431 research outputs found
A continuous-time diffusion limit theorem for dynamical decoupling and intrinsic decoherence
We discuss a few mathematical aspects of random dynamical decoupling, a key
tool procedure in quantum information theory. In particular, we place it in the
context of discrete stochastic processes, limit theorems and CPT semigroups on
matrix algebras. We obtain precise analytical expressions for expectation and
variance of the density matrix and fidelity over time in the continuum-time
limit depending on the system Lindbladian, which then lead to rough short-time
estimates depending only on certain coupling strengths. We prove that dynamical
decoupling does not work in the case of intrinsic (i.e., not
environment-induced) decoherence, and together with the above-mentioned
estimates this yields a novel method of partially identifying intrinsic
decoherence.Comment: 24 pages. Final published versio
Dynamic decoupling and homogenization of continuous variable systems
For finite-dimensional quantum systems, such as qubits, a well established
strategy to protect such systems from decoherence is dynamical decoupling.
However many promising quantum devices, such as oscillators, are infinite
dimensional, for which the question if dynamical decoupling could be applied
remained open. Here we first show that not every infinite-dimensional system
can be protected from decoherence through dynamical decoupling. Then we develop
dynamical decoupling for continuous variable systems which are described by
quadratic Hamiltonians. We identify a condition and a set of operations that
allow us to map a set of interacting harmonic oscillators onto a set of
non-interacting oscillators rotating with an averaged frequency, a procedure we
call homogenization. Furthermore we show that every quadratic
system-environment interaction can be suppressed with two simple operations
acting only on the system. Using a random dynamical decoupling or
homogenization scheme, we develop bounds that characterize how fast we have to
work in order to achieve the desired uncoupled dynamics. This allows us to
identify how well homogenization can be achieved and decoherence can be
suppressed in continuous variable systems.Comment: 14 page
Control of quantum noise: on the role of dilations
We show that every finite-dimensional quantum system with Markovian time
evolution has an autonomous unitary dilation which can be dynamically
decoupled. Since there is also always an autonomous unitary dilation which
cannot be dynamically decoupled, this highlights the role of dilations in the
control of quantum noise. We construct our dilation via a time-dependent
version of Stinespring in combination with Howland's clock Hamiltonian and
certain point-localised states, which may be regarded as a C*-algebraic
analogue of improper bra-ket position eigenstates and which are hence of
independent mathematical and physical interest.Comment: 17
Distinguishing decoherence from alternative quantum theories by dynamical decoupling
A longstanding challenge in the foundations of quantum mechanics is the
verification of alternative collapse theories despite their mathematical
similarity to decoherence. To this end, we suggest a novel method based on
dynamical decoupling. Experimental observation of nonzero saturation of the
decoupling error in the limit of fast decoupling operations can provide
evidence for alternative quantum theories. As part of the analysis we prove
that unbounded Hamiltonians can always be decoupled, and provide novel
dilations of Lindbladians.Comment: 8 pages. Final published versio
Women’s perspectives on Human Papillomavirus self-sampling in the context of the UK cervical screening programme
Background: Testing for human papillomavirus (HPV) is being incorporated into the cervical
screening programme, with the probable future introduction of HPV as a primary test
and a possibility of HPV self-sampling.
In anticipation of this development, we sought to
inform future policy and practice by identifying potential barriers to HPV self-sampling.
Methods: A cross-sectional
survey of 194 women aged 20-64
years was conducted.
Logistic regression analysis was used to identify determinants of self-sampling
intentions.
A purposive subsample of 19 women who reported low self-sampling
intentions
were interviewed. Interviews were framework-analysed.
Results: Most survey participants (N=133, 69.3%) intended to HPV self-sample.
Lower
intention was associated with lower self-efficacy
(OR=24.96, P≤.001), lower education
(OR=6.06, P≤.05) and lower perceived importance of HPV as a cause of cervical
cancer (OR=2.33, P≤.05). Interviews revealed personal and system-related
barriers.
Personal barriers included a lack of knowledge about HPV self-sampling,
women’s low
confidence in their ability to self-sample
correctly and low confidence in the subsequent
results. System-related
factors included a lack of confidence in the rationale for
modifying the current cervical screening programme, and concerns about sample
contamination
and identity theft.
Conclusions: Insights gained from this research can be used to guide further enquiry
into the possibility of HPV self-sampling
and to help inform future policy and practice.
Personal and system-related
barriers including low confidence in the reasons for
changing current cervical screening provision need to be addressed, should HPV self-sampling
be incorporated into the cervical screening programme
Stability and convergence of dynamical decoupling with finite amplitude controls
Dynamical decoupling is a key method to mitigate errors in a quantum mechanical system, and we studied it in a series of papers dealing, in particular, with the problems arising from unbounded Hamiltonians. The standard bangbang model of dynamical decoupling, which we also used in those papers, requires decoupling operations with infinite amplitude, which is, strictly speaking, unrealistic from a physical point of view. In this paper, we look at decoupling operations of finite amplitude, discuss under what assumptions dynamical decoupling works with such finite amplitude operations, and show how the bangbang description arises as a limit, hence justifying it as a reasonable approximation
Angular Sizes and Effective Temperatures of O-type Stars from Optical Interferometry with the CHARA Array
We present interferometric observations of six O-type stars that were made with the Precision Astronomical Visible Observations beam combiner at the Center for High Angular Resolution Astronomy (CHARA) Array. The observations include multiple brackets for three targets, λ Ori A, ζ Oph, and 10 Lac, but there are only preliminary, single observations of the other three stars, ξ Per, α Cam, and ζ Ori A. The stellar angular diameters range from 0.55 mas for ζ Ori A down to 0.11 mas for 10 Lac, the smallest star yet resolved with the CHARA Array. The rotational oblateness of the rapidly rotating star ζ Oph is directly measured for the first time. We assembled ultraviolet to infrared flux measurements for these stars, and then derived angular diameters and reddening estimates using model atmospheres and an effective temperature set by published results from analysis of the line spectrum. The model-based angular diameters are in good agreement with those observed. We also present estimates for the effective temperatures of these stars, derived by setting the interferometric angular size and fitting the spectrophotometry
Superconductivity and the upper critical field in the chiral noncentrosymmetric superconductor NbRh2B2
NbRh2B2 crystallises in a chiral noncentrosymmetric structure and exhibits bulk type-II superconductivity below 7.46(5) K. Here we show that the temperature dependence of the upper critical field deviates from the behaviour expected for both Werthamer-Helfand-Hohenberg and the Ginzburg-Landau models and that ÎĽ0Hc2 (0) ~ 18 T exceeds the Pauli paramagnetic limit, ÎĽ0HP = 13.9 T. We explore the reasons for this enhancement. Transverse-field muon spectroscopy measurements suggest that the superconducting gap is either s-wave or (s + s)-wave, a the pressure dependence of Tc reveals the superconducting gap is primarily s- wave in character. The magnetic penetration depth lambda(0) = 595(5) nm. Heat capacity measurements reveal the presence of a multigap (s + s)-wave superconducting order parameter and moderate electron-phonon coupling
Taming the rotating wave approximation
The interaction between light and matter is one of the oldest research areas of quantum mechanics, and a field that just keeps on delivering new insights and applications. With the arrival of cavity and circuit quantum electrodynamics we can now achieve strong light-matter couplings which form the basis of most implementations of quantum technology. But quantum information processing also has high demands requiring total error rates of fractions of percentage in order to be scalable (fault-tolerant) to useful applications. Since errors can also arise from modelling, this has brought into center stage one of the key approximations of quantum theory, the Rotating Wave Approximation (RWA) of the quantum Rabi model, leading to the Jaynes-Cummings Hamiltonian. While the RWA is often very good and incredibly useful to understand light-matter interactions, there is also growing experimental evidence of regimes where it is a bad approximation. Here, we ask and answer a harder question: for which experimental parameters is the RWA, although perhaps qualitatively adequate, already not good enough to match the demands of scalable quantum technology? For example, when is the error at least, and when at most, 1%? To answer this, we develop rigorous non-perturbative bounds taming the RWA. We find that these bounds not only depend, as expected, on the ratio of the coupling strength and the oscillator frequency, but also on the average number of photons in the initial state. This confirms recent experiments on photon-dressed Bloch-Siegert shifts. We argue that with experiments reporting controllable cavity states with hundreds of photons and with quantum error correcting codes exploring more and more of Fock space, this state-dependency of the RWA is increasingly relevant for the field of quantum computation, and our results pave the way towards a better understanding of those experiments
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