16 research outputs found
Quantum gates between capacitively coupled double quantum dot two-spin qubits
We study the two-qubit controlled-not gate operating on qubits encoded in the
spin state of a pair of electrons in a double quantum dot. We assume that the
electrons can tunnel between the two quantum dots encoding a single qubit,
while tunneling between the quantum dots that belong to different qubits is
forbidden. Therefore, the two qubits interact exclusively through the direct
Coulomb repulsion of the electrons. We find that entangling two-qubit gates can
be performed by the electrical biasing of quantum dots and/or tuning of the
tunneling matrix elements between the quantum dots within the qubits. The
entangling interaction can be controlled by tuning the bias through the
resonance between the singly-occupied and doubly-occupied singlet ground states
of a double quantum dot.Comment: 12 pages, 7 figure
Hyperfine-induced decoherence in triangular spin-cluster qubits
We investigate hyperfine-induced decoherence in a triangular spin-cluster for
different qubit encodings. Electrically controllable eigenstates of spin
chirality (C_z) show decoherence times that approach milliseconds, two orders
of magnitude longer than those estimated for the eigenstates of the total spin
projection (S_z) and of the partial spin sum (S_{12}). The robustness of
chirality is due to its decoupling from both the total- and individual-spin
components in the cluster. This results in a suppression of the effective
interaction between C_z and the nuclear spin bath
Interference of heavy holes in an Aharonov-Bohm ring
We study the coherent transport of heavy holes through a one-dimensional ring
in the presence of spin-orbit coupling. Spin-orbit interaction of holes, cubic
in the in-plane components of momentum, gives rise to an angular momentum
dependent spin texture of the eigenstates and influences transport. We analyze
the dependence of the resulting differential conductance of the ring on hole
polarization of the leads and the signature of the textures in the
Aharonov-Bohm oscillations when the ring is in a perpendicular magnetic field.
We find that the polarization-resolved conductance reveals whether the dominant
spin-orbit coupling is of Dresselhaus or Rashba type, and that the cubic
spin-orbit coupling can be distinguished from the conventional linear coupling
by observing the four-peak structure in the Aharonov-Bohm oscillations.Comment: 12 pages, 11 figure
Current-Conserving Aharonov-Bohm Interferometry with Arbitrary Spin Interactions
We propose a general scattering matrix formalism that guarantees the charge
conservation at junctions between conducting arms with arbitrary spin
interactions. By using our formalism, we find that the spin-flip scattering can
happen even at nonmagnetic junctions if the spin eigenstates in arms are not
orthogonal. We apply our formalism to the Aharonov-Bohm interferometer
consisting of -type semiconductor ring with both the Rashba spin-orbit
coupling and the Zeeman splitting. We discuss the characteristics of the
interferometer as conditional/unconditional spin switch in the
weak/strong-coupling limit, respectively.Comment: 14 pages, 13 figure
Exchange-based CNOT gates for singlet-triplet qubits with spin orbit interaction
We propose a scheme for implementing the CNOT gate over qubits encoded in a
pair of electron spins in a double quantum dot. The scheme is based on exchange
and spin orbit interactions and on local gradients in Zeeman fields. We find
that the optimal device geometry for this implementation involves effective
magnetic fields that are parallel to the symmetry axis of the spin orbit
interaction. We show that the switching times for the CNOT gate can be as fast
as a few nanoseconds for realistic parameter values in GaAs semiconductors.
Guided by recent advances in surface codes, we also consider the perpendicular
geometry. In this case, leakage errors due to spin orbit interaction occur but
can be suppressed in strong magnetic fields
Enhancement of electron spin coherence by optical preparation of nuclear spins
We study a large ensemble of nuclear spins interacting with a single electron
spin in a quantum dot under optical excitation and photon detection. When a
pair of applied laser fields satisfy two-photon resonance between the two
ground electronic spin states, detection of light scattering from the
intermediate exciton state acts as a weak quantum measurement of the effective
magnetic (Overhauser) field due to the nuclear spins. If the spin were driven
into a coherent population trapping state where no light scattering takes
place, then the nuclear state would be projected into an eigenstate of the
Overhauser field operator and electron decoherence due to nuclear spins would
be suppressed: we show that this limit can be approached by adapting the laser
frequencies when a photon is detected. We use a Lindblad equation to describe
the time evolution of the driven system under photon emission and detection.
Numerically, we find an increase of the electron coherence time from 5 ns to
500 ns after a preparation time of 10 microseconds.Comment: 5 pages, 4 figure
Spin-Electric Coupling in Molecular Magnets
We study the triangular antiferromagnet Cu in external electric fields,
using symmetry group arguments and a Hubbard model approach. We identify a
spin-electric coupling caused by an interplay between spin exchange, spin-orbit
interaction, and the chirality of the underlying spin texture of the molecular
magnet. This coupling allows for the electric control of the spin (qubit)
states, e.g. by using an STM tip or a microwave cavity. We propose an
experimental test for identifying molecular magnets exhibiting spin-electric
effects.Comment: 5 pages, 3 figure
Spin electric effects in molecular antiferromagnets
Molecular nanomagnets show clear signatures of coherent behavior and have a
wide variety of effective low-energy spin Hamiltonians suitable for encoding
qubits and implementing spin-based quantum information processing. At the
nanoscale, the preferred mechanism for control of quantum systems is through
application of electric fields, which are strong, can be locally applied, and
rapidly switched. In this work, we provide the theoretical tools for the search
for single molecule magnets suitable for electric control. By group-theoretical
symmetry analysis we find that the spin-electric coupling in triangular
molecules is governed by the modification of the exchange interaction, and is
possible even in the absence of spin-orbit coupling. In pentagonal molecules
the spin-electric coupling can exist only in the presence of spin-orbit
interaction. This kind of coupling is allowed for both and
spins at the magnetic centers. Within the Hubbard model, we find a relation
between the spin-electric coupling and the properties of the chemical bonds in
a molecule, suggesting that the best candidates for strong spin-electric
coupling are molecules with nearly degenerate bond orbitals. We also
investigate the possible experimental signatures of spin-electric coupling in
nuclear magnetic resonance and electron spin resonance spectroscopy, as well as
in the thermodynamic measurements of magnetization, electric polarization, and
specific heat of the molecules.Comment: 31 pages, 24 figure
Interference of heavy holes in an Aharonov-Bohm ring
12 pages, 11 figuresInternational audienceWe study the coherent transport of heavy holes through a one-dimensional ring in the presence of spin-orbit coupling. Spin-orbit interaction of holes, cubic in the in-plane components of momentum, gives rise to an angular momentum dependent spin texture of the eigenstates and influences transport. We analyze the dependence of the resulting differential conductance of the ring on hole polarization of the leads and the signature of the textures in the Aharonov-Bohm oscillations when the ring is in a perpendicular magnetic field. We find that the polarization-resolved conductance reveals whether the dominant spin-orbit coupling is of Dresselhaus or Rashba type, and that the cubic spin-orbit coupling can be distinguished from the conventional linear coupling by observing the four-peak structure in the Aharonov-Bohm oscillations