137 research outputs found
Parallel Information Transfer in a Multi-Node Quantum Information Processor
We describe a method for coupling disjoint quantum bits (qubits) in different
local processing nodes of a distributed node quantum information processor. An
effective channel for information transfer between nodes is obtained by moving
the system into an interaction frame where all pairs of cross-node qubits are
effectively coupled via an exchange interaction between actuator elements of
each node. All control is achieved via actuator-only modulation, leading to
fast implementations of a universal set of internode quantum gates. The method
is expected to be nearly independent of actuator decoherence and may be made
insensitive to experimental variations of system parameters by appropriate
design of control sequences. We show, in particular, how the induced cross-node
coupling channel may be used to swap the complete quantum states of the local
processors in parallel.Comment: revtex4-1; 7 pages; 5 figures. New version includes minor changes,
with updated Fig. 4 and new supplemental materia
Accelerated randomized benchmarking
© 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. Quantum information processing offers promising advances for a wide range of fields and applications, provided that we can efficiently assess the performance of the control applied in candidate systems. That is, we must be able to determine whether we have implemented a desired gate, and refine accordingly. Randomized benchmarking reduces the difficulty of this task by exploiting symmetries in quantum operations. Here, we bound the resources required for benchmarking and show that, with prior information, we can achieve several orders of magnitude better accuracy than in traditional approaches to benchmarking. Moreover, by building on state-of-the-art classical algorithms, we reach these accuracies with near-optimal resources. Our approach requires an order of magnitude less data to achieve the same accuracies and to provide online estimates of the errors in the reported fidelities. We also show that our approach is useful for physical devices by comparing to simulations
Practical adaptive quantum tomography
© 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. We introduce a fast and accurate heuristic for adaptive tomography that addresses many of the limitations of prior methods. Previous approaches were either too computationally intensive or tailored to handle special cases such as single qubits or pure states. By contrast, our approach combines the efficiency of online optimization with generally applicable and well-motivated data-processing techniques. We numerically demonstrate these advantages in several scenarios including mixed states, higher-dimensional systems, and restricted measurements
Bayesian quantum noise spectroscopy
© 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft. As commonly understood, the noise spectroscopy problem - characterizing the statistical properties of a noise process affecting a quantum system by measuring its response - is mathematically ill-posed, in the sense that no unique noise process leads to a set of responses unless extra assumptions are taken into account. Ad-hoc solutions assume an implicit structure, which is often never determined. Thus, it is unclear when the method will succeed or whether one should trust the solution obtained. Here, we propose to treat the problem from the point of view of statistical estimation theory. We develop a Bayesian solution to the problem which allows one to easily incorporate assumptions which render the problem solvable. We compare several numerical techniques for noise spectroscopy and find the Bayesian approach to be superior in many respects
Measurement of the Zero Crossing in a Feshbach Resonance of Fermionic 6-Li
We measure a zero crossing in the scattering length of a mixture of the two
lowest hyperfine states of 6-Li. To locate the zero crossing, we monitor the
decrease in temperature and atom number arising from evaporation in a CO2 laser
trap as a function of magnetic field B. The temperature decrease and atom loss
are minimized for B=528(4) G, consistent with no evaporation. We also present
preliminary calculations using potentials that have been constrained by the
measured zero crossing and locate a broad Feshbach resonance at approximately
860 G, in agreement with previous theoretical predictions. In addition, our
theoretical model predicts a second and much narrower Feshbach resonance near
550 G.Comment: Five pages, four figure
Bose-Einstein Condensation in a CO_2-laser Optical Dipole Trap
We report on the achieving of Bose-Einstein condensation of a dilute atomic
gas based on trapping atoms in tightly confining CO_2-laser dipole potentials.
Quantum degeneracy of rubidium atoms is reached by direct evaporative cooling
in both crossed and single beam trapping geometries. At the heart of these
all-optical condensation experiments is the ability to obtain high initial
atomic densities in quasistatic dipole traps by laser cooling techniques.
Finally, we demonstrate the formation of a condensate in a field insensitive
m_F=0 spin projection only. This suppresses fluctuations of the chemical
potential from stray magnetic fields.Comment: 8 pages, 5 figure
All-optical formation of a Bose-Einstein condensate for applications in scanning electron microscopy
We report on the production of a F=1 spinor condensate of 87Rb atoms in a
single beam optical dipole trap formed by a focused CO2 laser. The condensate
is produced 13mm below the tip of a scanning electron microscope employing
standard all-optical techniques. The condensate fraction contains up to 100,000
atoms and we achieve a duty cycle of less than 10s.Comment: 5 pages, 4 figure
Robust Online Hamiltonian Learning
In this work we combine two distinct machine learning methodologies,
sequential Monte Carlo and Bayesian experimental design, and apply them to the
problem of inferring the dynamical parameters of a quantum system. We design
the algorithm with practicality in mind by including parameters that control
trade-offs between the requirements on computational and experimental
resources. The algorithm can be implemented online (during experimental data
collection), avoiding the need for storage and post-processing. Most
importantly, our algorithm is capable of learning Hamiltonian parameters even
when the parameters change from experiment-to-experiment, and also when
additional noise processes are present and unknown. The algorithm also
numerically estimates the Cramer-Rao lower bound, certifying its own
performance.Comment: 24 pages, 12 figures; to appear in New Journal of Physic
Microscopic Structure of a Vortex Line in a Superfluid Fermi Gas
The microscopic properties of a single vortex in a dilute superfluid Fermi
gas at zero temperature are examined within the framework of self-consistent
Bogoliubov-de Gennes theory. Using only physical parameters as input, we study
the pair potential, the density, the energy, and the current distribution.
Comparison of the numerical results with analytical expressions clearly
indicates that the energy of the vortex is governed by the zero-temperature BCS
coherence length.Comment: 4 pages, 4 embedded figures. Added references. To be published in
Physical Review Letter
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