25 research outputs found
On fault-tolerance with noisy and slow measurements
It is not so well-known that measurement-free quantum error correction
protocols can be designed to achieve fault-tolerant quantum computing. Despite
the potential advantages of using such protocols in terms of the relaxation of
accuracy, speed and addressing requirements on the measurement process, they
have usually been overlooked because they are expected to yield a very bad
threshold as compared to error correction protocols which use measurements.
Here we show that this is not the case. We design fault-tolerant circuits for
the 9 qubit Bacon-Shor code and find a threshold for gates and preparation of
(30% of the best known result for the
same code using measurement based error correction) while admitting up to 1/3
error rates for measurements and allocating no constraints on measurement
speed. We further show that demanding gate error rates sufficiently below the
threshold one can improve the preparation threshold to .
We also show how these techniques can be adapted to other
Calderbank-Shor-Steane codes.Comment: 11 pages, 7 figures. v3 has an extended exposition and several
simplifications that provide for an improved threshold value and resource
overhea
Distinguishing separable and entangled states
We show how to design families of operational criteria that distinguish
entangled from separable quantum states. The simplest of these tests
corresponds to the well-known Peres-Horodecki positive partial transpose (PPT)
criterion, and the more complicated tests are strictly stronger. The new
criteria are tractable due to powerful computational and theoretical methods
for the class of convex optimization problems known as semidefinite programs.
We successfully applied the results to many low-dimensional states from the
literature where the PPT test fails. As a byproduct of the criteria, we provide
an explicit construction of the corresponding entanglement witnesses.Comment: 4 pages, Latex2e. Expanded discussion of numerical procedures.
Accepted for publication in Physical Review Letter
Pressurization and depressurization phases inside the plumbing system of Mount Etna volcano: Evidence from a multiparametric approach
During 2013 Mount Etna volcano experienced intense eruptive activity at the summit craters, foremost at the New Southeast Crater and to a minor degree at the Voragine and Bocca Nuova (BN), which took place in two cycles, February-April and September-December. In this work, we mainly focus on the period between these cycles, applying a multiparametric approach. The period from the end of April to 5 September showed a gradual increase in the amplitude of long-period (LP) events and volcanic tremor, a slight inflation testified by both tilt and GPS data, and a CO2 flux increase. Such variations were interpreted as due to a gradual pressurization of the plumbing system, from the shallowest part, where LP and volcanic tremor are located, down to about 3-9km below sea level, pressure source depths obtained by both geodetic and CO2 data. On 5 September, at the same time as a large explosion at BN, we observed an instantaneous variation of the aforementioned signals (decrease in amplitude of LP events and volcanic tremor, slight deflation, and CO2 flux decrease) and the activation of a new infrasonic source located at BN. In the light of it, the BN explosion probably caused the instantaneous end of the pressurization, and the opening of a new vent at BN, that has become a new steady source of infrasonic events. This apparently slight change in the plumbing system also led to the gradual resumption of activity at the New Southeast Crater, culminating with the second lava fountain cycle of 2013
Holonomic quantum computing in symmetry-protected ground states of spin chains
While solid-state devices offer naturally reliable hardware for modern
classical computers, thus far quantum information processors resemble vacuum
tube computers in being neither reliable nor scalable. Strongly correlated many
body states stabilized in topologically ordered matter offer the possibility of
naturally fault tolerant computing, but are both challenging to engineer and
coherently control and cannot be easily adapted to different physical
platforms. We propose an architecture which achieves some of the robustness
properties of topological models but with a drastically simpler construction.
Quantum information is stored in the symmetry-protected degenerate ground
states of spin-1 chains, while quantum gates are performed by adiabatic
non-Abelian holonomies using only single-site fields and nearest-neighbor
couplings. Gate operations respect the symmetry, and so inherit some protection
from noise and disorder from the symmetry-protected ground states.Comment: 19 pages, 4 figures. v2: published versio
Entanglement in a quantum annealing processor
Entanglement lies at the core of quantum algorithms designed to solve
problems that are intractable by classical approaches. One such algorithm,
quantum annealing (QA), provides a promising path to a practical quantum
processor. We have built a series of scalable QA processors consisting of
networks of manufactured interacting spins (qubits). Here, we use qubit
tunneling spectroscopy to measure the energy eigenspectrum of two- and
eight-qubit systems within one such processor, demonstrating quantum coherence
in these systems. We present experimental evidence that, during a critical
portion of QA, the qubits become entangled and that entanglement persists even
as these systems reach equilibrium with a thermal environment. Our results
provide an encouraging sign that QA is a viable technology for large-scale
quantum computing.Comment: 13 pages, 8 figures, contact corresponding author for Supplementary
Informatio
Magnetization Dynamics, Bennett Clocking and Associated Energy Dissipation in Multiferroic Logic
It has been recently shown that multiferroic logic - where logic bits are
encoded in the magnetization orientation of a nanoscale magnetostrictive layer
elastically coupled to a piezoelectric layer - can be Bennett clocked with
small electrostatic potentials of few tens of mV applied to the piezoelectric
layer. The potential generates stress in the magnetostrictive layer and rotates
its magnetization by a large angle to carry out Bennett clocking. This method
of clocking is far more energy-efficient than using spin transfer torque. In
order to assess if such a clocking scheme can be also reasonably fast, we have
studied the magnetization dynamics of a multiferroic logic array with nearest
neighbor dipole coupling using the Landau-Lifshitz-Gilbert (LLG) equation. We
find that switching delays of ~ 3 ns (clock rates of 0.33 GHz) can be achieved
with proper design provided we clock non-adiabatically and dissipate ~48,000 kT
(at room temperature) of energy per clock cycle per bit flip in the clocking
circuit. This dissipation far exceeds the energy barrier separating the two
logic states, which we assumed to be 32 kT to yield a bit error probability of
. Had we used spin transfer torque to switch with the same ~ 3 ns delay, the
energy dissipation would have been much larger (~ kT). This
shows that spin transfer torque, widely used in magnetic random access memory,
is an inefficient way to switch a magnet, and multiferroic logic clocked with
voltage-induced stress is a superior nanomagnetic logic scheme
Fault tolerant architectures for superconducting qubits
In this short review, I draw attention to new developments in the theory of
fault tolerance in quantum computation that may give concrete direction to
future work in the development of superconducting qubit systems. The basics of
quantum error correction codes, which I will briefly review, have not
significantly changed since their introduction fifteen years ago. But an
interesting picture has emerged of an efficient use of these codes that may put
fault tolerant operation within reach. It is now understood that two
dimensional surface codes, close relatives of the original toric code of
Kitaev, can be adapted to effectively perform logical gate operations in a very
simple planar architecture, with error thresholds for fault tolerant operation
simulated to be 0.75%. This architecture uses topological ideas in its
functioning, but it is not 'topological quantum computation' -- there are no
non-abelian anyons in sight. I offer some speculations on the crucial pieces of
superconducting hardware that could be demonstrated in the next couple of years
that would be clear stepping stones towards this surface-code architecture.Comment: 28 pages, 10 figures. For the Nobel Symposium on Qubits for Quantum
Information, submitted to Physica Scripta. v. 2 Corrections and small changes
to reference
High-rate quantum cryptography in untrusted networks
We extend the field of continuous-variable quantum cryptography to a network
formulation where two honest parties connect to an untrusted relay by insecure
quantum links. To generate secret correlations, they transmit coherent states
to the relay where a continuous-variable Bell detection is performed and the
outcome broadcast. Even though the detection could be fully corrupted and the
links subject to optimal coherent attacks, the honest parties can still extract
a secret key, achieving high rates when the relay is proximal to one party, as
typical in public networks with access points or proxy servers. Our theory is
confirmed by an experiment generating key-rates which are orders of magnitude
higher than those achievable with discrete-variable protocols. Thus, using the
cheapest possible quantum resources, we experimentally show the possibility of
high-rate quantum key distribution in network topologies where direct links are
missing between end-users and intermediate relays cannot be trusted.Comment: Theory and Experiment. Main article (6 pages) plus Supplementary
Information (additional 13 pages
Advances in photonic quantum sensing
Quantum sensing has become a mature and broad field. It is generally related
with the idea of using quantum resources to boost the performance of a number
of practical tasks, including the radar-like detection of faint objects, the
readout of information from optical memories or fragile physical systems, and
the optical resolution of extremely close point-like sources. Here we first
focus on the basic tools behind quantum sensing, discussing the most recent and
general formulations for the problems of quantum parameter estimation and
hypothesis testing. With this basic background in our hands, we then review
emerging applications of quantum sensing in the photonic regime both from a
theoretical and experimental point of view. Besides the state-of-the-art, we
also discuss open problems and potential next steps.Comment: Review in press on Nature Photonics. This is a preliminary version to
be updated after publication. Both manuscript and reference list will be
expande