We propose a scheme for nondistortion quantum interrogation defined as an interaction-free easurement 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.

We combine the ideas of qubit encoding and dispersive dynamics to enable robust and easy quantuminformation 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.

Most quantum computer realizations require the ability to apply local fields and tune the couplings between qubits, in order to realize single bit and two bit gates which are necessary for universal quantum computation.We present a scheme to remove the necessity of switching the couplings between qubits for two bit gates, which are more costly in many cases. Our strategy is to compute with encoded qubits in and out of carefully designed interaction free subspaces analogous to decoherence free subspaces.We give two examples to show how universal quantum computation is realized in our scheme with local manipulations to physical qubits only, for both diagonal and off diagonal interactions.

Abstract.-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 object's presence can be detected in an initial-state-preserving way. Meanwhile, a conditioned optimal interrogation probability is obtained.

We investigate how to concatenate different decoherence-free subspaces to realize scalable universal fault-tolerant quantum computation. Based on tunable XXZ interactions, we present an architecture for scalable quantum computers which can fault-tolerantly perform universal quantum computation by manipulating only a single type of parameter. By using the concept of interaction-free subspaces, we eliminate the need to tune the couplings between logical qubits, which further reduces the technical difficulties for implementing quantum computation.

Considering the dynamics of a two-qubit entangled system in the decoherence environment, we investigate the stability of pairwise entanglement under decoherence. We find that for different decoherence models, there exists some special class of entangled states of which the pairwise entanglement is the most stable. The lifetime of the entanglement in these states is larger than other states with the same initial entanglement. In addition, we also investigate the dynamics of pairwise entanglement in the ground state of spin models such as Heisenberg and XXY models.

Starting from local universal isotropic disentanglement, a threshold inequality for reduction factors is proposed, which is necessary and sufficient for this type of disentanglement processes. Furthermore, we give the conditions realizing ideal disentanglement processes provided that some information on quantum states is known. In addition, based on a fully entangled fraction, a concept called inseparability correlation is presented. Some properties on inseparability correlation coefficient are studied.

We consider the nondistortion quantum interrogation of an atom prepared in a quantum superposition. By manipulating the polarization of the probe photon and making connections to interaction-free measurements of opaque objects, we show that nondistortion interrogation of an atom in a quantum superposition can be done with efficiency approaching unity. However, if any component of the atom's superposition is completely transparent to the probe wave function, a nondistortion interrogation of the atom is impossible.

A scheme to implement a quantum computer subjected to decoherence and governed by an untunable qubit-qubit interaction is presented. By concatenating dynamical decoupling through bang-bang (BB) pulse with decoherence-free subspaces (DFSs) encoding, we protect the quantum computer from environmentinduced decoherence that results in quantum information dissipating into the environment. For the inherent qubit-qubit interaction that is untunable in the quantum system, BB control plus DFSs encoding will eliminate its undesired effect which spoils quantum information in qubits. We show how this quantum system can be used to implement universal quantum computation.

We propose a scheme for the quantum teleportation of an atomic state based on the detection of cavity decay. The internal state of an atom trapped in a cavity can be disembodiedly transferred to another atom trapped in a distant cavity by measuring interference of polarized photons through single-photon detectors. In comparison with the original proposal by Bose, Knight, Plenio, and Vedral [Phys. Rev. Lett. 83, 5158 (1999)], our protocol of teleportation has a high fidelity of almost unity, and inherent robustness, such as the insensitivity of fidelity to randomness in the atom's position, and to detection inefficiency. All these favorable features make the scheme feasible with the current experimental technology.