Random Control over Quantum Open Systems

Parametric fluctuations or stochastic signals are introduced into the control pulse sequence to investigate the feasibility of random control over quantum open systems. In a large parameter error region, the out-of-order control pulses work as well as the regular pulses for dynamical decoupling and dissipation suppression. Calculations and analysis are based on a non-perturbative control approach allowed by an exact quantum-state-diffusion equation. When the average frequency and duration of the pulse sequence take proper values, the random control sequence is robust, fault- tolerant, and insensitive to pulse strength deviations and interpulse temporal separation in the quasi-periodic sequence. This relaxes the operational requirements placed on quantum control experiments to a great deal.Comment: 7 pages, 6 firgure

Unambiguous determination of gravitational waveforms from binary black hole mergers

Gravitational radiation is properly defined only at future null infinity (\scri), but in practice it is estimated from data calculated at a finite radius. We have used characteristic extraction to calculate gravitational radiation at \scri for the inspiral and merger of two equal mass non-spinning black holes. Thus we have determined the first unambiguous merger waveforms for this problem. The implementation is general purpose, and can be applied to calculate the gravitational radiation, at \scri, given data at a finite radius calculated in another computation.Comment: 4 pages, 3 figures, published versio

Geometric phases in dressed state quantum computation

Geometric phases arise naturally in a variety of quantum systems with observable consequences. They also arise in quantum computations when dressed states are used in gating operations. Here we show how they arise in these gating operations and how one may take advantage of the dressed states producing them. Specifically, we show that that for a given, but arbitrary Hamiltonian, and at an arbitrary time {\tau}, there always exists a set of dressed states such that a given gate operation can be performed by the Hamiltonian up to a phase {\phi}. The phase is a sum of a dynamical phase and a geometric phase. We illustrate the new phase for several systems.Comment: 4 pages, 2 figure

High Fidelity State Transfer Over an Unmodulated Linear XY Spin Chain

We provide a class of initial encodings that can be sent with a high fidelity over an unmodulated, linear, XY spin chain. As an example, an average fidelity of ninety-six percent can be obtained using an eleven-spin encoding to transmit a state over a chain containing ten-thousand spins. An analysis of the magnetic field dependence is given, and conditions for field optimization are provided.Comment: Replaced with published version. 8 pages, 5 figure

Quantum state transfer through a spin chain in a multi-excitation subspace

We investigate the quality of quantum state transfer through a uniformly coupled antiferromagnetic spin chain in a multi-excitation subspace. The fidelity of state transfer using multi-excitation channels is found to compare well with communication protocols based on the ground state of a spin chain with ferromagnetic interactions. Our numerical results support the conjecture that the fidelity of state transfer through a multi-excitation subspace only depends on the number of initial excitations present in the chain and is independent of the excitation ordering. Based on these results, we describe a communication scheme which requires little effort for preparation.Comment: 5 pages, 4 figure