1,154 research outputs found
On the Interpretation of Energy as the Rate of Quantum Computation
Over the last few decades, developments in the physical limits of computing
and quantum computing have increasingly taught us that it can be helpful to
think about physics itself in computational terms. For example, work over the
last decade has shown that the energy of a quantum system limits the rate at
which it can perform significant computational operations, and suggests that we
might validly interpret energy as in fact being the speed at which a physical
system is "computing," in some appropriate sense of the word. In this paper, we
explore the precise nature of this connection. Elementary results in quantum
theory show that the Hamiltonian energy of any quantum system corresponds
exactly to the angular velocity of state-vector rotation (defined in a certain
natural way) in Hilbert space, and also to the rate at which the state-vector's
components (in any basis) sweep out area in the complex plane. The total angle
traversed (or area swept out) corresponds to the action of the Hamiltonian
operator along the trajectory, and we can also consider it to be a measure of
the "amount of computational effort exerted" by the system, or effort for
short. For any specific quantum or classical computational operation, we can
(at least in principle) calculate its difficulty, defined as the minimum effort
required to perform that operation on a worst-case input state, and this in
turn determines the minimum time required for quantum systems to carry out that
operation on worst-case input states of a given energy. As examples, we
calculate the difficulty of some basic 1-bit and n-bit quantum and classical
operations in an simple unconstrained scenario.Comment: Revised to address reviewer comments. Corrects an error relating to
time-ordering, adds some additional references and discussion, shortened in a
few places. Figures now incorporated into tex
Photo-dynamics of quantum emitters in aluminum nitride
Aluminum nitride is a technologically important wide bandgap semiconductor
which has been shown to host bright quantum emitters. In this paper, we probe
the photodynamics of quantum emitters in aluminum nitride using photon emission
correlations and time-resolved spectroscopy. We identify that each emitter
contains as many as 6 internal energy levels with distinct laser
power-dependent behaviors. Power-dependent shelving and de-shelving processes,
such as optically induced ionization and recombination are considered,
indicating complex optical dynamics associated with the spontaneous and
optically pumped transitions. State population dynamics simulations
qualitatively explain the temporal behaviours of the quantum emitters,
revealing that those with pump-dependent de-shelving processes can saturate at
significantly higher intensities, resulting in bright room-temperature quantum
light emission.Comment: 20 pages. 5 figures in main text, 3 in supplementary inf
Bullseye dielectric cavities for photon collection from a surface-mounted quantum-light-emitter
Coupling light from a point source to a propagating mode is an important
problem in nano-photonics and is essential for many applications in quantum
optics. Circular "bullseye" cavities, consisting of concentric rings of
alternating refractive index, are a promising technology that can achieve
near-unity coupling into a first lens. Here we design a bullseye structure
suitable for enhancing the emission from dye molecules, 2D materials and
nano-diamonds positioned on the surface of these cavities. A periodic design of
cavity, meeting the Bragg scattering condition, achieves a Purcell factor of
22.5 and collection efficiency of 80 %. We also tackle the more challenging
task of designing a cavity for coupling to a low numerical aperture fibre in
the near field. Using an iterative procedure, we show that apodized
(non-periodic) rings can achieve a collection efficiency that exceeds the
periodic Bragg cavity.Comment: 9 pages, 6 figure
Cavity-enhanced radiative emission rate in a single-photon-emitting diode operating at 0.5 GHz
We report the observation of a Purcell enhancement in the electroluminescence
decay rate of a single quantum dot, embedded in a microcavity
light-emitting-diode structure. Lateral confinement of the optical mode was
achieved using an annulus of low-refractive-index aluminium oxide, formed by
wet oxidation. The same layer acts as a current aperture, reducing the active
area of the device without impeding the electrical properties of the p-i-n
diode. This allowed single photon electroluminescence to be demonstrated at
repetition rates up to 0.5 GHz.Comment: 11 pages, 4 Figures. To be published in New Journal of Physic
A Controlled Study on the Characterisation of Bioaerosols Emissions from Compost
Bioaerosol emissions arising from biowaste treatment are an issue of public concern. To better characterise the bioaerosols, and to assess a range of measurement methods, we aerosolised green waste compost under controlled conditions. Viable and non-viable Andersen samplers, cyclone samplers and a real time bioaerosol detection system (Spectral Intensity Bioaerosol Sensor (SIBS)) were deployed simultaneously. The number-weighted fraction of fluorescent particles was in the range 22–26% of all particles for low and high emission scenarios. Overall fluorescence spectral profiles seen by the SIBS exhibited several peaks across the 16 wavelength bands from 298 to 735 nm. The size-fractionated endotoxin profile showed most endotoxin resided in the 2.1–9 μm aerodynamic diameter fraction, though up to 27% was found in a finer size fraction. A range of microorganisms were detected through culture, Matrix Assisted Laser Desorption and Ionisation Time of Flight Mass Spectrometry (MALDI-TOF) and quantitative polymerase chain reaction (qPCR), including Legionella pneumophila serogroup 1. These findings contribute to our knowledge of the physico-chemical and biological characteristics of bioaerosols from composting sites, as well as informing future monitoring approaches and data interpretation for bioaerosol measurement
Multipartite entanglement for continuous variables: A quantum teleportation network
We show that {\it one} single-mode squeezed state distributed among
parties using linear optics suffices to produce a truly -partite entangled
state for any nonzero squeezing and arbitrarily many parties. From this
-partite entangled state, via quadrature measurements of modes,
bipartite entanglement between any two of the parties can be `distilled',
which enables quantum teleportation with an experimentally determinable
fidelity better than could be achieved in any classical scheme.Comment: 4 pages, 2 figures, published version, paper shorter, title longe
High-speed metamagnetic resistive switching of FeRh through Joule heating
Due to its proximity to room temperature and demonstrated high degree of
temperature tunability, the metamagnetic ordering transition in FeRh is
attractive for novel high-performance computing devices seeking to use
magnetism as the state variable. We demonstrate electrical control of the
transition via Joule heating in FeRh wires. Finite element simulations based on
abrupt state transition within each domain result in a globally smooth
transition that agrees with the experimental findings and provides insight into
the thermodynamics involved. We measure a 150 K decrease in transition
temperature with currents up to 60 mA, limited only by the dimensions of the
device. The sizeable shift in transition temperature scales with current
density and wire length, suggesting the absolute resistance and heat
dissipation of the substrate are also important. The FeRh phase change is
evaluated by pulsed I-V using a variety of bias conditions. We demonstrate high
speed (~ ns) memristor-like behavior and report device performance parameters
such as switching speed and power consumption that compare favorably with
state-of-the-art phase change memristive technologies.Comment: 35 pages, 9 figure
Perfect quantum error correction coding in 24 laser pulses
An efficient coding circuit is given for the perfect quantum error correction
of a single qubit against arbitrary 1-qubit errors within a 5 qubit code. The
circuit presented employs a double `classical' code, i.e., one for bit flips
and one for phase shifts. An implementation of this coding circuit on an
ion-trap quantum computer is described that requires 26 laser pulses. A further
circuit is presented requiring only 24 laser pulses, making it an efficient
protection scheme against arbitrary 1-qubit errors. In addition, the
performance of two error correction schemes, one based on the quantum Zeno
effect and the other using standard methods, is compared. The quantum Zeno
error correction scheme is found to fail completely for a model of noise based
on phase-diffusion.Comment: Replacement paper: Lost two laser pulses gained one author; added
appendix with circuits easily implementable on an ion-trap compute
Unconditional teleportation of continuous-variable entanglement
We give a protocol and criteria for demonstrating unconditional teleportation
of continuous-variable entanglement (i.e., entanglement swapping). The initial
entangled states are produced with squeezed light and linear optics. We show
that any nonzero entanglement (any nonzero squeezing) in both of two
entanglement sources is sufficient for entanglement swapping to occur. In fact,
realization of continuous-variable entanglement swapping is possible using only
{\it two} single-mode squeezed states.Comment: 4 pages, 2 figures, published version, title change
Emission dynamics of optically driven aluminum nitride quantum emitters
Aluminum nitride is a technologically important wide band-gap semiconductor which has been shown to host bright quantum emitters. We use photon emission correlation spectroscopy (PECS), time-resolved photoluminescence (TRPL), and state-population dynamic simulations to probe the dynamics of emission under continuous wave (CW) and pulsed optical excitation. We infer that there are at least four dark shelving states, which govern the TRPL, bunching, and saturation of the optical transition. We study in detail the emission dynamics of two quantum emitters (QEs) with differing power-dependent shelving processes, hypothesized to result from charge ionization and recombination. These results demonstrate that photon bunching caused by shelving the system in a dark state inherently limits the saturation rate of the photon source. In emitters where increasing optical power deshelves the dark states, we observe an increased photon emission intensity
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