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Register a new userSpectral Conditions on the State of a Composite Quantum System Implying its Separability
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Authors
G.A. Raggio
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For any unitarily invariant convex function F on the states of a composite quantum system which isolates the trace there is a critical constant C such that F(w)<= C for a state w implies that w is not entangled; and for any possible D > C there are entangled states v with F(v)=D. Upper- and lower bounds on C are given. The critical values of some Fs for qubit/qubit and qubit/qutrit bipartite systems are computed. Simple conditions on the spectrum of a state guaranteeing separability are obtained. It is shown that the thermal equilbrium states specified by any Hamiltonian of an arbitrary compositum are separable if the temperature is high enough.
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For two qubits and for general bipartite quantum systems, we give a simple spectral condition in terms of the ordered eigenvalues of the density matrix which guarantees that the corresponding state is separable.
We numerically study a particle in a box with moving walls. In the case where the walls are oscillating sinusoidally with small amplitude, we show that states up to the fourth state can be populated with more than 80 percent population, while higher-lying states can also be selectively excited. This work introduces a way of controlling quantum systems which does not rely on (dipole) selection rules.
Quantum states are the key mathematical objects in quantum mechanics, and entanglement lies at the heart of the nascent fields of quantum information processing and computation. What determines whether an arbitrary quantum state is entangled or separable is therefore very important for investigating both fundamental physics and practical applications. Here we show that an arbitrary bipartite state can be divided into a unique purely entangled structure and a unique purely separable structure. We show that whether a quantum state is entangled or not is determined by the ratio of its weight of the purely entangled structure and its weight of the purely separable structure. We provide a general algorithm for the purely entangled structure and the purely separable structure, and a general algorithm for the best separable approximation (BSA) decomposition, that has been a long-standing open problem. Our result implies that quantum states exist as families in theory, and that the entanglement (separability) of family members can be determined by referring to a crucial member of the family.
Quantum teleportation, a way to transfer the state of a quantum system from one location to another, is central to quantum communication and plays an important role in a number of quantum computation protocols. Previous experimental demonstrations have been implemented with photonic or ionic qubits. Very recently long-distance teleportation and open-destination teleportation have also been realized. Until now, previous experiments have only been able to teleport single qubits. However, since teleportation of single qubits is insufficient for a large-scale realization of quantum communication and computation2-5, teleportation of a composite system containing two or more qubits has been seen as a long-standing goal in quantum information science. Here, we present the experimental realization of quantum teleportation of a two-qubit composite system. In the experiment, we develop and exploit a six-photon interferometer to teleport an arbitrary polarization state of two photons. The observed teleportation fidelities for different initial states are all well beyond the state estimation limit of 0.40 for a two-qubit system. Not only does our six-photon interferometer provide an important step towards teleportation of a complex system, it will also enable future experimental investigations on a number of fundamental quantum communication and computation protocols such as multi-stage realization of quantum-relay, fault-tolerant quantum computation, universal quantum error-correction and one-way quantum computation.
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