486 research outputs found
Non-linear spin to charge conversion in mesoscopic structures
Motivated by recent experiments [Vera-Marun et al., arXiv:1109.5969], we
formulate a non-linear theory of spin transport in quantum coherent conductors.
We show how a mesoscopic constriction with energy-dependent transmission can
convert a spin current injected by a spin accumulation into an electric signal,
relying neither on magnetic nor exchange fields. When the transmission through
the constriction is spin-independent, the spin-charge coupling is non-linear,
with an electric signal that is quadratic in the accumulation. We estimate that
gated mesoscopic constrictions have a sensitivity that allows to detect
accumulations much smaller than a percent of the Fermi energy.Comment: 4 pages, 3 figure
Fast Long-Distance Control of Spin Qubits by Photon Assisted Cotunneling
We investigate theoretically the long-distance coupling and spin exchange in
an array of quantum dot spin qubits in the presence of microwaves. We find that
photon assisted cotunneling is boosted at resonances between photon and
energies of virtually occupied excited states and show how to make it spin
selective. We identify configurations that enable fast switching and spin echo
sequences for efficient and non-local manipulation of spin qubits. We devise
configurations in which the near-resonantly boosted cotunneling provides
non-local coupling which, up to certain limit, does not diminish with distance
between the manipulated dots before it decays weakly with inverse distance.Comment: 17 pages (including 8 pages of Appendices), 2 figure
Theory of Spin Relaxation in Two-Electron Lateral Coupled Si/SiGe Quantum Dots
Highly accurate numerical results of phonon-induced two-electron spin
relaxation in silicon double quantum dots are presented. The relaxation,
enabled by spin-orbit coupling and the nuclei of Si (natural or purified
abundance), are investigated for experimentally relevant parameters, the
interdot coupling, the magnetic field magnitude and orientation, and the
detuning. We calculate relaxation rates for zero and finite temperatures (100
mK), concluding that our findings for zero temperature remain qualitatively
valid also for 100 mK. We confirm the same anisotropic switch of the axis of
prolonged spin lifetime with varying detuning as recently predicted in GaAs.
Conditions for possibly hyperfine-dominated relaxation are much more stringent
in Si than in GaAs. For experimentally relevant regimes, the spin-orbit
coupling, although weak, is the dominant contribution, yielding anisotropic
relaxation rates of at least two order of magnitude lower than in GaAs.Comment: 11 pages, 10 figure
Spin-orbit coupled particle in a spin bath
We consider a spin-orbit coupled particle confined in a quantum dot in a bath
of impurity spins. We investigate the consequences of spin-orbit coupling on
the interactions that the particle mediates in the spin bath. We show that in
the presence of spin-orbit coupling, the impurity-impurity interactions are no
longer spin-conserving. We quantify the degree of this symmetry breaking and
show how it relates to the spin-orbit coupling strength. We identify several
ways how the impurity ensemble can in this way relax its spin by coupling to
phonons. A typical resulting relaxation rate for a self-assembled Mn-doped ZnTe
quantum dot populated by a hole is 1 s. We also show that decoherence
arising from nuclear spins in lateral quantum dots is still removable by a spin
echo protocol, even if the confined electron is spin-orbit coupled.Comment: 18 pages, 1 figur
Coexistence of qubit effects
Two quantum events, represented by positive operators (effects), are coexistent if they can occur as possible outcomes in a single measurement scheme. Equivalently, the corresponding effects are coexistent if and only if they are contained in the ranges of a single (joint) observable. Here we give several equivalent characterizations of coexistent pairs of qubit effects. We also establish the equivalence between our results and those obtained independently by other authors. Our approach makes explicit use of the Minkowski space geometry inherent in the four-dimensional real vector space of selfadjoint operators in a two-dimensional complex Hilbert space
Quantum nondemolition measurement of an electron spin qubit
Measurement of quantum systems inevitably involves disturbance in various
forms. Within the limits imposed by quantum mechanics, however, one can design
an "ideal" projective measurement that does not introduce a back action on the
measured observable, known as a quantum nondemolition (QND) measurement. Here
we demonstrate an all-electrical QND measurement of a single electron spin in a
gate-defined quantum dot via an exchange-coupled ancilla qubit. The ancilla
qubit, encoded in the singlet-triplet two-electron subspace, is entangled with
the single spin and subsequently read out in a single shot projective
measurement at a rate two orders of magnitude faster than the spin relaxation.
The QND nature of the measurement protocol is evidenced by observing a
monotonic increase of the readout fidelity over one hundred repetitive
measurements against arbitrary input states. We extract information from the
measurement record using the method of optimal inference, which is tolerant to
the presence of the relaxation and dephasing. The QND measurement allows us to
observe spontaneous spin flips (quantum jumps) in an isolated system with small
disturbance. Combined with the high-fidelity control of spin qubits, these
results pave the way for various measurement-based quantum state manipulations
including quantum error correction protocols.Comment: This is a pre-print of an article published in Nature Nanotechnology.
The final authenticated version is available online at:
https://doi.org/10.1038/s41565-019-0426-
Robust Single-Shot Spin Measurement with 99.5% Fidelity in a Quantum Dot Array
We demonstrate a new method for projective single-shot measurement of two
electron spin states (singlet versus triplet) in an array of gate-defined
lateral quantum dots in GaAs. The measurement has very high fidelity and is
robust with respect to electric and magnetic fluctuations in the environment.
It exploits a long-lived metastable charge state, which increases both the
contrast and the duration of the charge signal distinguishing the two
measurement outcomes. This method allows us to evaluate the charge measurement
error and the spin-to-charge conversion error separately. We specify conditions
under which this method can be used, and project its general applicability to
scalable quantum dot arrays in GaAs or silicon.Comment: 13 pages, 3 figure
A fast quantum interface between different spin qubit encodings
Single-spin qubits in semiconductor quantum dots proposed by Loss and
DiVincenzo (LD qubits) hold promise for universal quantum computation with
demonstrations of a high single-qubit gate fidelity above 99.9 % and two-qubit
gates in conjunction with a long coherence time. However, initialization and
readout of a qubit is orders of magnitude slower than control, which is
detrimental for implementing measurement-based protocols such as
error-correcting codes. In contrast, a singlet-triplet (ST) qubit, encoded in a
two-spin subspace, has the virtue of fast readout with high fidelity and
tunable coupling to the electric field. Here, we present a hybrid system which
benefits from the different advantages of these two distinct spin-qubit
implementations. A quantum interface between the two codes is realized by
electrically tunable inter-qubit exchange coupling. We demonstrate a
controlled-phase (CPHASE) gate that acts within 5.5 ns, much faster than the
measured dephasing time of 211 ns. The presented hybrid architecture will be
useful to settle remaining key problems with building scalable spin-based
quantum computers
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