Two-qubit Conditional Phase Gate Research Articles

Microwave-activated conditional-phase gate for superconducting qubits

We introduce a new entangling gate between two fixed-frequency qubits statically coupled via a microwave resonator bus which combines the following desirable qualities: all-microwave control, appreciable qubit separation for reduction of crosstalk and leakage errors and the ability to function as a two-qubit conditional-phase gate. A fixed, always-on interaction is explicitly designed between higher energy (non-computational) states of two transmon qubits, and then a conditional-phase gate is ‘activated’ on the otherwise unperturbed qubit subspace via a microwave drive. We implement this microwave-activated conditional-phase gate with a fidelity from quantum process tomography of ∼ 87%.

Two-Qubit Conditional Phase Gate in Laser-Excited Semiconductor Quantum Dots Using the Quantum Zeno Effect

We propose a scheme for a two-qubit conditional quantum Zeno phase gate for semiconductor quantum dots. The proposed system consists of two charged dots and one ancillary neutral dot driven by a laser pulse tuned to the exciton resonance. The primary decoherence mechanism is phonon-assisted exciton relaxation, which can be viewed as continuous monitoring by the environment. Because of the Zeno effect, a strong possibility of emission is sufficient to strongly modify the coherent dynamics, with negligible probability of actual emission. We solve analytically the master equation and simulate the dynamics of the system using a realistic set of parameters. In contrast to standard schemes, larger phonon relaxation rates increase the fidelity of the operations.

Implementing a neutral atom Rydberg gate without populating the Rydberg state

In this paper, we propose a neutral atom implementation of the two-qubit conditional phase gate, based on the large Rydberg–Rydberg interaction induced energy shift. Contrary to previous proposals, this gate does not substantially populate the Rydberg levels. After describing the model in detail, we provide numerical simulations and discuss the performance of our gate. In particular we address the influence on the fidelity of spontaneous emission and atomic motion.

Alternative scheme for a two-qubit conditional phase gate by adiabatic passage under dissipation

We check a recent proposal (Goto and Ichimura 2004 Phys. Rev. A 70 012305) for a controlled phase gate through adiabatic passage under the influence of spontaneous emission and the cavity decay. We show a modification of the above proposal could be used to generate the necessary conditional phase gates in the two-qubit Grover search. Conditioned on no photon leakage either from the atomic excited state or from the cavity mode during the gating period, we numerically analyse the success probability and the fidelity of the two-qubit conditional phase gate by adiabatic passage. A comparison made between our proposed gating scheme and a previous one shows that Goto and Ichimura's scheme is an alternative and feasible way in the optical cavity regime for two-qubit gates and could be generalized in principle to multi-qubit gates.

Selective pulse implementation of two-qubit gates for spin-32-based fullerene quantum-information processing

We propose two potentially practical schemes to carry out two-qubit quantum gates on endohedral fullerenes $\mathrm{N}@{\mathrm{C}}_{60}$ or $\mathrm{P}@{\mathrm{C}}_{60}$. The qubits are stored in electronic spin degrees of freedom of the doped atom N or P. By means of the magnetic dipolar coupling between two neighboring fullerenes, the two-qubit controlled-NOT gate and the two-qubit conditional phase gate are performed by selective microwave pulses assisted by the refocusing technique. We will discuss the necessary additional steps for the universality of our proposal. We will also show that our proposal is useful for both quantum gating and the readout of quantum information from the spin-based qubit state.

Applications of coherent population transfer to quantum information processing

We develop a theoretical framework for the exploration of quantum mechanical coherent population transfer phenomena, with the ultimate goal of constructing faithful models of devices for classical and quantum information processing applications. We begin by outlining a general formalism for weak-field quantum optics in the Schrödinger picture, and we include a general phenomenological representation of Lindblad decoherence mechanisms. We use this formalism to describe the interaction of a single stationary multilevel atom with one or more propagating classical or quantum laser fields, and we describe in detail several manifestations and applications of electromagnetically induced transparency. In addition to providing a clear description of the nonlinear optical characteristics of electromagnetically transparent systems that lead to ‘ultraslow light’, we verify that—in principle—a multi-particle atomic or molecular system could be used as either a low power optical switch or a quantum phase shifter. However, we demonstrate that the presence of significant dephasing effects destroys the induced transparency, and that increasing the number of particles weakly interacting with the probe field only reduces the nonlinearity further. Finally, a detailed calculation of the relative quantum phase induced by a system of atoms on a superposition of spatially distinct Fock states predicts that a significant quasi-Kerr nonlinearity and a low entropy cannot be simultaneously achieved in the presence of arbitrary spontaneous emission rates. Within our model, we identify the constraints that need to be met for this system to act as a one-qubit and a two-qubit conditional phase gate.