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arXiv:quant-ph/0412184 · Physical Review A

Characterization of the nonclassical nature of conditionally prepared single photons

Alfred B. U’Ren, Christine Silberhorn, Jonathan L. Ball, Konrad Banaszek, Ian A. Walmsley

Published Aug 25, 2005• 60 citations
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Abstract

A reliable single photon source is a prerequisite for linear optical quantum computation and for secure quantum key distribution. A criterion yielding a conclusive test of the single photon character of a given source, attainable with realistic detectors, is therefore highly desirable. In the context of heralded single photon sources, such a criterion should be sensitive to the effects of higher photon number contributions, and to vacuum introduced through optical losses, which tend to degrade source performance. In this Rapid Communication we present, theoretically and experimentally, a criterion meeting the above requirements.

DOI
10.1103/physreva.72.021802

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A reliable single photon source is a prerequisite for linear optical quantum computation and for secure quantum key distribution. A criterion yielding a conclusive test of the single photon character of a given source, attainable with realistic detectors, is therefore highly desirable. In the context of heralded single photon sources, such a criterion should be sensitive to the effects of higher photon number contributions, and to vacuum introduced through optical losses, which tend to degrade source performance. In this Rapid Communication we present, theoretically and experimentally, a criterion meeting the above requirements.
The reliable generation of true single-photon wavepackets with well-defined modal structure is a crucial ingredient for the realization of linear optical quantum computing. In this paper we present experimental results indicating the efficient generation of conditionally prepared single photons from parametric downconversion in a KTP nonlinear waveguide. In addition we present, theoretically and experimentally, a novel criterion for the assessment of conditional single photon sources which takes into account the contributions of heralded vacuum due to optical losses, of higher photon numbers and of the binary response of typical single photon detectors. Utilizing this criterion, we show that our KTP nonlinear waveguide represents a high fidelity source of conditionally prepared single photons.
Abstract Single photon sources are necessary for optical quantum technology. The nonclassicality of emitted single photons manifests itself in diverse experiments and applications. The applications already require different nonclassical aspects of single photons, however, a suitable hierarchy of criteria is missing. We propose variable experimental tests, based on adjustable linear optical networks and single photon detectors, giving a hierarchy of such nonclassicality criteria. The hierarchy goes beyond Hanbury–Brown–Twiss test of photon antibunching and allows us to compare faithfully quality and performance of single photon sources via their nonclassical properties.
The creation of specified quantum states is important for most, if not all, applications in quantum computation and communication. The quality of the state preparation is therefore an essential ingredient in any assessment of a quantum-state gun. We show that the fidelity, under the standard definitions, is not sufficient to assess quantum sources, and we propose a measure of suitability that necessarily depends on the application for the source. We consider the performance of single-photon guns in the context of quantum key distribution (QKD) and linear optical quantum computation. Single-photon sources for QKD need radically different properties than sources for quantum computing. Furthermore, the suitability for single-photon guns is discussed explicitly in terms of experimentally accessible criteria.
This review surveys the physical principles and recent developments in manufacturing single-photon sources. Special emphasis is placed on important potential applications such as linear optical quantum computing (LOQC), quantum key distribution (QKD) and quantum metrology that drive the development of these sources of single photons. We discuss the quantum-mechanical properties of light prepared in a quantum state of definite photon number and compare it with coherent light that shows a Poissonian distribution of photon numbers. We examine how the single-photon fidelity directly influences the ability to transmit secure quantum bits over a predefined distance. The theoretical description of modified spontaneous decay, the main principle behind single-photon generation, provides the background for many experimental implementations such as those using microresonators or pillar microcavities. The main alternative way to generate single photons using postselection of entangled photon pairs from parametric down-conversion, will be discussed. We concentrate on describing the underlying physical principles and we will point out limitations and open problems associated with single-photon production.
The reliable generation of true single‐photon wavepackets with well‐defined modal structure is a crucial ingredient for the realization of linear optical quantum computing. We have implemented a highly efficient waveguided parametric downconversion source pumped by ultrashort pulses, which is characterized by the spatial decoupling of the signal and idler photon while exhibiting a conditional detection efficiency of 51%, corresponding to a preparation efficiency of 85%. We have performed various tests to confirm the single‐photon character of the conditionally prepared state.