Almut Beige

Quantum computing using dissipation to remain in a decoherence-free subspace

We propose a new approach to the implementation of quantum gates in which decoherence during the gate operations is strongly reduced. This is achieved by making use of an environment induced quantum Zeno effect that confines the dynamics effectively to a decoherence-free subspace.

CAVITY-LOSS-INDUCED GENERATION OF ENTANGLED ATOMS

Original article can be found at: http://pra.aps.org/ Copyright American Physical Society DOI : 10.1103/PhysRevA.59.2468

Repeat-Until-Success Linear Optics Distributed Quantum Computing

We demonstrate the possibility to perform distributed quantum computing using only single photon sources (atom-cavity-like systems), linear optics and photon detectors. The qubits are encoded in stable ground states of the sources. To implement a universal two-qubit gate, two photons should be generated simultaneously and pass through a linear optics network, where a measurement is performed on them. Gate operations can be repeated until a success is heralded without destroying the qubits at any stage of the operation. In contrast to other schemes, this does not require explicit qubit-qubit interactions, a priori entangled ancillas nor the feeding of photons into photon sources.

Secure communication with single-photon two-qubit states

A ball type valve for high pressure lines is designed so that the spherical valve member is offset from the centerline of the valve stem in such a manner that the spherical valve member in the open position has a cylindrical fluid passage in parallel alignment with the flow axis of the inlet and outlet passages, but in the closed position is transversely disposed across the valve chamber so that the cylindrical flow passage is perpendicular to the flow axis of the inlet and outlet passages. Due to the eccentric placement of the valve stem onto the spherical valve member, and the eccentric placement of the bore of the bonnet in which the valve stem rotates relative to the centerline of the bonnet, and the eccentric placement of the bonnet opening relative to the centerline of the inlet and outlet passages, the spherical valve member rotates in an arc from open position to closed position so as to approach the valve seat tangentially with a wiping motion and at an angle of approach so that the spherical valve member is self-locking. Further, the design of the valve is such that in open position, the surface of the spherical valve member engages the wall of the valve chamber to prevent further rotation. This eliminates the need for an external stop and prevents rotation of the spherical valve member beyond the full open position.

Repeat-until-success quantum computing using stationary and flying qubits

We introduce an architecture for robust and scalable quantum computation using both stationary qubits (e.g., single photon sources made out of trapped atoms, molecules, ions, quantum dots, or defect centers in solids) and flying qubits (e.g., photons). Our scheme solves some of the most pressing problems in existing nonhybrid proposals, which include the difficulty of scaling conventional stationary qubit approaches, and the lack of practical means for storing single photons in linear optics setups. We combine elements of two previous proposals for distributed quantum computing, namely the efficient photon-loss tolerant build up of cluster states by Barrett and Kok [Phys. Rev. A 71, 060310(R) (2005)] with the idea of repeat-until-success (RUS) quantum computing by Lim et al. [Phys. Rev. Lett. 95, 030505 (2005)]. This idea can be used to perform eventually deterministic two qubit logic gates on spatially separated stationary qubits via photon pair measurements. Under nonideal conditions, where photon loss is a possibility, the resulting gates can still be used to build graph states for one-way quantum computing. In this paper, we describe the RUS method, present possible experimental realizations, and analyze the generation of graph states.

Projection postulate and atomic quantum Zeno effect.

The projection postulate has been used to predict a slow-down of the time evolution of the state of a system under rapidly repeated measurements, and ultimately a freezing of the state. To test this so-called quantum Zeno effect an experiment was performed by Itano et al. (Phys. Rev. A 41, 2295 (1990)) in which an atomic-level measurement was realized by means of a short laser pulse. The relevance of the results has given rise to controversies in the literature. In particular the projection postulate and its applicability in this experiment have been cast into doubt. In this paper we show analytically that for a wide range of parameters such a short laser pulse acts as an effective level measurement to which the usual projection postulate applies with high accuracy. The corrections to the ideal reductions and their accumulation over n pulses are calculated. Our conclusion is that the projection postulate is an excellent pragmatic tool for a quick and simple understanding of the slow-down of time evolution in experiments of this type. However, corrections have to be included, and an actual freezing does not seem possible because of the finite duration of measurements.

Entangled-state preparation via dissipation-assisted adiabatic passages

The main obstacle for coherent control of open quantum systems is decoherence due to different dissipation channels and the inability to precisely control experimental parameters. To overcome these problems we propose to use dissipation-assisted adiabatic passages. These are relatively fast processes where the presence of spontaneous decay rates corrects for errors due to nonadiabaticity while the system remains in a decoherence-free state and behaves as predicted for an adiabatic passage. As a concrete example we present a scheme to entangle atoms by moving them in and out of an optical cavity.

Entangling atoms and ions in dissipative environments

Quantum information processing rests on our ability to manipulate quantum superpositions through coherent unitary transformations, and to establish entanglement between constituent quantum components of the processor. The quantum information processor (a linear ion trap, or a cavity confining the radiation field for example) exists in a dissipative environment. We discuss ways in which entanglement can be established within such dissipative environments. We can even make use of a strong interaction of the system with its environment to produce entanglement in a controlled way.

Generalized Hong–Ou–Mandel experiments with bosons and fermions

The Hong–Ou–Mandel (HOM) dip has played an important role in recent linear optics experiments. It is crucial for quantum computing with photons and can be used to characterize the quality of single photon sources and linear optics setups. In this paper, we consider generalized HOM experiments with N bosons or fermions passing simultaneously, i.e. within their coherence time, through a symmetric Bell multiport beam splitter. It is shown that for an even number of bosons, the HOM dip occurs naturally in the coincidence detection in the output ports. In contrast, fermions always leave the setup separately, exhibiting perfect coincidence detection. Our results can be used to verify or employ the quantum statistics of particles experimentally.

Robust Entanglement through Macroscopic Quantum Jumps

We propose an entanglement generation scheme that requires neither the coherent evolution of a quantum system nor the detection of single photons. Instead, the desired state is heralded by a macroscopic quantum jump. Macroscopic quantum jumps manifest themselves as a random telegraph signal with long intervals of intense fluorescence (light periods) interrupted by the complete absence of photons (dark periods). Here we show that a system of two atoms trapped inside an optical cavity can be designed such that a dark period prepares the atoms in a maximally entangled ground state. Achieving fidelities above 0.9 is possible even when the single-atom cooperativity parameter is as low as 10 and when using a photon detector with an efficiency as low as eta=0.2.