Two-qubit Gate Operations Research Articles

Exchange Coupling in Silicon Double Quantum Dots

An exchange coupling between localized electron spins is evaluated in silicon double quantum dots, taking account of electron-electron interaction exactly. A multivalley structure of conduction band is taken into account within the effective mass approximation. We show that the intervalley tunneling between the quantum dots plays an important role in the exchange coupling. The exchange coupling can be controlled by gating the central electrode of the double quantum dots, which is applicable to two-qubit gate operation for quantum computation using the electron spins.

Charge-based quantum computing using single donors in semiconductors

Solid-state quantum computer architectures with qubits encoded using single atoms are now feasible given recent advances in atomic doping of semiconductors. Here we present a charge qubit consisting of two dopant atoms in a semiconductor crystal, one of which is singly ionised. Surface electrodes control the qubit and a radio-frequency single electron transistor provides fast readout. The calculated single gate times, of order 50ps or less, are much shorter than the expected decoherence time. We propose universal one- and two-qubit gate operations for this system and discuss prospects for fabrication and scale up.

Ion-trap quantum computing in the presence of cooling

This paper discusses ways to implement two-qubit gate operations for quantum computing with cold trapped ions within one step. The proposed scheme is widely robust against parameter fluctuations and its simplicity might help to increase the number of qubits in present experiments. Basic idea is to use the quantum Zeno effect originating from continuous measurements on a common vibrational mode to realise gate operations with very high fidelities. The gate success rate can, in principle, be arbitrary high but operation times comparable to other schemes can only be obtained by accepting success rates below 80%.

Entanglement dynamics in one-dimensional quantum cellular automata

Several proposed schemes for the physical realization of a quantum computer consist of qubits arranged in a cellular array. In the quantum circuit model of quantum computation, an often complex series of two-qubit gate operations is required between arbitrarily distant pairs of lattice qubits. An alternative model of quantum computation based on quantum cellular automata (QCA) requires only homogeneous local interactions that can be implemented in parallel. This would be a huge simplification in an actual experiment. We find some minimal physical requirements for the construction of unitary QCA in a 1 dimensional Ising spin chain and demonstrate optimal pulse sequences for information transport and entanglement distribution. We also introduce the theory of non-unitary QCA and show by example that non-unitary rules can generate environment assisted entanglement.

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.

All-optical quantum dot implementation for quantum computing

We present an all-optical scheme for a solid-state implementation of quantum information processing. The combined effect of static electric fields and ultrafast sequences of multi-color laser pulses on a semiconductor quantum dot array is investigated. We show how these ingredients can be used to perform single- as well as two-qubit gate operations, by exploiting the ground-state excitonic transitions as computational degrees of freedom. We demonstrate that by these means it is possible to implement basic quantum-computation operations on a time scale much shorter than the exciton dephasing time.

Correlated errors in quantum-error corrections

We show that errors are not generated correlatedly provided that quantum bits do not directly interact with (or couple to) each other. Generally, this no-qubits-interaction condition is assumed except for the case where two-qubit gate operation is being performed. In particular, the no-qubits-interaction condition is satisfied in the collective decoherence models. Thus, errors are not correlated in the collective decoherence. Consequently, we can say that current quantum error correcting codes which correct single-qubit-errors will work in most cases including the collective decoherence.