115 research outputs found
Quantum Spin Dimers from Chiral Dissipation in Cold-Atom Chains
We consider the non-equilibrium dynamics of a driven dissipative spin chain
with chiral coupling to a 1D bosonic bath, and its atomic implementation with a
two-species mixture of cold quantum gases. The reservoir is represented by a
spin-orbit coupled 1D quasi-condensate of atoms in a magnetized phase, while
the spins are identified with motional states of a separate species of atoms in
an optical lattice. The chirality of reservoir excitations allows the spins to
couple differently to left and right moving modes, which in our atomic setup
can be tuned from bidirectional to purely unidirectional. Remarkably, this
leads to a pure steady state in which pairs of neighboring spins form dimers
that decouple from the remainder of the chain. Our results also apply to
current experiments with two-level emitters coupled to photonic waveguides.Comment: Replaced by published version (6 pages + 8 pages supplemental
material
Universal Quantum Computation in Globally Driven Rydberg Atom Arrays
We develop a model for quantum computation which only relies on global
driving, without the need of local addressing of the qubits. Our scheme is
based on dual-species processors, and we present it in the framework on neutral
atoms subjected to Rydberg blockade constraints. A circuit is imprinted in the
(static) trap positions of the atoms, and the algorithm is executed by a
sequence of global, resonant laser pulses; we show that this model for quantum
computation is universal and scalable
Continuous Coherent Quantum Feedback with Time Delays: Tensor Network Solution
In this paper we develop a novel method to solve problems involving quantum
optical systems coupled to coherent quantum feedback loops featuring time
delays. Our method is based on exact mappings of such non-Markovian problems to
equivalent Markovian driven dissipative quantum many-body problems. In this
work we show that the resulting Markovian quantum many-body problems can be
solved (numerically) exactly and efficiently using tensor network methods for a
series of paradigmatic examples, consisting of driven quantum systems coupled
to waveguides at several distant points. In particular, we show that our method
allows solving problems in so far inaccessible regimes, including problems with
arbitrary long time delays and arbitrary numbers of excitations in the delay
lines. We obtain solutions for the full real-time dynamics as well as the
steady state in all these regimes. Finally, motivated by our results, we
develop a novel mean-field approach, which allows us to find the solution
semi-analytically and identify parameter regimes where this approximation is in
excellent agreement with our exact tensor network results
Delayed Coherent Quantum Feedback from a Scattering Theory and a Matrix Product State Perspective
We study the scattering of photons propagating in a semi-infinite waveguide
terminated by a mirror and interacting with a quantum emitter. This paradigm
constitutes an example of coherent quantum feedback, where light emitted
towards the mirror gets redirected back to the emitter. We derive an analytical
solution for the scattering of two-photon states, which is based on an exact
resummation of the perturbative expansion of the scattering matrix, in a regime
where the time delay of the coherent feedback is comparable to the timescale of
the quantum emitter's dynamics. We compare the results with numerical
simulations based on matrix product state techniques simulating the full
dynamics of the system, and extend the study to the scattering of coherent
states beyond the low-power limit.Comment: 28 pages, 6 figure
Measurement Protocol for the Entanglement Spectrum of Cold Atoms
Entanglement, and, in particular the entanglement spectrum, plays a major
role in characterizing many-body quantum systems. While there has been a surge
of theoretical works on the subject, no experimental measurement has been
performed to date because of the lack of an implementable measurement scheme.
Here, we propose a measurement protocol to access the entanglement spectrum of
many-body states in experiments with cold atoms in optical lattices. Our scheme
effectively performs a Ramsey spectroscopy of the entanglement Hamiltonian and
is based on the ability to produce several copies of the state under
investigation together with the possibility to perform a global swap gate
between two copies conditioned on the state of an auxiliary qubit. We show how
the required conditional swap gate can be implemented with cold atoms, either
by using Rydberg interactions or coupling the atoms to a cavity mode. We
illustrate these ideas on a simple (extended) Bose-Hubbard model where such a
measurement protocol reveals topological features of the Haldane phase
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