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Journal of the Physical Society of Japan

Coherent Properties of the Two-Photon Coherent State

Yutaka Kano

Published May 1, 1981• 2 citations
Publisher

Abstract

Photon anti-bunching of the two-photon coherent state (TCS) is investigated under the variation of parameters. It appears that the effect of anti-bunching is less for stronger intensity of the radiation. Since two-photon laser has not been realized yet, detection or the TCS should be carried out for the radiation through matter in which as a result of interaction the TCS might be created, i.e., hybrid radiation containing the TCS. The second-order coherence of the hybrid radiation consisting of TCS and blackbody radiation is discussed. Numerical results are shown in several figures.

DOI
10.1143/jpsj.50.1631

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Photon anti-bunching of the two-photon coherent state (TCS) is investigated under the variation of parameters. It appears that the effect of anti-bunching is less for stronger intensity of the radiation. Since two-photon laser has not been realized yet, detection or the TCS should be carried out for the radiation through matter in which as a result of interaction the TCS might be created, i.e., hybrid radiation containing the TCS. The second-order coherence of the hybrid radiation consisting of TCS and blackbody radiation is discussed. Numerical results are shown in several figures.
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.
In Part I of this three-part study it was shown that the use of two-photon coherent state (TCS) radiation may yield siginificant performance gains in free-space optical communicatinn if the receiver makes a quantum measurement of a single field quadrature. In Part II it was shown that homodyne detection achieves the same signal-to-noise ratio as the quantum field quadrature measurement, thus providing a receiver which realizes the linear modulation TCS performance gain found in Part I. Furthermore, it was shown in Part il that ff homodyne detection does exactly correspond to the field quadrature measurement, then a large binary communication performance gain is afforded by homodyne detection of antipodal TCS signals. The full equivalence of honmdyne detection and single-quadrature field measurement, as well as that of heterodyne detection and two-quadrature field measurement, is established. Furthermore, a heterodyne configuration which uses a TCS image-band oscillator in addition to the usual coherent state local oscillator is studied. This coafiguration termed TCS heterodyne detection is shown to realize all the quantum measurements described by arbitrary TCS. The foregoing results are obtained by means of a representation theorem which shows that photoemissive detection realizes the photon flux density measurement.
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.
The statistic properties of photon of a light-atoms interaction system,and two atoms in Bell states interacting with two-mode entangled coherent field,were investigated by means of quantum theory and numerical method.The effect on the statistic properties of photon induced by the atomic initial state,mean photon numbers of field,the degree of the entanglement for the entangled coherent fields,and the coupling strength of dipole-dipole interaction between atoms were analyzed by numerical calculations.The results indicate that photon anti-bunching effect does not appear for the Bell state ｜β00〉 or ｜β10〉,however there can appear the photon anti-bunching effect under some conditions for the Bell state ｜β01〉 or ｜β11〉 appears.In the case,the photon anti-bunching effect depends on the mean photon numbers intensively.The influence of the degree of entanglement for the two-mode entangled coherent fields and the coupling strength of dipole-dipole interaction between atoms on the statistic properties of photon is small.
The concept of photon antibunching as a manifestation of nonclassical character in single-mode radiation is extended to two-mode radiation. A simple criterion for the existence of nonclassical effects in two-mode radiation is established. It implies that the intermode photon bunching, rather than antibunching, can play a key role in rendering two-mode radiation nonclassical. The two-mode squeezed vacuum states are used as simple examples to illustrate the essential point of the criterion. The criterion is then used to study the nonclassical properties of two types of two-mode squeezed states. A state generated by applying the squeeze operator to the two-mode vacuum first, followed by the displacement operator, is called a two-mode coherent squeezed state. If the order of the two operators is reversed, a two-mode squeezed coherent state is obtained. It is found that the latter has a much stronger tendency to become a nonclassical state than the former, assuming that the parameters involved are all the same. A measure of nonclassical depth of radiation is adopted. It is found that, according to this measure, the effect of squeezing in making two-mode coherent squeezed states nonclassical is not monotonic. Such phenomena do not occur in squeezed coherent states.