23,643 research outputs found
Witnessing eigenstates for quantum simulation of Hamiltonian spectra
The efficient calculation of Hamiltonian spectra, a problem often intractable
on classical machines, can find application in many fields, from physics to
chemistry. Here, we introduce the concept of an "eigenstate witness" and
through it provide a new quantum approach which combines variational methods
and phase estimation to approximate eigenvalues for both ground and excited
states. This protocol is experimentally verified on a programmable silicon
quantum photonic chip, a mass-manufacturable platform, which embeds entangled
state generation, arbitrary controlled-unitary operations, and projective
measurements. Both ground and excited states are experimentally found with
fidelities >99%, and their eigenvalues are estimated with 32-bits of precision.
We also investigate and discuss the scalability of the approach and study its
performance through numerical simulations of more complex Hamiltonians. This
result shows promising progress towards quantum chemistry on quantum computers.Comment: 9 pages, 4 figures, plus Supplementary Material [New version with
minor typos corrected.
Quantum Abacus for counting and factorizing numbers
We generalize the binary quantum counting algorithm of Lesovik, Suslov, and
Blatter [Phys. Rev. A 82, 012316 (2010)] to higher counting bases. The
algorithm makes use of qubits, qutrits, and qudits to count numbers in a base
2, base 3, or base d representation. In operating the algorithm, the number n <
N = d^K is read into a K-qudit register through its interaction with a stream
of n particles passing in a nearby wire; this step corresponds to a quantum
Fourier transformation from the Hilbert space of particles to the Hilbert space
of qudit states. An inverse quantum Fourier transformation provides the number
n in the base d representation; the inverse transformation is fully quantum at
the level of individual qudits, while a simpler semi-classical version can be
used on the level of qudit registers. Combining registers of qubits, qutrits,
and qudits, where d is a prime number, with a simpler single-shot measurement
allows to find the powers of 2, 3, and other primes d in the number n. We show,
that the counting task naturally leads to the shift operation and an algorithm
based on the quantum Fourier transformation. We discuss possible
implementations of the algorithm using quantum spin-d systems, d-well systems,
and their emulation with spin-1/2 or double-well systems. We establish the
analogy between our counting algorithm and the phase estimation algorithm and
make use of the latter's performance analysis in stabilizing our scheme.
Applications embrace a quantum metrological scheme to measure a voltage (analog
to digital converter) and a simple procedure to entangle multi-particle states.Comment: 23 pages, 15 figure
Experimental optical phase measurement approaching the exact Heisenberg limit
The use of quantum resources can provide measurement precision beyond the
shot-noise limit (SNL). The task of ab initio optical phase measurement---the
estimation of a completely unknown phase---has been experimentally demonstrated
with precision beyond the SNL, and even scaling like the ultimate bound, the
Heisenberg limit (HL), but with an overhead factor. However, existing
approaches have not been able---even in principle---to achieve the best
possible precision, saturating the HL exactly. Here we demonstrate a scheme to
achieve true HL phase measurement, using a combination of three techniques:
entanglement, multiple samplings of the phase shift, and adaptive measurement.
Our experimental demonstration of the scheme uses two photonic qubits, one
double passed, so that, for a successful coincidence detection, the number of
photon-passes is . We achieve a precision that is within of the HL,
surpassing the best precision theoretically achievable with simpler techniques
with . This work represents a fundamental achievement of the ultimate
limits of metrology, and the scheme can be extended to higher and other
physical systems.Comment: (12 pages, 6 figures), typos correcte
Integrated Photonic Sensing
Loss is a critical roadblock to achieving photonic quantum-enhanced
technologies. We explore a modular platform for implementing integrated
photonics experiments and consider the effects of loss at different stages of
these experiments, including state preparation, manipulation and measurement.
We frame our discussion mainly in the context of quantum sensing and focus
particularly on the use of loss-tolerant Holland-Burnett states for optical
phase estimation. In particular, we discuss spontaneous four-wave mixing in
standard birefringent fibre as a source of pure, heralded single photons and
present methods of optimising such sources. We also outline a route to
programmable circuits which allow the control of photonic interactions even in
the presence of fabrication imperfections and describe a ratiometric
characterisation method for beam splitters which allows the characterisation of
complex circuits without the need for full process tomography. Finally, we
present a framework for performing state tomography on heralded states using
lossy measurement devices. This is motivated by a calculation of the effects of
fabrication imperfections on precision measurement using Holland-Burnett
states.Comment: 19 pages, 7 figure
On Quantum Algorithms
Quantum computers use the quantum interference of different computational
paths to enhance correct outcomes and suppress erroneous outcomes of
computations. In effect, they follow the same logical paradigm as
(multi-particle) interferometers. We show how most known quantum algorithms,
including quantum algorithms for factorising and counting, may be cast in this
manner. Quantum searching is described as inducing a desired relative phase
between two eigenvectors to yield constructive interference on the sought
elements and destructive interference on the remaining terms.Comment: 15 pages, 8 figure
A novel haptic model and environment for maxillofacial surgical operation planning and manipulation
This paper presents a practical method and a new haptic model to support manipulations of bones and their segments during the planning of a surgical operation in a virtual environment using a haptic interface. To perform an effective dental surgery it is important to have all the operation related information of the patient available beforehand in order to plan the operation and avoid any complications. A haptic interface with a virtual and accurate patient model to support the planning of bone cuts is therefore critical, useful and necessary for the surgeons. The system proposed uses DICOM images taken from a digital tomography scanner and creates a mesh model of the filtered skull, from which the jaw bone can be isolated for further use. A novel solution for cutting the bones has been developed and it uses the haptic tool to determine and define the bone-cutting plane in the bone, and this new approach creates three new meshes of the original model. Using this approach the computational power is optimized and a real time feedback can be achieved during all bone manipulations. During the movement of the mesh cutting, a novel friction profile is predefined in the haptical system to simulate the force feedback feel of different densities in the bone
On the novelty, efficacy, and significance of weak measurements for quantum tomography
The use of weak measurements for performing quantum tomography is enjoying
increased attention due to several recent proposals. The advertised merits of
using weak measurements in this context are varied, but are generally
represented by novelty, increased efficacy, and foundational significance. We
critically evaluate two proposals that make such claims and find that weak
measurements are not an essential ingredient for most of their advertised
features.Comment: 12 pages, 10 figure
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