170,392 research outputs found
Electron Spins in Artificial Atoms and Molecules for Quantum Computing
Achieving control over the electron spin in quantum dots (artificial atoms)
or real atoms promises access to new technologies in conventional and in
quantum information processing. Here we review our proposal for quantum
computing with spins of electrons confined to quantum dots. We discuss the
basic requirements for implementing spin-qubits, and describe a complete set of
quantum gates for single- and two-qubit operations. We show how a quantum dot
attached to leads can be used for spin filtering and spin read-out, and as a
spin-memory device. Finally, we focus on the experimental characterization of
the quantum dot systems, and discuss transport properties of a double-dot and
show how Kondo correlations can be used to measure the Heisenberg exchange
interaction between the spins of two dots.Comment: 13 pages, 8 figures, Invited Review (Semiconductor Spintronics,
Special Issue of SST
Memory Effects In Nonequilibrium Quantum Impurity Models
Memory effects play a key role in the dynamics of strongly correlated systems
driven out of equilibrium. In the present study, we explore the nature of
memory in the nonequilibrium Anderson impurity model. The
Nakajima--Zwanzig--Mori formalism is used to derive an exact generalized
quantum master equation for the reduced density matrix of the interacting
quantum dot, which includes a non-Markovian memory kernel. A real-time path
integral formulation is developed, in which all diagrams are stochastically
sampled in order to numerically evaluate the memory kernel. We explore the
effects of temperature down to the Kondo regime, as well as the role of
source--drain bias voltage and band width on the memory. Typically, the memory
decays on timescales significantly shorter than the dynamics of the reduced
density matrix itself, yet under certain conditions it develops a smaller long
tail. In addition we address the conditions required for the existence,
uniqueness and stability of a steady-state.Comment: 4 pages, 3 figure
Strong quantum memory at resonant Fermi edges revealed by shot noise
Studies of non-equilibrium current fluctuations enable assessing correlations
involved in quantum transport through nanoscale conductors. They provide
additional information to the mean current on charge statistics and the
presence of coherence, dissipation, disorder, or entanglement. Shot noise,
being a temporal integral of the current autocorrelation function, reveals
dynamical information. In particular, it detects presence of non-Markovian
dynamics, i.e., memory, within open systems, which has been subject of many
current theoretical studies. We report on low-temperature shot noise
measurements of electronic transport through InAs quantum dots in the
Fermi-edge singularity regime and show that it exhibits strong memory effects
caused by quantum correlations between the dot and fermionic reservoirs. Our
work, apart from addressing noise in archetypical strongly correlated system of
prime interest, discloses generic quantum dynamical mechanism occurring at
interacting resonant Fermi edges.Comment: 6 pages, 3 figure
Storage of multiple single-photon pulses emitted from a quantum dot in a solid-state quantum memory
Quantum repeaters are critical components for distributing entanglement over
long distances in presence of unavoidable optical losses during transmission.
Stimulated by Duan-Lukin-Cirac-Zoller protocol, many improved quantum-repeater
protocols based on quantum memories have been proposed, which commonly focus on
the entanglement-distribution rate. Among these protocols, the elimination of
multi-photons (multi-photon-pairs) and the use of multimode quantum memory are
demonstrated to have the ability to greatly improve the
entanglement-distribution rate. Here, we demonstrate the storage of
deterministic single photons emitted from a quantum dot in a
polarization-maintaining solid-state quantum memory; in addition,
multi-temporal-mode memory with , and narrow single-photon pulses
is also demonstrated. Multi-photons are eliminated, and only one photon at most
is contained in each pulse. Moreover, the solid-state properties of both
sub-systems make this configuration more stable and easier to be scalable. Our
work will be helpful in the construction of efficient quantum repeaters based
on all-solid-state devicesComment: Published version, including supplementary materia
Co-tunneling current and shot noise in quantum dots
We derive general expressions for the current and shot noise, taking into
account non-Markovian memory effects. In generalization of previous approaches
our theory is valid for arbitrary Coulomb interaction and coupling strength and
is applicable to quantum dots and more complex systems like molecules. A
diagrammatic expansion up to second-order in the coupling strength, taking into
account co-tunneling processes, allows for a study of transport in a regime
relevant to many experiments. As an example, we consider a single-level quantum
dot, focusing on the Coulomb-blockade regime. We find super-Poissonian shot
noise due to spin-flip co-tunneling processes at an energy scale different from
the one expected from first-order calculations, with a sensitive dependence on
the coupling strength.Comment: 4 pages, three figures, submitted to PR
Quantum interface of an electron and a nuclear ensemble.
Coherent excitation of an ensemble of quantum objects underpins quantum many-body phenomena and offers the opportunity to realize a memory that stores quantum information. Thus far, a deterministic and coherent interface between a spin qubit and such an ensemble has remained elusive. In this study, we first used an electron to cool the mesoscopic nuclear spin ensemble of a semiconductor quantum dot to the nuclear sideband-resolved regime. We then implemented an all-optical approach to access individual quantized electronic-nuclear spin transitions. Lastly, we performed coherent optical rotations of a single collective nuclear spin excitation-a spin wave. These results constitute the building blocks of a dedicated local memory per quantum-dot spin qubit and promise a solid-state platform for quantum-state engineering of isolated many-body systems
- …