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Annual Meeting Optical Society of America

Communicating with quantum interference

John Rarity, P. R. Tapster

Published Jan 1, 1991• 0 citations
Publisher

Abstract

In recent experiments1 the phenomenon of two-photon interference has been studied, and apparent nonlocal interference effects have been convincingly demonstrated.2 In all these experiments interference effects are not manifest in the singles count rates but appear in the coincidence rate or rate of detection of pair events and are a result of entanglement of the pair photon wavefunction. Modulation of the coincidence rate provides the basis of various covert communications schemes. We have previously demonstrated a free-space propagation communication system using a classical time shift coding out to ranges of 200 m using light levels 1/1000th of ambient daylight levels. This has been extended to a fiber (200-m) based system that exploits nonlocal interference using a scheme first proposed by Franson? With high visibility interference effects single photons can be used to code individual bits of information leading to extremely secure communication.

DOI
10.1364/oam.1991.mo2

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In recent experiments1 the phenomenon of two-photon interference has been studied, and apparent nonlocal interference effects have been convincingly demonstrated.2 In all these experiments interference effects are not manifest in the singles count rates but appear in the coincidence rate or rate of detection of pair events and are a result of entanglement of the pair photon wavefunction. Modulation of the coincidence rate provides the basis of various covert communications schemes. We have previously demonstrated a free-space propagation communication system using a classical time shift coding out to ranges of 200 m using light levels 1/1000th of ambient daylight levels. This has been extended to a fiber (200-m) based system that exploits nonlocal interference using a scheme first proposed by Franson? With high visibility interference effects single photons can be used to code individual bits of information leading to extremely secure communication.
Although we tend to think of optical interference as a classical wave phenomenon, recent experiments have revealed a number of effects that are not describable in classical terms. This is particularly true of interference effects involving the detection of a photon pair. We shall refer to them as fourth order interference, on the grounds that the joint probability density for the detection of one photon at r sub 1 at time t and another r sub 2 at time t is proportional to the fourth order correlation function of the field. This probability is readily measured when two photodetectors are positioned at r sub 1 and r sub 2 and the signals from the two detectors are fed to a coincidence counter that registers 'simultaneous' detections by the two detectors in coincidence. The topics covered include: fourth order interference measurements; the Franson experiment; and experimental test of the de Broglie guided wave theory.
We demonstrate the suitability of fiber-generated entangled photon pairs for practical quantum communications in the telecom band by measuring their properties with superconducting single-photon detectors that produce negligible dark counts. The photon pairs are created in approximately 5-ps duration windows at 50 MHz rate while the detectors are operated in ungated free running mode. We obtain a coincidence to accidental-coincidence ratio >80 with raw photon-counting data, i.e., without making any post-measurement corrections. Using a previously demonstrated counter-propagating scheme we also produce polarization-entangled photon pairs at 50-MHz rate, which in coincidence detection directly yield two-photon interference with a fringe visibility >98%.
Quantum communication and quantum computation are novel methods of information transfer and information processing, all fundamentally based on the principles of quantum physics. The performances outdo their classical counterparts in many aspects [1,2]. In almost all quantum communication and quantum computation schemes, quantum entanglement [3] plays a decisive role. In essence, an entangled system can carry all information (e.g., on their polarization properties) only in their correlations, while no individual subsystem carries any information. This leads to correlations that are much stronger than classically allowed [89, 100], which is a powerful resource for information processing. It is therefore important to be able to generate, manipulate, and distribute entanglement as accurately and as efficiently as possible.
We present an experimental demonstration of a fourth-order interference effect first suggested by Franson [Phys. Rev. Lett. 62, 2205 (1989)], where pair photons are directed to two separated Mach-Zehnder interferometers. Apparent nonlocal control of the output ports in which coincident photons are detected can be demonstrated. We show that this effect is present when one of the photons has passed through 170 m of multimode optical fiber. Such a system could form the basis of an interception-proof communication system.
Conventional quantum key distribution (QKD) typically uses binary encoding based on photon polarization or time-bin degrees of freedom and achieves a key capacity of at most one bit per photon. Under photon-starved conditions the rate of detection events is much lower than the photon generation rate, because of losses in long distance propagation and the relatively long recovery times of available single-photon detectors. Multi-bit encoding in the photon arrival times can be beneficial in such photon-starved situations. Recent security proofs indicate high-dimensional encoding in the photon arrival times is robust and can be implemented to yield high secure throughput. In this work we demonstrate entanglement-based QKD with high-dimensional encoding whose security against collective Gaussian attacks is provided by a high-visibility Franson interferometer. We achieve unprecedented key capacity and throughput for an entanglement-based QKD system because of four principal factors: Franson interferometry that does not degrade with loss; error correction coding that can tolerate high error rates; optimized time–energy entanglement generation; and highly efficient WSi superconducting nanowire single-photon detectors. The secure key capacity yields as much as 8.7 bits per coincidence. When optimized for throughput we observe a secure key rate of 2.7 Mbit s −1 after 20 km fiber transmission with a key capacity of 6.9 bits per photon coincidence. Our results demonstrate a viable approach to high-rate QKD using practical photonic entanglement and single-photon detection technologies.