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Contemporary Physics

Quantum information processing and precise optical measurement with entangled-photon pairs

А. В. Сергиенко, Gregg S. Jaeger

Published Jul 1, 2003• 39 citations
Publisher

Abstract

Two photons in a pair generated in the nonlinear optical process of spontaneous parametric down-conversion are, in general, strongly quantum entangled. Accordingly, they contain extremely strong energy, time, polarization and momentum quantum correlations. This entanglement involves more than one quantum variable and has served as a powerful tool in fundamental studies of quantum theory. It is now playing a large role in the development of novel information processing techniques and new optical measurement technologies. Here we review some of these technologies and their origins.

DOI
10.1080/0010751031000102711

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Two photons in a pair generated in the nonlinear optical process of spontaneous parametric down-conversion are, in general, strongly quantum entangled. Accordingly, they contain extremely strong energy, time, polarization and momentum quantum correlations. This entanglement involves more than one quantum variable and has served as a powerful tool in fundamental studies of quantum theory. It is now playing a large role in the development of novel information processing techniques and new optical measurement technologies. Here we review some of these technologies and their origins.
One of the most intriguing concepts of quantum mechanics is quantum entanglement. Two physical systems are said to be entangled with respect to a certain variable, if their individual outcomes of the variable are undetermined before measurement, but strictly correlated. Measurement of the variable for only one system immediately determines the outcome for the other system, irrespective of the distance between the two systems. It is this strong correlation that provides a basis for perspective, powerful applications like quantum cryptography and quantum computation. The most studied entangled systems, often called EPR systems (after the pioneers Einstein, Podolsky and Rosen), are pairs of light particles or photon pairs. These photon pairs, usually generated via the optical process of a spontaneous parametric down-conversion, are, in principal, simultaneously entangled in three variables, being polarization (polarization entanglement), energy or time (time entanglement), and momentum or transverse space (spatial entanglement). The contents of this thesis cover new insights in the polarization and spatial entanglement of photons. We have investigated both the entanglement quality and the yield of entangled photons for different experimental geometries that are associated with the production and detection of the photons.
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The parametric generation of quantum states of light holds a central position across various fields of quantum technologies. Starting from the seminal experiments in the 1970’s, we retrace the history of this powerful approach for creating and tailoring entangled photon pairs or squeezed states of light from the interaction of strong laser fields with optically nonlinear media. From its inception to its latest applications, this technique not only continues to play a pivotal role in the burgeoning fields of quantum computation, communication, and metrology, but shares also fundamental links with general concepts of quantum field theories in curved space-times.
Quantum states of light, such as single photons, pairs of entangled photons, and squeezed light are a key resource for photonic quantum technologies. All these states can be obtained via the nonlinear optical effect of spontaneous parametric down-conversion (SPDC). In SPDC, a higher-energy photon of a coherent beam (pump) splits in two lower-energy daughter photons, called signal and idler. The photons of such a pair can be entangled in polarization, time/frequency, and position/momentum. Efficient generation of photon pairs requires a high second-order susceptibilityχ(2)\chi^{(2)}χ(2)of the material where it occurs, and the phase matching between the pump and the down-converted photons.
Nonclassical photon sources are key components in quantum information science and technology. Here, the basic principles and progresses for single photon generation and their further manipulation based on second- or third-order nonlinear processes in various degrees of freedom are briefly reviewed and discussed. Based on spontaneous parametric down-conversion and spontaneous four-wave mixing, various nonlinear materials such as quasi-phase-matching crystals, dispersion-shifted fibers, and silicon-on-insulator waveguides are used for single photon generation. The kinds of entanglement generated include polarization, time-energy, time-bin, and orbital angular momentum. The key ingredient for photon pair generation in nonlinear processes is described and discussed. Besides, we also introduce quantum frequency conversion for converting a single photon from one wavelength to another wavelength, while keeping its quantum properties unchanged. Finally, we give a comprehensive conclusion and discussion about future perspectives for single photon generation and manipulation in nonlinear processes. This chapter will provide an overview about the status, current challenge, and future perspectives about single photon generation and processing in nonlinear processes.