533 research outputs found
Probing quantum coherence in qubit arrays
We discuss how the observation of population localization effects in
periodically driven systems can be used to quantify the presence of quantum
coherence in interacting qubit arrays. Essential for our proposal is the fact
that these localization effects persist beyond tight-binding Hamiltonian
models. This result is of special practical relevance in those situations where
direct system probing using tomographic schemes becomes infeasible beyond a
very small number of qubits. As a proof of principle, we study analytically a
Hamiltonian system consisting of a chain of superconducting flux qubits under
the effect of a periodic driving. We provide extensive numerical support of our
results in the simple case of a two-qubits chain. For this system we also study
the robustness of the scheme against different types of noise and disorder. We
show that localization effects underpinned by quantum coherent interactions
should be observable within realistic parameter regimes in chains with a larger
number o
Circuit QED and engineering charge based superconducting qubits
The last two decades have seen tremendous advances in our ability to generate
and manipulate quantum coherence in mesoscopic superconducting circuits. These
advances have opened up the study of quantum optics of microwave photons in
superconducting circuits as well as providing important hardware for the
manipulation of quantum information. Focusing primarily on charge-based qubits,
we provide a brief overview of these developments and discuss the present state
of the art. We also survey the remarkable progress that has been made in
realizing circuit quantum electrodynamics (QED) in which superconducting
artificial atoms are strongly coupled to individual microwave photons.Comment: Proceedings of Nobel Symposium 141: Qubits for Future Quantum
Informatio
Quantum state transfer in arrays of flux qubits
In this work, we describe a possible experimental realization of Bose's idea
to use spin chains for short distance quantum communication [S. Bose, {\it
Phys. Rev. Lett.} {\bf 91} 207901]. Josephson arrays have been proposed and
analyzed as transmission channels for systems of superconducting charge qubits.
Here, we consider a chain of persistent current qubits, that is appropriate for
state transfer with high fidelity in systems containing flux qubits. We
calculate the fidelity of state transfer for this system. In general, the
Hamiltonian of this system is not of XXZ-type, and we analyze the magnitude and
the effect of the terms that don't conserve the z-component of the total spin.Comment: 10 pages, 8 figure
Topologically protected quantum bits from Josephson junction arrays
All physical implementations of quantum bits (qubits), carrying the
information and computation in a putative quantum computer, have to meet the
conflicting requirements of environmental decoupling while remaining
manipulable through designed external signals. Proposals based on quantum
optics naturally emphasize the aspect of optimal isolation, while those
following the solid state route exploit the variability and scalability of
modern nanoscale fabrication techniques. Recently, various designs using
superconducting structures have been successfully tested for quantum coherent
operation, however, the ultimate goal of reaching coherent evolution over
thousands of elementary operations remains a formidable task. Protecting qubits
from decoherence by exploiting topological stability, a qualitatively new
proposal due to Kitaev, holds the promise for long decoherence times, but its
practical physical implementation has remained unclear so far. Here, we show
how strongly correlated systems developing an isolated two-fold degenerate
quantum dimer liquid groundstate can be used in the construction of
topologically stable qubits and discuss their implementation using Josephson
junction arrays.Comment: 6 pages, 4 figure
Quantum Computing
Quantum mechanics---the theory describing the fundamental workings of
nature---is famously counterintuitive: it predicts that a particle can be in
two places at the same time, and that two remote particles can be inextricably
and instantaneously linked. These predictions have been the topic of intense
metaphysical debate ever since the theory's inception early last century.
However, supreme predictive power combined with direct experimental observation
of some of these unusual phenomena leave little doubt as to its fundamental
correctness. In fact, without quantum mechanics we could not explain the
workings of a laser, nor indeed how a fridge magnet operates. Over the last
several decades quantum information science has emerged to seek answers to the
question: can we gain some advantage by storing, transmitting and processing
information encoded in systems that exhibit these unique quantum properties?
Today it is understood that the answer is yes. Many research groups around the
world are working towards one of the most ambitious goals humankind has ever
embarked upon: a quantum computer that promises to exponentially improve
computational power for particular tasks. A number of physical systems,
spanning much of modern physics, are being developed for this task---ranging
from single particles of light to superconducting circuits---and it is not yet
clear which, if any, will ultimately prove successful. Here we describe the
latest developments for each of the leading approaches and explain what the
major challenges are for the future.Comment: 26 pages, 7 figures, 291 references. Early draft of Nature 464, 45-53
(4 March 2010). Published version is more up-to-date and has several
corrections, but is half the length with far fewer reference
Magnetic qubits as hardware for quantum computers
We propose two potential realisations for quantum bits based on nanometre
scale magnetic particles of large spin S and high anisotropy molecular
clusters. In case (1) the bit-value basis states |0> and |1> are the ground and
first excited spin states Sz = S and S-1, separated by an energy gap given by
the ferromagnetic resonance (FMR) frequency. In case (2), when there is
significant tunnelling through the anisotropy barrier, the qubit states
correspond to the symmetric, |0>, and antisymmetric, |1>, combinations of the
two-fold degenerate ground state Sz = +- S. In each case the temperature of
operation must be low compared to the energy gap, \Delta, between the states
|0> and |1>. The gap \Delta in case (2) can be controlled with an external
magnetic field perpendicular to the easy axis of the molecular cluster. The
states of different molecular clusters and magnetic particles may be entangled
by connecting them by superconducting lines with Josephson switches, leading to
the potential for quantum computing hardware.Comment: 17 pages, 3 figure
Two-dimensional Ising model with competing interactions and its application to clusters and arrays of -rings and adiabatic quantum computing
We study planar clusters consisting of loops including a Josephson
-junction (-rings). Each -ring carries a persistent current and
behaves as a classical orbital moment. The type of particular state associated
with the orientation of orbital moments at the cluster depends on the
interaction between these orbital moments and can be easily controlled, i.e. by
a bias current or by other means. We show that these systems can be described
by the two-dimensional Ising model with competing nearest-neighbor and diagonal
interactions and investigate the phase diagram of this model. The
characteristic features of the model are analyzed based on the exact solutions
for small clusters such as a 5-site square plaquette as well as on a mean-field
type approach for the infinite square lattice of Ising spins. The results are
compared with spin patterns obtained by Monte Carlo simulations for the 100
100 square lattice and with experiment. We show that the -ring
clusters may be used as a new type of superconducting memory elements. The
obtained results may be verified in experiments and are applicable to adiabatic
quantum computing where the states are switched adiabatically with the slow
change of coupling constants.Comment: 32 pages, 22 figures, RevTe
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