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Chinese Physics B

Effects of photon addition on the quantum nonlocality of squeezed entangled coherent states

Benyuan Zhou, Lei Deng, Yong-Fa Duan, Li Zhi Yu, Gao‐xiang Li

Published Sep 1, 2012• 5 citations
Publisher

Abstract

Effects of photon addition on the quantum nonlocality of squeezed entangled coherent states for Bell-inequality tests are studied theoretically. By utilizing the method of photon-parity measurement, it is found that photon addition can always increase the degrees of Bell violations within a certain parameter range. A possible scheme for generating photon-added squeezed entangled coherent states is proposed.

DOI
10.1088/1674-1056/21/9/090302

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Effects of photon addition on the quantum nonlocality of squeezed entangled coherent states for Bell-inequality tests are studied theoretically. By utilizing the method of photon-parity measurement, it is found that photon addition can always increase the degrees of Bell violations within a certain parameter range. A possible scheme for generating photon-added squeezed entangled coherent states is proposed.
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We show that the coherent superposition ta^+ra^† of photon subtraction and addition applied to each local mode of a two-mode entangled state can enhance the nonlocality manifested by the violation of a Bell inequality. A two-mode squeezed state is used as an input state for this demonstration with four different Bell inequalities employed: Bell inequalities adopting displaced parity operator, pseudospin operator, homodyne measurement, and conditional entropy, respectively. We find that the coherent operation significantly enhances the nonlocality remarkably in the weak squeezing limit, compared with other possible non-Gaussian operations. It can also give a maximal Bell violation with a very small squeezing for the inequalities with pseudospin operator and conditional entropy.
We introduce a class of the photon-added Bell-type entangled coherent states (PABECSs) obtained from Bell-type entangled coherent states by applying creation operators. We investigate their entanglement characteristics by analyzing the entropy of entanglement and discuss the influence of photon excitations on quantum entanglement. It is shown that applying a creation operator can increase the amount of entanglement. We also study the statistical properties of such states by discussing the behavior of the quasi-probability functions graphically. In addition, a possible scheme is presented to produce the PABECSs by using cavity quantum electrodynamics and quantum measurements.
Quantum entanglement and quantum nonlocality of N -photon entangled states |ψ Nm ⟩ = C m [cos γ| N − m ⟩ 1 | m ⟩ 2 +e iθ m sin γ| m ⟩ 1 | N − m ⟩ 2 ] and their superpositions are studied. We point out that the relative phase θ m affects the quantum nonlocality but not the quantum entanglement for the state |ψ Nm ⟩. We show that quantum nonlocality can be controlled and manipulated by adjusting the state parameters of |ψ Nm ⟩, superposition coefficients and the azimuthal angles of the Bell operator. We also show that the violation of the Bell inequality can reach its maximal value under certain conditions. It is found that quantum superpositions based on |ψ Nm ⟩ can increase the amount of entanglement and give more ways to reach the maximal violation of the Bell inequality.
Violation of Bell inequality is a prominent detection method for quantum correlations present in composite quantum systems, both in finite and infinite dimensions. We investigate the consequence of the violation of local realism based on pseduospin operators when photons are added or subtracted in a single mode or in both the modes of the two-mode squeezed states of light in the presence of noise. In the noiseless situation, we show that for addition (subtraction) of photons in a single mode, there is an overall enhancement in the maximal violation, although we observe an interplay between monotonicity and nonmonotonicity in the violation of Bell inequality depending on the squeezing strength. Moreover, we report that for low squeezing or low number of photons added or subtracted, subtraction in both the modes can lead to higher violation of local realism than that in the case of addition. For any choice of parameters, such ordering is not seen if one compares their entanglement contents. In the event of a faulty twin-beam generator, we obtain a lower than expected squeezing in the state. In such a case, or in imperfect photon addition (subtraction) or under local noise, we find that the violation of local realism by the noise-affected two-mode squeezed states always decreases. Interestingly, however, we notice that photon addition (subtraction) can in general help to conquer the ill effects of noise by enhancing the violation of local realism or by transforming nonviolating states to violating ones, thereby acting as an activating agent.