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CRC Press eBooks

Nonclassical Light

Horace P. Yuen

Published Aug 12, 2020• 0 citations
Publisher

Abstract

This chapter presents a synopsis on nonclassical light, emphasizes the roles of different quantum amplifiers, and discusses the problem of overcoming the detrimental effect of loss. The generation of two-photon coherent state (TCS) and near-number state (NNS) via nonlinear optical processes are, in some sense, surprisingly similar. This similarity is in addition to that between TCS and number-phase minimum uncertainty states when the nonclassical effects are small. It seems to arise from an underlying intrinsic quantum correlation. Nonclassical light do not propagate well. This is because significant loss, be it radiative, absorption, or otherwise, is often incurred during propagation. Nonclassical light with matching detection leads to significant improvement in signal-to-noise (SNR) ratio. Even in situations where SNR is not the most appropriate performance measure, it is clear that the low noise characteristics of TCS and NNS, or simply their difference from classical states, could be useful.

DOI
10.1201/9781003069539-1

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This chapter presents a synopsis on nonclassical light, emphasizes the roles of different quantum amplifiers, and discusses the problem of overcoming the detrimental effect of loss. The generation of two-photon coherent state (TCS) and near-number state (NNS) via nonlinear optical processes are, in some sense, surprisingly similar. This similarity is in addition to that between TCS and number-phase minimum uncertainty states when the nonclassical effects are small. It seems to arise from an underlying intrinsic quantum correlation. Nonclassical light do not propagate well. This is because significant loss, be it radiative, absorption, or otherwise, is often incurred during propagation. Nonclassical light with matching detection leads to significant improvement in signal-to-noise (SNR) ratio. Even in situations where SNR is not the most appropriate performance measure, it is clear that the low noise characteristics of TCS and NNS, or simply their difference from classical states, could be useful.
Recent theoretical work has shown that novel quantum states, called two-photon coherent states (TCS), have significant potential for improving free-space optical communications. The first part of a three-part study of the communication theory of TCS radiation is presented. The issues of quantum-field propagation and optimum quantum-state generation are addressed. In particular, the quantum analog of the classical paraxial diffraction theory for quasimonochromatic scalar waves is developed. This result, which describes the propagation of arbitrary quantum states as a boundary-value problem suitable for communication system analysis, is used to treat a number of quantum transmitter optimization problems. It is shown that, under near-field propagation conditions, a TCS transmitter maximizes field-measurement signal-to-noise ratio among all transmitter quantum states; the performance of the TCS system exceeds that for a conventional (coherent state) transmitter by a factor ofN_{s} + 1, whereN_{s}is the average number of signal photons (transmitter energy constraint). Under far-field propagation conditions, it is shown that use of a TCS local oscillator in the receiver can, in principle, attenuate field-measurement quantum noise by a factor equal to the diffraction loss of the channel, if appropriate spatial mode mixing can be achieved. These communcation results are derived by assuming that field-quadrature quantum measurement is performed. In part II of this study, photoemissive reception of TCS radiation will be considered; it will be shown therein that homodyne detection of TCS fields can realize the field-quadrature signal - to-noise ratio performance of part I. In part III, the relationships between photoemissive detection and general quantum measurements will be explored. In particular, a synthesis procedure will be obtained for realizing all the measurements described by arbitrary TCS.
In Part I of this three-part study, it was shown that novel quantum states, called two-photon coherent states (TCS), have significant potential for improving free-space optical communications. Because TCS radiation does not possess a classical analog, i.e., its diagonalP-representation is highly singular, the semiclassical conditional Poisson process model for direct detection is not applicable to TCS reception. In this paper, photoemissive detection of arbitrary quantized radiation fields is studied with incorporation of the nontrivial effects of detector quantum efficiency. General theorems are derived permitting the application of classical point process results to the detection and estimation of signals in arbitrary quantum states. These general theorems are applied to determining the performance of TCS optical communication systems that employ direct, heterodyne, or homodyne detection in binary decision as well as in linear modulation problems. It is shown that the use of TCS radiation with direct detection or heterodyne detection results in minimal performance increments over comparable coherent-state systems. Homodyne detection, however, can achieve the full TCS signal-to-noise ratio improvement predicted in Part I of this study. The increase in homodyne signal-to-noise ratio obtained by use of TCS radiation yields significant performance gains in both linear modulation and antipodal signal detection.
The performance of two-photon coherent-state (TCS) signals and integral-quantum signals in unimodal and bimodal free-space optical communications is compared with that of ordinary ,coherent signals under the constraint of fixed average error probability. For the unimodal channel of known phase the minimum attainable error probability for both on-off and antipodal TCS signals received in thermal noise is calculated by applying perturbation theory to the detection operator equation. For unimodal antipodal TCS signaling the threshold receiver is also considered. For channels of random phase the optimum photon-counting receiver is analyzed. In all cases the advantage of the nonclassical signal over an ordinary coherent signal vanishes as the transmittance of the channel goes to zero.
High quality light, whenever coherent light and nonclassical light, is more and more widely used in many filed.The coherent state behave minimum classical noise and better coherence, is mostly important light in quantum information, precise measurement and quantum metrology etc.In quantum information and quantum optics, a coherent light is used to entanglement generation (Ou, Z. et al. 1992), quantum teleportation (Furusawa, A. et al. 1998A.et al. ), quantum computation (Menicucci, N. et al. 2006) and quantum secret sharing (Lance, A. et al. 2004).The quiet laser beam, shot noise limited laser beam is important in above application.In the past 20 years there has been much interest in the generation and application of various nonclassical light sources, such as squeezed-state entanglement, quantum-correlated twin beams, and sub-poissonian light, for which improved signal-to-noise ratios and measurement precision beyond the quantum-noise limit are anticipated.In particular, subpoissonian light is a promising light source in quantum information and optical communication because of its communication capacity (
Nonclassical states of light may be generated by processes involving the creation or annihilation of photons in pairs. A quadratic coupling, characteristic of a parametric amplifier, generates a squeezed vacuum from a normal vacuum, and a two-photon absorber can also generate a squeezed state (though not a minimum-uncertainty state) even though it is a purely dissipative process. We consider here the simultaneous action of a quadratic pump on a two-photon absorber and demonstrate how superpositions of distinct coherent states may be generated by their combined effects. We use standard master equations to describe the time development, employing split operators and direct numerical integrations to determine the field density-matrix elements and quasiprobabilities. The purities of the nonclassical states are determined by evaluating the field entropy. Provided one-photon dissipative processes may be ignored, a pure superposition state is formed in the steady state. This superposition is destroyed if one-photon loss processes are important.