242 research outputs found
Theory of solid state quantum information processing
Recent theoretical work on solid-state proposals for the implementation of
quantum computation and quantum information processing is reviewed. The
differences and similarities between microscopic and macroscopic qubits are
highlighted and exemplified by the spin qubit proposal on one side and the
superconducting qubits on the other. Before explaining the spin and
supercondcuting qubits in detail, some general concepts that are relevant for
both types of solid-state qubits are reviewed. The controlled production of
entanglement in solid-state devices, the transport of carriers of entanglement,
and entanglement detection will be discussed in the final part of this review.Comment: 57 pages, 33 figures, review article, prepared for Handbook of
Theoretical and Computational Nanotechnology. v.2: minor revision; references
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Non-Markovian qubit dynamics in the presence of 1/f noise
Within the lowest-order Born approximation, we calculate the exact dynamics
of a single qubit in the presence of 1/f noise, without making any Markov
approximation. We show that the non-Markovian qubit time-evolution exhibits
asymmetries and beatings that cannot be explained within a Markovian theory.
The present theory for 1/f noise is relevant for both spin- and superconducting
qubit realizations in solid-state devices, where 1/f noise is ubiquitous.Comment: v2: 6.2 pages, 5 figures, published versio
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
Long-range photon-mediated gate scheme between nuclear spin qubits in diamond
Defect centers in diamond are exceptional solid-state quantum systems that
can have exceedingly long electron and nuclear spin coherence times. So far,
single-qubit gates for the nitrogen nuclear spin, a two-qubit gate with a
nitrogen-vacancy (NV) center electron spin, and entanglement between nearby
nitrogen nuclear spins have been demonstrated. Here, we develop a scheme to
implement a universal two-qubit gate between two distant nitrogen nuclear
spins. Virtual excitation of an NV center that is embedded in an optical cavity
can scatter a laser photon into the cavity mode; we show that this process
depends on the nuclear spin state of the nitrogen atom. If two NV centers are
simultaneously coupled to a common cavity mode and individually excited,
virtual cavity photon exchange can mediate an effective interaction between the
nuclear spin qubits, conditioned on the spin state of both nuclei, which
implements a universal controlled- gate. We predict operation times
below 100 nanoseconds, which is several orders of magnitude faster than the
decoherence time of nuclear spin qubits in diamond.Comment: 6 pages (including 2 appendices), 3 figures, 1 tabl
Decoherence in Solid State Qubits
Interaction of solid state qubits with environmental degrees of freedom
strongly affects the qubit dynamics, and leads to decoherence. In quantum
information processing with solid state qubits, decoherence significantly
limits the performances of such devices. Therefore, it is necessary to fully
understand the mechanisms that lead to decoherence. In this review we discuss
how decoherence affects two of the most successful realizations of solid state
qubits, namely, spin-qubits and superconducting qubits. In the former, the
qubit is encoded in the spin 1/2 of the electron, and it is implemented by
confining the electron spin in a semiconductor quantum dot. Superconducting
devices show quantum behavior at low temperatures, and the qubit is encoded in
the two lowest energy levels of a superconducting circuit. The electron spin in
a quantum dot has two main decoherence channels, a (Markovian) phonon-assisted
relaxation channel, due to the presence of a spin-orbit interaction, and a
(non-Markovian) spin bath constituted by the spins of the nuclei in the quantum
dot that interact with the electron spin via the hyperfine interaction. In a
superconducting qubit, decoherence takes place as a result of fluctuations in
the control parameters, such as bias currents, applied flux, and bias voltages,
and via losses in the dissipative circuit elements.Comment: review article, 66 pages, 10 figure
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