Xingxiang Zhou

Dispersive manipulation of paired superconducting qubits

We combine the ideas of qubit encoding and dispersive dynamics to enable robust and easy quantum-information processing on paired superconducting charge boxes sharing a common bias lead. We establish a decoherence free subspace on these and introduce universal gates by dispersive interaction with a LC resonator and inductive couplings between the encoded qubits. These gates preserve the code space and only require the established local symmetry and the control of the voltage bias. The principle of dispersive manipulation of encoded qubits is general and beneficial to other physical systems.

Isolation Structures for the Solid-State Quantum-to-Classical Interface

Solid-state qubits confront a much higher density of states than atomic-scale systems. This is deleterious for quantum information processing. We analyze various superconducting structures for isolating qubit circuits to increase their coherence time.

Conditions for nondistortion interrogation of quantum system

Under some physical considerations, we present a universal formulation to study the possibility of localizing a quantum object in a given region without disturbing its unknown internal state. When the interaction between the object and probe wave function takes place only once, we prove the necessary and sufficient condition that the objects presence can be detected in an initial-state–preserving way. Meanwhile, a conditioned optimal interrogation probability is obtained.

A tipping pulse scheme for a rf-SQUID qubit

We present a technique to control the quantum state of a rf-SQUID qubit. We propose to employ a stream of single flux quantum (SFQ) pulses magnetically coupled to the qubit junction to momentarily suppress its critical current. This effectively lowers the barrier in the double-well rf SQUID potential thereby increasing the tunneling oscillation frequency between the wells. By carefully choosing the time interval between SFQ pulses one may accelerate the interwell tunneling rate. Thus it is possible to place the qubit into a chosen superposition of flux states and then effectively to freeze the qubit state. We present both numerical simulations and analytical time-dependent perturbation theory calculations that demonstrate the technique. Using this strategy one may control the quantum state of the rf SQUID in a way analogous to the /spl pi/ pulses in other qubit schemes.

Nondistortion quantum interrogation using Einstein-Podolsky-Rosen entangled photons

We propose a scheme for nondistortion quantum interrogation defined as an interaction-free measurement that preserves the internal state of the object being detected. In our scheme, two Einstein-Podolsky-Rosen entangled photons are used as the probe and polarization-sensitive measurements are performed at the four ports of the Mach-Zehnder interferometer. In comparison with the previous single-photon scheme, it is shown that the two-photon approach has a higher probability of initial state preserving interrogation of an atom prepared in a quantum superposition. In the case that the presence of the atom is not successfully detected, the experiment can be repeated since the initial state of the atom is unperturbed.

SUPERCONDUCTING QUANTUM COMPUTING WITHOUT SWITCHES

This Chapter presents a very simple architecture for a large-scale superconducting quantum computer. All of the SQUID qubits are fixed-coupled to a single large superconducting loop.

A tipping pulse scheme for an rf-SQUID qubit

We present a technique of using a stream of single flux quantum (SFQ) pulses magnetically coupled to the qubit junction to control the state of a rf-SQUID qubit. This momentarily suppresses the critical current of the junction and lowers the potential barrier so the tunneling oscillation frequency is increased.