23 research outputs found
Unconventional quantum optics in topological waveguide QED
The discovery of topological materials has challenged our understanding of
condensed matter physics and led to novel and unusual phenomena. This has
motivated recent developments to export topological concepts into photonics to
make light behave in exotic ways. Here, we predict several unconventional
quantum optical phenomena that occur when quantum emitters interact with a
topological waveguide QED bath, namely, the photonic analogue of the
Su-Schrieffer-Hegger model. When the emitters frequency lies within the
topological band-gap, a chiral bound state emerges, which is located at just
one side (right or left) of the emitter. In the presence of several emitters,
it mediates topological, long-range tunable interactions between them, that can
give rise to exotic phases such as double N\'eel ordered states. On the
contrary, when the emitters' optical transition is resonant with the bands, we
find unconventional scattering properties and different super/subradiant states
depending on the band topology. We also investigate the case of a bath with
open boundary conditions to understand the role of topological edge states.
Finally, we propose several implementations where these phenomena can be
observed with state-of-the-art technology.Comment: 17 pages, 10 figure
Probing Thermalization through Spectral Analysis with Matrix Product Operators
We combine matrix product operator techniques with Chebyshev polynomial expansions and present a method that is able to explore spectral properties of quantum many-body Hamiltonians. In particular, we show how this method can be used to probe thermalization of large spin chains without explicitly simulating their time evolution, as well as to compute full and local densities of states. The performance is illustrated with the examples of the Ising and PXP spin chains. For the nonintegrable Ising chain, our findings corroborate the presence of thermalization for several initial states, well beyond what direct timedependent simulations have been able to achieve so far
Key distillation from Gaussian states by Gaussian operations
We study the secrecy properties of Gaussian states under Gaussian operations.
Although such operations are useless for quantum distillation, we prove that it
is possible to distill a secret key secure against any attack from sufficiently
entangled Gaussian states with non-positive partial transposition. Moreover,
all such states allow for key distillation, when Eve is assumed to perform
finite-size coherent attacks before the reconciliation process.Comment: 2 figures, REVTEX
Quantum East model: localization, non-thermal eigenstates and slow dynamics
We study in detail the properties of the quantum East model, an interacting
quantum spin chain inspired by simple kinetically-constrained models of
classical glasses. Through a combination of analytics, exact diagonalization
and tensor-network methods we show the existence of a transition, from a fast
to a slow thermalization regime, which manifests itself throughout the
spectrum. On the slow side, by exploiting the localization of the ground state
and the form of the Hamiltonian, we explicitly construct a large (exponential
in size) number of non-thermal states which become exact finite-energy-density
eigenstates in the large-size limit, as expected for a true phase transition. A
``super-spin'' generalization allows us to find a further large class of
area-law states proved to display very slow relaxation. These states retain
memory of their initial conditions for extremely long times. Our numerical
analysis reveals that the localization properties are not limited to the ground
state and that} many eigenstates have large overlap with product states and can
be approximated well by matrix product states at arbitrary energy densities.
The mechanism that induces localization to the ground state, and hence the
non-thermal behavior of the system, can be extended to a wide range of models
including a number of simple spin chains. We discuss implications of our
results for slow thermalization and non-ergodicity more generally in
disorder-free systems with constraints and we give numerical evidence that
these results may be extended to two dimensional systems.Comment: 21 pages, 16 figure
Quantum memories based on engineered dissipation
Storing quantum information for long times without disruptions is a major
requirement for most quantum information technologies. A very appealing
approach is to use self-correcting Hamiltonians, i.e. tailoring local
interactions among the qubits such that when the system is weakly coupled to a
cold bath the thermalization process takes a long time. Here we propose an
alternative but more powerful approach in which the coupling to a bath is
engineered, so that dissipation protects the encoded qubit against more general
kinds of errors. We show that the method can be implemented locally in four
dimensional lattice geometries by means of a toric code, and propose a simple
2D set-up for proof of principle experiments.Comment: 6 +8 pages, 4 figures, Includes minor corrections updated references
and aknowledgement
Toward Quantum Superposition of Living Organisms
The most striking feature of quantum mechanics is the existence of
superposition states, where an object appears to be in different situations at
the same time. The existence of such states has been tested with small objects,
like atoms, ions, electrons and photons, and even with molecules. More
recently, it has been possible to create superpositions of collections of
photons, atoms, or Cooper pairs. Current progress in optomechanical systems may
soon allow us to create superpositions of even larger objects, like micro-sized
mirrors or cantilevers, and thus to test quantum mechanical phenomena at larger
scales. Here we propose a method to cool down and create quantum superpositions
of the motion of sub-wavelength, arbitrarily shaped dielectric objects trapped
inside a high--finesse cavity at a very low pressure. Our method is ideally
suited for the smallest living organisms, such as viruses, which survive under
low vacuum pressures, and optically behave as dielectric objects. This opens up
the possibility of testing the quantum nature of living organisms by creating
quantum superposition states in very much the same spirit as the original
Schr\"odinger's cat "gedanken" paradigm. We anticipate our essay to be a
starting point to experimentally address fundamental questions, such as the
role of life and consciousness in quantum mechanics.Comment: 9 pages, 4 figures, published versio
Spin dynamics for bosons in an optical lattice
We study the internal dynamics of bosonic atoms in an optical lattice. Within
the regime in which the atomic crystal is a Mott insulator with one atom per
well, the atoms behave as localized spins which interact according to some spin
Hamiltonian. The type of Hamiltonian (Heisenberg, Ising), and the sign of
interactions may be tuned by changing the properties of the optical lattice, or
applying external magnetic fields. When, on the other hand, the number of atoms
per lattice site is unknown, we can still use the bosons to perform general
quantum computation
Simulating lattice gauge theories within quantum technologies
Abstract: Lattice gauge theories, which originated from particle physics in the context of Quantum Chromodynamics (QCD), provide an important intellectual stimulus to further develop quantum information technologies. While one long-term goal is the reliable quantum simulation of currently intractable aspects of QCD itself, lattice gauge theories also play an important role in condensed matter physics and in quantum information science. In this way, lattice gauge theories provide both motivation and a framework for interdisciplinary research towards the development of special purpose digital and analog quantum simulators, and ultimately of scalable universal quantum computers. In this manuscript, recent results and new tools from a quantum science approach to study lattice gauge theories are reviewed. Two new complementary approaches are discussed: first, tensor network methods are presented – a classical simulation approach – applied to the study of lattice gauge theories together with some results on Abelian and non-Abelian lattice gauge theories. Then, recent proposals for the implementation of lattice gauge theory quantum simulators in different quantum hardware are reported, e.g., trapped ions, Rydberg atoms, and superconducting circuits. Finally, the first proof-of-principle trapped ions experimental quantum simulations of the Schwinger model are reviewed. Graphical abstract
Von Neumann, informática y física cuántica
El mundo microscópico está plagado de fenómenos que parecen sacados de una película de ciencia fi cción. Todos ellos son explicados por la Física Cuántica, una teoría que surgió hace un siglo y en cuyo desarrollo participaron los más ilustres científicos. Esta teoría nos proporciona,
además, una nueva visión sobre la Naturaleza, en donde nosotros de finimos la realidad según realizamos observaciones. En esta conferencia explicaré de una manera sencilla algunos de los fenómenos cuánticos más impactantes, la posibilidad de construir nuevos sistemas informáticos y de comunicación basados en estos fenómenos, así como algunas implicaciones filosófi cas