115 research outputs found
Josephson junction qubit network with current-controlled interaction
We design and evaluate a scalable charge qubit chain network with
controllable current-current coupling of neighbouring qubit loops via local
dc-current gates. The network allows construction of general N-qubit gates. The
proposed design is in line with current main stream experiments.Comment: 4 pages, 4 figure
Implementation of the three-qubit phase-flip error correction code with superconducting qubits
We investigate the performance of a three qubit error correcting code in the
framework of superconducting qubit implementations. Such a code can recover a
quantum state perfectly in the case of dephasing errors but only in situations
where the dephasing rate is low. Numerical studies in previous work have
however shown that the code does increase the fidelity of the encoded state
even in the presence of high error probability, during both storage and
processing. In this work we give analytical expressions for the fidelity of
such a code. We consider two specific schemes for qubit-qubit interaction
realizable in superconducting systems; one -coupling and one
cavity mediated coupling. With these realizations in mind, and considering
errors during storing as well as processing, we calculate the maximum operation
time allowed in order to still benefit from the code. We show that this limit
can be reached with current technology.Comment: 10 pages, 8 figure
A Variational Approach for Minimizing Lennard-Jones Energies
A variational method for computing conformational properties of molecules
with Lennard-Jones potentials for the monomer-monomer interactions is
presented. The approach is tailored to deal with angular degrees of freedom,
{\it rotors}, and consists in the iterative solution of a set of deterministic
equations with annealing in temperature. The singular short-distance behaviour
of the Lennard-Jones potential is adiabatically switched on in order to obtain
stable convergence. As testbeds for the approach two distinct ensembles of
molecules are used, characterized by a roughly dense-packed ore a more
elongated ground state. For the latter, problems are generated from natural
frequencies of occurrence of amino acids and phenomenologically determined
potential parameters; they seem to represent less disorder than was previously
assumed in synthetic protein studies. For the dense-packed problems in
particular, the variational algorithm clearly outperforms a gradient descent
method in terms of minimal energies. Although it cannot compete with a careful
simulating annealing algorithm, the variational approach requires only a tiny
fraction of the computer time. Issues and results when applying the method to
polyelectrolytes at a finite temperature are also briefly discussed.Comment: 14 pages, uuencoded compressed postscript fil
Tunable resonators for quantum circuits
We have designed, fabricated and measured high-Q coplanar
waveguide microwave resonators whose resonance frequency is made tunable with
magnetic field by inserting a DC-SQUID array (including 1 or 7 SQUIDs) inside.
Their tunability range is 30% of the zero field frequency. Their quality factor
reaches up to 3. We present a model based on thermal fluctuations
that accounts for the dependance of the quality factor with magnetic field.Comment: subm. to JLTP (Proc. of LTD12 conference
Dynamics of one-dimensional tight-binding models with arbitrary time-dependent external homogeneous fields
The exact propagators of two one-dimensional systems with time-dependent
external fields are presented by following the path-integral method. It is
shown that the Bloch acceleration theorem can be generalized to the
impulse-momentum theorem in quantum version. We demonstrate that an evolved
Gaussian wave packet always keeps its shape in an arbitrary time-dependent
homogeneous driven field. Moreover, that stopping and accelerating of a wave
packet can be achieved by the pulsed field in a diabatic way.Comment: 8 pages, 6 figure
Spectral compression of single photons
Photons are critical to quantum technologies since they can be used for
virtually all quantum information tasks: in quantum metrology, as the
information carrier in photonic quantum computation, as a mediator in hybrid
systems, and to establish long distance networks. The physical characteristics
of photons in these applications differ drastically; spectral bandwidths span
12 orders of magnitude from 50 THz for quantum-optical coherence tomography to
50 Hz for certain quantum memories. Combining these technologies requires
coherent interfaces that reversibly map centre frequencies and bandwidths of
photons to avoid excessive loss. Here we demonstrate bandwidth compression of
single photons by a factor 40 and tunability over a range 70 times that
bandwidth via sum-frequency generation with chirped laser pulses. This
constitutes a time-to-frequency interface for light capable of converting
time-bin to colour entanglement and enables ultrafast timing measurements. It
is a step toward arbitrary waveform generation for single and entangled
photons.Comment: 6 pages (4 figures) + 6 pages (3 figures
Two-dimensional cavity grid for scalable quantum computation with superconducting circuits
Superconducting circuits are among the leading contenders for quantum
information processing. This promising avenue has been strengthened with the
advent of circuit quantum electrodynamics, underlined by recent experiments
coupling on-chip microwave resonators to superconducting qubits. However,
moving towards more qubits will require suitable novel architectures. Here, we
propose a scalable setup for quantum computing where such resonators are
arranged in a two-dimensional grid with a qubit at each intersection. Its
versatility allows any two qubits on the grid to be coupled at a swapping
overhead independent of their distance and yields an optimal balance between
reducing qubit transition frequency spread and spurious cavity-induced
couplings. These features make this setup unique and distinct from existing
proposals in ion traps, optical lattices, or semiconductor spins. We
demonstrate that this approach encompasses the fundamental elements of a
scalable fault-tolerant quantum computing architecture.Comment: version as published in EPL 95 No 5 (March 2009) 50007, 5 page
Coherent optical wavelength conversion via cavity-optomechanics
We theoretically propose and experimentally demonstrate coherent wavelength
conversion of optical photons using photon-phonon translation in a
cavity-optomechanical system. For an engineered silicon optomechanical crystal
nanocavity supporting a 4 GHz localized phonon mode, optical signals in a 1.5
MHz bandwidth are coherently converted over a 11.2 THz frequency span between
one cavity mode at wavelength 1460 nm and a second cavity mode at 1545 nm with
a 93% internal (2% external) peak efficiency. The thermal and quantum limiting
noise involved in the conversion process is also analyzed, and in terms of an
equivalent photon number signal level are found to correspond to an internal
noise level of only 6 and 4x10-3 quanta, respectively.Comment: 11 pages, 7 figures, appendi
Quantum Transduction of Telecommunications-band Single Photons from a Quantum Dot by Frequency Upconversion
The ability to transduce non-classical states of light from one wavelength to
another is a requirement for integrating disparate quantum systems that take
advantage of telecommunications-band photons for optical fiber transmission of
quantum information and near-visible, stationary systems for manipulation and
storage. In addition, transducing a single-photon source at 1.3 {\mu}m to
visible wavelengths for detection would be integral to linear optical quantum
computation due to the challenges of detection in the near-infrared. Recently,
transduction at single-photon power levels has been accomplished through
frequency upconversion, but it has yet to be demonstrated for a true
single-photon source. Here, we transduce the triggered single-photon emission
of a semiconductor quantum dot at 1.3 {\mu}m to 710 nm with a total detection
(internal conversion) efficiency of 21% (75%). We demonstrate that the 710 nm
signal maintains the quantum character of the 1.3 {\mu}m signal, yielding a
photon anti-bunched second-order intensity correlation, g^(2)(t), that shows
the optical field is composed of single photons with g^(2)(0) = 0.165 < 0.5.Comment: 7 pages, 4 figure
Hybrid Mechanical Systems
We discuss hybrid systems in which a mechanical oscillator is coupled to
another (microscopic) quantum system, such as trapped atoms or ions,
solid-state spin qubits, or superconducting devices. We summarize and compare
different coupling schemes and describe first experimental implementations.
Hybrid mechanical systems enable new approaches to quantum control of
mechanical objects, precision sensing, and quantum information processing.Comment: To cite this review, please refer to the published book chapter (see
Journal-ref and DOI). This v2 corresponds to the published versio
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