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Fortschritte der Physik

Quantum Interference between Macroscopically Distinct Optical States

F. T. Arecchi, A. Montina

Published May 1, 2000• 1 citation
Publisher

Abstract

Thus far, all proposals for an optical evidence of macroscopic quantum interference have been frustrated by the low detector efficiencies. Two innovations permit such an evidence with currently available equipment. The availability of optical superposition states with large separation suggests a practical way of transforming them into entangled coherent states associated with different paths, thus providing a new convenient source for a “which path” experiment.

DOI
10.1002/(sici)1521-3978(200005)48:5/7<423::aid-prop423>3.0.co;2-#

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Thus far, all proposals for an optical evidence of macroscopic quantum interference have been frustrated by the low detector efficiencies. Two innovations permit such an evidence with currently available equipment. The availability of optical superposition states with large separation suggests a practical way of transforming them into entangled coherent states associated with different paths, thus providing a new convenient source for a “which path” experiment.
Thus far, all proposals for an optical evidence of macroscopic quantum interference have been frustrated by the low detector efficiencies. Two innovations permit such an evidence with currently available equipment. We refer to the generation scheme of entangled states by parametric amplification proposed by S. Song, C. M. Caves, and B. Yurke [Phys. Rev. A 41, 5261 (1990)]. Setting a low gain for the parametric amplifier introduces a drastic cutoff in the generated photon-number probabilities that compensates for the broadening of the detection probability in the first detector. Furthermore, we supplement the final homodyne detector with a degenerate type-I parametric amplifier which brings nearer the two previously distant states of the superposition. The availability of optical superposition states with large separation suggests a practical way of transforming them into entangled coherent states associated with different paths, thus providing a new convenient source for a ``which path'' experiment.
We discuss several methods to produce superpositions of optical coherent states (also known as “cat states”). Cat states have remarkable properties that could allow them to be powerful tools for quantum information processing and metrology. A number of proposals for how one can produce cat states have appeared in the literature in recent years. We describe these proposals and present a new simulation and analysis of them incorporating practical issues such as photon loss, detector inefficiency, and limited strength of nonlinear interactions. We also examine how each would perform in a realistic experiment.
Quantum state exchange between light and matter is an important ingredient for future quantum information networks as well as other applications. Photons are the fastest and simplest carriers of information for transmission but in general, it is difficult to localize and store photons, so usually one prefers choosing matter as quantum memory elements. Macroscopic superposition and nonlocal quantum interactions have received considerable interest for this purpose over recent years in fields ranging from quantum computers to cryptography, in addition to providing major insights into physical laws. However, these experiments are generally performed either with equipment or under conditions that are unrealistic for practical applications. Ideally, the two can be combined using conventional equipment and conditions to generate a "quantum teleportation"-like state, particularly with a very small amount of purity existing in an overall highly mixed thermal state (relatively low decoherence at high temperatures). In this study we used an experimental design to demonstrate these principles. We performed optical coherence tomography (OCT) using a thermal source at room temperatures of a specifically designed target in the sample arm. Here, position uncertainty (i.e., dispersion) was induced in the reference arm. In the sample arm (target) we placed two glass plates separated by a different medium while altering position uncertainty in the reference arm. This resulted in a chirped signal between the glass plate reflective surfaces in the combined interferogram. The chirping frequency, as measured by the fast Fourier transform (FFT), varies with the medium between the plates, which is a nonclassical phenomenon. These results are statistically significant and occur from a superposition between the glass surface and the medium with increasing position uncertainty, a true quantum-mechanical phenomenon produced by photon pressure from two-photon interference. The differences in chirping frequency with medium disappears when second-order correlations are removed by dual balanced detection, confirming the proposed mechanism. We demonstrated that increasing position uncertainty at one site leads to position uncertainty (quantum position probability amplitude) nonlocally via second-order correlations (two-photon probability amplitude) from a low coherence thermal source (low purity, high local entropy). The implications, first, are that the phenomenon cannot be explained through classical mechanisms but can be explained within the context of quantum mechanics, particularly relevant to the second-order correlations where controversy exists. More specifically, we provide the theoretical framework that these results indicate a nonlocal macroscopic superposition is occurring through a two-photon probability amplitude-induced increase in the target position probability amplitude uncertainty. In addition, as the experiments were performed with a classical source at room temperature, it supports both the quantum-mechanical properties of second-order correlations and that macroscopic superposition is obtainable in a target not in a single coherent state (mixed state). Future work will focus on generalizing the observations outside the current experimental design and creating embodiments that allow practical application of the phenomenon.