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Journal of Optics B Quantum and Semiclassical Optics

Single-mode squeezed vacuum states as approximate Schr dinger phase cats: relation toSU(1, 1) phase operators

Christopher C. Gerry, Adil Benmoussa, Kimberley M. Bruno

Published Feb 17, 2003• 4 citations
Publisher

Abstract

We study the eigenstates of the square of the phase operator for a single-mode field, and show that they are given as superpositions of macroscopically distinguishable phase states, which we call Schrödinger phase cats, the phase states being eigenstates of the phase operator. Those solutions that are also eigenstates of the parity operator with even parity are shown to be very similar to the squeezed vacuum states over a range of relevant parameters. Thus, it is possible to consider some squeezed vacuum states as approximate Schrödinger phase-cat states. We discuss the connections to the phase states of the SU (1, 1) phase operators for various realizations and representations.

DOI
10.1088/1464-4266/5/2/301

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We study the eigenstates of the square of the phase operator for a single-mode field, and show that they are given as superpositions of macroscopically distinguishable phase states, which we call Schrödinger phase cats, the phase states being eigenstates of the phase operator. Those solutions that are also eigenstates of the parity operator with even parity are shown to be very similar to the squeezed vacuum states over a range of relevant parameters. Thus, it is possible to consider some squeezed vacuum states as approximate Schrödinger phase-cat states. We discuss the connections to the phase states of the SU (1, 1) phase operators for various realizations and representations.
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Phase properties of states consisting of superpositions of the states mod a) and mod -a), coherent states of a single mode quantized electromagnetic field, are studied. These states are also known as macroscopic superposition states and 'Schrodinger cat' states. After first reviewing the squeezing and antibunching properties of these states, from the density operator and the phase states we derive a probability distribution for the phase. We also compare with this distributions obtained from the associated Q-function and the Wigner function.
We extend our previous study of squeezed two-component Schrödinger cat states to a phase-referenced single-mode family with an arbitrary number NNN of coherent components and general relative phases. Within the established circular-state construction, we derive a closed complex overlap kernel for the ordered action of displacement, phase rotation, and squeezing, retaining exact finite-amplitude normalization. We distinguish finite-displacement orthogonality from local phase-estimation sensitivity quantified by quantum Fisher information (QFI). For equal-phase states, we compare N=2,…,9N=2,\ldots,9N=2,…,9 at a common mean photon number and include coherent-state and squeezed-vacuum benchmarks. A bound on the higher photon-number sectors establishes the fixed-energy asymptote FQ,N=4nˉ(N−nˉ)+o(1)F_{Q,N}=4\bar n(N-\bar n)+o(1)FQ,N​=4nˉ(N−nˉ)+o(1). Its leading term agrees with the known QFI of a vacuum--NNN-photon superposition. The resulting decrease of the ideal local quantum Cramér--Rao bound does not establish unlimited attainable precision: prior phase information, sample size, phase-reference resources, optical loss, state preparation, and measurement choice remain relevant.
Two-mode entangled-state wavefunctions and with complex variables z and y are introduced and investigated. They are direct analogues of the wavefunctions and in the single-mode case or are linearly related to each other by two-dimensional Fourier transformation. The states and are eigenstates of two pairs of commuting operators ( Z , Z † ) (or ( Q + , P − )) and ( Y , Y † ) (or ( P + , Q − )) to eigenvalues ( z , z * ) and ( y , y * ), respectively, and are normalized by means of the two-dimensional delta function. The entangled states and are represented as two limiting cases of two-mode squeezed coherent states. Different representations of these states, in particular, the analogue of the Agarwal–Simon representation of squeezed vacuum states, are derived and the properties of these states are discussed. The transition from the single-mode to the two-mode case is made using the common property of the squeezing operators to belong to different realizations of the abstract SU (1,1) group. The Wigner quasiprobability in representation of the states and is discussed and is explicitly calculated for two-mode squeezed vacuum (and squeezed coherent) states.