7,353 research outputs found
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
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
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
Nucleation and Growth of GaN/AlN Quantum Dots
We study the nucleation of GaN islands grown by plasma-assisted
molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In
particular, we assess the variation of their height and density as a function
of GaN coverage. We show that the GaN growth passes four stages: initially, the
growth is layer-by-layer; subsequently, two-dimensional precursor islands form,
which transform into genuine three-dimensional islands. During the latter
stage, island height and density increase with GaN coverage until the density
saturates. During further GaN growth, the density remains constant and a
bimodal height distribution appears. The variation of island height and density
as a function of substrate temperature is discussed in the framework of an
equilibrium model for Stranski-Krastanov growth.Comment: Submitted to PRB, 10 pages, 15 figure
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
On the electromagnetic properties of active media
Several results concerning active media or metamaterials are proved and
discussed. In particular, we consider the permittivity, permeability, wave
vector, and refractive index, and discuss stability, refraction, gain, and
fundamental limitations resulting from causality
Coherence of Spin Qubits in Silicon
Given the effectiveness of semiconductor devices for classical computation
one is naturally led to consider semiconductor systems for solid state quantum
information processing. Semiconductors are particularly suitable where local
control of electric fields and charge transport are required. Conventional
semiconductor electronics is built upon these capabilities and has demonstrated
scaling to large complicated arrays of interconnected devices. However, the
requirements for a quantum computer are very different from those for classical
computation, and it is not immediately obvious how best to build one in a
semiconductor. One possible approach is to use spins as qubits: of nuclei, of
electrons, or both in combination. Long qubit coherence times are a
prerequisite for quantum computing, and in this paper we will discuss
measurements of spin coherence in silicon. The results are encouraging - both
electrons bound to donors and the donor nuclei exhibit low decoherence under
the right circumstances. Doped silicon thus appears to pass the first test on
the road to a quantum computer.Comment: Submitted to J Cond Matter on Nov 15th, 200
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