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Symmetric Autocompensating Quantum Key Distribution

Zachary Walton, Alexander V. Sergienko, Lev B. Levitin, Bahaa E. A. Saleh, Teich, M C

Published Sep 4, 2003• 1 citation
OpenAlex

Abstract

We present quantum key distribution schemes which are autocompensating (require no alignment) and symmetric (Alice and Bob receive photons from a central source) for both polarization and time-bin qubits. The primary benefit of the symmetric configuration is that both Alice and Bob may have passive setups (neither Alice nor Bob is required to make active changes for each run of the protocol). We show that both the polarization and the time-bin schemes may be implemented with existing technology. The new schemes are related to previously described schemes by the concept of advanced waves.

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Journal of the Optical Society of America B•Sep 16, 2014•3 citations
We present quantum key distribution schemes which are autocompensating (require no alignment) and symmetric (Alice and Bob receive photons from a central source) for both polarization and time-bin qubits. The primary benefit of the symmetric configuration is that both Alice and Bob may have passive setups (neither Alice nor Bob is required to make active changes for each run of the protocol). We show that both the polarization and the time-bin schemes may be implemented with existing technology. The new schemes are related to previously described schemes by the concept of advanced waves.
We present quantum key distribution schemes which are autocompensating (require no alignment) and symmetric (Alice and Bob receive photons from a central source) for both polarization and time-bin qubits. The primary benefit of the symmetric configuration is that both Alice and Bob may have passive setups (neither Alice nor Bob is required to make active changes for each run of the protocol). We show that both the polarization and the time-bin schemes may be implemented with existing technology. The new schemes are related to previously described schemes by the concept of advanced waves.
Quantum cryptographic key distribution (QKD) uses extremely faint light pulses to carry quantum information between two parties (Alice and Bob), allowing them to generate a shared, secret cryptographic key. Autocompensating QKD systems automatically and passively compensate for uncontrolled time-dependent variations of the optical fibre properties by coding the information as a differential phase between orthogonally polarized components of a light pulse sent on a round trip through the fibre, reflected at mid-course using a Faraday mirror. We have built a prototype system based on standard telecom technology that achieves a privacy-amplified bit generation rate of ~1000 bits s -1 over a 10 km optical fibre link. Quantum cryptography is an example of an application that, by using quantum states of individual particles to represent information, accomplishes a practical task that is impossible using classical means.
In so-called private-key cryptographic systems, secure transmission of confi dential information through unprotected channels relies on the exchange of secret keys, known only to the sender, Alice, and the receiver, Bob.Quantum cryptography is based on the properties of quantum mechanics to obtain a provably secure key distribution.Most existing implement ations rely on either the polarization or the phase of very weak pulses of light sent through optical fibers.Recent results by research groups in British Telecom, Los Alamos National Laboratory and at the University of Geneva, have shown that quantum key distribution is possible over distances of tens of kilometers, using only standard telecom cables.However, due to birefringence and the effects of the environment, the polarization at the output of a fiber fluctuates randomly.Therefore, all existing systems need active polarization control.Systems based on phase also require interferometric detection, which necessitates active stabilization of the interferometers.To eliminate these problems, our team, led by Prof. N. Gisin in the Group of Applied Physics at the University of Geneva, is currently developing a new interferometric system implementing phase-encoded quantum key distribution (US patent pending).A schematic of the setup is given in Fig. 1.Its main features are:1) The system is based on time multiplexing, the interfering pulses following exactly the same spatial path, albeit with a small time delay.The interferometer is thus automatically aligned, and needs neither adjustments nor stabilization of the path lengths.2) Use of Faraday mirrors enables all birefringence effects in the fibers to be cancelled out.Therefore, the system does not require any polarization control.With this new system, Alice and Bob could exchange their cryptographic keys through standard telecom systems.They would be provided with a sending kit and
Quantum key distribution allows for a provably secure transmission of cryptographic keys over an optical channel. Encoded polarization states or time-bin degree of freedom have been used for successful demonstrations. However, photon losses in long fibers, slow single photon detectors, and detector dark counts significantly limit the overall bit rate. Improving key throughput and reducing the overhead of key reconciliation remain as major challenges. Methods which utilize multiple time bins allow for multiple key bits to be encoded in a single photon, thus increasing the fidelity of transmitted keys and decreasing the overhead of key reconciliation in real-world conditions. Previous implementations of these methods required that Alice and Bob share a time reference by sharing a dedicated classical channel used for synchronization. This work presents a technique that allows two parties to exchange time-bin encoded photons without the need for synchronized time references. Our technique uses a framing protocol which allows Alice to encode a time reference along with a key which is determined by Alice before transmission. Security can be achieved by monitoring the visibility of a pair of Franson interferometers, using decoy pulses and measuring the round trip time between Alice and Bob. The bit rate of this technique is limited only by the recovery time of the detector and the speed of the modulation electronics. We experimentally demonstrate a raw bit rate of 5Mb/s over an optical channel with 55dB of loss, which is competitive with current research. We also demonstrate absolute timing synchronization with an accuracy of 20ps.
We propose an alignment-free photonic qubit transmission scheme and apply it in quantum key distribution schemes, in which the authorized parties do not have to calibrate their Cartesian reference frames in advance. The polarization states are labeled by time, and the receiver can obtain the original state in his own coordinate by postselecting a determinate time of arrival. The scheme does not resort to decoherence-free subspace based on multiple photon states, and it can be implemented with only passive linear optics, which make it efficient and practical with current technology. We demonstrate its application in quantum key distributions, and the scheme should find applications in long-distance quantum communication without frame alignment.