2,011 research outputs found
Coarse-graining in retrodictive quantum state tomography
Quantum state tomography often operates in the highly idealised scenario of
assuming perfect measurements. The errors implied by such an approach are
entwined with other imperfections relating to the information processing
protocol or application of interest. We consider the problem of retrodicting
the quantum state of a system, existing prior to the application of random but
known phase errors, allowing those errors to be separated and removed. The
continuously random nature of the errors implies that there is only one click
per measurement outcome -- a feature having a drastically adverse effect on
data-processing times. We provide a thorough analysis of coarse-graining under
various reconstruction algorithms, finding dramatic increases in speed for only
modest sacrifices in fidelity
Classical noise assists the flow of quantum energy by `momentum rejuvenation'
An important challenge in quantum science is to fully understand the
efficiency of energy flow in networks. Here we present a simple and intuitive
explanation for the intriguing observation that optimally efficient networks
are not purely quantum, but are assisted by some interaction with a `noisy'
classical environment. By considering the system's dynamics in both the
site-basis and the momentum-basis, we show that the effect of classical noise
is to sustain a broad momentum distribution, countering the depletion of high
mobility terms which occurs as energy exits from the network. This picture
predicts that the optimal level of classical noise is reciprocally related to
the linear dimension of the lattice; our numerical simulations verify this
prediction to high accuracy for regular 1D and 2D networks over a range of
sizes up to thousands of sites. This insight leads to the discovery that
dramatic further improvements in performance occur when a driving field targets
noise at the low mobility components
Quantum process tomography via completely positive and trace-preserving projection
We present an algorithm for projecting superoperators onto the set of
completely positive, trace-preserving maps. When combined with gradient descent
of a cost function, the procedure results in an algorithm for quantum process
tomography: finding the quantum process that best fits a set of sufficient
observations. We compare the performance of our algorithm to the diluted
iterative algorithm as well as second-order solvers interfaced with the popular
CVX package for MATLAB, and find it to be significantly faster and more
accurate while guaranteeing a physical estimate.Comment: 13pp, 8 fig
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
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
Quantum sensors based on weak-value amplification cannot overcome decoherence
Sensors that harness exclusively quantum phenomena (such as entanglement) can
achieve superior performance compared to those employing only classical
principles. Recently, a technique based on postselected, weakly-performed
measurements has emerged as a method of overcoming technical noise in the
detection and estimation of small interaction parameters, particularly in
optical systems. The question of which other types of noise may be combatted
remains open. We here analyze whether the effect can overcome decoherence in a
typical field sensing scenario. Benchmarking a weak, postselected measurement
strategy against a strong, direct strategy we conclude that no advantage is
achievable, and that even a small amount of decoherence proves catastrophic to
the weak-value amplification technique.Comment: Published version with improvements to presentation, including
clarifying our understanding of technical noise and quantum nois
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
Spin Amplification for Magnetic Sensors Employing Crystal Defects
Recently there have been several theoretical and experimental studies of the
prospects for magnetic field sensors based on crystal defects, especially
nitrogen vacancy (NV) centres in diamond. Such systems could potentially be
incorporated into an AFM-like apparatus in order to map the magnetic properties
of a surface at the single spin level. In this Letter we propose an augmented
sensor consisting of an NV centre for readout and an `amplifier' spin system
that directly senses the local magnetic field. Our calculations show that this
hybrid structure has the potential to detect magnetic moments with a
sensitivity and spatial resolution far beyond that of a simple NV centre, and
indeed this may be the physical limit for sensors of this class
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
The type of adjuvant in whole inactivated influenza a virus vaccines impacts vaccine-associated enhanced respiratory disease
Influenza A virus (IAV) causes a disease burden in the swine industry in the US and is a challenge to prevent due to substantial genetic and antigenic diversity of IAV that circulate in pig populations. Whole inactivated virus (WIV) vaccines formulated with oil-in-water (OW) adjuvant are commonly used in swine. However, WIV-OW are associated with vaccine-associated enhanced respiratory disease (VAERD) when the hemagglutinin and neuraminidase of the vaccine strain are mismatched with the challenge virus. Here, we assessed if different types of adjuvant in WIV vaccine formulations impacted VAERD outcome. WIV vaccines with a swine δ1-H1N2 were formulated with different commercial adjuvants: OW1, OW2, nano-emulsion squalene-based (NE) and gel polymer (GP). Pigs were vaccinated twice by the intramuscular route, 3 weeks apart, then challenged with an H1N1pdm09 three weeks post-boost and necropsied at 5 days post infection. All WIV vaccines elicited antibodies detected using the hemagglutination inhibition (HI) assay against the homologous vaccine virus, but not against the heterologous challenge virus; in contrast, all vaccinated groups had cross-reactive IgG antibody and IFN-γ responses against H1N1pdm09, with a higher magnitude observed in OW groups. Both OW groups demonstrated robust homologous HI titers and cross-reactivity against heterologous H1 viruses in the same genetic lineage. However, both OW groups had severe immunopathology consistent with VAERD after challenge when compared to NE, GP, and non-vaccinated challenge controls. None of the WIV formulations protected pigs from heterologous virus replication in the lungs or nasal cavity. Thus, although the type of adjuvant in the WIV formulation played a significant role in the magnitude of immune response to homologous and antigenically similar H1, none tested here increased the breadth of protection against the antigenically-distinct challenge virus, and some impacted immunopathology after challenge
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