803 research outputs found
Quantum computation with cold bosonic atoms in an optical lattice
We analyse an implementation of a quantum computer using bosonic atoms in an
optical lattice. We show that, even though the number of atoms per site and the
tunneling rate between neighbouring sites is unknown, one may perform a
universal set of gates by means of adiabatic passage
Switchable ultrastrong coupling in circuit QED
Superconducting quantum circuits possess the ingredients for quantum
information processing and for developing on-chip microwave quantum optics.
From the initial manipulation of few-level superconducting systems (qubits)
to their strong coupling to microwave resonators, the time has come to consider
the generation and characterization of propagating quantum microwaves. In this
paper, we design a key ingredient that will prove essential in the general
frame: a swtichable coupling between qubit(s) and transmission line(s) that can
work in the ultrastrong coupling regime, where the coupling strength approaches
the qubit transition frequency. We propose several setups where two or more
loops of Josephson junctions are directly connected to a closed (cavity) or
open transmission line. We demonstrate that the circuit induces a coupling that
can be modulated in strength and type. Given recent studies showing the
accessibility to the ultrastrong regime, we expect our ideas to have an
immediate impact in ongoing experiments
Hall response of interacting bosonic atoms in strong gauge fields: from condensed to FQH states
Interacting bosonic atoms under strong gauge fields undergo a series of phase
transitions that take the cloud from a simple Bose-Einstein condensate all the
way to a family of fractional-quantum-Hall-type states [M. Popp, B. Paredes,
and J. I. Cirac, Phys. Rev. A 70, 053612 (2004)]. In this work we demonstrate
that the Hall response of the atoms can be used to locate the phase transitions
and characterize the ground state of the many-body state. Moreover, the same
response function reveals within some regions of the parameter space, the
structure of the spectrum and the allowed transitions to excited states. We
verify numerically these ideas using exact diagonalization for a small number
of atoms, and provide an experimental protocol to implement the gauge fields
and probe the linear response using a periodically driven optical lattice.
Finally, we discuss our theoretical results in relation to recent experiments
with condensates in artificial magnetic fields [ L. J. LeBlanc, K.
Jimenez-Garcia, R. A. Williams, M. C. Beeler, A. R. Perry, W. D. Phillips, and
I. B. Spielman, Proc. Natl. Acad. Sci. USA 109, 10811 (2012)] and we analyze
the role played by vortex states in the Hall response.Comment: 10 pages, 7 figure
Deep Strong Coupling Regime of the Jaynes-Cummings model
We study the quantum dynamics of a two-level system interacting with a
quantized harmonic oscillator in the deep strong coupling regime (DSC) of the
Jaynes-Cummings model, that is, when the coupling strength g is comparable or
larger than the oscillator frequency w (g/w > 1). In this case, the
rotating-wave approximation cannot be applied or treated perturbatively in
general. We propose an intuitive and predictive physical frame to describe the
DSC regime where photon number wavepackets bounce back and forth along parity
chains of the Hilbert space, while producing collapse and revivals of the
initial population. We exemplify our physical frame with numerical and
analytical considerations in the qubit population, photon statistics, and
Wigner phase space.Comment: Published version, note change of title: DSC regime of the JC mode
Seeing Majorana fermions in time-of-flight images of spinless fermions coupled by s-wave pairing
The Chern number, nu, as a topological invariant that identifies the winding
of the ground state in the particle-hole space, is a definitive theoretical
signature that determines whether a given superconducting system can support
Majorana zero modes. Here we show that such a winding can be faithfully
identified for any superconducting system (p-wave or s-wave with spin-orbit
coupling) through a set of time-of-flight measurements, making it a diagnostic
tool also in actual cold atom experiments. As an application, we specialize the
measurement scheme for a chiral topological model of spinless fermions. The
proposed model only requires the experimentally accessible s-wave pairing and
staggered tunnelling that mimics spin-orbit coupling. By adiabatically
connecting this model to Kitaev's honeycomb lattice model, we show that it
gives rise to nu = \pm 1 phases, where vortices bind Majorana fermions, and
nu=\pm 2 phases that emerge as the unique collective state of such vortices.
Hence, the preparation of these phases and the detection of their Chern numbers
provide an unambiguous signature for the presence of Majorana modes. Finally,
we demonstrate that our detection procedure is resilient against most
inaccuracies in experimental control parameters as well as finite temperature.Comment: 9+4 pages, 11 figures, expanded versio
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