171 research outputs found
Harnessing Fluctuations in Thermodynamic Computing via Time-Reversal Symmetries
We experimentally demonstrate that highly structured distributions of work
emerge during even the simple task of erasing a single bit. These are
signatures of a refined suite of time-reversal symmetries in distinct
functional classes of microscopic trajectories. As a consequence, we introduce
a broad family of conditional fluctuation theorems that the component work
distributions must satisfy. Since they identify entropy production, the
component work distributions encode both the frequency of various mechanisms of
success and failure during computing, as well giving improved estimates of the
total irreversibly-dissipated heat. This new diagnostic tool provides strong
evidence that thermodynamic computing at the nanoscale can be constructively
harnessed. We experimentally verify this functional decomposition and the new
class of fluctuation theorems by measuring transitions between flux states in a
superconducting circuit
Gigahertz Sub-Landauer Momentum Computing
We introduce a fast and highly-efficient physically-realizable bit swap.
Employing readily available and scalable Josephson junction microtechnology,
the design implements the recently introduced paradigm of momentum computing.
Its nanosecond speeds and sub-Landauer thermodynamic efficiency arise from
dynamically storing memory in momentum degrees of freedom. As such, during the
swap, the microstate distribution is never near equilibrium and the
memory-state dynamics fall far outside of stochastic thermodynamics that
assumes detailed-balanced Markovian dynamics. The device implements a bit-swap
operation -- a fundamental operation necessary to build reversible universal
computing. Extensive, physically-calibrated simulations demonstrate that device
performance is robust and that momentum computing can support
thermodynamically-efficient, high-speed, large-scale general-purpose computing
that circumvents Landauer's bound.Comment: 18 pages, 11 figures, 5 appendices;
http://csc.ucdavis.edu/~cmg/compmech/pubs/gslmc.ht
Keldysh Field Theory for Driven Open Quantum Systems
Recent experimental developments in diverse areas - ranging from cold atomic
gases over light-driven semiconductors to microcavity arrays - move systems
into the focus, which are located on the interface of quantum optics, many-body
physics and statistical mechanics. They share in common that coherent and
driven-dissipative quantum dynamics occur on an equal footing, creating genuine
non-equilibrium scenarios without immediate counterpart in condensed matter.
This concerns both their non-thermal flux equilibrium states, as well as their
many-body time evolution. It is a challenge to theory to identify novel
instances of universal emergent macroscopic phenomena, which are tied
unambiguously and in an observable way to the microscopic drive conditions. In
this review, we discuss some recent results in this direction. Moreover, we
provide a systematic introduction to the open system Keldysh functional
integral approach, which is the proper technical tool to accomplish a merger of
quantum optics and many-body physics, and leverages the power of modern quantum
field theory to driven open quantum systems.Comment: 73 pages, 13 figure
Thermodynamic Computing
The hardware and software foundations laid in the first half of the 20th
Century enabled the computing technologies that have transformed the world, but
these foundations are now under siege. The current computing paradigm, which is
the foundation of much of the current standards of living that we now enjoy,
faces fundamental limitations that are evident from several perspectives. In
terms of hardware, devices have become so small that we are struggling to
eliminate the effects of thermodynamic fluctuations, which are unavoidable at
the nanometer scale. In terms of software, our ability to imagine and program
effective computational abstractions and implementations are clearly challenged
in complex domains. In terms of systems, currently five percent of the power
generated in the US is used to run computing systems - this astonishing figure
is neither ecologically sustainable nor economically scalable. Economically,
the cost of building next-generation semiconductor fabrication plants has
soared past $10 billion. All of these difficulties - device scaling, software
complexity, adaptability, energy consumption, and fabrication economics -
indicate that the current computing paradigm has matured and that continued
improvements along this path will be limited. If technological progress is to
continue and corresponding social and economic benefits are to continue to
accrue, computing must become much more capable, energy efficient, and
affordable. We propose that progress in computing can continue under a united,
physically grounded, computational paradigm centered on thermodynamics. Herein
we propose a research agenda to extend these thermodynamic foundations into
complex, non-equilibrium, self-organizing systems and apply them holistically
to future computing systems that will harness nature's innate computational
capacity. We call this type of computing "Thermodynamic Computing" or TC.Comment: A Computing Community Consortium (CCC) workshop report, 36 page
Light-Driven Nanoscale Vectorial Currents
Controlled charge flows are fundamental to many areas of science and
technology, serving as carriers of energy and information, as probes of
material properties and dynamics, and as a means of revealing or even inducing
broken symmetries. Emerging methods for light-based current control offer
promising routes beyond the speed and adaptability limitations of conventional
voltage-driven systems. However, optical manipulation of currents at nanometer
spatial scales remains a basic challenge and a key step toward scalable
optoelectronic systems and local probes. Here, we introduce vectorial
optoelectronic metasurfaces as a new class of metamaterial in which ultrafast
charge flows are driven by light pulses, with actively-tunable directionality
and arbitrary patterning down to sub-diffractive nanometer scales. In the
prototypical metasurfaces studied herein, asymmetric plasmonic nanoantennas
locally induce directional, linear current responses within underlying
graphene. Nanoscale unit cell symmetries are read out via polarization- and
wavelength-sensitive currents and emitted terahertz (THz) radiation. Global
vectorial current distributions are revealed by spatial mapping of the THz
field polarization, also demonstrating the direct generation of elusive
broadband THz vector beams. We show that a detailed interplay between
electrodynamic, thermodynamic, and hydrodynamic degrees of freedom gives rise
to these currents through rapidly-evolving nanoscale forces and charge flows
under extreme spatial and temporal localization. These results set the stage
for versatile patterning and optical control over nanoscale currents in
materials diagnostics, nano-magnetism, microelectronics, and ultrafast
information science
Nonequilibrium thermodynamics of erasure with superconducting flux logic
We implement a thermal-fluctuation-driven logical bit reset on a superconducting flux logic cell. We show that the logical state of the system can be continuously monitored with only a small perturbation to the thermally activated dynamics at 500 mK. We use the trajectory information to derive a single-shot estimate of the work performed on the system per logical cycle. We acquire a sample of 10⁵ erasure trajectories per protocol and show that the work histograms agree with both microscopic theory and global fluctuation theorems. The results demonstrate how to design and diagnose complex, high-speed, and thermodynamically efficient computing using superconducting technology
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