3,147 research outputs found
Demonstration of non-Markovian process characterisation and control on a quantum processor
In the scale-up of quantum computers, the framework underpinning
fault-tolerance generally relies on the strong assumption that environmental
noise affecting qubit logic is uncorrelated (Markovian). However, as physical
devices progress well into the complex multi-qubit regime, attention is turning
to understanding the appearance and mitigation of correlated -- or
non-Markovian -- noise, which poses a serious challenge to the progression of
quantum technology. This error type has previously remained elusive to
characterisation techniques. Here, we develop a framework for characterising
non-Markovian dynamics in quantum systems and experimentally test it on
multi-qubit superconducting quantum devices. Where noisy processes cannot be
accounted for using standard Markovian techniques, our reconstruction predicts
the behaviour of the devices with an infidelity of . Our results show
this characterisation technique leads to superior quantum control and extension
of coherence time by effective decoupling from the non-Markovian environment.
This framework, validated by our results, is applicable to any controlled
quantum device and offers a significant step towards optimal device operation
and noise reduction
Filtering crosstalk from bath non-Markovianity via spacetime classical shadows
From an open system perspective non-Markovian effects due to a nearby bath or
neighbouring qubits are dynamically equivalent. However, there is a conceptual
distinction to account for: neighbouring qubits may be controlled. We combine
recent advances in non-Markovian quantum process tomography with the framework
of classical shadows to characterise spatiotemporal quantum correlations.
Observables here constitute operations applied to the system, where the free
operation is the maximally depolarising channel. Using this as a causal break,
we systematically erase causal pathways to narrow down the progenitors of
temporal correlations. We show that one application of this is to filter out
the effects of crosstalk and probe only non-Markovianity from an inaccessible
bath. It also provides a lens on spatiotemporally spreading correlated noise
throughout a lattice from common environments. We demonstrate both examples on
synthetic data. Owing to the scaling of classical shadows, we can erase
arbitrarily many neighbouring qubits at no extra cost. Our procedure is thus
efficient and amenable to systems even with all-to-all interactions.Comment: 5 pages, 4 figure
On the sampling complexity of open quantum systems
Open quantum systems are ubiquitous in the physical sciences, with widespread
applications in the areas of chemistry, condensed matter physics, material
science, optics, and many more. Not surprisingly, there is significant interest
in their efficient simulation. However, direct classical simulation quickly
becomes intractable with coupling to an environment whose effective dimension
grows exponentially. This raises the question: can quantum computers help model
these complex dynamics? A first step in answering this question requires
understanding the computational complexity of this task. Here, we map the
temporal complexity of a process to the spatial complexity of a many-body state
using a computational model known as the process tensor framework. With this,
we are able to explore the simulation complexity of an open quantum system as a
dynamic sampling problem: a system coupled to an environment can be probed at
successive points in time -- accessing multi-time correlations. The complexity
of multi-time sampling, which is an important and interesting problem in its
own right, contains the complexity of master equations and stochastic maps as a
special case. Our results show how the complexity of the underlying quantum
stochastic process corresponds to the complexity of the associated family of
master equations for the dynamics. We present both analytical and numerical
examples whose multi-time sampling is as complex as sampling from a many-body
state that is classically hard. This also implies that the corresponding family
of master equations are classically hard. Our results pave the way for studying
open quantum systems from a complexity-theoretic perspective, highlighting the
role quantum computers will play in our understanding of quantum dynamics
Non-Markovian Quantum Process Tomography
Characterisation protocols have so far played a central role in the
development of noisy intermediate-scale quantum (NISQ) computers capable of
impressive quantum feats. This trajectory is expected to continue in building
the next generation of devices: ones that can surpass classical computers for
particular tasks -- but progress in characterisation must keep up with the
complexities of intricate device noise. A missing piece in the zoo of
characterisation procedures is tomography which can completely describe
non-Markovian dynamics. Here, we formally introduce a generalisation of quantum
process tomography, which we call process tensor tomography. We detail the
experimental requirements, construct the necessary post-processing algorithms
for maximum-likelihood estimation, outline the best-practice aspects for
accurate results, and make the procedure efficient for low-memory processes.
The characterisation is the pathway to diagnostics and informed control of
correlated noise. As an example application of the technique, we improve
multi-time circuit fidelities on IBM Quantum devices for both standalone qubits
and in the presence of crosstalk to a level comparable with the fault-tolerant
noise threshold in a variety of different noise conditions. Our methods could
form the core for carefully developed software that may help hardware
consistently pass the fault-tolerant noise threshold
White Dwarfs in Globular Clusters: HST Observations of M4
Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258
white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial
distances from the cluster center were observed and sizeable WD populations
were found in all three. The location of these WDs in the color-magnitude
diagram, their mean mass of 0.51()M, and their luminosity
function confirm basic tenets of stellar evolution theory and support the
results from current WD cooling theory. The WDs are used to extend the cluster
main-sequence mass function upward to stars that have already completed their
nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and
found to be at most a few percent of all the main-sequence stars. The most
ancient WDs found are about 9 Gyr old, a level which is set solely by the
photometric limits of our data. Even though this is less than the age of M4, we
discuss how these cooling WDs can eventually be used to check the turnoff ages
of globular clusters and hence constrain the age of the Universe.Comment: 46 pages, latex, no figures included, figures available at
ftp://ftp.astro.ubc.ca/pub/richer/wdfig.uu size 2.7Mb. To be published in the
Astrophysical Journa
Thick domain wall universes
We investigate the spacetime of a thick gravitating domain wall for a general
potential . Using general analytical arguments we show that all
nontrivial solutions fall into two categories: those interpretable as an
isolated domain wall with a cosmological event horizon, and those which are
pure false vacuum de Sitter solutions. Although this latter solution is always
unstable to the field rolling coherently to its true vacuum, we show that there
is an additional instability to wall formation if the scalar field does not
couple too strongly to gravity. Using the and sine-Gordon
models as illustrative examples, we investigate the phase space of the
gravitating domain wall in detail numerically, following the solutions from
weak to strong gravity. We find excellent agreement with the analytic work.
Then, we analyse the domain wall in the presence of a cosmological constant
finding again the two kinds of solutions, wall and de Sitter, even in the
presence of a negative cosmological constant.Comment: 20 pages revtex, epsfig, references added, some conclusions altere
Non stationary Einstein-Maxwell fields interacting with a superconducting cosmic string
Non stationary cylindrically symmetric exact solutions of the
Einstein-Maxwell equations are derived as single soliton perturbations of a
Levi-Civita metric, by an application of Alekseev inverse scattering method. We
show that the metric derived by L. Witten, interpreted as describing the
electrogravitational field of a straight, stationary, conducting wire may be
recovered in the limit of a `wide' soliton. This leads to the possibility of
interpreting the solitonic solutions as representing a non stationary
electrogravitational field exterior to, and interacting with, a thin, straight,
superconducting cosmic string. We give a detailed discussion of the
restrictions that arise when appropiate energy and regularity conditions are
imposed on the matter and fields comprising the string, considered as `source',
the most important being that this `source' must necessarily have a non-
vanishing minimum radius. We show that as a consequence, it is not possible,
except in the stationary case, to assign uniquely a current to the source from
a knowledge of the electrogravitational fields outside the source. A discussion
of the asymptotic properties of the metrics, the physical meaning of their
curvature singularities, as well as that of some of the metric parameters, is
also included.Comment: 14 pages, no figures (RevTex
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