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Optical Fiber Communication Conference

Quantum physics in optical communication systems

Colin J. McKinstrie

Published Jan 1, 2011• 0 citations
Publisher

Abstract

The quantum properties of optical devices are important • In conventional optical communication systems, quantum effects are responsible for noise inherent in the si (by amplifiers and attenuators) and noise in the detectors. gnals, noise added during transmission • Noise obscures the signals and lowers the information capacities. • There are many schemes for quantum communication and computation that rely on the quantum properties of light. • • Examples include quantum key distribution (QKD) and linear optical quantum computation (LOQC). • Common requirements for quantum information experiments include photon generators (singlet or pair) and (distortionless) photon frequency-convertors.

DOI
10.1364/ofc.2011.owx1

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The quantum properties of optical devices are important • In conventional optical communication systems, quantum effects are responsible for noise inherent in the si (by amplifiers and attenuators) and noise in the detectors. gnals, noise added during transmission • Noise obscures the signals and lowers the information capacities. • There are many schemes for quantum communication and computation that rely on the quantum properties of light. • • Examples include quantum key distribution (QKD) and linear optical quantum computation (LOQC). • Common requirements for quantum information experiments include photon generators (singlet or pair) and (distortionless) photon frequency-convertors.
Quantum communications is fast becoming an important component of many applications in quantum information science. Sharing quantum information over a distance among geographically separated nodes using photonic qubits requires a reconfigurable transparent networking infrastructure that can support quantum information services. Using quantum key distribution (QKD) as an example of a quantum communications service, we investigate the ability of fiber networks to support both conventional optical traffic and single-photon quantum communications signals on a shared infrastructure. The effect of Raman scattering from conventional channels on the quantum bit error rate (QBER) of a QKD system is analyzed. Additionally, the potential impact and mitigation strategies of other transmission impairments such as four-wave mixing, cross-phase modulation, and noise from mid-span optical amplifiers are discussed. We also review recent trends toward the development of automated and integrated QKD systems which are important steps toward reliable and manufacturable quantum communications systems.
New methods in optical communications rely on the quantum properties of photons to assure link security and data privacy. However, optical quantum communications make several assumptions, among them that the quantum properties of photons are preserved over many-kilometer long continuous fiber, which additionally is free from absorbing and scattering centers. In this paper we review quantum communication methods based on photon polarization, single and entangled, and we investigate the impact of noise sources on quantum signal propagation.
Photon pair sources are an important building block for optics based Quantum Information Processing (QIP) such as Quantum Key Distribution (QKD) and Linear Optics Quantum Computing (LOQC) [1]. Beyond the demand for reliable and cheap photon pair sources for QKD, LOQC also requires interferometers that are mechanically stable and loss-free for which integrated photonics is particularly attractive [2]. In this context, photon pair generation (PPG) has been demonstrated via the third order nonlinear interaction between a silicon straight waveguide (Si-w) and a pulsed beam [3]. Such a source cannot be directly followed by other integrated components as photon pairs would be generated throughout the silicon chip. The solution we propose to suppress the pump beam consists of a Sagnac Loop Interferometer (SLI) in which photon pairs are generated and pump beam destructively interferes with itself, thereby allowing photon pairs to be generated in a localised part of the silicon chip. Furthermore, our study has been performed in a continuous instead of a pulsed regime, thereby avoiding any synchronization between detectors and laser and allowing for much cheaper laser sources. Our experimental setup is presented in Fig. 1. The experiment is performed at different pump power both in the a Si-w and the SLI. Results are presented in Fig. 2. The flux of generated pairs and the Signal-to-noise ratio are limited by the outcoupling loss and detector inefficiency. The photon flux is less than expected because the emitted spectrum is around 8nm (FWHM) broad instead of the expected 70nm. The performance of the SLI is not yet perfect: the pump suppression efficiency decreases with the pump power (from −22dB to −15dB) due to unexpected nonlinear behaviour in the directional coupler that changes the coupling ratio; and the Signal-to-Noise ratio is lower than in the Si-w which might be due to PPG in the broad (800nm) waveguide before the SLI.
In a previous paper [Opt. Express 13, 4986 (2005)], formulas were derived for the field-quadrature and photon-number variances produced by multiple-mode parametric processes. In this paper, formulas are derived for the quadrature and number correlations. The number formulas are used to analyze the properties of basic devices, such as two-mode amplifiers, attenuators and frequency convertors, and composite systems made from these devices, such as cascaded parametric amplifiers and communication links. Amplifiers generate idlers that are correlated with the amplified signals, or correlate pre-existing pairs of modes, whereas attenuators decorrelate pre-existing modes. Both types of device modify the signal-to-noise ratios (SNRs) of the modes on which they act. Amplifiers decrease or increase the mode SNRs, depending on whether they are operated in phase-insensitive (PI) or phase-sensitive (PS) manners, respectively, whereas attenuators always decrease these SNRs. Two-mode PS links are sequences of transmission fibers (attenuators) followed by two-mode PS amplifiers. Not only do these PS links have noise figures that are 6-dB lower than those of the corresponding PI links, they also produce idlers that are (almost) completely correlated with the signals. By detecting the signals and idlers, one can eliminate the effects of electronic noise in the detectors.
The natural choice of physical qubit for quantum communication is the photon: a photon can be transmitted quickly from the sender to a distant receiver and the technology for creating, manipulating, distributing, and measuring light pulses is well established. Many of these classical techniques can also be employed for quantum communication protocols. We will therefore focus our attention on optical setups for quantum communication. We will study a number of optical setups in the next few sections and use the conventions introduced in Chapter 4 to analyze them. In particular, we will assume that the qubits are encoded in the degrees of freedom of a single photon. Some of the main technical challenges in quantum communication arise from this need to work with single photon pulses. For instance, photons do not interact with each other in vacuum or linear media, and interactions between two photons in non-linear optical media are relatively small, so that realizing entangling two photon gates via coherent interactions is a challenging task. The quantum communication schemes discussed here circumvent this technical problem to a large extent. They only use non-linear optical materials for parametric down-conversion to create Bell-pairs of photons and then exploit standard linear optical devices and photo-detectors to manipulate them. No further non-linear entangling gates are required. Parametric down-conversion In non-linear optical media a single photon can be down-converted into a pair of photons. In this coherent process the incoming photon is destroyed and two photons of lower energy are created.