403 research outputs found
Breaking the entanglement barrier: Tensor network simulation of quantum transport
The recognition that large classes of quantum many-body systems have limited
entanglement in the ground and low-lying excited states led to dramatic
advances in their numerical simulation via so-called tensor networks. However,
global dynamics elevates many particles into excited states, and can lead to
macroscopic entanglement and the failure of tensor networks. Here, we show that
for quantum transport -- one of the most important cases of this failure -- the
fundamental issue is the canonical basis in which the scenario is cast: When
particles flow through an interface, they scatter, generating a "bit" of
entanglement between spatial regions with each event. The frequency basis
naturally captures that -- in the long-time limit and in the absence of
inelastic scattering -- particles tend to flow from a state with one frequency
to a state of identical frequency. Recognizing this natural structure yields a
striking -- potentially exponential in some cases -- increase in simulation
efficiency, greatly extending the attainable spatial- and time-scales, and
broadening the scope of tensor network simulation to hitherto inaccessible
classes of non-equilibrium many-body problems.Comment: Published version; 6+9 pages; 4+4 figures; Added: an example of
interacting reservoirs, further evidence on performance scaling, and extended
discussion of the numerical detail
Quantum data gathering
Measurement of a quantum system – the process by which an observer gathers information about it – provides a link between the quantum and classical worlds. The nature of this process is the central issue for attempts to reconcile quantum and classical descriptions of physical processes. Here, we show that the conventional paradigm of quantum measurement is directly responsible for a well-known disparity between the resources required to extract information from quantum and classical systems. We introduce a simple form of quantum data gathering, “coherent measurement”, that eliminates this disparity and restores a pleasing symmetry between classical and quantum statistical inference. To illustrate the power of quantum data gathering, we demonstrate that coherent measurements are optimal and strictly more powerful than conventional one-at-a-time measurements for the task of discriminating quantum states, including certain entangled many-body states (e.g., matrix product states)
Defects in Quantum Computers
The shift of interest from general purpose quantum computers to adiabatic
quantum computing or quantum annealing calls for a broadly applicable and easy
to implement test to assess how quantum or adiabatic is a specific hardware.
Here we propose such a test based on an exactly solvable many body system --
the quantum Ising chain in transverse field -- and implement it on the D-Wave
machine. An ideal adiabatic quench of the quantum Ising chain should lead to an
ordered broken symmetry ground state with all spins aligned in the same
direction. An actual quench can be imperfect due to decoherence, noise, flaws
in the implemented Hamiltonian, or simply too fast to be adiabatic.
Imperfections result in topological defects: Spins change orientation, kinks
punctuating ordered sections of the chain. The number of such defects
quantifies the extent by which the quantum computer misses the ground state,
and is, therefore, imperfect.Comment: 8 pages, 7 figures, to appear in Scientific Reports, authors' list
complete
Quantum Darwinism requires an extra-theoretical assumption of encoding redundancy
Observers restricted to the observation of pointer states of apparatus cannot
conclusively demonstrate that the pointer of an apparatus A registers the state
of a system of interest S without perturbing S. Observers cannot, therefore,
conclusively demonstrate that the states of a system S are redundantly encoded
by pointer states of multiple independent apparatus without destroying the
redundancy of encoding. The redundancy of encoding required by quantum
Darwinism must, therefore, be assumed from outside the quantum-mechanical
formalism and without the possibility of experimental demonstration.Comment: 5 pages, 1 figure. Comments on foundational assumptions of W. Zurek
(2009) Nat Phys 5 181 (arXiv 0903.5082). v2 significant revision to improve
clarit
Mixed-state dynamics in one-dimensional quantum lattice systems: a time-dependent superoperator renormalization algorithm
We present an algorithm to study mixed-state dynamics in one-dimensional
quantum lattice systems. The algorithm can be used, e.g., to construct thermal
states or to simulate real time evolutions given by a generic master equation.
Its two main ingredients are (i) a superoperator renormalization scheme to
efficiently describe the state of the system and (ii) the time evolving block
decimation (TEBD) technique to efficiently update the state during a time
evolution. The computational cost of a simulation increases significantly with
the amount of correlations between subsystems but it otherwise depends only
linearly in the system size. We present simulations involving quantum spins and
fermions in one spatial dimension.Comment: See also F. Verstraete et al. cond-mat/040642
QFlow lite dataset: A machine-learning approach to the charge states in quantum dot experiments
Over the past decade, machine learning techniques have revolutionized how
research is done, from designing new materials and predicting their properties
to assisting drug discovery to advancing cybersecurity. Recently, we added to
this list by showing how a machine learning algorithm (a so-called learner)
combined with an optimization routine can assist experimental efforts in the
realm of tuning semiconductor quantum dot (QD) devices. Among other
applications, semiconductor QDs are a candidate system for building quantum
computers. The present-day tuning techniques for bringing the QD devices into a
desirable configuration suitable for quantum computing that rely on heuristics
do not scale with the increasing size of the quantum dot arrays required for
even near-term quantum computing demonstrations. Establishing a reliable
protocol for tuning that does not rely on the gross-scale heuristics developed
by experimentalists is thus of great importance. To implement the machine
learning-based approach, we constructed a dataset of simulated QD device
characteristics, such as the conductance and the charge sensor response versus
the applied electrostatic gate voltages. Here, we describe the methodology for
generating the dataset, as well as its validation in training convolutional
neural networks. We show that the learner's accuracy in recognizing the state
of a device is ~96.5 % in both current- and charge-sensor-based training. We
also introduce a tool that enables other researchers to use this approach for
further research: QFlow lite - a Python-based mini-software suite that uses the
dataset to train neural networks to recognize the state of a device and
differentiate between states in experimental data. This work gives the
definitive reference for the new dataset that will help enable researchers to
use it in their experiments or to develop new machine learning approaches and
concepts.Comment: 18 pages, 6 figures, 3 table
Role of heating and current-induced forces in the stability of atomic wires
We investigate the role of local heating and forces on ions in the stability
of current-carrying aluminum wires. We find that heating increases with wire
length due to a red shift of the frequency spectrum. Nevertheless, the local
temperature of the wire is relatively low for a wide range of biases provided
good thermal contact exists between the wire and the bulk electrodes. On the
contrary, current-induced forces increase substantially as a function of bias
and reach bond-breaking values at about 1 V. These results suggest that local
heating promotes low-bias instabilities if dissipation into the bulk electrodes
is not efficient, while current-induced forces are mainly responsible for the
wire break-up at large biases. We compare these results to experimental
observations.Comment: 4 pages, 4 figure
Simulating adiabatic evolution of gapped spin systems
We show that adiabatic evolution of a low-dimensional lattice of quantum
spins with a spectral gap can be simulated efficiently. In particular, we show
that as long as the spectral gap \Delta E between the ground state and the
first excited state is any constant independent of n, the total number of
spins, then the ground-state expectation values of local operators, such as
correlation functions, can be computed using polynomial space and time
resources. Our results also imply that the local ground-state properties of any
two spin models in the same quantum phase can be efficiently obtained from each
other. A consequence of these results is that adiabatic quantum algorithms can
be simulated efficiently if the spectral gap doesn't scale with n. The
simulation method we describe takes place in the Heisenberg picture and does
not make use of the finitely correlated state/matrix product state formalism.Comment: 13 pages, 2 figures, minor change
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