23 research outputs found
Post hoc verification of quantum computation
With recent progress on experimental quantum information processing, an
important question has arisen as to whether it is possible to verify arbitrary
computation performed on a quantum processor. A number of protocols have been
proposed to achieve this goal, however all are interactive in nature, requiring
that the computation be performed in an interactive manner with back and forth
communication between the verifier and one or more provers. Here we propose two
methods for verifying quantum computation in a non-interactive manner based on
recent progress in the understanding of the local Hamiltonian problem. Provided
that the provers compute certain witnesses for the computation, this allows the
result of a quantum computation to be verified after the fact, a property not
seen in current verification protocols.Comment: 4 pages, 2 figure
Architecture and protocols for all-photonic quantum repeaters
An all-photonic repeater scheme based on a type of graph state called a
repeater graph state (RGS) promises tolerance to photon losses as well as
operational errors, and offers a fast Bell pair generation rate, limited only
by the RGS creation time (rather than enforced round-trip waits). Prior
research on the topic has focused on the RGS generation and analyzing the
secret key sharing rate, but there is a need to extend to use cases such as
distributed computation or teleportation as will be used in a general-purpose
Quantum Internet. Here, we propose a protocol and architecture that consider
how end nodes participate in the connection; the capabilities and
responsibilities of each node; the classical communications between nodes; and
the Pauli frame correction information per end-to-end Bell pair. We give
graphical reasoning on the correctness of the protocol via graph state
manipulation rules. We then show that the RGS scheme is well suited to use in a
link architecture connecting memory-based repeaters and end nodes for
applications beyond secret sharing. Finally, we discuss the practicality of
implementing our proposed protocol on quantum network simulators and how it can
be integrated into an existing proposed quantum network architecture.Comment: 10 pages, 8 figures, comments welcom
Measurement Induced Continuous Time Crystals
Strong measurements usually restrict the dynamics of measured finite
dimensional systems to the Zeno subspace, where subsequent evolution is unitary
due to the suppression of dissipative terms. Here we show qualitatively
different behaviour due to the competition between strong measurements and the
thermodynamic limit, inducing a time-translation symmetry breaking phase
transition resulting in a continuous time crystal. We consider a spin star
model, where the central spin is subject to a strong continuous measurement,
and qualify the dynamic behaviour of the system in various parameter regimes.
We show that above a critical value of measurement strength, the magnetization
of the thermodynamically large ancilla spins develops limit cycle oscillations.
Our result also demonstrates that a coherent drive is not necessary in order to
induce continuous time-translation symmetry breaking.Comment: 9 pages total (5 pages plus appendix
Role of Coherence and Degeneracies in Quantum Synchronisation
Progress on the study of synchronisation in quantum systems has been largely
driven by specific examples which resulted in several examples of frequency
entrainment as well as mutual synchronisation. Here we study quantum
synchronisation by utilising Liouville space perturbation theory. We begin by
clarifying the role of centers, symmetries and oscillating coherences in the
context of quantum synchronisation. We then analyse the eigenspectrum of the
Liouville superoperator generating the dynamics of the quantum system and
determine the conditions under which synchronisation arises. We apply our
framework to derive a powerful relationship between energy conservation,
degeneracies and synchronisation in quantum systems. Finally, we demonstrate
our approach by analysing two mutually coupled thermal machines and the close
relationship between synchronisation and thermodynamic quantities.Comment: 9 total pages, comments welcom
Quantum Synchronisation in Nanoscale Heat Engines
Owing to the ubiquity of synchronization in the classical world, it is
interesting to study its behavior in quantum systems. Though quantum
synchronisation has been investigated in many systems, a clear connection to
quantum technology applications is lacking. We bridge this gap and show that
nanoscale heat engines are a natural platform to study quantum synchronization
and always possess a stable limit cycle. Furthermore, we demonstrate an
intimate relationship between the power of a heat engine and its phase-locking
properties by proving that synchronization places an upper bound on the
achievable steady-state power of the engine. Finally, we show that the
efficiency of the engine sets a point in terms of the bath temperatures where
synchronization vanishes. We link the physical phenomenon of synchronization
with the emerging field of quantum thermodynamics by establishing quantum
synchronization as a mechanism of stable phase coherence.Comment: 5 pages, 3 pages appendix, 2 figure
Symmetries and Synchronization Blockade
Synchronization blockade refers to an interferometric cancellation of quantum
synchronization. In this manuscript, we show how the choice of synchronization
measure and Hamiltonian symmetries affect the discussion of synchronization
blockade. Using counting principles, we prove a general theorem that
synchronization blockade cannot be observed in an level system when the
coherent state used to define the diagonal limit-cycle state is in the full
group. We present several illustrative examples of synchronization
blockade in multi-level systems and prove that information-theoretic measures
of synchronization can also observe synchronization blockade-like behavior by
an appropriate choice of the set of limit cycle states.Comment: 10 pages, comments welcom
Generalized Measure of Quantum synchronization
We present a generalized information-theoretic measure of synchronization in
quantum systems. This measure is applicable to dynamics of anharmonic
oscillators, few-level atoms, and coupled oscillator networks. Furthermore, the
new measure allows us to discuss synchronization of disparate physical systems
such as coupled hybrid quantum systems and coupled systems undergoing mutual
synchronization that are also driven locally. In many cases of interest, we
find a closed-form expression for the proposed measure.Comment: 8 pages, 4 figures, comments welcom