3,387 research outputs found
The quantum-classical crossover of a field mode
We explore the quantum-classical crossover in the behaviour of a quantum
field mode. The quantum behaviour of a two-state system - a qubit - coupled to
the field is used as a probe. Collapse and revival of the qubit inversion form
the signature for quantum behaviour of the field and continuous Rabi
oscillations form the signature for classical behaviour of the field. We
demonstrate both limits in a single model for the full coupled system, for
states with the same average field strength, and so for qubits with the same
Rabi frequency.Comment: 6 pages, 3 figures (in this version the figures, text and references
have all been expanded
Integrating process and factor understanding of environmental innovation by water utilities
Innovations in technology and organisations are central to enabling the water sector to adapt to major environmental changes such as climate change, land degradation or drinking water pollution. While there are literatures on innovation as a process and on the factors that influence it, there is little research that integrates these. Development of such an integrated understanding of innovation is central to understanding how policy makers and organisations can stimulate and direct environmental innovation. In the research reported here a framework is developed that enables such an integrated analysis of innovation process and factors. From research interviews and the literature twenty factors were identified that affect the five stages of the environmental innovation process in English and Welsh water utilities. The environmental innovations investigated are measures taken by water utilities to reduce or prevent pollution in drinking water catchments rather than technical measures to treat water. These Source Control Interventions are similar to other environmental innovations, such as ecosystem and species conservation, in that they emphasise the mix of technology, management and engagement with multiple actors. Results show that in water utilities direct performance regulation and regulation that raises awareness of a ‘performance’ gap as a ‘problem’ can stimulate innovation, but only under particular organisational, natural physical and regulatory conditions. The integrated framework also suggests that while flexible or framework legislation (e.g. Water Framework Directive) does not stimulate innovation in itself, it has shaped the option spaces and characteristics of innovations selected towards source control instead of technical end-of-pipe solutions
Freezing distributed entanglement in spin chains
We show how to freeze distributed entanglement that has been created from the
natural dynamics of spin chain systems. The technique that we propose simply
requires single-qubit operations and isolates the entanglement in specific
qubits at the ends of branches. Such frozen entanglement provides a useful
resource, for example for teleportation or distributed quantum processing. The
scheme can be applied to a wide range of systems -- including actual spin
systems and alternative qubit embodiments in strings of quantum dots, molecules
or atoms.Comment: 5 pages, to appear in Phys. Rev. A (Rapid Communication
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
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
Engineering entanglement for metrology with rotating matter waves
Entangled states of rotating, trapped ultracold bosons form a very promising scenario for quantum metrology. In order to employ such states for metrology, it is vital to understand their detailed form and the enhanced accuracy with which they could measure phase, in this case generated through rotation. In this work, we study the rotation of ultracold bosons in an asymmetric trapping potential beyond the lowest Landau level (LLL) approximation. We demonstrate that while the LLL can identify reasonably the critical frequency for a quantum phase transition and entangled state generation, it is vital to go beyond the LLL to identify the details of the state and quantify the quantum Fisher information (which bounds the accuracy of the phase measurement). We thus identify a new parameter regime for useful entangled state generation, amenable to experimental investigation
Radiative corrections and quantum gates in molecular systems
We propose a method for quantum information processing using molecules
coupled to an external laser field. This utilizes molecular interactions,
control of the external field and an effective energy shift of the
doubly-excited state of two coupled molecules. Such a level shift has been seen
in the two-photon resonance experiments recently reported in Ref. [1]. Here we
show that this can be explained in terms of the QED Lamb shift. We quantify the
performance of the proposed quantum logic gates in the presence of dissipative
mechanisms. The unitary transformations required for performing one- and
two-qubit operations can be implemented with present day technology.Comment: 4 pages, 3 figures, Updated to correct important missing referenc
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
Entanglement of superconducting charge qubits by homodyne measurement
We present a scheme by which projective homodyne measurement of a microwave
resonator can be used to generate entanglement between two superconducting
charge qubits coupled to this resonator. The non-interacting qubits are
initialised in a product of their ground states, the resonator is initialised
in a coherent field state, and the state of the system is allowed to evolve
under a rotating wave Hamiltonian. Making a homodyne measurement on the
resonator at a given time projects the qubits into an state of the form (|gg> +
exp(-i phi)|ee>)/sqrt(2). This protocol can produce states with a fidelity as
high as required, with a probability approaching 0.5. Although the system
described is one that can be used to display revival in the qubit oscillations,
we show that the entanglement procedure works at much shorter timescales.Comment: 17 pages, 7 figure
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