89 research outputs found
Restrictions on the coherence of the ultrafast optical emission from an electron-hole pairs condensate
We report on the transfer of coherence from a quantum-well electron-hole
condensate to the light it emits. As a function of density, the coherence of
the electron-hole pair system evolves from being full for the low density
Bose-Einstein condensate to a chaotic behavior for a high density BCS-like
state. This degree of coherence is transfered to the light emitted in a damped
oscillatory way in the ultrafast regime. Additionally, the photon field
exhibits squeezing properties during the transfer time. We analyze the effect
of light frequency and separation between electron and hole layers on the
optical coherence. Our results suggest new type of ultrafast experiments for
detecting electron-hole pair condensation.Comment: 4 pages,3 figures, to be published in Physical Review Letters. Minor
change
Transient synchronisation and quantum coherence in a bio-inspired vibronic dimer
Synchronisation is a collective phenomenon widely investigated in classical oscillators and, more recently, in quantum systems. However it remains unclear what features distinguish synchronous behaviour in these two scenarios. Recent works have shown that investigating synchronisation dynamics in open quantum systems can give insight into this issue. Here we study transient synchronisation in a bio-inspired vibronic dimer, where electronic excitation dynamics is mediated by coherent interactions with intramolecular vibrational modes. We show that the synchronisation dynamics of local mode displacements exhibit a rich behaviour which arises directly from the distinct time-evolutions of different vibronic quantum coherences. Furthermore, our study shows that coherent energy transport in this bio-inspired system is concomitant with the emergence of positive synchronisation between mode displacements. Our work provides further understanding of the relations between quantum coherence and synchronisation in open quantum systems and suggests an interesting role for coherence in biomolecules, that of promoting synchronisation of vibrational motions driven out of thermal equilibrium
Generation of three-qubit entangled states using coupled multi-quantum dots
We discuss a mechanism for generating a maximum entangled state (GHZ) in a
coupled quantum dots system, based on analytical techniques. The reliable
generation of such states is crucial for implementing solid-state based quantum
information schemes. The signature originates from a remarkably weak field
pulse or a far off-resonance effects which could be implemented using
technology that is currently being developed. The results are illustrated with
an application to a specific wide-gap semiconductor quantum dots system, like
Zinc Selenide (ZnSe) based quantum dots.Comment: 8 pages, 2 figure
Strong Quantum Darwinism and Strong Independence are Equivalent to Spectrum Broadcast Structure
How the objective everyday world emerges from the underlying quantum behavior of its microscopic constituents is an open question at the heart of the foundations of quantum mechanics. Quantum Darwinism and spectrum broadcast structure are two different frameworks providing key insight into this question. Recent works, however, indicate these two frameworks can lead to conflicting predictions on the objectivity of the state of a system interacting with an environment. Here, we provide a resolution to this issue by defining strong quantum Darwinism and proving that it is equivalent to spectrum broadcast structure when combined with strong independence of the subenvironments. We further show that strong quantum Darwinism is sufficient and necessary to signal state objectivity without the requirement of strong independence. Our Letter unveils the deep connection between strong quantum Darwinism and spectrum broadcast structure, thereby making fundamental progress toward understanding and solving the emergence of classicality from the quantum world. Together they provide us a sharper understanding of the transition in terms of state structure, geometry, and quantum and classical information
Objectivity (or lack thereof): Comparison between predictions of quantum Darwinism and spectrum broadcast structure
Quantum Darwinism and spectrum broadcast structure describe the emergence of objectivity in quantum
systems. However, it is unclear whether these two frameworks lead to consistent predictions on the objectivity
of the state of a quantum system in a given scenario. In this paper, we jointly investigate quantum Darwinism
and spectrum broadcasting, as well as the subdivision of quantum Darwinism into accessible information and
quantum discord, in a two-level system interacting with an N-level environment via a random matrix coupling.
We propose a partial trace method to suitably and consistently partition the effective N-level environment and
compare the predictions with those obtained using the partitioning method proposed by Perez [Phys. Rev.
A 81, 052326 (2010)]. We find that quantum Darwinism can apparently emerge under the Perez trace even
when spectrum broadcast structure does not emerge, and the majority of the quantum mutual information
between system and environment fractions is in fact quantum in nature. This work therefore shows there can
be discrepancies between quantum Darwinism and the nature of information and spectrum broadcast structure
Non-classicality of the molecular vibrations assisting exciton energy transfer at room temperature
Advancing the debate on quantum effects in light-initiated reactions in biology requires clear identification of non-classical features that these processes can exhibit and utilize. Here we show that in prototype dimers present in a variety of photosynthetic antennae, efficient vibration-assisted energy transfer in the sub-picosecond timescale and at room temperature can manifest and benefit from non-classical fluctuations of collective pigment motions. Non-classicality of initially thermalized vibrations is induced via coherent exciton-vibration interactions and is unambiguously indicated by negativities in the phase-space quasi-probability distribution of the effective collective mode coupled to the electronic dynamics. These quantum effects can be prompted upon incoherent input of excitation. Our results therefore suggest that investigation of the non-classical properties of vibrational motions assisting excitation and charge transport, photoreception and chemical sensing processes could be a touchstone for revealing a role for non-trivial quantum phenomena in biology
Entanglement of a microcanonical ensemble
We replace time-averaged entanglement by ensemble-averaged entanglement and
derive a simple expression for the latter. We show how to calculate the
ensemble average for a two-spin system and for the Jaynes-Cummings model. In
both cases the time-dependent entanglement is known as well so that one can
verify that the time average coincides with the ensemble average.Comment: 10 page
Ensemble averaged entanglement of two-particle states in Fock space
Recent results, extending the Schmidt decomposition theorem to wavefunctions
of identical particles, are reviewed. They are used to give a definition of
reduced density operators in the case of two identical particles. Next, a
method is discussed to calculate time averaged entanglement. It is applied to a
pair of identical electrons in an otherwise empty band of the Hubbard model,
and to a pair of bosons in the the Bose-Hubbard model with infinite range
hopping. The effect of degeneracy of the spectrum of the Hamiltonian on the
average entanglement is emphasised.Comment: 19 pages Latex, changed title, references added in the conclusion
Electronic excitation dynamics in multichromophoric systems described via a polaron-representation master equation
We derive a many-site version of the non-Markovian time-convolutionless
polaron master equation [S. Jang et al., J. Chem Phys. 129, 101104 (2008)] to
describe electronic excitation dynamics in multichromophoric systems. By
treating electronic and vibrational degrees of freedom in a combined frame
(polaron frame), this theory is capable of interpolating between weak and
strong exciton-phonon coupling and is able to account for initial
non-equilibrium bath states and spatially correlated environments. Besides
outlining a general expression for the expected value of any electronic system
observable in the original frame, we also discuss implications of the Markovian
and secular approximations highlighting that they need not hold in the
untransformed frame despite being strictly satisfied in the polaron frame. The
key features of the theory are illustrated using as an example a four-site
subsystem of the Fenna-Mathew-Olson light-harvesting complex. For a spectral
density including a localised high-energy mode, we show that oscillations of
site populations may only be observed when non-equilibrium bath effects are
taken into account. Furthermore, we illustrate how this formalism allows us to
identify the electronic or vibrational origin of the oscillatory dynamics.Comment: 13 pages, 6 figures; minor corrections made; accepted for publication
in Journal of Chemical Physic
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