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Quantum Information Processing

Nonclassical photon statistics and bipartite entanglement generation of excited coherent states

Ram Soorat, S. Nitharshini, M. Anil Kumar, S. K. Singh

Published Aug 24, 2020• 18 citations
Publisher
DOI
10.1007/s11128-020-02756-9

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Abstract We theoretically investigate the emergence of nonclassical features, two-mode squeezing, sub-Poissonian photon statistics, and bipartite entanglement, in the third harmonic generation (THG) process, employing a first-order interaction Hamiltonian within the short-time approximation. The study reveals that both first- and second-order squeezing exhibit strong dependence on the input field amplitudes, relative phase, and interaction strength. Asymmetric coherent inputs, where the harmonic mode carries greater intensity than the fundamental, enhance the degree of squeezing and lead to deeper nonclassical minima. The squeezing parameters display periodic phase-dependent oscillations, indicating phase-sensitive control of quantum correlations. Sub-Poissonian photon statistics in the fundamental mode further confirm the quantum nature of the generated light. Entanglement is analyzed through the Duan et al. inseparability criterion and the Hillery–Zubairy (HZ) criteria. While the Duan criterion fails to detect entanglement under purely coherent excitation in the short-time regime, both the HZ-1 and HZ-2 criteria confirm entanglement under specific phase conditions, with HZ-2 exhibiting greater sensitivity at elevated field strengths. These results underscore the potential of THG as a viable platform for the controllable generation of nonclassical light, with implications for continuous-variable quantum optics and integrated quantum photonics.
In this paper, after a brief review of the single-mode excited entangled coherent states, two new classes of continuous-variable entangled pure states, namely, single-mode nonlinear excited entangled coherent states, have been produced by using the f -deformed creation operator instead of . In order to establish the nonclassicality of the states, some of the physical properties of the corresponding entangled states, such as entanglement degree, photon statistics, second-order correlation function and quadrature squeezing have been studied in detail by choosing a particular f -deformation function. In addition, the influence of photon excitations on the properties of our introduced states has been analyzed and the results compared with those of a single-mode excited entangled coherent state. In continuation, a theoretical scheme for their generation based on the interaction of a two-level atom with a quantized cavity field has been proposed.
We investigate how post-selected von Neumann measurements modify the nonclassical properties of two-mode entangled coherent states (ECSs). From the conditional pointer state, we characterize squeezing, photon statistics, intermode correlations, entanglement, fidelity, and post-selection probability. Anomalous weak values with moderate coupling enhance squeezing, antibunching, sub-Poissonian statistics, intermode correlations, and bipartite entanglement, while fidelity captures the crossover from weak perturbation to strong distinguishability. These results demonstrate post-selection as a tunable approach to engineering ECSs.
The nonlinear χ (2) Mach-Zehnder interferometer is proposed as a device for conditional generation of a modified coherent nonclassical state. We show that the generated macroscopic state exhibits nonclassical effects, such as squeezing, photon antibunching, and sub-Poissonian statistics. The modified coherent state generates a macroscopic entangled state. The scheme works without the photon number resolving detection but requires high-efficiency photodetectors. We explain the mechanism of generation of the modified coherent non-classical state.