7,006 research outputs found
Universal Quantum Computing with Spin and Valley
We investigate a two-electron double quantum dot with both spin and valley
degrees of freedom as they occur in graphene, carbon nanotubes, or silicon, and
regard the 16-dimensional space with one electron per dot as a four-qubit logic
space. In the spin-only case, it is well known that the exchange coupling
between the dots combined with arbitrary single-qubit operations is sufficient
for universal quantum computation. The presence of the valley degeneracy in the
electronic band structure alters the form of the exchange coupling and in
general leads to spin-valley entanglement. Here, we show that universal quantum
computation can still be performed by exchange interaction and single-qubit
gates in the presence of the additional (valley) degree of freedom. We present
an explicit pulse sequence for a spin-only controlled-NOT consisting of the
generalized exchange coupling and single-electron spin and valley rotations. We
also propose state preparations and projective measurements with the use of
adiabatic transitions between states with (1,1) and (0,2) charge distributions
similar to the spin-only case, but with the additional requirement of
controlling the spin and the valley Zeeman energies by an external magnetic
field. Finally, we demonstrate a universal two-qubit gate between a spin and a
valley qubit, allowing universal gate operations on the combined spin and
valley quantum register.Comment: 18 pages, 3 figures, 1 tabl
Adiabatic and Hamiltonian computing on a 2D lattice with simple 2-qubit interactions
We show how to perform universal Hamiltonian and adiabatic computing using a
time-independent Hamiltonian on a 2D grid describing a system of hopping
particles which string together and interact to perform the computation. In
this construction, the movement of one particle is controlled by the presence
or absence of other particles, an effective quantum field effect transistor
that allows the construction of controlled-NOT and controlled-rotation gates.
The construction translates into a model for universal quantum computation with
time-independent 2-qubit ZZ and XX+YY interactions on an (almost) planar grid.
The effective Hamiltonian is arrived at by a single use of first-order
perturbation theory avoiding the use of perturbation gadgets. The dynamics and
spectral properties of the effective Hamiltonian can be fully determined as it
corresponds to a particular realization of a mapping between a quantum circuit
and a Hamiltonian called the space-time circuit-to-Hamiltonian construction.
Because of the simple interactions required, and because no higher-order
perturbation gadgets are employed, our construction is potentially realizable
using superconducting or other solid-state qubits.Comment: 33 pages, 5 figure
Robust adiabatic approach to optical spin entangling in coupled quantum dots
Excitonic transitions offer a possible route to ultrafast optical spin
manipulation in coupled nanostructures. We perform here a detailed study of the
three principal exciton-mediated decoherence channels for optically-controlled
electron spin qubits in coupled quantum dots: radiative decay of the excitonic
state, exciton-phonon interactions, and Landau-Zener transitions between
laser-dressed states. We consider a scheme to produce an entangling
controlled-phase gate on a pair of coupled spins which, in its simplest dynamic
form, renders the system subject to fast decoherence rates associated with
exciton creation during the gating operation. In contrast, we show that an
adiabatic approach employing off-resonant laser excitation allows us to
suppress all sources of decoherence simultaneously, significantly increasing
the fidelity of operations at only a relatively small gating time cost. We find
that controlled-phase gates accurate to one part in 10^2 can realistically be
achieved with the adiabatic approach, whereas the conventional dynamic approach
does not appear to support a fidelity suitable for scalable quantum
computation. Our predictions could be demonstrated experimentally in the near
future.Comment: 26 pages, 9 figure
What is a quantum computer, and how do we build one?
The DiVincenzo criteria for implementing a quantum computer have been seminal
in focussing both experimental and theoretical research in quantum information
processing. These criteria were formulated specifically for the circuit model
of quantum computing. However, several new models for quantum computing
(paradigms) have been proposed that do not seem to fit the criteria well. The
question is therefore what are the general criteria for implementing quantum
computers. To this end, a formal operational definition of a quantum computer
is introduced. It is then shown that according to this definition a device is a
quantum computer if it obeys the following four criteria: Any quantum computer
must (1) have a quantum memory; (2) facilitate a controlled quantum evolution
of the quantum memory; (3) include a method for cooling the quantum memory; and
(4) provide a readout mechanism for subsets of the quantum memory. The criteria
are met when the device is scalable and operates fault-tolerantly. We discuss
various existing quantum computing paradigms, and how they fit within this
framework. Finally, we lay out a roadmap for selecting an avenue towards
building a quantum computer. This is summarized in a decision tree intended to
help experimentalists determine the most natural paradigm given a particular
physical implementation
Quantum computation with cold bosonic atoms in an optical lattice
We analyse an implementation of a quantum computer using bosonic atoms in an
optical lattice. We show that, even though the number of atoms per site and the
tunneling rate between neighbouring sites is unknown, one may perform a
universal set of gates by means of adiabatic passage
Effect of noise on geometric logic gates for quantum computation
We introduce the non-adiabatic, or Aharonov-Anandan, geometric phase as a
tool for quantum computation and show how it could be implemented with
superconducting charge qubits. While it may circumvent many of the drawbacks
related to the adiabatic (Berry) version of geometric gates, we show that the
effect of fluctuations of the control parameters on non-adiabatic phase gates
is more severe than for the standard dynamic gates. Similarly, fluctuations
also affect to a greater extent quantum gates that use the Berry phase instead
of the dynamic phase.Comment: 8 pages, 4 figures; published versio
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