2,442 research outputs found
Quantum dynamics in a tiered non-Markovian environment
We introduce a new analytical method for studying the open quantum systems
problem of a discrete system weakly coupled to an environment of harmonic
oscillators. Our approach is based on a phase space representation of the
density matrix for a system coupled to a two-tiered environment. The dynamics
of the system and its immediate environment are resolved in a non-Markovian
way, and the environmental modes of the inner environment can themselves be
damped by a wider `universe'. Applying our approach to the canonical cases of
the Rabi and spin-boson models we obtain new analytical expressions for an
effective thermalisation temperature and corrections to the environmental
response functions as direct consequences of considering such a tiered
environment. A comparison with exact numerical simulations confirms that our
approximate expressions are remarkably accurate, while their analytic nature
offers the prospect of deeper understanding of the physics which they describe.
A unique advantage of our method is that it permits the simultaneous inclusion
of a continuous bath as well as discrete environmental modes, leading to wide
and versatile applicability.Comment: Video abstract available at
http://iopscience.iop.org/1367-2630/17/2/023063. 15 pages, 6 figure
High fidelity all-optical control of quantum dot spins: detailed study of the adiabatic approach
Confined electron spins are preferred candidates for embodying quantum
information in the solid state. A popular idea is the use of optical excitation
to achieve the ``best of both worlds'', i.e. marrying the long spin decoherence
times with rapid gating. Here we study an all-optical adiabatic approach to
generating single qubit phase gates. We find that such a gate can be extremely
robust against the combined effect of all principal sources of decoherence,
with an achievable fidelity of 0.999 even at finite temperature. Crucially this
performance can be obtained with only a small time cost: the adiabatic gate
duration is within about an order of magnitude of a simple dynamic
implementation. An experimental verification of these predictions is
immediately feasible with only modest resources
Practicality of spin chain 'wiring' in diamond quantum technologies
Coupled spin chains are promising candidates for 'wiring up' qubits in
solid-state quantum computing (QC). In particular, two nitrogen-vacancy centers
in diamond can be connected by a chain of implanted nitrogen impurities; when
driven by a suitable global fields the chain can potentially enable quantum
state transfer at room temperature. However, our detailed analysis of error
effects suggests that foreseeable systems may fall far short of the fidelities
required for QC. Fortunately the chain can function in the more modest role as
a mediator of noisy entanglement, enabling QC provided that we use subsequent
purification. For instance, a chain of 5 spins with inter-spin distances of 10
nm has finite entangling power as long as the T2 time of the spins exceeds 0.55
ms. Moreover we show that re-purposing the chain this way can remove the
restriction to nearest-neighbor interactions, so eliminating the need for
complicated dynamical decoupling sequences.Comment: 5 pages (plus 5-page supplement
Measurement-based approach to entanglement generation in coupled quantum dots
Measurements provide a novel mechanism for generating the entanglement
resource necessary for performing scalable quantum computation. Recently, we
proposed a method for performing parity measurements in a coupled quantum dot
system. In this paper we generalise this scheme and perform a comprehensive
analytic and numerical study of environmental factors. We calculate the effects
of possible error sources including non-ideal photon detectors, ineffective
spin-selective excitation and dot distinguishability (both spatial and
spectral). Furthermore, we present an experimental approach for verifying the
success of the parity measurement
Modeling physical and chemical climate of the northeastern United States for a geographic information system
A model of physical and chemical climate was developed for New York and New England that can be used in a GIs for integration with ecosystem models. The variables included are monthly average maximum and minimum daily temperatures, precipitation, humidity, and solar radiation, as well as annual atmospheric deposition of sulfur and nitrogen. Equations generated from regional data bases were combined with a digital elevation model of the region to generate digital coverages of each variable
Optical Quantum Computation with Perpetually Coupled Spins
The possibility of using strongly and continuously interacting spins for
quantum computation has recently been discussed. Here we present a simple
optical scheme that achieves this goal while avoiding the drawbacks of earlier
proposals. We employ a third state, accessed by a classical laser field, to
create an effective barrier to information transfer. The mechanism proves to be
highly efficient both for continuous and pulsed laser modes; moreover it is
very robust, tolerating high decay rates for the excited states. The approach
is applicable to a broad range of systems, in particular dense structures such
as solid state self-assembled (e.g., molecular) devices. Importantly, there are
existing structures upon which `first step' experiments could be immediately
performed.Comment: 5 pages including 3 figures. Updated to published versio
Superabsorption of light via quantum engineering
Almost 60 years ago Dicke introduced the term superradiance to describe a
signature quantum effect: N atoms can collectively emit light at a rate
proportional to N^2. Even for moderate N this represents a significant increase
over the prediction of classical physics, and the effect has found applications
ranging from probing exciton delocalisation in biological systems, to
developing a new class of laser, and even in astrophysics. Structures that
super-radiate must also have enhanced absorption, but the former always
dominates in natural systems. Here we show that modern quantum control
techniques can overcome this restriction. Our theory establishes that
superabsorption can be achieved and sustained in certain simple nanostructures,
by trapping the system in a highly excited state while extracting energy into a
non-radiative channel. The effect offers the prospect of a new class of quantum
nanotechnology, capable of absorbing light many times faster than is currently
possible; potential applications of this effect include light harvesting and
photon detection. An array of quantum dots or a porphyrin ring could provide an
implementation to demonstrate this effect
Ensemble based quantum metrology
The field of quantum metrology promises measurement devices that are
fundamentally superior to conventional technologies. Specifically, when quantum
entanglement is harnessed the precision achieved is supposed to scale more
favourably with the resources employed, such as system size and the time
required. Here we consider measurement of magnetic field strength using an
ensemble of spins, and we identify a third essential resource: the initial
system polarisation, i.e. the low entropy of the original state. We find that
performance depends crucially on the form of decoherence present; for a
plausible dephasing model, we describe a quantum strategy which can indeed beat
the standard quantum limit
Quantum metrology with molecular ensembles
This work was supported by the EPSRC through QIP IRC (Grants No. GR/S82176/01 and No. GR/S15808/01), the National Research Foundation and Ministry of Education, Singapore, the DAAD, and the Royal Society.The field of quantum metrology promisesmeasurement devices that are fundamentally superior to conventional technologies. Specifically, when quantum entanglement is harnessed, the precision achieved is supposed to scale more favorably with the resources employed, such as system size and time required. Here, we consider measurement of magnetic-field strength using an ensemble of spin-active molecules. We identify a third essential resource: the change in ensemble polarization (entropy increase) during the metrology experiment. We find that performance depends crucially on the form of decoherence present; for a plausible dephasing model, we describe a quantum strategy, which can indeed beat the standard strategy.Publisher PDFPeer reviewe
Global Optical Control of a Quantum Spin Chain
Quantum processors which combine the long decoherence times of spin qubits
together with fast optical manipulation of excitons have recently been the
subject of several proposals. I show here that arbitrary single- and entangling
two-qubit gates can be performed in a chain of perpetually coupled spin qubits
solely by using laser pulses to excite higher lying states. It is also
demonstrated that universal quantum computing is possible even if these pulses
are applied {\it globally} to a chain; by employing a repeating pattern of four
distinct qubit units the need for individual qubit addressing is removed. Some
current experimental qubit systems would lend themselves to implementing this
idea.Comment: 5 pages, 3 figure
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