452 research outputs found
Proposed optical realisation of a two photon, four-qubit entangled state
The four-qubit states , exhibiting genuinely
multi-partite entanglement have been shown to have many interesting properties
and have been suggested for novel applications in quantum information
processing. In this work we propose a simple quantum circuit and its
corresponding optical embodiment with which to prepare photon pairs in the
states. Our approach uses hyper-entangled photon
pairs, produced by the type-I spontaneous parametric down-conversion (SPDC)
process in two contiguous nonlinear crystals, together with a set of simple
linear-optical transformations. Our photon pairs are maximally hyper-entangled
in both their polarisation and orbital angular momentum (OAM). After one of
these daughter photons passes through our optical setup, we obtain photon pairs
in the hyper-entangled state , and the
states can be achieved by further simple
transformations.Comment: Submitted to Journal of Optic
Local monotonicity of Riemannian and Finsler volume with respect to boundary distances
We show that the volume of a simple Riemannian metric on is locally
monotone with respect to its boundary distance function. Namely if is a
simple metric on and is sufficiently close to and induces
boundary distances greater or equal to those of , then . Furthermore, the same holds for Finsler metrics and the
Holmes--Thompson definition of volume. As an application, we give a new proof
of the injectivity of the geodesic ray transform for a simple Finsler metric.Comment: 13 pages, v3: minor corrections and clarifications, to appear in
Geometriae Dedicat
Gleason-Busch theorem for sequential measurements
Gleason's theorem is a statement that, given some reasonable assumptions, the Born rule used to calculate probabilities in quantum mechanics is essentially unique [A. M. Gleason, Indiana Univ. Math. J. 6, 885 (1957)]. We show that Gleason's theorem contains within it also the structure of sequential measurements, and along with this the state update rule. We give a small set of axioms, which are physically motivated and analogous to those in Busch's proof of Gleason's theorem [P. Busch, Phys. Rev. Lett. 91, 120403 (2003)], from which the familiar Kraus operator form follows. An axiomatic approach has practical relevance as well as fundamental interest, in making clear those assumptions which underlie the security of quantum communication protocols. Interestingly, the two-time formalism is seen to arise naturally in this approach
Quantum data gathering
Measurement of a quantum system – the process by which an observer gathers information about it – provides a link between the quantum and classical worlds. The nature of this process is the central issue for attempts to reconcile quantum and classical descriptions of physical processes. Here, we show that the conventional paradigm of quantum measurement is directly responsible for a well-known disparity between the resources required to extract information from quantum and classical systems. We introduce a simple form of quantum data gathering, “coherent measurement”, that eliminates this disparity and restores a pleasing symmetry between classical and quantum statistical inference. To illustrate the power of quantum data gathering, we demonstrate that coherent measurements are optimal and strictly more powerful than conventional one-at-a-time measurements for the task of discriminating quantum states, including certain entangled many-body states (e.g., matrix product states)
Optimal sequential measurements for bipartite state discrimination
State discrimination is a useful test problem with which to clarify the power and limitations of different classes of measurement. We consider the problem of discriminating between given states of a bipartite quantum system via sequential measurement of the subsystems, with classical feed-forward of measurement results. Our aim is to understand when sequential measurements, which are relatively easy to implement experimentally, perform as well, or almost as well, as optimal joint measurements, which are in general more technologically challenging. We construct conditions that the optimal sequential measurement must satisfy, analogous to the well-known Helstrom conditions for minimum error discrimination in the unrestricted case. We give several examples and compare the optimal probability of correctly identifying the state via global versus sequential measurement strategies
Optimal discrimination of single-qubit mixed states
We consider the problem of minimum-error quantum state discrimination for
single-qubit mixed states. We present a method which uses the Helstrom
conditions constructively and analytically; this algebraic approach is
complementary to existing geometric methods, and solves the problem for any
number of arbitrary signal states with arbitrary prior probabilities.Comment: 8 pages, 1 figur
Is coherence catalytic?
Quantum coherence, the ability to control the phases in superposition states
is a resource, and it is of crucial importance, therefore, to understand how it
is consumed in use. It has been suggested that catalytic coherence is possible,
that is repeated use of the coherence without degradation or reduction in
performance. The claim has particular relevance for quantum thermodynamics
because, were it true, it would allow free energy that is locked in coherence
to be extracted . We address this issue directly with a
careful analysis of the proposal by berg. We find that coherence
be used catalytically, or even repeatedly without limit.Comment: 23 pages with 2 figure
Fabrication and Characterization of Electrostatic Quantum Dots in a Si/SiGe 2D Electron Gas, Including an Integrated Read-out Channel
A new fabrication technique is used to produce quantum dots with read-out
channels in silicon/silicon-germanium two-dimensional electron gases. The
technique utilizes Schottky gates, placed on the sides of a shallow etched
quantum dot, to control the electronic transport process. An adjacent quantum
point contact gate is integrated to the side gates to define a read-out channel
and thus allow for noninvasive detection of the electronic occupation of the
quantum dot. Reproducible and stable Coulomb oscillations and the corresponding
jumps in the read-out channel resistance are observed at low temperatures. The
fabricated dot combined with the read-out channel represent a step towards the
spin-based quantum bit in Si/SiGe heterostructures.Comment: 3 pages, 4 fig
Sputtered Gold as an Effective Schottky Gate for Strained Si/SiGe Nanostructures
Metallization of Schottky surface gates by sputtering Au on strained Si/SiGe
heterojunctions enables the depletion of the two dimensional electron gas
(2DEG) at a relatively small voltage while maintaining an extremely low level
of leakage current. A fabrication process has been developed to enable the
formation of sub-micron Au electrodes sputtered onto Si/SiGe without the need
of a wetting layer.Comment: 3 pages, 3 figure
- …