase of an individual pulse. This applied phase gets affected by random fluctuations and results in higher quantum bit error rate (QBER). For more spatial paths, i.e., n > 3, DPS MDI leads to increased complexity in key reconciliation scheme as well as in its implementation. However, in the proposed system there is an equivalence between spatial and temporal paths and it is possible to use n>3 if we use temporal time-bins without the use of an optical spitter.

[0010] These objectives and advantages of the present invention will become more evident from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS:

[ 0011 ] The objective of the present invention will now be described in more detail with reference to the accompanying drawing, wherein:

[0012] Fig. 1 shows the proposed plug-and-play DPS MDI QKD system with one transmitter and two receiver sub-systems;

[0013] Fig. 2 shows a simplified embodiment of the proposed plug-and-play DPS MDI QKD system; and

[0014] Fig. 3 shows an embodiment of the proposed plug-and-play DPS MDI QKD system. REFERENCE NUMERALS:

1 - Continuous Wave Laser Source

2a, 2b - Pair of Polarization controllers at Transmitter end

3 - Intensity Modulator at Transmitter end

4 - Delay Line Interferometer at Transmitter end

5, 6, 8 - Polarization Beam Splitters at Transmitter end

7 - Half-Wave Plate at Transmitter end

9a - Beam Splitter at first Receiver sub-system

9b - Beam Splitter at second Receiver sub-system

10a - Photo-Detector at first Receiver sub-system

10b - Photo-Detector at second Receiver sub-system

I la - Intensity Modulator at first Receiver sub-system

11b - Intensity Modulator at second Receiver sub-system

12a - Phase Modulator at first Receiver sub-system

12b - Phase Modulator at second Receiver sub-system

13a - Phase Randomizer at first Receiver sub-system

13b - Phase Randomizer at second Receiver sub-system

14a - Faraday Mirror at first Receiver sub-system

14b - Faraday Mirror at second Receiver sub-system

15 - Beam Splitter at measurement unit

16a, 16b - Acousto-Optic Deflectors

17a, 17b, 17c, 17d - Single-photon detectors

18a - Attenuator at first Receiver sub- system 18b - Atenuator at second Receiver sub-system

19a, 19b - Beam Splitters used at measurement unit instead of AODs

DETAILED DESCRIPTION OF THE INVENTION:

[0015] The present invention provides a plug-and-play differential phase-encoded measurement device independent (DPS MDI) quantum key distribution (QKD) system that uses single photons in a linear superposition of three orthogonal time-bin states. The plug-and- play architecture implemented in the proposed system eliminates the requirement of optical phase-locked-loop (OPLL) which is a typical subsystem used in MDI implementations. The proposed system guarantees indistinguishability between transmiter and receiver photons in terms of frequency, time and polarization degree of freedoms. The key information is encoded as phase difference between two time-bins which makes protocol robust against random phase fluctuations.

[0016] According to the present invention, the system comprises a transmitter, a pair of receiver sub-systems and a measurement unit. The transmiter comprises a laser source, a pair of polarization controllers, an intensity modulator, a delay line interferometer, plurality of polarization beam splitters and a half-wave plate. Each receiver sub-system comprises a beam splitter, an intensity modulator, a phase modulator, a phase randomizer, a photo-detector and a faraday mirror. The measurement unit comprises a beam splitter, two acousto-optic deflectors and four single-photon detectors.

[0017] In accordance with present invention, the method for securely transmiting key using a plug-and-play differential phase-encoded measurement device independent quantum key distribution (DPS MDI QKD) system comprises the steps of: generating a high-powered optical signal using a continuous wave laser in the transmitter, transmiting the generated signal through a pair of polarization controllers for polarizing the signal to 45°, passing the signal through an intensity modulator for obtaining a pulsed signal, passing the pulsed signal through a delay line interferometer for obtaining three time-bins, transmitting the signal from the delay line interferometer to a polarization beam splitter for splitting into horizontally polarized and vertically polarized signals, transmitting the horizontally polarized signal from the polarization beam splitter to another polarization beam splitter and reflecting the vertically polarized signal to the polarized beam splitter. The horizontally polarized signal is transmitted from the polarization beam splitter to a receiver sub-system through an optical channel and converted into a phase encoded key. The vertically reflected signal is again horizontally polarised using half-wave plate and transmitted to another receiver sub-system through an optical channel. This signal is also converted into a phase encoded key in the receiver sub-system. The phase encoded keys from both the receiver sub-systems are transmitted to measurement unit for photon detection.

[0018] The detailed working of the system is explained below:

[0019] Fig. 1 shows the plug-and-play DPS MDI QKD system comprising a transmitter and two receiver sub-systems. The transmitter (Tx) is separated from the two receiver sub-systems (Rxl, Rx2) by an optical channel which is usually a single mode optical fibre. The transmitter (Tx) generates a high-powered optical signal using a continuous wave (CW) laser (1). The operator (Charles) of the transmitter uses a pair of polarization controllers (PC) (2a, 2b) to ensure that the generated signal has 45° polarization. This ensures that the polarization beam splitter (PBS) (5) outputs horizontally (H) and vertically (V) polarized beams having equal power. The high-powered CW signal is then passed to an intensity modulator (IM) (3) so as to obtain a pulsed signal. This pulsed signal is then passed through a delay line interferometer (DLI) (4) to obtain three time-bins. The output from DLI (4) is then passed through a PBS (5). The PBS (5) transmits horizontally (H) polarized signal and reflects the vertically (V) polarized signal. The PBS (5) outputs equally powered horizontally (H) and vertically (V) polarized signals. The horizontally (H) polarized signal passes through another polarization beam splitter (PBS 1) (6). The PBS 1 (6) transmits all the incident signal as the photons are already horizontally polarized. This horizontally (H) polarized signal is transmitted to one receiver subsystem (Rxl) through an optical channel. The vertically (V) polarized signal coming out of PBS (5) is then passed through a half-wave plate (HWP) (7) which makes the signal horizontally (H) polarized. This horizontally (H) polarized signal is transmitted to another polarization beam splitter (PBS 2) (8). The PBS 2 (8) transmits this signal to a second receiver sub-system (Rx2) through an optical channel. The signals transmitted from Tx to Rxl and Tx to Rx2 are not at single-photon levels. Also, these signals transmitted to Rxl and Rx2 are both horizontally (H) polarized.

[0020] In the Rxl sub-system, a beam splitter (BS) (9a) reflects a fraction of incoming signal from Tx towards a photo- detector (PD) (10a). The detection of the incoming signal in the photodetector (10a) is used for synchronization and generating necessary timing signals. The remaining fraction of the incoming signal is passed through an intensity modulator (IM) (I la). The IM (I la) brings down the power of incoming signal to single-photon levels (i.e. active attenuation). The same IM (1 la) is also used to generate decoy states. This weak coherent signal (WCS) is now passed through a phase modulator (PM) (12a), and the key bit information is encoded as the phase difference between two successive time bins. The differential phase- encoded signal is then passed through a phase randomizer (PR) (13a) to randomize the overall global phase. This random phase is applied by the PR (13a) and it is known to the operator of Rxl. But if the PR (13a) is removed, the use of random phase fluctuations of the channel itself as a randomizer, the operator will not have any information regarding this random phase. This result in errors while sifting. Hence, this phase randomization is a necessary step to establish decoy states. The encoded signal then gets reflected using a Faraday mirror (FM) (14a). The FM (14a) rotates the H polarization to V polarization. The operator (Alice) must ensure that the IM (I la), PM (12a), PR (13a) in the receiver sub-system are off when this V polarized reflected signal travels back through them. In an embodiment, the active components of the operator are kept off while the signal is incoming, and turn them on after it gets reflected by the FM (14a). This weak coherent signal travels through an optical channel and falls on PBS 2 (8) as shown in Fig. 1. The PBS 2 (8) reflects the vertically (V) polarized photons, and hence these photons fall on the non-polarizing beam splitter. This BS (15) forms the main component of the measurement unit, and is used for Bell state measurements wherein the quantum correlations between two qubits are measured and quantified as being stronger than in a classical system. A similar process is carried out by the second operator (Bob) in receiver sub-system (Rx2) to ensure that the encoded signal coming back to the measurement unit is vertically (V) polarized.

[0021] The measurement unit comprises of a beam splitter (BS) (15), two acousto-optic deflectors (AOD) (16a, 16b), and four single-photon detectors (SPD) (17a, 17b, 17c, 17d). In DPS MDI QKD system, successful events correspond to the detection of two photons which are a few nanoseconds apart. However, a single photon detector (SPD) has a dead time of few microseconds, thereby making such a detection event impossible. Hence, AODs (16a, 16b) are used to route these two photons to two different SPDs.

[0022] In an embodiment, a pair of beam splitters (BS) (19a, 19b) are used instead of the two acousto-optic deflectors (AOD) (16a, 16b) as shown in Fig. 3. The use of beam splitters (BS) (19a, 19b) provides lower complexity to the system.

[0023] Fig. 2 shows a simplified embodiment of plug-and-play DPS MDI QKD system. In this embodiment, the two polarization controllers (PC) in the transmitter (Tx) are removed and polarization maintaining (PM) fiber is used to maintain the 45° polarization. This arrangement just requires a few meters of PM fiber and removes the use of bulky PCs. The BS (9a, 9b) and the photo- detector (10a, 10b) setup present in the receiver sub-systems (Rxl, Rx2) used to generate the synchronization and timing signals are also removed. This simplified embodiment uses phase modulation in a free running mode. The two operators (Alice and Bob) use an optical or RF delay while calibrating the implementation to synchronize the optical pulses with RF signal. In this embodiment a passive attenuator (18a, 18b) is used for attenuating the signal to single-photon levels. The signal passes through this attenuator twice and so, the attenuation value should be fixed. For example, if Alice wants a mean photon number of 0.2, she should fix her attenuator at level which would attenuate the signal to 0.4 mean photon number. In this embodiment, the intensity modulator (I la, 11b) and phase randomizer (13a, 13b) are also removed from the receiver sub-systems Rxl and Rx2. The simplification of the sub-systems is possible if a decoy state version of DPS MDI QKD is not implemented. [0024] In typical DPS MDI QKD implementations, the two operators (Alice and Bob) use independent lasers and requires the use of an optical phase locked loops (OPLL) in one of the sub-systems, to establish a common phase reference. The use of a single laser in plug-and-play architecture, in the transmitter sub-system removes the requirement of OPLL. Further, in conventional DPS MDI QKD system, the two operators (Alice and Bob) use independent delay lines to create three time-bins. The time-bins of Alice and Bob is not identical due to variations in optical components that form their respective delay line interferometers (DLIs). The use of a single DLI in the transmitter, in the proposed plug-and-play DPS MDI QKD system solves this issue. Also, differential phase encoding offers protection against random phase fluctuations within the optical channel, making this system more robust against environmental disturbances.

[0025] While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope of the invention as claimed.

Claims

E CLAIM: A plug-and-play differential phase-encoded measurement device independent quantum key distribution (DPS MDI QKD) system for transmitting key in a secure way, wherein the DPS MDI QKD system comprises: a. a transmitter (Tx) for transmitting the key wherein, the transmitter (Tx) comprises i. a continuous wave (CW) laser source (1) for generating a high-powered optical signal; ii. a pair of polarization controllers (PC) (2a, 2b) for polarizing the high- powered optical signal; iii. an intensity modulator (IM) (3) for obtaining a pulsed signal from the received high-powered optical signal; iv. a delay line interferometer (DLI) (4) for obtaining three time-bins from the pulsed signal received from the intensity modulator (IM); v. a plurality of polarization beam splitters (PBS, PBS1, PBS2) (5, 6, 8) for transmitting horizontally (H) polarized signal and reflecting vertically (V) polarized signal received from the delay line interferometer (DLI); and vi. a half-wave plate (HWP) (7) for converting the vertically (V) polarized signal to horizontally polarized signal. b. a pair of receiver sub-systems (Rxl, Rx2) for receiving the signal transmitted from the transmitter (Tx), wherein each receiver sub-system comprises i. a beam splitter (BS) (9a, 9b) for reflecting a fraction of signal received from the transmitter (Tx) towards a photo-detector (PD) (10a, 10b) for synchronization and generating necessary timing signals; ii. an intensity modulator (IM) (I la, 11b) for receiving remaining fraction of signal from the beam splitter (9a, 9b) and bringing down the power of signal to single-photon level; iii. a phase modulator (PM) (12a, 12b) for encoding key as phase difference between two successive time bins; iv. a phase randomizer (PR) (13a, 13b) for randomizing the overall global phase of phase encoded key; and v. a faraday mirror (FM) (14a, 14b) for reflecting the received phase encoded key c. a measurement unit for measuring the signal from the pair receiver sub-systems (Rxl, Rx2), wherein the measurement unit comprises a beam splitter (BS) (15), a pair of acousto-optic deflectors (AOD) (16a, 16b), and a plurality of single-photon detectors (SPD) (17a, 17b, 17c, 17d).

2. The system as claimed in claim 1 , wherein the transmitter (Tx) and the pair of receiver subsystems (Rxl, Rx2) are connected by an optical channel.

3. The system as claimed in claim 2, wherein the optical channel is a single mode optical fibre.

4. The system as claimed in claim 1, wherein the polarization is maintained to 45° in the transmitter (Tx) using polarization maintaining (PM) fibre.

5. The system as claimed in claim 1, wherein the pair of receiver sub-systems (Rxl, Rx2) includes an optical or RF delay for calibration and to synchronize the pulses with the signal.

6. The system as claimed in claim 1, wherein the pair of receiver sub-systems (Rxl, Rx2) includes a passive attenuator (18a, 18b) for attenuating the signal to single-photon levels.

7. The system as claimed in claim 1 , wherein the measurement unit includes a pair of beam splitters (19a, 19b) in place of acousto-optic deflectors (AOD) (16a, 16b). A method for securely transmitting key using a plug-and-play differential phase-encoded measurement device independent quantum key distribution (DPS MDI QKD) system, wherein the method comprises the steps of: a. generating a high-powered optical signal using a continuous wave (CW) laser (1) in the transmitter (Tx); b. transmitting the signal through a pair of polarization controllers (PC) (2a, 2b) for polarizing the signal to 45°; c. passing the polarized signal through an intensity modulator (IM) (3) for obtaining a pulsed signal; d. passing the pulsed signal through a delay line interferometer (DLI) (4) for obtaining three time-bins; e. transmitting the pulsed signal from the DLI (4) to a polarization beam splitter (PBS) (5) for transmitting horizontally (H) polarized signal and reflecting vertically (V) polarized signal; f. transmitting the horizontally (H) polarized signal from the PBS (5) to a polarization beam splitter (PBS 1) (6) and reflecting the vertically (V) polarized signal, wherein i. the horizontally (H) polarized signal is transmitted from the PBS 1 (6) to a receiver sub-system (Rxl) through an optical channel; and ii. the reflected vertically (V) polarized signal is passed to a half-wave plate (HWP) (7) for horizontally (H) polarizing and transmitting it to a receiver sub-system (Rx2) through an optical channel g. reflecting a fraction of the received signal from the transmitter (Tx) towards a photodetector (10a, 10b) (PD) for synchronization and generating necessary timing signals; h. transmitting other fraction of the received signal from the transmitter (Tx) through an intensity modulator (IM) (I la, 11b) to bring down the power of the signal to single-photon levels; i. passing the signal from the IM (I la, 11b) through a phase modulator (PM) (12a, 12b) for encoding key as phase difference between two successive time bins; j. transmitting the phase encoded key through a phase randomizer (PR) (13a, 13b) for randomizing to overall global phase; and k. passing the randomized phase encoded key through a faraday mirror (FM) (14a,

14b) for reflecting it to the PBS 1 (6), PBS 2 (8) and

1. measuring the signals received from the receiver sub-systems (Rxl, Rx2) using measurement unit.