1,039 research outputs found
Lower bound for electron spin entanglement from beamsplitter current correlations
We determine a lower bound for the entanglement of pairs of electron spins
injected into a mesoscopic conductor. The bound can be expressed in terms of
experimentally accessible quantities, the zero-frequency current correlators
(shot noise power or cross-correlators) after transmission through an
electronic beam splitter. The effect of spin relaxation (T_1 processes) and
decoherence (T_2 processes) during the ballistic coherent transmission of the
carriers in the wires is taken into account within Bloch theory. The presence
of a variable inhomogeneous magnetic field allows the determination of a useful
lower bound for the entanglement of arbitrary entangled states. The decrease in
entanglement due to thermally mixed states is studied. Both the entanglement of
the output of a source (entangler) and the relaxation (T_1) and decoherence
(T_2) times can be determined.Comment: 4 pages, 3 figure
Nuclear State Preparation via Landau-Zener-Stueckelberg transitions in Double Quantum Dots
We theoretically model a nuclear-state preparation scheme that increases the
coherence time of a two-spin qubit in a double quantum dot. The two-electron
system is tuned repeatedly across a singlet-triplet level-anticrossing with
alternating slow and rapid sweeps of an external bias voltage. Using a
Landau-Zener-Stueckelberg model, we find that in addition to a small nuclear
polarization that weakly affects the electron spin coherence, the slow sweeps
are only partially adiabatic and lead to a weak nuclear spin measurement and a
nuclear-state narrowing which prolongs the electron spin coherence. This
resolves some open problems brought up by a recent experiment [D. J. Reilly et
al., Science 321, 817 (2008).]. Based on our description of the weak
measurement, we simulate a system with up to n=200 nuclear spins per dot.
Scaling in n indicates a stronger effect for larger n.Comment: 4.1 pages, 2 figure
Bias-controllable intrinsic spin polarization in a quantum dot
We propose a novel scheme to efficiently polarize and manipulate the electron
spin in a quantum dot. This scheme is based on the spin-orbit interaction and
it possesses following advantages: (1) The direction and the strength of the
spin polarization is well controllable and manipulatable by simply varying the
bias or the gate voltage. (2) The spin polarization is quite large even with a
weak spin-orbit interaction. (3) Both electron-electron interaction and
multi-energy levels do not weaken but strengthen the spin polarization. (4) It
has the short spin flip time. (5) The device is free of a magnetic field or a
ferromagnetic material. (6) It can be easily realized with present technology.Comment: 9 pages, 5 figure
Coherent Adiabatic Spin Control in the Presence of Charge Noise Using Tailored Pulses
We study finite-time Landau-Zener transitions at a singlet-triplet level
crossing in a GaAs double quantum dot, both experimentally and theoretically.
Sweeps across the anticrossing in the high driving speed limit result in
oscillations with a small visibility. Here we demonstrate how to increase the
oscillation visibility while keeping sweep times shorter than T2* using a
tailored pulse with a detuning dependent level velocity. Our results show an
improvement of a factor ~2.9 for the oscillation visibility. In particular, we
were able to obtain a visibility of ~0.5 for St\"uckelberg oscillations, which
demonstrates the creation of an equally weighted superposition of the qubit
states.Comment: Related papers at http://pettagroup.princeton.ed
Double-Occupancy Errors, Adiabaticity, and Entanglement of Spin-Qubits in Quantum Dots
Quantum gates that temporarily increase singlet-triplet splitting in order to
swap electronic spins in coupled quantum dots, lead inevitably to a finite
double-occupancy probability for both dots. By solving the time-dependent
Schr\"odinger equation for a coupled dot model, we demonstrate that this does
not necessarily lead to quantum computation errors. Instead, the coupled dot
ground state evolves quasi-adiabatically for typical system parameters so that
the double-occupancy probability at the completion of swapping is negligibly
small. We introduce a measure of entanglement which explicitly takes into
account the possibilty of double occupancies and provides a necessary and
sufficient criterion for entangled states.Comment: 9 pages, 4 figures include
Spin exchange interaction with tunable range between graphene quantum dots
We study the spin exchange between two electrons localized in separate
quantum dots in graphene. The electronic states in the conduction band are
coupled indirectly by tunneling to a common continuum of delocalized states in
the valence band. As a model, we use a two-impurity Anderson Hamiltonian which
we subsequently transform into an effective spin Hamiltonian by way of a
two-stage Schrieffer-Wolff transformation. We then compare our result to that
from a Coqblin-Schrieffer approach as well as to fourth order perturbation
theory.Comment: 8 pages, 3 figure
Electronic Hong-Ou-Mandel interferometer for multi-mode entanglement detection
We show that multi-mode entanglement of electrons in a mesoscopic conductor
can be detected by a measurement of the zero-frequency current correlations in
an electronic Hong-Ou-Mandel interferometer. By this mean, one can further
establish a lower bound to the entanglement of formation of two-electron input
states. Our results extend the work of Burkard and Loss [Phys. Rev. Lett. 91,
087903 (2003)] to many channels and provide a way to test the existence of
entangled states involving both orbital and spin degrees of freedom.Comment: 6 pages. Revised version. Ref. adde
Relaxation and Dephasing in a Flux-qubit
We report detailed measurements of the relaxation and dephasing time in a
flux-qubit measured by a switching DC SQUID. We studied their dependence on the
two important circuit bias parameters: the externally applied magnetic flux and
the bias current through the SQUID in two samples. We demonstrate two
complementary strategies to protect the qubit from these decoherence sources.
One consists in biasing the qubit so that its resonance frequency is stationary
with respect to the control parameters ({\it optimal point}) ; the second
consists in {\it decoupling} the qubit from current noise by chosing a proper
bias current through the SQUID. At the decoupled optimal point, we measured
long spin-echo decay times of up to .Comment: 4 pages, 4 figures, submitted to Phys. Rev. Letter
Dephasing of a superconducting qubit induced by photon noise
We have studied the dephasing of a superconducting flux-qubit coupled to a
DC-SQUID based oscillator. By varying the bias conditions of both circuits we
were able to tune their effective coupling strength. This allowed us to measure
the effect of such a controllable and well-characterized environment on the
qubit coherence. We can quantitatively account for our data with a simple model
in which thermal fluctuations of the photon number in the oscillator are the
limiting factor. In particular, we observe a strong reduction of the dephasing
rate whenever the coupling is tuned to zero. At the optimal point we find a
large spin-echo decay time of .Comment: New version of earlier paper arXiv/0507290 after in-depth rewritin
- …