Two-qubit SWAP Research Articles

Quantum SWAP gate in an optical cavity with an atomic cloud

A scheme is presented for realizing a two-qubit quantum SWAP gate for intracavity fields. In the scheme, a $\mathit{V}$-type atomic ensemble prepared in its ground state mediates the interaction between the two-cavity modes. Under certain conditions, the cavity-field degree of freedom is decoupled from the atomic degrees of freedom, and the effective coupling strength between the two-cavity modes scales up with $\sqrt{N}$ ($\mathit{N}$ is the number of effective atoms). The numerical simulation shows that the quantum SWAP gate may have a high fidelity under the influence of the atomic spontaneous emission and the decay of the cavity modes.

One-Step Realization of SWAP Gate with Superconducting Quantum-Interference Devices and Atoms in Cavity QED

We put forward a simple scheme for one-step realization of a two-qubit SWAP gate with SQUIDs (super-conducting quantum-interference devices) in cavity QED via Raman transition. In this scheme, the cavity field is only virtually excited and thus the cavity decay is suppressed. The SWAP gate is realized by using only two lower flux states of the SQUID system and the excited state would not be excited. Therefore, the effect of decoherence caused from the levels of the SQUID system is possibly minimized. The scheme can also be used to implement the SWAP gate with atoms.

Quantum cloning of a pair of orthogonally polarized photons with linear optics

A linear optical probabilistic scheme for the optimal cloning of a pair of orthogonally polarized photons is devised, based on single- and two-photon interferences. It consists in a partial symmetrization device realized with a modified unbalanced Mach-Zehnder interferometer, followed by two balanced beam splitters where the Hong-Ou-Mandel photon bunching occurs. This scheme has the advantage that it enables quantum cloning without the need for stimulated amplification in a nonlinear medium. It can also be modified so to make an optical two-qubit partial SWAP gate, thereby providing a potentially useful tool to linear optics quantum computing.

Experimental Realization of Linear-Optical Partial swap Gates

We present a linear-optical implementation of a class of two-qubit partial SWAP gates for polarization states of photons. Different gate operations, including the SWAP and entangling sqrt[SWAP], can be obtained by changing a classical control parameter, namely, the path difference in the interferometer. Reconstruction of output states, full quantum process tomography, and an evaluation of entanglement of formation prove very good performance of the gates.

Selective recoupling and stochastic dynamical decoupling

An embedded selective recoupling method is proposed which is based on the idea of embedding the recently proposed deterministic selective recoupling scheme of Yamaguchi et al. into a stochastic dynamical decoupling method, such as the recently proposed Pauli-random-error-correction scheme. The recoupling scheme enables the implementation of elementary quantum gates in a quantum information processor by partial suppression of the unwanted interactions. The random dynamical decoupling method cancels a significant part of the residual interactions. Thus the time scale of reliable quantum computation is increased significantly. Numerical simulations are presented for a conditional two-qubit swap gate and for a complex iterative quantum algorithm.

Single-photon two-qubit SWAP gate for entanglement manipulation

A SWAP operation between different types of qubits of single photons is essential for manipulating hyperentangled photons for a variety of applications. We have implemented an efficient SWAP gate for the momentum and polarization degrees of freedom of single photons. The SWAP gate was utilized in a single-photon two-qubit quantum logic circuit to deterministically transfer momentum entanglement between a pair of down-converted photons to polarization entanglement. The polarization entanglement thus obtained violates Bell's inequality by more than 150 standard deviations.

Dissipation-assisted quantum gates with cold trapped ions

It is shown that a two-qubit phase gate and SWAP operation between ground states of cold trapped ions can be realized in one step by simultaneously applying two laser fields. Cooling during gate operations is possible without perturbing the computation. On the contrary, the cooling lasers even stabilize the desired time evolution of the system. This affords gate operation times of nearly the same order of magnitude as the inverse coupling constant of the ions to a common vibrational mode.

Decoherence of the quantum gate in Milburn’s model of decoherence

We study the decoherence properties of a two-qubit quantum swap gate in Milburn's model of decoherence where the gate is realized by the quantum-dot system. We examine how the so-called intrinsic decoherence may degrade the gate purity and gate fidelity of the quantum swap gate and also present a potential source for the occurrence of Milburn's equation in the system of concern.