183 research outputs found
When do weak-coupling approaches accurately capture the dynamics of complex quantum systems?
Understanding the dynamics of higher-dimensional quantum systems embedded in
a complex environment remains a significant theoretical challenge. While
several approaches yielding numerically converged solutions exist, these are
computationally expensive and often provide only limited physical insight. Here
we address the question when more intuitive and simpler to compute
weak-coupling approaches still provide adequate accuracy. We develop a simple
analytical criterion and verify its validity for the case of the much-studied
FMO dynamics as well as the canonical spin-boson model.Comment: 10 pages, 5 figures, comments are very welcome
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
Creating nuclear spin entanglement using an optical degree of freedom
Funding: Marie Curie Early Stage Training network QIPEST (Grant No. MESTCT-2005-020505), the EPSRC through QIP IRC (Grant Nos. GR/S82176/01 and GR/S15808/01), the National Research Foundation and Ministry of Education, Singapore, the DAAD (German Academic Exchange Service), Linacre College, Oxford, and the Royal Society.Molecular nanostructures are promising building blocks for future quantum technologies, provided methods of harnessing their multiple degrees of freedom can be identified and implemented. Due to low decoherence rates, nuclear spins are considered ideal candidates for storing quantum information, while optical excitations can give rise to fast and controllable interactions for information processing. A recent paper [M. Schaffry et al., Phys. Rev. Lett. 104, 200501 (2010)] proposed a method for entangling two nuclear spins through their mutual coupling to a transient optically excited electron spin. Building on the same idea, we present here an extended and much more detailed theoretical framework, showing that this method is in fact applicable to a much wider class of molecular structures than previously discussed in the original proposal.Publisher PDFPeer reviewe
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
Quantum thermometry using the ac Stark shift within the Rabi model
This work was supported by the EPSRC, the National Research Foundation and Ministry of Education, Singapore, and the Royal Society.A quantum two-level system coupled to a harmonic oscillator represents a ubiquitous physical system. New experiments in circuit QED and nanoelectromechanical systems (NEMS) achieve unprecedented coupling strength at large detuning between qubit and oscillator, thus requiring a theoretical treatment beyond the Jaynes-Cummings model. Here we present a new method for describing the qubit dynamics in this regime, based on an oscillator correlation function expansion of a non-Markovian master equation in the polaron frame. Our technique yields a new numerical method as well as a succinct approximate expression for the qubit dynamics. These expressions are valid in the experimentally interesting regime of strong coupling at low temperature. We obtain a new expression for the ac Stark shift and show that this enables practical and precise qubit thermometry of an oscillator.Peer reviewe
Light-harvesting with guide-slide superabsorbing condensed-matter nanostructures
We establish design principles for light-harvesting antennae whose energy
capture scales superlinearly with system size. Controlling the absorber dipole
orientations produces sets of `guide-slide' states which promote steady-state
superabsorbing characteristics in noisy condensed-matter nanostructures.
Inspired by natural photosynthetic complexes, we discuss the example of
ring-like dipole arrangements and show that, in our setup, vibrational
relaxation enhances rather than impedes performance. Remarkably, the
superabsorption effect proves robust to O(5%) disorder simultaneously for all
relevant system parameters, showing promise for experimental exploration across
a broad range of platforms.Comment: Updated to include supporting results in the polaron frame for strong
vibrational couplin
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
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
Frequency-encoded linear cluster states with coherent Raman photons
Entangled multi-qubit states are an essential resource for quantum
information and computation. Solid-state emitters can mediate interactions
between subsequently emitted photons via their spin, thus offering a route
towards generating entangled multi-photon states. However, existing schemes
typically rely on the incoherent emission of single photons and suffer from
severe practical limitations, for self-assembled quantum dots most notably the
limited spin coherence time due to Overhauser magnetic field fluctuations. We
here propose an alternative approach of employing spin-flip Raman scattering
events of self-assembled quantum dots in Voigt geometry. We argue that weakly
driven hole spins constitute a promising platform for the practical generation
of frequency-entangled photonic cluster states
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