17 research outputs found
Band Gaps and Localization in Acoustic Propagation in Water with Air-cylinders
Multiple scattering of waves leads to many peculiar phenomena such as
complete band gaps in periodic structures and wave localization in disordered
media. Within a band gap excitations are evanescent; when localized they remain
confined in space until dissipated. Here we report acoustic band gap and
localization in a 2D system of air-cylinders in water. Exact numerical
calculations reveal the unexpected result that localization is relatively
independent of the precise location or organization of the scatterers.
Localization occurs within a finite region of frequencies, coincident with the
complete band gap predicted by a conventional band structure calculation for a
periodic lattice of scatterers. Inside the gap or localization regime, a
previously uninvestigated stable collective behavior of the cylinders appears.Comment: 10 pages, 4 figure
A superfluid 4He interferometer operating near 2 K
Matter-wave interferometers reveal some of the most fascinating phenomena of
the quantum world. Phase shifts due to rotation (the Sagnac effect) for
neutrons, free atoms and superfluid 3He reveal the connection of matter waves
to a non-rotating inertial frame. In addition, phase shifts in electron waves
due to magnetic vector potentials (the Aharonov-Bohm effect) show that physical
states can be modified in the absence of classical forces. We report here the
observation of interference induced by the Earth's rotation in superfluid 4He
at 2 K, a temperature 2000 times higher than previously achieved with 3He. This
interferometer, an analog of a dc-SQUID, employs a recently reported phenomenon
wherein superfluid 4He exhibits quantum oscillations in an array of sub-micron
apertures. We find that the interference pattern persists not only when the
aperture array current-phase relation is a sinusoidal function characteristic
of the Josephson effect, but also at lower temperatures where it is linear and
oscillations occur by phase slips. The modest requirements for the
interferometer (2 K cryogenics and fabrication of apertures at the level of
100nm) and its potential resolution suggest that, when engineering challenges
such as vibration isolation are met, superfluid 4He interferometers could
become important scientific probes.Comment: 8 pages, 2 figure
Probing flux and charge noise with macroscopic resonant tunneling
We report on measurements of flux and charge noise in an rf-SQUID flux qubit
using macroscopic resonant tunneling (MRT). We measure rates of incoherent
tunneling from the lowest energy state in the initial well to the ground and
first excited states in the target well. The result of the measurement consists
of two peaks. The first peak corresponds to tunneling to the ground state of
the target well, and is dominated by flux noise. The second peak is due to
tunneling to the excited state and is wider due to an intrawell relaxation
process dominated by charge noise. We develop a theoretical model that allows
us to extract information about flux and charge noise within one experimental
setup. The model agrees very well with experimental data over a wide dynamic
range and provides parameters that characterize charge and flux noise.Comment: 11 pages, 5 figure
A Phase transition in acoustic propagation in 2D random liquid media
Acoustic wave propagation in liquid media containing many parallel air-filled
cylinders is considered. A self-consistent method is used to compute rigorously
the propagation, incorporating all orders of multiple scattering. It is shown
that under proper conditions, multiple scattering leads to a peculiar phase
transition in acoustic propagation. When the phase transition occurs, a
collective behavior of the cylinders appears and the acoustic waves are
confined in a region of space in the neighborhood of the transmission source. A
novel phase diagram is used to describe such phase transition.
Originally submitted on April 6, 99.Comment: 5 pages, 5 color figure
Quantum critical dynamics in a 5000-qubit programmable spin glass
Experiments on disordered alloys suggest that spin glasses can be brought
into low-energy states faster by annealing quantum fluctuations than by
conventional thermal annealing. Due to the importance of spin glasses as a
paradigmatic computational testbed, reproducing this phenomenon in a
programmable system has remained a central challenge in quantum optimization.
Here we achieve this goal by realizing quantum critical spin-glass dynamics on
thousands of qubits with a superconducting quantum annealer. We first
demonstrate quantitative agreement between quantum annealing and time-evolution
of the Schr\"odinger equation in small spin glasses. We then measure dynamics
in 3D spin glasses on thousands of qubits, where simulation of many-body
quantum dynamics is intractable. We extract critical exponents that clearly
distinguish quantum annealing from the slower stochastic dynamics of analogous
Monte Carlo algorithms, providing both theoretical and experimental support for
a scaling advantage in reducing energy as a function of annealing time
Quantum error mitigation in quantum annealing
Quantum Error Mitigation (QEM) presents a promising near-term approach to
reduce error when estimating expectation values in quantum computing. Here, we
introduce QEM techniques tailored for quantum annealing, using Zero-Noise
Extrapolation (ZNE). We implement ZNE through zero-temperature extrapolation as
well as energy-time rescaling. We conduct experimental investigations into the
quantum critical dynamics of a transverse-field Ising spin chain, demonstrating
the successful mitigation of thermal noise through both of these techniques.
Moreover, we show that energy-time rescaling effectively mitigates control
errors in the coherent regime where the effect of thermal noise is minimal. Our
ZNE results agree with exact calculations of the coherent evolution over a
range of annealing times that exceeds the coherent annealing range by almost an
order of magnitude.Comment: 10 pages, 5 figure