21,635 research outputs found
Spin-Cooling of the Motion of a Trapped Diamond
Observing and controlling macroscopic quantum systems has long been a driving
force in research on quantum physics. In this endeavor, strong coupling between
individual quantum systems and mechanical oscillators is being actively
pursued. While both read-out of mechanical motion using coherent control of
spin systems and single spin read-out using pristine oscillators have been
demonstrated, temperature control of the motion of a macroscopic object using
long-lived electronic spins has not been reported. Here, we observe both a
spin-dependent torque and spin-cooling of the motion of a trapped microdiamond.
Using a combination of microwave and laser excitation enables the spin of
nitrogen-vacancy centers to act on the diamond orientation and to cool the
diamond libration via a dynamical back-action. Further, driving the system in
the non-linear regime, we demonstrate bistability and self-sustained coherent
oscillations stimulated by the spin-mechanical coupling, which offers prospects
for spin-driven generation of non-classical states of motion. Such a levitating
diamond operated as a compass with controlled dissipation has implications in
high-precision torque sensing, emulation of the spin-boson problem and probing
of quantum phase transitions. In the single spin limit and employing ultra-pure
nano-diamonds, it will allow quantum non-demolition read-out of the spin of
nitrogen-vacancy centers under ambient conditions, deterministic entanglement
between distant individual spins and matter-wave interferometry.Comment: New version with a calibration of angular resolution and sensitivity.
Fig. 1 is also replaced to show an ODMR when the diamond is static to avoid
spin-torque induced distortion
Optimal Control for Open Quantum Systems: Qubits and Quantum Gates
This article provides a review of recent developments in the formulation and
execution of optimal control strategies for the dynamics of quantum systems. A
brief introduction to the concept of optimal control, the dynamics of of open
quantum systems, and quantum information processing is followed by a
presentation of recent developments regarding the two main tasks in this
context: state-specific and state-independent optimal control. For the former,
we present an extension of conventional theory (Pontryagin's principle) to
quantum systems which undergo a non-Markovian time-evolution. Owing to its
importance for the realization of quantum information processing, the main body
of the review, however, is devoted to state-independent optimal control. Here,
we address three different approaches: an approach which treats dissipative
effects from the environment in lowest-order perturbation theory, a general
method based on the time--evolution superoperator concept, as well as one based
on the Kraus representation of the time-evolution superoperator. Applications
which illustrate these new methods focus on single and double qubits (quantum
gates) whereby the environment is modeled either within the Lindblad equation
or a bath of bosons (spin-boson model). While these approaches are widely
applicable, we shall focus our attention to solid-state based physical
realizations, such as semiconductor- and superconductor-based systems. While an
attempt is made to reference relevant and representative work throughout the
community, the exposition will focus mainly on work which has emerged from our
own group.Comment: 27 pages, 18 figure
Strongly correlated Fermi-Bose mixtures in disordered optical lattices
We investigate theoretically the low-temperature physics of a two-component
ultracold mixture of bosons and fermions in disordered optical lattices. We
focus on the strongly correlated regime. We show that, under specific
conditions, composite fermions, made of one fermion plus one bosonic hole,
form. The composite picture is used to derive an effective Hamiltonian whose
parameters can be controlled via the boson-boson and the boson-fermion
interactions, the tunneling terms and the inhomogeneities. We finally
investigate the quantum phase diagram of the composite fermions and we show
that it corresponds to the formation of Fermi glasses, spin glasses, and
quantum percolation regimes.Comment: Proceedings of the 3rd International Workshop on `Theory of Quantum
Gases and Quantum Coherence
The dynamical role of initial correlation in the exactly solvable dephasing model
We investigate the effects of the initial correlation on the dynamics of open
system in the exactly solvable pure dephasing model. We show that the role of
the initial correlation come into play through a phase function and a weight
factor, which would perform oscillations during time evolution, and find that
the decoherence of a qubit coupled to a boson bath is more enhanced with
respect to a spin bath in the short time. We also demonstrate that the trace
distance between two states of a qubit can increase above its initial value,
and that the initial correlation can provide another resource for the damply
oscillation and revival of the entanglement of two qubits. We finally
investigate the dependence of the crossover of decoherence from the dynamical
enhancement to suppression under the bang-bang pulse control on the initial
correlation and the statistics of the bath constituents.Comment: revised final versio
Magnetic vortex crystals in frustrated Mott insulator
Quantum fluctuations become particularly relevant in highly frustrated
quantum magnets and can lead to new states of matter. We provide a simple and
robust scenario for inducing magnetic vortex crystals in frustrated Mott
insulators. By considering a quantum paramagnet that has a gapped spectrum with
six-fold degenerate low energy modes, we study the magnetic field induced
condensation of these modes. We use a dilute gas approximation to demonstrate
that a plethora of multi- condensates are stabilized for different
combinations of exchange interactions. This rich quantum phase diagram includes
magnetic vortex crystals, which are further stabilized by symmetric exchange
anisotropies. Because magnetic skyrmion and domain wall crystals have already
been predicted and experimentally observed, this novel vortex phase completes
the picture of emergent crystals of topologically nontrivial spin
configurations.Comment: 12 pages, 12 figures; published in Phys. Rev.
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