944 research outputs found
Violating the Shannon capacity of metric graphs with entanglement
The Shannon capacity of a graph G is the maximum asymptotic rate at which
messages can be sent with zero probability of error through a noisy channel
with confusability graph G. This extensively studied graph parameter disregards
the fact that on atomic scales, Nature behaves in line with quantum mechanics.
Entanglement, arguably the most counterintuitive feature of the theory, turns
out to be a useful resource for communication across noisy channels. Recently,
Leung, Mancinska, Matthews, Ozols and Roy [Comm. Math. Phys. 311, 2012]
presented two examples of graphs whose Shannon capacity is strictly less than
the capacity attainable if the sender and receiver have entangled quantum
systems. Here we give new, possibly infinite, families of graphs for which the
entangled capacity exceeds the Shannon capacity.Comment: 15 pages, 2 figure
Entanglement flow in multipartite systems
We investigate entanglement dynamics in multipartite systems, establishing a
quantitative concept of entanglement flow: both flow through individual
particles, and flow along general networks of interacting particles. In the
former case, the rate at which a particle can transmit entanglement is shown to
depend on that particle's entanglement with the rest of the system. In the
latter, we derive a set of entanglement rate equations, relating the rate of
entanglement generation between two subsets of particles to the entanglement
already present further back along the network. We use the rate equations to
derive a lower bound on entanglement generation in qubit chains, and compare
this to existing entanglement creation protocols.Comment: 13 pages, 5 figures, REVTeX format. Proof of lemma 3 corrected.
Restructured and expande
Engineering correlation and entanglement dynamics in spin systems
We show that the correlation and entanglement dynamics of spin systems can be
understood in terms of propagation of spin waves. This gives a simple, physical
explanation of the behaviour seen in a number of recent works, in which a
localised, low-energy excitation is created and allowed to evolve. But it also
extends to the scenario of translationally invariant systems in states far from
equilibrium, which require less local control to prepare. Spin-wave evolution
is completely determined by the system's dispersion relation, and the latter
typically depends on a small number of external, physical parameters.
Therefore, this new insight into correlation dynamics opens up the possibility
not only of predicting but also of controlling the propagation velocity and
dispersion rate, by manipulating these parameters. We demonstrate this
analytically in a simple, example system.Comment: 4 pages, 4 figures, REVTeX4 forma
Uncomputability of phase diagrams
The phase diagram of a material is of central importance in describing the properties and behaviour of a condensed matter system. In this work, we prove that the task of determining the phase diagram of a many-body Hamiltonian is in general uncomputable, by explicitly constructing a continuous one-parameter family of Hamiltonians H(Ï), where Ïâ R, for which this is the case. The H(Ï) are translationally-invariant, with nearest-neighbour couplings on a 2D spin lattice. As well as implying uncomputablity of phase diagrams, our result also proves that undecidability can hold for a set of positive measure of a Hamiltonianâs parameter space, whereas previous results only implied undecidability on a zero measure set. This brings the spectral gap undecidability results a step closer to standard condensed matter problems, where one typically studies phase diagrams of many-body models as a function of one or more continuously varying real parameters, such as magnetic field strength or pressure
The Absence of Vortex Lattice Melting in a Conventional Superconductor
The state of the vortex lattice extremely close to the superconducting to
normal transition in an applied magnetic field is investigated in high purity
niobium. We observe that thermal fluctuations of the order parameter broaden
the superconducting to normal transition into a crossover but no sign of a
first order vortex lattice melting transition is detected in measurements of
the heat capacity or the small angle neutron scattering (SANS) intensity.
Direct observation of the vortices via SANS always finds a well ordered vortex
lattice. The fluctuation broadening is considered in terms of the Lowest Landau
Level theory of critical fluctuations and scaling is found to occur over a
large H_{c2}(T) range
Compact fermion to qubit mappings
Mappings between fermions and qubits are valuable constructions in physics. To date only a handful exist. In addition to revealing dualities between fermionic and spin systems, such mappings are indispensable in any quantum simulation of fermionic physics on quantum computers. The number of qubits required per fermionic mode, and the locality of mapped fermionic operators strongly impact the cost of such simulations. We present a fermion to qubit mapping that outperforms all previous local mappings in both the qubit to mode ratio and the locality of mapped operators. In addition to these practically useful features, the mapping bears an elegant relationship to the toric code, which we discuss. Finally, we consider the error mitigating properties of the mappingâwhich encodes fermionic states into the code space of a stabilizer code. Although there is an implicit tradeoff between low weight representations of local fermionic operators, and high distance code spaces, we argue that fermionic encodings with low-weight representations of local fermionic operators can still exhibit error mitigating properties which can serve a similar role to that played by high code distances. In particular, when undetectable errors correspond to ânaturalâ fermionic noise. We illustrate this point explicitly both for this encoding and the Verstraete-Cirac encoding
Separable states can be used to distribute entanglement
We show that no entanglement is necessary to distribute entanglement; that
is, two distant particles can be entangled by sending a third particle that is
never entangled with the other two. Similarly, two particles can become
entangled by continuous interaction with a highly mixed mediating particle that
never itself becomes entangled. We also consider analogous properties of
completely positive maps, in which the composition of two separable maps can
create entanglement.Comment: 4 pages, 2 figures. Slight modification
Hamiltonian simulation algorithms for near-term quantum hardware
The quantum circuit model is the de-facto way of designing quantum algorithms. Yet any level of abstraction away from the underlying hardware incurs overhead. In this work, we develop quantum algorithms for Hamiltonian simulation "one level belowâ the circuit model, exploiting the underlying control over qubit interactions available in most quantum hardware and deriving analytic circuit identities for synthesising multi-qubit evolutions from two-qubit interactions. We then analyse the impact of these techniques under the standard error model where errors occur per gate, and an error model with a constant error rate per unit time. To quantify the benefits of this approach, we apply it to time-dynamics simulation of the 2D spin Fermi-Hubbard model. Combined with new error bounds for Trotter product formulas tailored to the non-asymptotic regime and an analysis of error propagation, we find that e.g. for a 5âĂâ5 Fermi-Hubbard lattice we reduce the circuit depth from 1,â243,â586 using the best previous fermion encoding and error bounds in the literature, to 3,â209 in the per-gate error model, or the circuit-depth-equivalent to 259 in the per-time error model. This brings Hamiltonian simulation, previously beyond reach of current hardware for non-trivial examples, significantly closer to being feasible in the NISQ era
Assessing non-Markovian dynamics
We investigate what a snapshot of a quantum evolution - a quantum channel
reflecting open system dynamics - reveals about the underlying continuous time
evolution. Remarkably, from such a snapshot, and without imposing additional
assumptions, it can be decided whether or not a channel is consistent with a
time (in)dependent Markovian evolution, for which we provide computable
necessary and sufficient criteria. Based on these, a computable measure of
`Markovianity' is introduced. We discuss how the consistency with Markovian
dynamics can be checked in quantum process tomography. The results also clarify
the geometry of the set of quantum channels with respect to being solutions of
time (in)dependent master equations.Comment: 5 pages, RevTex, 2 figures. (Except from typesetting) version to be
published in the Physical Review Letter
The Perioperative Nursing Workforce Program in NSW: How a professional perioperative nursing association meets one of its mandates Part 1
The need to review and change the way nursing care is delivered in perioperative settings is predicated on nursing workforce shortages, the changing, increasingly technologised and risk-prone OR practice milieu, and increasing demand for surgery. In responding to members' concerns about these issues, the NSW Operating Theatre Association Inc. (NSW OTA) in conjunction with and with sponsorship from the NSW Chief Nursing and Midwifery Officer, initiated and oversaw the development of a program, called the Perioperative Nursing Workforce Program (PNWP). The aims of this program are to make better use of human resources, to improve the way care is provided and thus improve patient outcomes; and to empower perioperative nurses so they are capable of independently improving their working environment. The program, which takes a practice development approach, program participants and some of their projects are presented in this paper. What is known about the topic The role of perioperative professional nursing associations is to write standards for practice and to assist in the professional development of their members. Practice development is hypothesised to assist clinical nurses to 'see' their work contexts afresh and to implement changes to improve patient care by focusing on patient-centredness and the use of credible evidence. What this article contributes It describes the contents and the implementation of a perioperative nursing workforce program, initiated by the NSW OTA and auspiced by the Chief Nursing and Midwifery Officer, NSW Health. The PNWP uses the tenets of practice development (PD) to achieve its aims and this is possibly the first use of PD in perioperative settings, and on a statewide basis
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