No Arabic abstract
In many quantum information processing applications, it is important to be able to transfer a quantum state from one location to another - even within a local device. Typical approaches to implement the quantum state transfer rely on unitary evolutions or measurement feedforward operations. However, these existing schemes require accurate pulse operations and/or precise timing controls. Here, we propose a one-way transfer of the quantum state with near unit efficiency using dissipation from a tailored environment. After preparing an initial state, the transfer can be implemented without external time dependent operations. Moreover, our scheme is irreversible due to the non-unitary evolution, and so the transferred state remains in the same site once the system reaches the steady state. This is in stark contrast to the unitary state transfer where the quantum states continue to oscillate between different sites. Our novel quantum state transfer via the dissipation paves the way towards robust and practical quantum control.
The initial states which minimize the predictability loss for a damped harmonic oscillator are identified as quasi-free states with a symmetry dictated by the environments diffusion coefficients. For an isotropic diffusion in phase space, coherent states (or mixtures of coherent states) are selected as the most stable ones.
Quantum state transfer and teleportation, with qubits encoded in internal states of the atoms in cavities, among spatially separated nodes of a quantum network in decoherence-free subspace are proposed, based on a cavity-assisted interaction by single-photon pulses. We show in details the implementation of a logic-qubit Hadamard gate and a two-logic-qubit conditional gate, and discuss the experimental feasibility of our scheme.
In the framework of Lindblad theory for open quantum systems, we calculate the entropy of a damped quantum harmonic oscillator which is initially in a quasi-free state. The maximally predictable states are identified as those states producing the minimum entropy increase after a long enough time. In general, the states with a squeezing parameter depending on the environments diffusion coefficients and friction constant are singled out, but if the friction constant is much smaller than the oscillators frequency, coherent states (or thermalized coherent states) are obtained as the preferred classical states.
We describe in detail the application of four qubit cluster states, built on the simultaneous entanglement of two photons in the degrees of freedom of polarization and linear momentum, for the realization of a complete set of basic one-way quantum computation operations. These consist of arbitrary single qubit rotations, either probabilistic or deterministic, and simple two qubit gates, such as a c-not gate for equatorial qubits and a universal c-phase (CZ) gate acting on arbitrary target qubits. Other basic computation operations, such as the Grovers search and the Deutschs algorithms, have been realized by using these states. In all the cases we obtained a high value of the operation fidelities. These results demonstrate that cluster states of two photons entangled in many degrees of freedom are good candidates for the realization of more complex quantum computation operations based on a larger number of qubits.
A multi-slit interference experiment, with which-way detectors, in the presence of environment induced decoherence, is theoretically analyzed. The effect of environment is modeled via a coupling to a bath of harmonic oscillators. Through an exact analysis, an expression for $mathcal{C}$, a recently introduced measure of coherence, of the particle at the detecting screen is obtained as a function of the parameters of the environment. It is argued that the effect of decoherence can be quantified using the measured coherence value which lies between zero and one. For the specific case of two slits, it is shown that the decoherence time can be obtained from the measured value of the coherence, $mathcal{C}$, thus providing a novel way to quantify the effect of decoherence via direct measurement of quantum coherence. This would be of significant value in many current studies that seek to exploit quantum superpositions for quantum information applications and scalable quantum computation.