We demonstrate a robust implementation of a deterministic linear-optical controlled-not gate for single-photon two-qubit quantum logic. A polarization Sagnac interferometer with an embedded 45 degrees -oriented dove prism is used to enable the polarization control qubit to act on the momentum (spatial) target qubit of the same photon. The optical controlled-not gate requires no active stabiliza...

We consider a two-level quantum system ~qubit! that is continuously measured by a detector. The information provided by the detector is taken into account to describe the evolution during a particular realization of the measurement process. We discuss the Bayesian formalism for such ‘‘selective’’ evolution of an individual qubit and apply it to several solid-state setups. In particular, we show...

Recent advances in nanofabrication and optical control imply that multi-qubit-cavity systems can now be engineered with pre-designed couplings. Here we propose optical realizations of spin-glass systems which exploit these new nanoscale technologies. By contrast with traditional realizations using magnetic solids, phase transition phenomena can now arise in both the matter and radiation subsyst...

Recently proposed quantum key distribution protocols are shown to be vulnerable to a classic manin-the-middle attack using entangled pairs created by Eve. It appears that the attack could be applied to any protocol that relies on manipulation and return of entangled qubits to create a shared key. The protocols that are cryptana lyzed in this paper were proven secure with respect to some eavesdr...

We study the decohering effect of qubit-qubit interactions on entanglement. Modeling such qubit cross talk by nearest-neighbor exchange interactions, we explore the Heisenberg spin chain as a system in which decoherence occurs dynamically, and we obtain an analytic description of the process. We find expressions for the short-time and long-time transfer and decay of entanglement in a long chain...

We describe a fast quantum computer based on optically controlled electron spins in charged quantum dots that are coupled to microcavities. This scheme uses broadband optical pulses to rotate electron spins and provide the clock signal to the system. Nonlocal two-qubit gates are performed by phase shifts induced by electron spins on laser pulses propagating along a shared waveguide. Numerical s...

We show how, given any set of generators of the stabilizer of a quantum code, an efficient gate array that computes the codewords can be constructed. For an n-qubit code whose stabilizer has d generators, the resulting gate array consists of O(nd) operations, and converts k-qubit data (where k = n − d) into n-qubit codewords. 02.70.Rw, 03.65.Bz, 89.80.+h Typeset using REVTEX ∗[email protected]...

We present the visible entangled states of 4-qubit system which can be observed easily in physical laboratories. This was motivated from the fact that the entangled state of 2-qubit system comes from singlet and triplet states which are constructed through the angular momentum addition formalism. We show that 4-qubit system has the new entangled states different from GHZ or W types entangled st...

We present a simple formalism describing evolution of a qubit in the process of its measurement in a circuit QED setup. When a phase-sensitive amplifier is used, the evolution depends on only one output quadrature, and the formalism is the same as for a broadband setup. When a phasepreserving amplifier is used, the qubit evolution depends on two output quadratures. In both cases a perfect monit...

Additivity of the Holevo capacity is proved for product channels, under the condition that one of the channels is in a certain class of unital qubit channels, with the other completely arbitrary. This qubit class includes the depolarizing channel. As a byproduct this proves that the Holevo bound is the ultimate information capacity of such qubit channels (assuming no prior entanglement between ...