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Physical Review A

Simultaneous fourth-order squeezing of both quadrature components

Robert Lynch

Published Apr 1, 1994• 30 citations
Publisher

Abstract

It is shown that a particular combination of so-called even coherent state dubbed the orthogonal-even coherent state, while exhibiting no ``ordinary'' or second-order squeezing, can furnish near-optimal simultaneous-quadrature fourth-order squeezing [where the latter term is defined in the sense of Hong and Mandel, Phys. Rev. Lett. 54, 323 (1985)]. The Wigner function of the particular quantum-mechanical superposition of states is calculated and various plots compared to the Wigner function of a simple statistical mixture of the same states. Rather uninteresting behavior is found in the latter case, which illustrates that the higher-order squeezing effect is due to nonclassical, quantum-mechanical ``interference in phase space.''

DOI
10.1103/physreva.49.2800

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Physical Review A•Aug 1, 1992•38 citations
It is shown that a particular combination of so-called even coherent state dubbed the orthogonal-even coherent state, while exhibiting no ordinary'' or second-order squeezing, can furnish near-optimal simultaneous-quadrature fourth-order squeezing [where the latter term is defined in the sense of Hong and Mandel, Phys. Rev. Lett. 54, 323 (1985)]. The Wigner function of the particular quantum-mechanical superposition of states is calculated and various plots compared to the Wigner function of a simple statistical mixture of the same states. Rather uninteresting behavior is found in the latter case, which illustrates that the higher-order squeezing effect is due to nonclassical, quantum-mechanical interference in phase space.''
In a previous Comment on recent papers by Hong and Mandel, I was able to show that the ``Jackiw state,'' which is infinitesimally different from the coherent state, displays fourth-order squeezing (as defined by Hong and Mandel's criterion) of both quadrature field components; the magnitude of such squeezing is, however, infinitesimal. In this paper an explicit perturbation calculation is given which results in a state with finite fourth-order squeezing of both quadrature components.
In this paper we discuss the nonclassical properties of quantum superpositions of coherent states of light. Using general expressions for the Wigner functions of superposition states we analyze the consequences of quantum interference between coherent states. We describe in detail nonclassical properties of a superposition of two coherent states. In particular, we study the oscillatory behavior of the photon number distribution of the even and odd coherent states. We show under which conditions a superposition of two coherent states can exhibit second- and fourth-order squeezing or sub-Poissonian photon statistics. We examine the sensitivity of nonclassical effects such as oscillations in the photon number distribution or second-order squeezing to dissipation. We demonstrate that quantities such as the photon number distribution and interferences in phase space are highly sensitive to even a quite small dissipative coupling, because they depend on all moments of the field observables, and higher moments decay more rapidly than lower moments. Quantities such as quadrature squeezing, on the other hand, are more robust against dissipation because they involve only lower moments. Finally, we find a remarkable effect whereby fourth-order squeezing is generated by damping.
We define and study the properties of ``squeezed quantum multiplets''. Ordinary multiplets are sets of DDD-orthonormal quantum states formed by superpositions of states squeezed along DDD equally spaced directions in quadrature space. More generally, we also discuss superpositions of ``higher-order squeezed states'', including tri-squeezed and quad-squeezed states. All these states involve superpositions of multiples of ppp photons. We compare states in ordinary (p=2p=2p=2) multiplets and higher-order ones (p>2p>2p>2) in the most relevant cases, showing that ordinary squeezed multiplets and higher-order ones share some important similarities, as well as some differences. Finally, we present analytical expressions for phase-space distributions (Wigner and characteristic functions) representing ordinary squeezed multiplets. We use this to show that some squeezed multiplets are highly sensitive to perturbations in all phase-space directions, making them interesting for metrological applications.
Hong and Mandel have recently suggested a scheme which allows generalization of ``squeezing'' to higher-order (even) moments of the electromagnetic field. The new criterion for squeezing is that the expectation value of the moment be less than that for the coherent state. In this paper it is explicitly shown that both electric-field quadrature components are squeezed to fourth order in a ``Jackiw state,'' thus proving that the coherent state does not minimize the fourth-order uncertainty product. The implications of this result for Hong and Mandel's proposal are briefly considered.
An investigation is carried out of higher-order squeezing in a class of three- and four-wave mixing processes that can be modeled by a master equation. In (2N)th-order squeezing, as defined by Hong and Mandel, the Nth-power of the variance of a field quadrature falls below the value it would have for a coherent state. Higher-order squeezing is of interest because (i) it occurs in many processes in which ordinary (or second-order) squeezing occurs, and (ii) it allows a much larger fractional noise reduction to be achieved than ordinary squeezing. The master equation we use models a degenerate three- or four-wave process and takes into account the losses in the signal mode. We convert the master equation into a Fokker-Planck equation for the Wigner distribution function and solve it for a wide range of initial signal states (including thermal, coherent, Fock states, and mixtures of these). Results are presented for the enhancement of fourth- and sixth-order squeezing over second-order squeezing. Differences in the character of higher-order squeezing for different input states are pointed out. The role of losses in limiting the amount of achievable squeezing in all orders is especially investigated.