Cool for Cats

The iconic Schr\"odinger's cat state describes a system that may be in a superposition of two macroscopically distinct states, for example two clearly separated oscillator coherent states. Quite apart from their role in understanding the quantum classical boundary, such states have been suggested as offering a quantum advantage for quantum metrology, quantum communication and quantum computation. As is well known these applications have to face the difficulty that the irreversible interaction with an environment causes the superposition to rapidly evolve to a mixture of the component states in the case that the environment is not monitored. Here we show that by engineering the interaction with the environment there exists a large class of systems that can evolve irreversibly to a cat state. To be precise we show that it is possible to engineer an irreversible process so that the steady state is close to a pure Schr\"odinger's cat state by using double well systems and an environment comprising two-photon (or phonon) absorbers. We also show that it should be possible to prolong the lifetime of a Schr\"odinger's cat state exposed to the destructive effects of a conventional single-photon decohering environment. Our protocol should make it easier to prepare and maintain Schr\"odinger cat states which would be useful in applications of quantum metrology and information processing as well as being of interest to those probing the quantum to classical transition.Comment: 10 pages, 7 figures. Significantly updated version with supplementary informatio

Selective spin coupling through a single exciton

We present a novel scheme for performing a conditional phase gate between two spin qubits in adjacent semiconductor quantum dots through delocalized single exciton states, formed through the inter-dot Foerster interaction. We consider two resonant quantum dots, each containing a single excess conduction band electron whose spin embodies the qubit. We demonstrate that both the two-qubit gate, and arbitrary single-qubit rotations, may be realized to a high fidelity with current semiconductor and laser technology.Comment: 5 pages, 3 figures; published version, equation formatting improved, references adde

Weak nonlinearities: A new route to optical quantum computation

Quantum information processing (QIP) offers the promise of being able to do things that we cannot do with conventional technology. Here we present a new route for distributed optical QIP, based on generalized quantum non-demolition measurements, providing a unified approach for quantum communication and computing. Interactions between photons are generated using weak non-linearities and intense laser fields--the use of such fields provides for robust distribution of quantum information. Our approach requires only a practical set of resources, and it uses these very efficiently. Thus it promises to be extremely useful for the first quantum technologies, based on scarce resources. Furthermore, in the longer term this approach provides both options and scalability for efficient many-qubit QIP.Comment: 7 Pages, 4 Figure

Efficient optical quantum information processing

Quantum information offers the promise of being able to perform certain communication and computation tasks that cannot be done with conventional information technology (IT). Optical Quantum Information Processing (QIP) holds particular appeal, since it offers the prospect of communicating and computing with the same type of qubit. Linear optical techniques have been shown to be scalable, but the corresponding quantum computing circuits need many auxiliary resources. Here we present an alternative approach to optical QIP, based on the use of weak cross-Kerr nonlinearities and homodyne measurements. We show how this approach provides the fundamental building blocks for highly efficient non-absorbing single photon number resolving detectors, two qubit parity detectors, Bell state measurements and finally near deterministic control-not (CNOT) gates. These are essential QIP devicesComment: Accepted to the Journal of optics B special issue on optical quantum computation; References update

The efficiencies of generating cluster states with weak non-linearities

We propose a scalable approach to building cluster states of matter qubits using coherent states of light. Recent work on the subject relies on the use of single photonic qubits in the measurement process. These schemes can be made robust to detector loss, spontaneous emission and cavity mismatching but as a consequence the overhead costs grow rapidly, in particular when considering single photon loss. In contrast, our approach uses continuous variables and highly efficient homodyne measurements. We present a two-qubit scheme, with a simple bucket measurement system yielding an entangling operation with success probability 1/2. Then we extend this to a three-qubit interaction, increasing this probability to 3/4. We discuss the important issues of the overhead cost and the time scaling. This leads to a "no-measurement" approach to building cluster states, making use of geometric phases in phase space.Comment: 21 pages, to appear in special issue of New J. Phys. on "Measurement-Based Quantum Information Processing

Model for an irreversible bias current in the superconducting qubit measurement process

The superconducting charge-phase "quantronium" qubit is considered in order to develop a model for the measurement process used in the experiment of Vion [Science 296, 886 (2002)]. For this model we propose a method for including the bias current in the readout process in a fundamentally irreversible way, which to first order is approximated by the Josephson junction tilted-washboard potential phenomenology. The decohering bias current is introduced in the form of a Lindblad operator and the Wigner function for the current-biased readout Josephson junction is derived and analyzed. During the readout current pulse used in the quantronium experiment we find that the coherence of the qubit initially prepared in a symmetric superposition state is lost at a time of 0.2 ns after the bias current pulse has been applied, a time scale that is much shorter than the experimental readout time. Additionally we look at the effect of Johnson-Nyquist noise with zero mean from the current source during the qubit manipulation and show that the decoherence due to the irreversible bias current description is an order of magnitude smaller than that found through adding noise to the reversible tilted-washboard potential model. Our irreversible bias current model is also applicable to persistent-current-based qubits where the state is measured according to its flux via a small-inductance direct-current superconducting quantum interference device

Signatures of chaotic and non-chaotic-like behaviour in a non-linear quantum oscillator through photon detection

The driven non-linear duffing osillator is a very good, and standard, example of a quantum mechanical system from which classical-like orbits can be recovered from unravellings of the master equation. In order to generated such trajectories in the phase space of this oscillator in this paper we use a the quantum jumps unravelling together with a suitable application of the correspondence principle. We analyse the measured readout by considering the power spectra of photon counts produced by the quantum jumps. Here we show that localisation of the wave packet from the measurement of the oscillator by the photon detector produces a concomitant structure in the power spectra of the measured output. Furthermore, we demonstrate that this spectral analysis can be used to distinguish between different modes of the underlying dynamics of the oscillator.Comment: 7 pages, 6 figure

Applications of Coherent Population Transfer to Quantum Information Processing

We develop a theoretical framework for the exploration of quantum mechanical coherent population transfer phenomena, with the ultimate goal of constructing faithful models of devices for classical and quantum information processing applications. We begin by outlining a general formalism for weak-field quantum optics in the Schr\"{o}dinger picture, and we include a general phenomenological representation of Lindblad decoherence mechanisms. We use this formalism to describe the interaction of a single stationary multilevel atom with one or more propagating classical or quantum laser fields, and we describe in detail several manifestations and applications of electromagnetically induced transparency. In addition to providing a clear description of the nonlinear optical characteristics of electromagnetically transparent systems that lead to ``ultraslow light,'' we verify that -- in principle -- a multi-particle atomic or molecular system could be used as either a low power optical switch or a quantum phase shifter. However, we demonstrate that the presence of significant dephasing effects destroys the induced transparency, and that increasing the number of particles weakly interacting with the probe field only reduces the nonlinearity further. Finally, a detailed calculation of the relative quantum phase induced by a system of atoms on a superposition of spatially distinct Fock states predicts that a significant quasi-Kerr nonlinearity and a low entropy cannot be simultaneously achieved in the presence of arbitrary spontaneous emission rates. Within our model, we identify the constraints that need to be met for this system to act as a one-qubit and a two-qubit conditional phase gate.Comment: 25 pages, 14 figure

Single photon quantum non-demolition in the presence of inhomogeneous broadening

Electromagnetically induced transparency (EIT) has been often proposed for generating nonlinear optical effects at the single photon level; in particular, as a means to effect a quantum non-demolition measurement of a single photon field. Previous treatments have usually considered homogeneously broadened samples, but realisations in any medium will have to contend with inhomogeneous broadening. Here we reappraise an earlier scheme [Munro \textit{et al.} Phys. Rev. A \textbf{71}, 033819 (2005)] with respect to inhomogeneities and show an alternative mode of operation that is preferred in an inhomogeneous environment. We further show the implications of these results on a potential implementation in diamond containing nitrogen-vacancy colour centres. Our modelling shows that single mode waveguide structures of length $200 \mu\mathrm{m}$ in single-crystal diamond containing a dilute ensemble of NV$^-$ of only 200 centres are sufficient for quantum non-demolition measurements using EIT-based weak nonlinear interactions.Comment: 21 pages, 9 figures (some in colour) at low resolution for arXiv purpose

The Depth Resolution of Secondary Ion Mass Spectrometers: A Critical Evaluation

The ability of five Secondary Ion Mass Spectrometry (SIMS) instruments to resolve thin layer and modulated dopant structures by depth profiling has been assessed. Three magnetic sector instruments (two Cameca IMS 3f\u27s and one 4f), which use optical gating and a high extraction field, were used, together with two different quadrupole based instruments (EVA 2000 and Atomika) , which use electronic gating and a low extraction field. The test structure, a thirty-one peak boron-in-silicon modulating dopant structure, was grown by Molecular Beam Epitaxy (MBE). In all the depth profiles the near surface peaks appeared narrow and asymmetric, being broadened only by fundamental processes (e.g., atomic mixing and recoil implantation). As the profiles proceeded, however, further broadening was observed. This phenomena varied markedly both from one instrument to another and from one experiment to another on the same instrument. In some cases the loss of depth resolution with depth was manifested by broadening mainly in the leading edge, in others the trailing edge, of successive boron peaks. The \u27order of merit\u27 of the instruments thus depended on the parameter used to define depth resolution. The loss of peak (depth) resolution with depth was due to variations in primary ion beam density across the gated area of the crater, which led to uneven etching. The changes in peak shape with depth can be explained by a numerical model of the etching process. These observations dictate that the depth resolution of a SIMS instrument should not be measured in terms of a single interface width, such as the leading or trailing edge