740 research outputs found
An Ising-Glauber Spin Cluster Model for Temperature Dependent Magnetization Noise in SQUIDs
Clusters of interacting two-level-systems (TLS),likely due to centers
at the metal-insulator interface, are shown to self consistently lead to
magnetization noise in SQUIDs. By introducing a
correlation-function calculation method and without any a priori assumptions on
the distribution of fluctuation rates, it is shown why the flux noise is only
weakly temperature dependent with , while the inductance
noise has a huge temperature dependence seen in experiment, even though the
mechanism producing both spectra is the same. Though both ferromagnetic- RKKY
and short-range-interactions (SRI) lead to strong flux-inductance-noise
cross-correlations seen in experiment, the flux noise varies a lot with
temperature for SRI. Hence it is unlikely that the TLS's time reversal symmetry
is broken by the same mechanism which mediates surface ferromagnetism in
nanoparticles and thin films of the same insulator materials
1/f Flux Noise in low-T SQUIDs due to Superparamagnetic Phase Transitions in Defect Clusters
It is shown here that flux noise in conventional low-T
SQUIDs is a result of low temperature superparamagnetic phase transitions in
small clusters of strongly correlated color center defects. The spins in each
cluster interact via long-range ferromagnetic interactions. Due to its small
size, the cluster behaves like a 'random-telegraphic' macro-spin when
transitioning to the superparamagnetic phase. This results in
noise when ensemble averaged over a random distribution of clusters. This model
is self-consistent and explains all related experimental results which includes
independent of system-size. The experimental
flux-inductance-noise spectrum is explained through three-point correlation
calculations and time reversal symmetry breaking arguments. Also, unlike the
flux noise, it is shown why the second-spectrum inductance noise is inherently
temperature dependent due to the fluctuation-dissipation theorem. A
correlation-function calculation methodology using Ising-Glauber dynamics was
key for obtaining these results
A Couped-Qubit Tavis Cummings Scheme for Prolonging Quantum Coherence
Qubit-qubit interactions can significantly boost quantum coherence times for
Bell states. The coherence-time-enhancements are however not monotonic and
there exists a phase where further increasing the interaction is unhelpful. A
resonator in a suggested circuit QED type implementation of the
Tavis-Cummings(Dicke) model, is shown to shift this transition point depending
on the number of loaded photons. This allows the resonator to amplify the
coherence enhancements in certain regimes. The interactions also induce unusual
collapse and revival type behavior for the entanglement dynamics. A new and
exact open quantum systems formalism -- the quasi-Hamiltonians for the Dicke
model thus reveals how a Bell state in a resonator can be protected against
noise from randomly fluctuating two level systems. Simple circuit level
details are given for flux qubits
Fast Quantum Control for Weakly Nonlinear Qubits: On Two-Quadrature Adiabatic Gates
Adiabatic or slowly varying gate operations are typically required in order
to remain within the qubit subspace in an anharmonic oscillator. However
significant speed ups are possible by using the two quadrature
derivative-removal-by-adiabatic-gate(DRAG) technique, where a second time
derivative pulse component burns a spectral hole near an unwanted transition.
It is shown here, that simultaneous optimization of the detuning and the pulse
norm in addition, further reduces leakage errors and significantly improve gate
fidelities. However, with this optimization accounting for the AC Stark shift,
there is a low spectral weight pulse envelope regime, where DRAG is almost not
needed and where the two state error fidelities are stable against pulse
jitter. Explicit time evolution calculations are carried out in the lab frame
for truncated multi-level Transmon qubit models obtained from a tight-binding
model
Strong Cavity-Pseudospin Coupling in Monolayer Transition Metal Dichalcogenides: Spontaneous Spin-Oscillations and Magnetometry
Strong coupling between the electronic states of monolayer transition metal
dichalcogenides (TMDC) such as MoS, MoSe, WS, or WSe, and a
two-dimensional (2D) photonic cavity gives rise to several exotic effects. The
Dirac type Hamiltonian for a 2D gapped semiconductor with large spin-orbit
coupling facilitates pure Jaynes-Cummings type coupling in the presence of a
single mode electric field. The presence of an additional circularly polarized
beam of light gives rise to valley and spin dependent cavity-QED properties.
The cavity causes the TMDC monolayer to act as an on-chip coherent light source
and a spontaneous spin-oscillator. In addition, a TMDC monolayer in a cavity is
a sensitive magnetic field sensor for an in-plane magnetic field
Magnetization Noise Induced Collapse and Revival of Rabi Oscillations in circuit QED
We use a quasi Hamiltonian formalism to describe the dissipative dynamics of
a circuit QED qubit that is affected by several fluctuating two level systems
with a 1/f noise power spectrum. The qubit-resonator interactions are described
by the Jaynes Cummings model. We argue that the presence of pure dephasing
noise in such a qubit-resonator system will also induce an energy relaxation
mechanism via a fluctuating dipole coupling term. This random modulation of the
coupling is seen to lead to rich physical behavior. For non-Markovian noise,
the coupling can either worsen or alleviate decoherence depending on the
initial conditions. The magnetization noise leads to behavior resembling the
collapse and revival of Rabi oscillations. For a broad distribution of noise
couplings, the frequency of these oscillations depends on the mean noise
strength. We describe this behavior semi-analytically and find it to be
independent of the number of fluctuators. This phenomenon could be used as an
in situ probe of the noise characteristics
Electronic Structure and Optical Properties of the Lonsdaleite Phase of Si, Ge and diamond
Crystalline semiconductors may exist in different polytypic phases with
significantly different electronic and optical properties. In this paper, we
calculate the electronic structure and optical properties of diamond, Si and Ge
in the lonsdaleite (hexagonal-diamond) phase. We use an empirical
pseudopotentials method based on transferable model potentials, including
spin-orbit interactions. We obtain band structures, densities of states and
complex dielectric functions calculated in the dipole approximation for light
polarized perpendicular and parallel to the c-axis of the crystal. We find
strong polarization dependent optical anisotropy. Simple analytical expressions
are provided for the dispersion relations. We find that in the lonsdaleite
phase, diamond and Si remain indirect gap semiconductors while Ge is
transformed into a direct gap semiconductor with a significantly smaller band
gap
Dynamically corrected gates for qubits with always-on Ising couplings: Error model and fault-tolerance with the toric code
We describe how a universal set of dynamically-corrected quantum gates can be
implemented using sequences of shaped decoupling pulses on any qubit network
forming a sparse bipartite graph with always-on Ising interactions. These
interactions are constantly decoupled except when they are needed for two-qubit
gates. We analytically study the error operators associated with the
constructed gates up to third order in the Magnus expansion, analyze these
errors numerically in the unitary time evolution of small qubit clusters, and
give a bound on high-order errors for qubits on a large square lattice. We
prove that with a large enough toric code the present gate set can be used to
implement a fault-tolerant quantum memory
Universal set of Dynamically Protected Gates for Bipartite Qubit Networks II: Soft Pulse Implementation of the [[5,1,3]] Quantum Error Correcting Code
We model repetitive quantum error correction (QEC) with the
single-error-correcting five-qubit code on a network of individually-controlled
qubits with always-on Ising couplings, using our previously designed universal
set of quantum gates based on sequences of shaped decoupling pulses. In
addition to serving as accurate quantum gates, the sequences also provide
dynamical decoupling (DD) of low-frequency phase noise. The simulation involves
integrating unitary dynamics of six qubits over the duration of tens of
thousands of control pulses, using classical stochastic phase noise as a source
of decoherence. The combined DD/QEC protocol dramatically improves the
coherence, with the QEC alone responsible for more than an order of magnitude
infidelity reduction.Comment: 12 pages, 9 figure
Control of Majorana Edge Modes by a g-factor Engineered Nanowire Spin Transistor
We propose the manipulation of Majorana edge states via hybridization and
spin currents in a nanowire spin transistor. The spin transistor is based on a
heterostructure nanowire comprising of semiconductors with large and small
g-factors that form the topological and non-topological regions respectively.
The hybridization of bound edge states results in spin currents and
-periodic torques, as a function of the relative magnetic field angle --
an effect which is dual to the fractional Josephson effect. We establish
relation between torques and spin-currents in the non-topological region where
the magnetic field is almost zero and spin is conserved along the spin-orbit
field direction. The angular momentum transfer could be detected by sensitive
magnetic resonance force microscopy techniques
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