514 research outputs found
Quantum Computation and Spin Electronics
In this chapter we explore the connection between mesoscopic physics and
quantum computing. After giving a bibliography providing a general introduction
to the subject of quantum information processing, we review the various
approaches that are being considered for the experimental implementation of
quantum computing and quantum communication in atomic physics, quantum optics,
nuclear magnetic resonance, superconductivity, and, especially, normal-electron
solid state physics. We discuss five criteria for the realization of a quantum
computer and consider the implications that these criteria have for quantum
computation using the spin states of single-electron quantum dots. Finally, we
consider the transport of quantum information via the motion of individual
electrons in mesoscopic structures; specific transport and noise measurements
in coupled quantum dot geometries for detecting and characterizing
electron-state entanglement are analyzed.Comment: 28 pages RevTeX, 4 figures. To be published in "Quantum Mesoscopic
Phenomena and Mesoscopic Devices in Microelectronics," eds. I. O. Kulik and
R. Ellialtioglu (NATO Advanced Study Institute, Turkey, June 13-25, 1999
Entanglement of Assistance is not a bipartite measure nor a tripartite monotone
The entanglement of assistance quantifies the entanglement that can be
generated between two parties, Alice and Bob, given assistance from a third
party, Charlie, when the three share a tripartite state and where the
assistance consists of Charlie initially performing a measurement on his share
and communicating the result to Alice and Bob through a one-way classical
channel. We argue that if this quantity is to be considered an operational
measure of entanglement, then it must be understood to be a tripartite rather
than a bipartite measure. We compare it with a distinct tripartite measure that
quantifies the entanglement that can be generated between Alice and Bob when
they are allowed to make use of a two-way classical channel with Charlie. We
show that the latter quantity, which we call the entanglement of collaboration,
can be greater than the entanglement of assistance. This demonstrates that the
entanglement of assistance (considered as a tripartite measure of
entanglement), and its multipartite generalizations such as the localizable
entanglement, are not entanglement monotones, thereby undermining their
operational significance.Comment: 5 pages, revised, title changed, added a discussion explaining why
entanglement of assistance can not be considered as a bipartite measure, to
appear in Phys. Rev.
Quantum Speed Limit for Perfect State Transfer in One Dimension
The basic idea of spin chain engineering for perfect quantum state transfer
(QST) is to find a set of coupling constants in the Hamiltonian, such that a
particular state initially encoded on one site will evolve freely to the
opposite site without any dynamical controls. The minimal possible evolution
time represents a speed limit for QST. We prove that the optimal solution is
the one simulating the precession of a spin in a static magnetic field. We also
argue that, at least for solid-state systems where interactions are local, it
is more realistic to characterize the computation power by the couplings than
the initial energy.Comment: 5 pages, no figure; improved versio
Quantum logic gates using Stark shifted Raman transitions in a cavity
We present a scheme to realise the basic two-quibit logic gates such as
quantum phase gate and controlle-NOT gate using a detuned optical cavity
interacting with a three-level Raman system. We discuss the role of Stark
shifts which are as important as the terms leading to two-photon transition.
The operation of the proposed logic gates involves metastable states of the
atom and hence is not affected by spontaneous emission. These ideas can be
extended to produce multiparticle entanglement.Comment: 5 pages, 1 figure, RevTeX4, Text is modifie
Physical Optimization of Quantum Error Correction Circuits
Quantum error correcting codes have been developed to protect a quantum
computer from decoherence due to a noisy environment. In this paper, we present
two methods for optimizing the physical implementation of such error correction
schemes. First, we discuss an optimal quantum circuit implementation of the
smallest error-correcting code (the three bit code). Quantum circuits are
physically implemented by serial pulses, i.e. by switching on and off external
parameters in the Hamiltonian one after another. In contrast to this, we
introduce a new parallel switching method that allows faster gate operation by
switching all external parameters simultaneously. These two methods are applied
to electron spins in coupled quantum dots subject to a Heisenberg coupling
H=J(t) S_1*S_2 which can generate the universal quantum gate
`square-root-of-swap'. Using parallel pulses, the encoding for three-bit
quantum error correction in a Heisenberg system can be accelerated by a factor
of about two. We point out that parallel switching has potential applications
for arbitrary quantum computer architectures.Comment: 13 pages, 6 figure
Spin-Orbit Coupling and Time-Reversal Symmetry in Quantum Gates
We study the effect of spin-orbit coupling on quantum gates produced by
pulsing the exchange interaction between two single electron quantum dots.
Spin-orbit coupling enters as a small spin precession when electrons tunnel
between dots. For adiabatic pulses the resulting gate is described by a unitary
operator acting on the four-dimensional Hilbert space of two qubits. If the
precession axis is fixed, time-symmetric pulsing constrains the set of possible
gates to those which, when combined with single qubit rotations, can be used in
a simple CNOT construction. Deviations from time-symmetric pulsing spoil this
construction. The effect of time asymmetry is studied by numerically
integrating the Schr\"odinger equation using parameters appropriate for GaAs
quantum dots. Deviations of the implemented gate from the desired form are
shown to be proportional to dimensionless measures of both spin-orbit coupling
and time asymmetry of the pulse.Comment: 10 pages, 3 figure
Deterministic Entanglement of Assistance and Monogamy Constraints
Certain quantum information tasks require entanglement of assistance, namely
a reduction of a tripartite entangled state to a bipartite entangled state via
local measurements. We establish that 'concurrence of assistance' (CoA)
identifies capabilities and limitations to producing pure bipartite entangled
states from pure tripartite entangled states and prove that CoA is an
entanglement monotone for -dimensional pure states.
Moreover, if the CoA for the pure tripartite state is at least as large as the
concurrence of the desired pure bipartite state, then the former may be
transformed to the latter via local operations and classical communication, and
we calculate the maximum probability for this transformation when this
condition is not met.Comment: 5 pages, no figure
Simple Realization Of The Fredkin Gate Using A Series Of Two-body Operators
The Fredkin three-bit gate is universal for computational logic, and is
reversible. Classically, it is impossible to do universal computation using
reversible two-bit gates only. Here we construct the Fredkin gate using a
combination of six two-body reversible (quantum) operators.Comment: Revtex 3.0, 7 pages, 3 figures appended at the end, please refer to
the comment lines at the beginning of the manuscript for reasons of
replacemen
High Kinetic Inductance Superconducting Nanowire Resonators for Circuit QED in a Magnetic Field
We present superconducting microwave-frequency resonators based on NbTiN
nanowires. The small cross section of the nanowires minimizes vortex
generation, making the resonators resilient to magnetic fields. Measured
intrinsic quality factors exceed in a T in-plane magnetic
field, and in a mT perpendicular magnetic field. Due to
their high characteristic impedance, these resonators are expected to develop
zero-point voltage fluctuations one order of magnitude larger than in standard
coplanar waveguide resonators. These properties make the nanowire resonators
well suited for circuit QED experiments needing strong coupling to quantum
systems with small electric dipole moments and requiring a magnetic field, such
as electrons in single and double quantum dots
Realization of Topological Quantum Computation with surface codes
In this paper, the degenerate ground states of Z2 topological order on a
plane with holes (the so-called surface codes) are used as the protected code
subspace to build a topological quantum computer by tuning their quantum
tunneling effect. Using a designer Hamiltonian - the Kitaev toric-code model as
an example, we study quantum tunneling effects of the surface codes and obtain
its effective theory. Finally, we show how to do topological quantum
computation including the initialization, the unitary transformation and the
measurement
- …