82 research outputs found
On Small Beams with Large Topological Charge II: Photons, Electrons and Gravitational Waves
Beams of light with a large topological charge significantly change their
spatial structure when they are focused strongly. Physically, it can be
explained by an emerging electromagnetic field component in the direction of
propagation, which is neglected in the simplified scalar wave picture in
optics. Here we ask: Is this a specific photonic behavior, or can similar
phenomena also be predicted for other species of particles? We show that the
same modification of the spatial structure exists for relativistic electrons as
well as for focused gravitational waves. However, this is for different
physical reasons: For electrons, which are described by the Dirac equation, the
spatial structure changes due to a Spin-Orbit coupling in the relativistic
regime. In gravitational waves described with linearized general relativity,
the curvature of space-time between the transverse and propagation direction
leads to the modification of the spatial structure. Thus, this universal
phenomenon exists for both massive and massless elementary particles with Spin
1/2, 1 and 2. It would be very interesting whether other types of particles
such as composite systems (neutrons or C) or neutrinos show a similar
behaviour and how this phenomenon can be explained in a unified physical way.Comment: 8 pages, 3 figure
Quantum gate description for induced coherence without induced emission and related phenomena
We introduce unitary quantum gates for photon pair creation in spontaneous
parametric down-conversion nonlinear crystals (NLs) and for photon path
alignment. These are the two key ingredients for the method of "induced
coherence without induced emission" and many ensuing variations thereof. The
difficulty in doing so stems from an apparent mixing of the mode picture (such
as the polarization of photons) and the Fock picture (such as the existence of
the photons). We illustrate utility of these gates by obtaining quantum
circuits for the experimental setups of the frustrated generation of photon
pairs, identification of a point-like object with undetected photons, and
creation of a Bell state. We also introduce an effective nonunitary description
for the action of NLs in experiments where all the NLs are pumped coherently.
As an example, by using this simplifying picture, we show how NLs can be used
to create superposition of given quantum states in a modular fashion.Comment: 4+3 page
Quantum Experiments and Graphs: Multiparty States as coherent superpositions of Perfect Matchings
We show a surprising link between experimental setups to realize
high-dimensional multipartite quantum states and Graph Theory. In these setups,
the paths of photons are identified such that the photon-source information is
never created. We find that each of these setups corresponds to an undirected
graph, and every undirected graph corresponds to an experimental setup. Every
term in the emerging quantum superposition corresponds to a perfect matching in
the graph. Calculating the final quantum state is in the complexity class
#P-complete, thus cannot be done efficiently. To strengthen the link further,
theorems from Graph Theory -- such as Hall's marriage problem -- are rephrased
in the language of pair creation in quantum experiments. We show explicitly how
this link allows to answer questions about quantum experiments (such as which
classes of entangled states can be created) with graph theoretical methods, and
potentially simulate properties of Graphs and Networks with quantum experiments
(such as critical exponents and phase transitions).Comment: 6+5 pages, 4+7 figure
On Small Beams with Large Topological Charge
Light beams can carry a discrete, in principle unbounded amount of angular
momentum. Examples of such beams, the Laguerre-Gauss modes, are frequently
expressed as solutions of the paraxial wave equation. There, they are
eigenstates of the orbital angular momentum (OAM) operator. The paraxial
solutions predict that beams with large OAM could be used to resolve
arbitrarily small distances - a dubious situation. Here we show how to solve
that situation by calculating the properties of beams free from the paraxial
approximation. We find the surprising result that indeed one can resolve
smaller distances with larger OAM, although with decreased visibility. If the
visibility is kept constant (for instance at the Rayleigh criterion, the limit
where two points are reasonably distinguishable), larger OAM does not provide
an advantage. The drop in visibility is due to a field in the direction of
propagation, which is neglected within the paraxial limit.Comment: 6 pages, 2 figures; + supplementary informatio
Quantum Experiments and Graphs III: High-Dimensional and Multi-Particle Entanglement
Quantum entanglement plays an important role in quantum information
processes, such as quantum computation and quantum communication. Experiments
in laboratories are unquestionably crucial to increase our understanding of
quantum systems and inspire new insights into future applications. However,
there are no general recipes for the creation of arbitrary quantum states with
many particles entangled in high dimensions. Here, we exploit a recent
connection between quantum experiments and graph theory and answer this
question for a plethora of classes of entangled states. We find experimental
setups for Greenberger-Horne-Zeilinger states, W states, general Dicke states,
and asymmetrically high-dimensional multipartite entangled states. This result
sheds light on the producibility of arbitrary quantum states using photonic
technology with probabilistic pair sources and allows us to understand the
underlying technological and fundamental properties of entanglement.Comment: 7 pages, 7 figures; Appendix 3 pages, 5 figure
Gouy Phase Radial Mode Sorter for Light: Concepts and Experiments
We present an in principle lossless sorter for radial modes of light, using
accumulated Gouy phases. The experimental setups have been found by a computer
algorithm, and can be intuitively understood in a geometric way. Together with
the ability to sort angular-momentum modes, we now have access to the complete
2-dimensional transverse plane of light. The device can readily be used in
multiplexing classical information. On a quantum level, it is an analog of the
Stern-Gerlach experiment -- significant for the discussion of fundamental
concepts in quantum physics. As such, it can be applied in high-dimensional and
multi-photonic quantum experiments.Comment: main text: 7 pages, 5 figures. Supplementary Information: 5 pages, 4
figure
Orbital angular momentum of photons and the entanglement of Laguerre-Gaussian modes
The identification of orbital angular momentum (OAM) as a fundamental
property of a beam of light nearly twenty-five years ago has led to an
extensive body of research around this topic. The possibility that single
photons can carry OAM has made this degree of freedom an ideal candidate for
the investigation of complex quantum phenomena and their applications. Research
in this direction has ranged from experiments on complex forms of quantum
entanglement to the interaction between light and quantum states of matter.
Furthermore, the use of OAM in quantum information has generated a lot of
excitement, as it allows for encoding large amounts of information on a single
photon. Here we explain the intuition that led to the first quantum experiment
with OAM fifteen years ago. We continue by reviewing some key experiments
investigating fundamental questions on photonic OAM and the first steps into
applying these properties in novel quantum protocols. In the end, we identify
several interesting open questions that could form the subject of future
investigations with OAM.Comment: 17 pages, 7 figures; close to accepted versio
Twisted Photons: New Quantum Perspectives in High Dimensions
Quantum information science and quantum information technology have seen a
virtual explosion world-wide. It is all based on the observation that
fundamental quantum phenomena on the individual particle or system-level lead
to completely novel ways of encoding, processing and transmitting information.
Quantum mechanics, a child of the first third of the 20th century, has found
numerous realizations and technical applications, much more than was thought at
the beginning. Decades later, it became possible to do experiments with
individual quantum particles and quantum systems. This was due to technological
progress, and for light in particular, the development of the laser. Hitherto,
nearly all experiments and also nearly all realizations in the fields have been
performed with qubits, which are two-level quantum systems. We suggest that
this limitation is again mainly a technological one, because it is very
difficult to create, manipulate and measure more complex quantum systems. Here,
we provide a specific overview of some recent developments with
higher-dimensional quantum systems. We mainly focus on Orbital Angular Momentum
(OAM) states of photons and possible applications in quantum information
protocols. Such states form discrete higher-dimensional quantum systems, also
called qudits. Specifically, we will first address the question what kind of
new fundamental properties exist and the quantum information applications which
are opened up by such novel systems. Then we give an overview of recent
developments in the field by discussing several notable experiments over the
past 2-3 years. Finally, we conclude with several important open questions
which will be interesting for investigations in the future.Comment: 15 pages, 7 figure
Predicting Research Trends with Semantic and Neural Networks with an application in Quantum Physics
The vast and growing number of publications in all disciplines of science
cannot be comprehended by a single human researcher. As a consequence,
researchers have to specialize in narrow sub-disciplines, which makes it
challenging to uncover scientific connections beyond the own field of research.
Thus access to structured knowledge from a large corpus of publications could
help pushing the frontiers of science. Here we demonstrate a method to build a
semantic network from published scientific literature, which we call SemNet. We
use SemNet to predict future trends in research and to inspire new,
personalized and surprising seeds of ideas in science. We apply it in the
discipline of quantum physics, which has seen an unprecedented growth of
activity in recent years. In SemNet, scientific knowledge is represented as an
evolving network using the content of 750,000 scientific papers published since
1919. The nodes of the network correspond to physical concepts, and links
between two nodes are drawn when two physical concepts are concurrently studied
in research articles. We identify influential and prize-winning research topics
from the past inside SemNet thus confirm that it stores useful semantic
knowledge. We train a deep neural network using states of SemNet of the past,
to predict future developments in quantum physics research, and confirm high
quality predictions using historic data. With the neural network and
theoretical network tools we are able to suggest new, personalized,
out-of-the-box ideas, by identifying pairs of concepts which have unique and
extremal semantic network properties. Finally, we consider possible future
developments and implications of our findings.Comment: 9+6 pages, 6 figure
Automated Search for new Quantum Experiments
Quantum mechanics predicts a number of at first sight counterintuitive
phenomena. It is therefore a question whether our intuition is the best way to
find new experiments. Here we report the development of the computer algorithm
Melvin which is able to find new experimental implementations for the creation
and manipulation of complex quantum states. And indeed, the discovered
experiments extensively use unfamiliar and asymmetric techniques which are
challenging to understand intuitively. The results range from the first
implementation of a high-dimensional Greenberger-Horne-Zeilinger (GHZ) state,
to a vast variety of experiments for asymmetrically entangled quantum states --
a feature that can only exist when both the number of involved parties and
dimensions is larger than 2. Additionally, new types of high-dimensional
transformations are found that perform cyclic operations. Melvin autonomously
learns from solutions for simpler systems, which significantly speeds up the
discovery rate of more complex experiments. The ability to automate the design
of a quantum experiment can be applied to many quantum systems and allows the
physical realization of quantum states previously thought of only on paper.Comment: 5+8 pages, 4+1 figures (main text + supplementary
- …