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Physical Review A

Fock-space inseparability criteria of bipartite continuous-variable quantum states

Xiaoyu Chen

Published Aug 10, 2007• 15 citations
Publisher

Abstract

We prove that previous various forms of the inseparability of a Gaussian state are equivalent to the negativity of a partial transposed density matrix in the one-photon Fock subspace. We give an infinite series of inequalities; the fulfillment of each inequality shows the presence of entanglement in bipartite systems. For many non-Gaussian states, we show that Fock-space inseparability criteria are theoretically more efficient and simpler than Shchukin-Vogel criteria, which are based on the moments of the states.

DOI
10.1103/physreva.76.022309

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We prove that previous various forms of the inseparability of a Gaussian state are equivalent to the negativity of a partial transposed density matrix in the one-photon Fock subspace. We give an infinite series of inequalities; the fulfillment of each inequality shows the presence of entanglement in bipartite systems. For many non-Gaussian states, we show that Fock-space inseparability criteria are theoretically more efficient and simpler than Shchukin-Vogel criteria, which are based on the moments of the states.
Inseparability criteria for continuous and discrete bipartite quantum states based on moments of annihilation and creation operators are studied by developing the idea of Shchukin-Vogel criterion [Phys. Rev. Lett. 95, 230502 (2005)]. If a state is separable, then the corresponding matrix of moments is separable too. Thus, we derive generalized criteria based on the separability properties of the matrix of moments. In particular, a criterion based on realignment of moments in the matrix is proposed as an analog of the standard realignment criterion for density matrices. Other inseparability inequalities are obtained by applying positive maps to the matrix of moments. Usefulness of the Shchukin-Vogel criterion to describe bipartite-entanglement of more than two modes is demonstrated: we obtain various three-mode inseparability criteria, including some previously known ones, which were originally derived from the Cauchy-Schwarz inequality.
There are several examples of bipartite entangled states of continuous variables for which the existing criteria for entanglement using the inequalities involving the second order moments are insufficient. We derive new inequalities involving higher order correlation, for testing entanglement in non-Gaussian states. In this context we study an example of a non-Gaussian state, which is a bipartite entangled state of the form psi(xrma,xrmb)propto(alphaxrma+betaxrmb)e−(xrma2+xrmb2)/2\\psi(x_{\\rm a},x_{\\rm b})\\propto (\\alpha x_{\\rm a}+\\beta x_{\\rm b})e^{-(x_{\\rm a}^2+x_{\\rm b}^2)/2}psi(xrma​,xrmb​)propto(alphaxrma​+betaxrmb​)e−(xrma2​+xrmb2​)/2. Our results open up an avenue to search for new inequalities to test entanglement in non-Gaussian states.
There are several examples of bipartite entangled states of continuous variables for which the existing criteria for entanglement using the inequalities involving the second-order moments are insufficient. We derive new inequalities involving higher order correlation, for testing entanglement in non-Gaussian states. In this context, we study an example of a non-Gaussian state, which is a bipartite entangled state of the form . Our results open up an avenue to search for new inequalities to test entanglement in non-Gaussian states.
The partial transposition (PT) operation is an efficient tool in detecting the inseparability of a mixed state. We give an explicit formula for the PT operation for the continuous variable states in Fock space. We then give the necessary and sufficient condition for the positivity of Gaussian operators. Based on this, a number of criteria on the inseparability and distillability for the multimode Gaussian states are naturally drawn. We finally give an explicit formula for the state in a subspace of a global Gaussian state. This formula, together with the known results for Gaussian states, gives the criteria for the inseparability and distillability in a subspace of the global Gaussian state.