645 research outputs found
Universal quantum computation in decoherence-free subspaces with hot trapped-ions
We consider interactions that generate a universal set of quantum gates on
logical qubits encoded in a collective-dephasing-free subspace, and discuss
their implementations with trapped ions. This allows for the removal of the
by-far largest source of decoherence in current trapped-ion experiments,
collective dephasing. In addition, an explicit parametrization of all two-body
Hamiltonians able to generate such gates without the system's state ever
exiting the protected subspace is provided.Comment: 8 pages, 1 figur
Energy Transport in Trapped Ion Chains
We experimentally study energy transport in chains of trapped ions. We use a
pulsed excitation scheme to rapidly add energy to the local motional mode of
one of the ions in the chain. Subsequent energy readout allows us to determine
how the excitation has propagated throughout the chain. We observe energy
revivals that persist for many cycles. We study the behavior with an increasing
number of ions of up to 37 in the chain, including a zig-zag configuration. The
experimental results agree well with the theory of normal mode evolution. The
described system provides an experimental toolbox for the study of
thermodynamics of closed systems and energy transport in both classical and
quantum regimes
Antiferromagnetic phase transition in a nonequilibrium lattice of Rydberg atoms
We study a driven-dissipative system of atoms in the presence of laser
excitation to a Rydberg state and spontaneous emission. The atoms interact via
the blockade effect, whereby an atom in the Rydberg state shifts the Rydberg
level of neighboring atoms. We use mean-field theory to study how the Rydberg
population varies in space. As the laser frequency changes, there is a
continuous transition between the uniform and antiferromagnetic phases. The
nonequilibrium nature also leads to a novel oscillatory phase and bistability
between the uniform and antiferromagnetic phases.Comment: 4 pages + appendi
Polarization of electric field noise near metallic surfaces
Electric field noise in proximity to metallic surfaces is a poorly understood
phenomenon that appears in different areas of physics. Trapped ion quantum
information processors are particular susceptible to this noise, leading to
motional decoherence which ultimately limits the fidelity of quantum
operations. On the other hand they present an ideal tool to study this effect,
opening new possibilities in surface science. In this work we analyze and
measure the polarization of the noise field in a micro-fabricated ion trap for
various noise sources. We find that technical noise sources and noise emanating
directly from the surface give rise to different degrees of polarization which
allows us to differentiate between the two noise sources. Based on this, we
demonstrate a method to infer the magnitude of surface noise in the presence of
technical noise
Two mode coupling in a single ion oscillator via parametric resonance
Atomic ions, confined in radio-frequency Paul ion traps, are a promising
candidate to host a future quantum information processor. In this letter, we
demonstrate a method to couple two motional modes of a single trapped ion,
where the coupling mechanism is based on applying electric fields rather than
coupling the ion's motion to a light field. This reduces the design constraints
on the experimental apparatus considerably. As an application of this
mechanism, we cool a motional mode close to its ground state without accessing
it optically. As a next step, we apply this technique to measure the mode's
heating rate, a crucial parameter determining the trap quality. In principle,
this method can be used to realize a two-mode quantum parametric amplifier.Comment: 8 pages, 5 figure
High-fidelity ion-trap quantum computing with hyperfine clock states
We propose the implementation of a geometric-phase gate on
magnetic-field-insensitive qubits with -dependent forces for
trapped ion quantum computing. The force is exerted by two laser beams in a
Raman configuration. Qubit-state dependency is achieved by a small frequency
detuning from the virtually-excited state. Ion species with excited states of
long radiative lifetimes are used to reduce the chance of a spontaneous photon
emission to less than 10 per gate-run. This eliminates the main source
of gate infidelity of previous implementations. With this scheme it seems
possible to reach the fault tolerant threshold.Comment: 4 pages, 1 figur
Revealing quantum statistics with a pair of distant atoms
Quantum statistics have a profound impact on the properties of systems
composed of identical particles. In this Letter, we demonstrate that the
quantum statistics of a pair of identical massive particles can be probed by a
direct measurement of the exchange symmetry of their wave function even in
conditions where the particles always remain spatially well separated and thus
the exchange contribution to their interaction energy is negligible. We present
two protocols revealing the bosonic or fermionic nature of a pair of particles
and discuss possible implementations with a pair of trapped atoms or ions.Comment: 4+13 pages, v2 corresponds to the version published by PR
Collective generation of quantum states of light by entangled atoms
We present a theoretical framework to describe the collective emission of
light by entangled atomic states. Our theory applies to the low excitation
regime, where most of the atoms are initially in the ground state, and relies
on a bosonic description of the atomic excitations. In this way, the problem of
light emission by an ensemble of atoms can be solved exactly, including
dipole-dipole interactions and multiple light scattering. Explicit expressions
for the emitted photonic states are obtained in several situations, such as
those of atoms in regular lattices and atomic vapors. We determine the
directionality of the photonic beam, the purity of the photonic state, and the
renormalization of the emission rates. We also show how to observe collective
phenomena with ultracold atoms in optical lattices, and how to use these ideas
to generate photonic states that are useful in the context of quantum
information.Comment: 15 pages, 10 figure
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