907 research outputs found
Highly efficient energy excitation transfer in light-harvesting complexes: The fundamental role of noise-assisted transport
Excitation transfer through interacting systems plays an important role in
many areas of physics, chemistry, and biology. The uncontrollable interaction
of the transmission network with a noisy environment is usually assumed to
deteriorate its transport capacity, especially so when the system is
fundamentally quantum mechanical. Here we identify key mechanisms through which
noise such as dephasing, perhaps counter intuitively, may actually aid
transport through a dissipative network by opening up additional pathways for
excitation transfer. We show that these are processes that lead to the
inhibition of destructive interference and exploitation of line broadening
effects. We illustrate how these mechanisms operate on a fully connected
network by developing a powerful analytical technique that identifies the
invariant (excitation trapping) subspaces of a given Hamiltonian. Finally, we
show how these principles can explain the remarkable efficiency and robustness
of excitation energy transfer from the light-harvesting chlorosomes to the
bacterial reaction center in photosynthetic complexes and present a numerical
analysis of excitation transport across the Fenna-Matthew-Olson (FMO) complex
together with a brief analysis of its entanglement properties. Our results show
that, in general, it is the careful interplay of quantum mechanical features
and the unavoidable environmental noise that will lead to an optimal system
performance.Comment: 16 pages, 9 figures; See Video Abstract at
http://www.quantiki.org/video_abstracts/09014454 . New revised version;
discussion of entanglement properties enhance
The classical-quantum boundary for correlations: discord and related measures
One of the best signatures of nonclassicality in a quantum system is the
existence of correlations that have no classical counterpart. Different methods
for quantifying the quantum and classical parts of correlations are amongst the
more actively-studied topics of quantum information theory over the past
decade. Entanglement is the most prominent of these correlations, but in many
cases unentangled states exhibit nonclassical behavior too. Thus distinguishing
quantum correlations other than entanglement provides a better division between
the quantum and classical worlds, especially when considering mixed states.
Here we review different notions of classical and quantum correlations
quantified by quantum discord and other related measures. In the first half, we
review the mathematical properties of the measures of quantum correlations,
relate them to each other, and discuss the classical-quantum division that is
common among them. In the second half, we show that the measures identify and
quantify the deviation from classicality in various
quantum-information-processing tasks, quantum thermodynamics, open-system
dynamics, and many-body physics. We show that in many cases quantum
correlations indicate an advantage of quantum methods over classical ones.Comment: Close to the published versio
Coupled activity-current fluctuations in open quantum systems under strong symmetries
We acknowledge the Spanish Ministry and Agencia Estatal de Investigacion (AEI) through Grant FIS2017-84256-P (European Regional Development Fund), as well as the Consejeria de Conocimiento, Investigacion y Universidad, Junta de Andalucia and European Regional Development Fund, Ref. A-FQM-175-UGR18 and SOMM17/6105/UGR for financial support. We are also grateful for the computational resources and assistance provided by PROTEUS, the supercomputing center of Institute Carlos I for Theoretical and Computational Physics at the University of Granada, Spain.Strong symmetries in open quantum systems lead to broken ergodicity and the emergence of
multiple degenerate steady states. From a quantum jump (trajectory) perspective, the appearance
of multiple steady states is related to underlying dynamical phase transitions (DPTs) at the
fluctuating level, leading to a dynamical coexistence of different transport channels classified by
symmetry. In this paper we investigate how strong symmetries affect both the transport properties
and the activity patterns of a particular class of Markovian open quantum system, a three-qubit
model under the action of a magnetic field and in contact with a thermal bath.We find a pair of
twin DPTs in exciton current statistics, induced by the strong symmetry and related by time
reversibility, where a zero-current exchange-antisymmetric phase coexists with a symmetric phase
of negative exciton current. On the other hand, the activity statistics exhibits a single DPT where
the symmetric and antisymmetric phases of different but nonzero activities dynamically coexists.
Interestingly, the maximum current and maximum activity phases do not coincide for this
three-qubits system. We also investigate how symmetries are reflected in the joint large deviation
statistics of the activity and the current, a central issue in the characterization of the complex
quantum jump dynamics. The presence of a strong symmetry under nonequilibrium conditions
implies non-analyticities in the dynamical free energy in the dual activity-current plane (or
equivalently in the joint activity-current large deviation function), including an activity-driven
current lockdown phase for activities below some critical threshold. Remarkably, the DPT
predicted around the steady state and its Gallavotti–Cohen twin dual are extended into lines of
first-order DPTs in the current-activity plane, with a nontrivial structure which depends on the
transport and activity properties of each of the symmetry phases. Finally, we also study the effect
of a symmetry-breaking, ergodicity-restoring dephasing channel on the coupled activity-current
statistics for this model. Interestingly, we observe that while this dephasing noise destroys the
symmetry-induced DPTs, the underlying topological symmetry leaves a dynamical fingerprint in
the form of an intermittent, bursty on/off dynamics between the different symmetry sectors.Spanish Ministry and Agencia Estatal de Investigacion (AEI) FIS2017-84256-PJunta de AndaluciaEuropean Commission A-FQM-175-UGR18
SOMM17/6105/UG
Intrinsic decoherence in superconducting quantum circuits
Decoherence and parameter fluctuations are two of the mayor obstacles for solid-state quantum computing.
In this work, decoherence in superconducting qubits of the transmon type is investigated. For this purpose, a time-multiplexed measurement protocol was developed and applied in long-term measurements. The resulting simultaneous measurement of the qubit\u27s relaxation and dephasing rate, as well as its resonance frequency enables analysis of correlations between these parameters. A spectral noise analysis complements these measurements. Together, the results agree well with the interacting defect model of two-level-systems and yield information about the microscopic origin of the intrinsic decoherence mechanisms in Josephson qubits.
Our measurements show inherent correlations between dephasing and fluctuations in qubit frequency on the timescale of seconds to days, which is attributed to the influence of individual defects, located close to conductor edges. Cross-correlation and spectral noise analysis confirm this interpretation and ascribe the source of fluctuation to interactions between thermal fluctuators and surface defects. Single defects reducing the coherence of qubits by up to one order of magnitude are a major challenge for future quantum computers.
Non-tunable qubits are intrinsically insensitive to some decoherence channels and thus ideal for this fundamental analysis. However, to widen the focus and contrast the results of different material systems, we pursue the fabrication of voltage controlled gatemon qubits. In the course of this work, the theoretical foundation and technical implementation of transmon qubits based on regular Josephson weak links, and semiconducting nanowires is given. The experimental design and measurement setup are explained in detail.
Our findings make continuous re-calibration a necessity in today\u27s solid-state qubits, although new materials or processing techniques might mitigate the problem. However, the results of this work imply that fundamental improvements of qubit parameter stability are necessary in order to realize scalable and coherent qubit circuits
Decoherence in Solid State Qubits
Interaction of solid state qubits with environmental degrees of freedom
strongly affects the qubit dynamics, and leads to decoherence. In quantum
information processing with solid state qubits, decoherence significantly
limits the performances of such devices. Therefore, it is necessary to fully
understand the mechanisms that lead to decoherence. In this review we discuss
how decoherence affects two of the most successful realizations of solid state
qubits, namely, spin-qubits and superconducting qubits. In the former, the
qubit is encoded in the spin 1/2 of the electron, and it is implemented by
confining the electron spin in a semiconductor quantum dot. Superconducting
devices show quantum behavior at low temperatures, and the qubit is encoded in
the two lowest energy levels of a superconducting circuit. The electron spin in
a quantum dot has two main decoherence channels, a (Markovian) phonon-assisted
relaxation channel, due to the presence of a spin-orbit interaction, and a
(non-Markovian) spin bath constituted by the spins of the nuclei in the quantum
dot that interact with the electron spin via the hyperfine interaction. In a
superconducting qubit, decoherence takes place as a result of fluctuations in
the control parameters, such as bias currents, applied flux, and bias voltages,
and via losses in the dissipative circuit elements.Comment: review article, 66 pages, 10 figure
Decoherence and Measurement of Charge Qubits in Double Quantum Dots
In this thesis, theoretical studies of decoherence properties and the measurement process of a charge qubit defined in a double quantum dot structure are summarized. There have been three experimental realizations of charge qubits in double quantum dots already, where the quality factors have been quite small. Therefore, the theoretical analysis of possible decoherence mechanisms and how measurements can ideally be performed could be beneficial for the further development in new experiments
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