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arXiv:1706.03136 · Journal of Modern Optics

Verifying cross-Kerr induced number squeezing: a case study

David Schmid, Kevin Marshall, Daniel F. V. James

Published Aug 4, 2017• 4 citations
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Abstract

We analyse an experimental method for creating interesting nonclassical states by processing the entanglement generated when two large coherent states interact in a cross-Kerr medium. We specifically investigate the effects of loss and noise in every mode of the experiment, as well as the effect of ‘binning’ the post-selection outcomes. Even with these imperfections, we find an optimal set of currently achievable parameters which would allow a proof-of-principle demonstration of number squeezing in states with large mean photon number. We discuss other useful states which can be generated with the same experimental tools, including a class of states which contain coherent superpositions of differing photon numbers, e.g. good approximations to the state 12(|0⟩+|20⟩). Finally, we suggest one possible application of this state in the field of optomechanics.

DOI
10.1080/09500340.2017.1357852

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We analyse an experimental method for creating interesting nonclassical states by processing the entanglement generated when two large coherent states interact in a cross-Kerr medium. We specifically investigate the effects of loss and noise in every mode of the experiment, as well as the effect of ‘binning’ the post-selection outcomes. Even with these imperfections, we find an optimal set of currently achievable parameters which would allow a proof-of-principle demonstration of number squeezing in states with large mean photon number. We discuss other useful states which can be generated with the same experimental tools, including a class of states which contain coherent superpositions of differing photon numbers, e.g. good approximations to the state 12(|0⟩+|20⟩). Finally, we suggest one possible application of this state in the field of optomechanics.
We construct a distinct class of nonlinear displaced Kerr states by application of the displacement operator upon a state which is prepared by sending the well-known photon-added coherent state through a normal Kerr medium. A sketch for the experimental setup for preparing the state is suggested. We evaluate some statistical properties such as the photon number distribution, Mandel’s Q parameter, Husimi-Q and Wigner functions, and quadrature squeezing for the nonlinear displaced Kerr state and then analyze the nonclassicality in terms of these standard parameters. We reduce the infinite-level problem to a truncated discrete two-level system by using a low Kerr parameter approximation and then convert the generated nonclassicality into bipartite entanglement between the two modes of an output state of a linear optical device.
We propose two experimental schemes for producing coherent-state superpositions which approximate different nonclassical states conditionally in traveling optical fields. Although these setups are constructed of a small number of linear optical elements and homodyne measurements, they can be used to generate various photon number superpositions in which the number of constituent states can be higher than the number of measurements in the schemes. We determine numerically the parameters to achieve maximal fidelity of the preparation for a large variety of nonclassical states, such as amplitude squeezed states, binomial states, squeezed cat states, and various photon number superpositions. The proposed setups can generate these states with high fidelities and with success probabilities that can be sufficient for practical applications.
We consider the classical and quantum-mechanical processes of three-wave interactions in different phase regimes and present numerical calculations for the quantum case, where all three modes are sizably excited from the beginning. These excitations are coherent so that various important phase regimes can be adjusted. In addition, one mode can also be prepared in a squeezed or Kerr state. The classical solutions are well known and are briefly summarized, but certain phase regimes are classically unexplored and we show here that they give interesting and surprising results. In the out-of-phase regime (where the photon numbers do not change in the first order of time) we get, with an initial Kerr state, strongly sub-Poissonian photon statistics in the signal after a short interaction time. This effect is limited by the classically described phase shifts that are present even in the parametric approximation. This nonclassical phenomenon (due to the Kerr state) helps us to understand similar nonclassical effects generated by entangled states of the pump and signal during sum-frequency generation. \textcopyright{} 1996 The American Physical Society.
As a subset of quantum optics, single photons are one of the competing physical resources for quantum information processing. When used as carriers of quantum information, they have no equal. For the processing of quantum information, single photons have proven difficult to scale beyond the order of ~10 photons. The lack of single-photon-level interaction has led to creative approaches which rely on post-selection to filter the possible measured outcomes to those which appear as though interaction occurred. This approach of post-selection leans heavily on the ability to not only generate but also detect scores of single photons simultaneously and with near perfect efficiency. Our work relaxes the emphasis which has been placed on single photons for quantum information processing to that of states with, in principle, an arbitrary number of photons. Central moment expectations on two-mode squeezed states are shown to exhibit post-selection behavior which reflects the single-photon counterpart. These measures are proven to be robust to loss and return entangled state statistics on average. With naive estimation of the central moment, states with ~20 modes are within reach with current technology, closing the gap between quantum states which can and cannot be classically simulated.
Quantum mechanics allows us to use nonclassical states of light to make measurements with a greater precision than comparable classical states. Here an experiment is presented that squeezes the polarization state of three photons. We demonstrate the deep connection that exists between squeezing and entanglement, unifying the squeezed state and multi-photon entangled state approaches to quantum metrology. For the first time we observe the phenomenon of over-squeezing where a system is squeezed to the point that further squeezing leads to a counter-intuitive increase in measurement uncertainty. Quasi-probability distributions on the surface of a Poincaré sphere are the most natural way to represent the topology of our polarization states. Using this representation it is easy to observe the squeezing and over-squeezing behaviour of our photon states.\nWork is also presented on two different technologies for generating nonclassical states of light. The first is based on the nonlinear process of spontaneous parametric downconversion to produce pairs of photons. With this source up to 200,000 pairs of photons/s have been collected into single-mode fibre, and over 100 double pairs/s have been detected. This downconversion source is suitable for use in a wide variety of multi-qubit quantum information applications. The second source presented is a single-photon source based on semiconductor quantum dots. The single-photon character of the source is verified using a Hanbury Brown-Twiss interferometer.
An analytical solution for a dissipative cavity field interacting with two qubits (TQs) in presence of Kerr-like medium is obtained. The super-operator method is used to obtaining the field density matrix elements under the dispersive approximation. Linear entropy is calculated to determine the entanglement between the cavity field and the TQs. Moreover, the Wehrl entropy and Wehrl phase distribution are discussed to discover the nonclassicality dynamics of a dissipative cavity field. It is found that the Kerr medium and the dissipation reduce the generated quantum coherence due to the qubit–cavity interactions. It is possible to control the ability of cavity–qubit interactions to generate quantum coherence by the mean photon number of the initial coherent cavity state. Moreover, the nonclassicality of the initial coherent state reduces the dissipative and the Kerr medium effects.