494 research outputs found
Electrically driven spin resonance in a bent disordered carbon nanotube
Resonant manipulation of carbon nanotube valley-spin qubits by an electric
field is investigated theoretically. We develop a new analysis of electrically
driven spin resonance exploiting fixed physical characteristics of the
nanotube: a bend and inhomogeneous disorder. The spectrum is simulated for an
electron valley-spin qubit coupled to a hole valley-spin qubit and an impurity
electron spin, and features that coincide with a recent measurement are
identified. We show that the same mechanism allows resonant control of the full
four-dimensional spin-valley space.Comment: 11 pages, 7 figure
Quantum sensors based on weak-value amplification cannot overcome decoherence
Sensors that harness exclusively quantum phenomena (such as entanglement) can
achieve superior performance compared to those employing only classical
principles. Recently, a technique based on postselected, weakly-performed
measurements has emerged as a method of overcoming technical noise in the
detection and estimation of small interaction parameters, particularly in
optical systems. The question of which other types of noise may be combatted
remains open. We here analyze whether the effect can overcome decoherence in a
typical field sensing scenario. Benchmarking a weak, postselected measurement
strategy against a strong, direct strategy we conclude that no advantage is
achievable, and that even a small amount of decoherence proves catastrophic to
the weak-value amplification technique.Comment: Published version with improvements to presentation, including
clarifying our understanding of technical noise and quantum nois
Seeing opportunity in every difficulty: protecting information with weak value techniques
A weak value is an effective description of the influence of a pre and
post-selected 'principal' system on another 'meter' system to which it is
weakly coupled. Weak values can describe anomalously large deflections of the
meter, and deflections in otherwise unperturbed variables: this motivates
investigation of the potential benefits of the protocol in precision metrology.
We present a visual interpretation of weak value experiments in phase space,
enabling an evaluation of the effects of three types of detector noise as
'Fisher information efficiency' functions. These functions depend on the
marginal distribution of the Wigner function of the meter, and give a unified
view of the weak value protocol as a way of protecting Fisher information from
detector imperfections. This approach explains why weak value techniques are
more effective for avoiding detector saturation than for mitigating detector
jitter or pixelation.Comment: 17 pp, 4 figs, Quantum Stud.: Math. Found. (2018
Creating excitonic entanglement in quantum dots through the optical Stark effect
We show that two initially non-resonant quantum dots may be brought into
resonance by the application of a single detuned laser. This allows for control
of the inter-dot interactions and the generation of highly entangled excitonic
states on the picosecond timescale. Along with arbitrary single qubit
manipulations, this system would be sufficient for the demonstration of a
prototype excitonic quantum computer.Comment: 4 pages, 3 figures; published version, figure 3 improved, corrections
to RWA derive
Electron spin relaxation of N@C60 in CS2
We examine the temperature dependence of the relaxation times of the
molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a
carbon cage) in liquid CS2 solution. The results are inconsistent with the
fluctuating zero field splitting (ZFS) mechanism, which is commonly invoked to
explain electron spin relaxation for S > 1/2 spins in liquid solution, and is
the mechanism postulated in the literature for these systems. Instead, we find
a clear Arrhenius temperature dependence for N@C60, indicating the spin
relaxation is driven primarily by an Orbach process. For the asymmetric N@C70
molecule, which has a permanent non-zero ZFS, we resolve an additional
relaxation mechanism caused by the rapid reorientation of its ZFS. We also
report the longest coherence time (T2) ever observed for a molecular electron
spin, being 0.25 ms at 170K.Comment: 6 pages, 6 figures V2: Updated to published versio
Switchable ErSc2N rotor within a C80 fullerene cage: An EPR and photoluminescence excitation study
Systems exhibiting both spin and orbital degrees of freedom, of which Er3+ is
one, can offer mechanisms for manipulating and measuring spin states via
optical excitations. Motivated by the possibility of observing
photoluminescence and electron paramagnetic resonance from the same species
located within a fullerene molecule, we initiated an EPR study of Er3+ in
ErSc2N@C80. Two orientations of the ErSc2N rotor within the C80 fullerene are
observed in EPR, consistent with earlier studies using photoluminescence
excitation (PLE) spectroscopy. For some crystal field orientations, electron
spin relaxation is driven by an Orbach process via the first excited electronic
state of the 4I_15/2 multiplet. We observe a change in the relative populations
of the two ErSc2N configurations upon the application of 532 nm illuminations,
and are thus able to switch the majority cage symmetry. This
photoisomerisation, observable by both EPR and PLE, is metastable, lasting many
hours at 20 K.Comment: 4 pages, 4 figure
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