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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Sending quantum bits with rotating photons

Songhao Liu, Changjun Liao

Published Sep 11, 2002• 0 citations
Publisher

Abstract

Linearly polarized photon can be encoded by phase modulation of half wavelets composing the photon to form rotating photon that is circularly polarized clockwise or counterclockwise. Quantum key distribution can be performed by the rotating photons. Theoretical analysis indicates that this new scheme of quantum key distribution system uses less optical components and therefore less insert loses. The rotating photon is insensitive to birefringence. The experimental procedure is also described.

DOI
10.1117/12.483022

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Linearly polarized photon can be encoded by phase modulation of half wavelets composing the photon to form rotating photon that is circularly polarized clockwise or counterclockwise. Quantum key distribution can be performed by the rotating photons. Theoretical analysis indicates that this new scheme of quantum key distribution system uses less optical components and therefore less insert loses. The rotating photon is insensitive to birefringence. The experimental procedure is also described.
The rapid increase on the information sharing around the world, leads to an utmost requirement for capacity and bandwidth. However, the need for security in the transmission and storage of information is also of major importance. The use of quantum technologies provides a practical solution for secure communications systems. Quantum key distribution (QKD) was the first practical application of quantum mechanics, and nowadays it is the most developed one. In order to share secret keys between two parties can be used several methods of encoding. Due to its simplicity, the encoding into polarization is one of the most used. However, when we use optical fibers as transmission channels, the polarization suffers random rotations that may change the state of polarization (SOP) of the light initially sent to the fiber to a new one at the output. Thus, in order to enable real-time communication using this encoding method it is required the use of a dynamic control system. We describe a scheme of transmission of quantum information, which is based in the polarization encoding, and that allows to share secret keys through optical fibers without interruption. The dynamic polarization control system used in such scheme is described, both theoretically and experimentally. Their advantages and limitations for the use in quantum communications are presented and discussed.
An analysis of the most critical issues in quantum key distribution systems with polarization encoded photons is presented. Effects like the fiber birefringence, fiber losses and some technical aspects can difficult the state of polarization control in such kind of systems, leading to an undesired quantum bit-error-rate increment. It is shown that a system with a long fiber, presenting a typical PMD value, only assures a strong correlation between two Stokes vectors for narrow wavelength separations. In terms of time correlation, it is shown that the typical drift times are much longer than delays between reference and data signals. The problematic of the reference pulse leakage to the data detector is also analyzed; a good agreement between experimental and theoretical data is observed.
We present two polarization-based protocols for quantum key distribution. The protocols encode key bits in noiseless subspaces or subsystems and so can function over a quantum channel subjected to an arbitrary degree of collective noise, as occurs, for instance, due to rotation of polarizations in an optical fiber. These protocols can be implemented using only entangled photon-pair sources, single-photon rotations, and single-photon detectors. Thus, our proposals offer practical and realistic alternatives to existing schemes for quantum key distribution over optical fibers without resorting to interferometry or two-way quantum communication, thereby circumventing, respectively, the need for high precision timing and the threat of Trojan horse attacks.
We propose a scheme for quantum key distribution, in which single photons are encoded with polarization states and then divided and delayed to encode phase differences between two sequential pulses. In an example, the BB84 and DPS (differential phase shift) protocols are combined to achieve single-photon two-qubit quantum key distribution. This scheme can increase the key generation efficiency up to 7/6, higher than that of the conventional single-photon one-qubit quantum key distribution scheme.
Polarization-multiplexed phase modulation was demonstrated in a Sagnac interferometer for polarization-auto-compensating quantum key distribution. The Sagnac interferometer was composed of a polarization beam splitter to split the signal pulses into clockwise and counter-clockwise beams of different polarizations, a Faraday rotator to rotate the polarizations of the split clockwise and counter-clockwise beams, and two phase-modulators asymmetrically set in the Sagnac ring. With this structure, the delays of the electronic triggering signals for the two phase-modulators could be set separately, reducing the difficulty in the setup and achieving more precise phase modulation on different polarized beams. Such polarization-multiplexed phase modulation was appropriately gated in sequential time gates to ensure the long-term stability of automatic polarization compensation in the long-distance quantum key distribution system. Polarization-multiplexed phase modulation was implemented in a protocol quantum key distribution system of 25-km communicating fiber length, generating 20-kHz sifted key rate with a qubit error rate lower than 5%.
This paper reports on the results of experimental investigations into the distribution of a quantum key on a setup designed for quantum cryptography with single photons. The quantum key is transmitted with pulsed semiconductor lasers by coding polarization states of photons in two alternative nonorthogonal bases. Silicon avalanche photodiodes (C30902S) serve as single-photon detectors. The rate of key generation is equal to 1.8 kbit/s when the clock frequency of laser pulse repetition is 100 kHz and the mean number of photons per pulse is approximately equal to 0.2. The number of errors in the key does not exceed 1%.