95 research outputs found
Shaken not stirred: Creating exotic angular momentum states by shaking an optical lattice
We propose a method to create higher orbital states of ultracold atoms in the
Mott regime of an optical lattice. This is done by periodically modulating the
position of the trap minima (known as shaking) and controlling the interference
term of the lasers creating the lattice. These methods are combined with
techniques of shortcuts to adiabaticity. As an example of this, we show
specifically how to create an anti-ferromagnetic type ordering of angular
momentum states of atoms. The specific pulse sequences are designed using
Lewis-Riesenfeld invariants and a four-level model for each well. The results
are compared with numerical simulations of the full Schroedinger equation.Comment: 20 pages, 8 figure
Quantum state transfer via invariant based shortcuts to adiabaticity
Adiabatic processes in quantum mechanics are very useful to prepare and manipulate quantum states but have the drawback of requiring long operation times. Hence there is a long time for the system to interact with its environment which can lead to a loss of coherence of the final state. This decoherence is problematic for implementing future quantum technologies which require the state's quantum mechanical features. “Shortcuts to Adiabaticity"(STA) provides a toolbox of methods to improve on adiabatic processes. Using these methods one can derive alternative processes which work for much shorter times with perfect fidelity. Since adiabatic processes are ubiquitous in atomic, molecular and optical physics, there is a broad scope of application for STA. In this thesis, STA (especially those using Lewis-Riesenfeld invariants) are applied to a variety of quantum systems for the purpose of quantum state transfer. In particular I show that STA control schemes in two- and three-level systems can be optimised to be more stable against unwanted uncontrollable transitions than adiabatic methods with the same operation time. I also show that STA methods can be applied in a triple well ring system with complex tunnelling, in optical lattices for the purposes of generating a higher orbital state of neutral atoms and in Penning traps to quickly compress or expand the trapped ion wavefunction. Finally I also investigate the effect of classical Poisson white noise on adiabatic processes
Classical dissipative cost of quantum control
Protocols for non-adiabatic quantum control often require the use of
classical time varying fields. Assessing the thermodynamic cost of such
protocols, however, is far from trivial. In this letter we study the
irreversible entropy produced by the classical apparatus generating the control
fields, thus providing a direct link between the cost of a control protocol and
dissipation. We focus, in particular, on the case of time-dependent magnetic
fields and shortcuts to adiabaticity. Our results are showcased with two
experimentally realisable case studies: the Landau-Zener model of a spin-1/2
particle in a magnetic field and an ion confined in a Penning trap
Universally Robust Quantum Control
We study the robustness of the evolution of a quantum system against small
uncontrolled variations in parameters in the Hamiltonian. We show that the
fidelity susceptibility, which quantifies the perturbative error to leading
order, can be expressed in superoperator form and use this to derive control
pulses which are robust to any class of systematic unknown errors. The proposed
optimal control protocol is equivalent to searching for a sequence of unitaries
that mimics the first-order moments of the Haar distribution, i.e. it
constitutes a 1-design. We highlight the power of our results for error
resistant single- and two-qubit gates.Comment: 14 pages, 6 figure
First Passage Times for Continuous Quantum Measurement Currents
The First Passage Time (FPT) is the time taken for a stochastic process to
reach a desired threshold. It finds wide application in various fields and has
recently become particularly important in stochastic thermodynamics, due to its
relation to kinetic uncertainty relations (KURs). In this letter we address the
FPT of the stochastic measurement current in the case of continuously measured
quantum systems. Our approach is based on a charge-resolved master equation,
which is related to the Full-Counting statistics of charge detection. In the
quantum jump unravelling we show that this takes the form of a coupled system
of master equations, while for quantum diffusion it becomes a type of quantum
Fokker-Planck equation. In both cases, we show that the FPT can be obtained by
introducing absorbing boundary conditions, making their computation extremely
efficient. The versatility of our framework is demonstrated with two relevant
examples. First, we show how our method can be used to study the tightness of
recently proposed KURs for quantum jumps. Second, we study the homodyne
detection of a single two-level atom, and show how our approach can unveil
various non-trivial features in the FPT distribution.Comment: 8 pages, 2 figure
A systematic review and meta-regression analysis of the vitamin D intake-serum 25-hydroxyvitamin D relationship to inform European recommendations
The present study used a systematic review approach to identify relevant randomised control trials (RCT) with vitamin D and then apply meta-regression to explore the most appropriate model of the vitamin D intake–serum 25-hydroxyvitamin D (25(OH)D) relationship to underpin setting reference intake values. Methods included an updated structured search on Ovid MEDLINE; rigorous inclusion/exclusion criteria; data extraction; and meta-regression (using different model constructs).
In particular, priority was given to data from winter-based RCT performed at latitudes >49•58°N (n 12). A combined weighted linear model meta-regression analyses of natural log (Ln) total vitamin D intake
(i.e. diet and supplemental vitamin D)
versus achieved serum 25(OH)D in winter (that used by the North American Dietary Reference
Intake Committee) produced a curvilinear relationship (mean (95% lower CI) serum 25(OH)D (nmol/l) = 9•2 (8•5) Ln
(total vitamin D)).
Use of non-transformed total vitamin D intake data (maximum 1400 IU/d; 35µg/d) provided for a more linear relationship
(mean serum 25(OH)D (nmol/l) = 0•044 × (total vitamin D) + 33•035).
Although inputting an intake of 600 IU/d (i.e. the RDA) into the 95% lower CI curvilinear and linear models
predicted a serum 25(OH)D of 54•4 and 55•2 nmol/l, respectively, the total vitamin D intake that would
achieve 50 (and 40) nmol/l serum 25(OH)D was 359 (111) and 480 (260) IU/d, respectively.
Inclusion of 95% range in the model to account for inter-individual variability increased the predicted intake of
vitamin D needed to maintain serum 25(OH)D ≥50 nmol/l to 930 IU/d.
The model used to describe the vitamin D intake–status relationship needs to be considered carefully when
setting new reference intake values in Europe
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