101 research outputs found
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
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
Real-Time Imaging of Quantum Entanglement
Quantum Entanglement is widely regarded as one of the most prominent features
of quantum mechanics and quantum information science. Although, photonic
entanglement is routinely studied in many experiments nowadays, its signature
has been out of the grasp for real-time imaging. Here we show that modern
technology, namely triggered intensified charge coupled device (ICCD) cameras
are fast and sensitive enough to image in real-time the effect of the
measurement of one photon on its entangled partner. To quantitatively verify
the non-classicality of the measurements we determine the detected photon
number and error margin from the registered intensity image within a certain
region. Additionally, the use of the ICCD camera allows us to demonstrate the
high flexibility of the setup in creating any desired spatial-mode
entanglement, which suggests as well that visual imaging in quantum optics not
only provides a better intuitive understanding of entanglement but will improve
applications of quantum science.Comment: Two supplementary movies available at the data conservancy projec
Cyclic transformation of orbital angular momentum modes
The spatial modes of photons are one realization of a QuDit, a quantum system
that is described in a D-dimensional Hilbert space. In order to perform quantum
information tasks with QuDits, a general class of D-dimensional unitary
transformations is needed. Among these, cyclic transformations are an important
special case required in many high-dimensional quantum communication protocols.
In this paper, we experimentally demonstrate a cyclic transformation in the
high-dimensional space of photonic orbital angular momentum (OAM). Using simple
linear optical components, we show a successful four-fold cyclic transformation
of OAM modes. Interestingly, our experimental setup was found by a computer
algorithm. In addition to the four-cyclic transformation, the algorithm also
found extensions to higher-dimensional cycles in a hybrid space of OAM and
polarization. Besides being useful for quantum cryptography with QuDits, cyclic
transformations are key for the experimental production of high-dimensional
maximally entangled Bell-states.Comment: 18 pages, 6 figure
Divergence of an orbital-angular-momentum-carrying beam upon propagation
There is recent interest in the use of light beams carrying orbital angular
momentum (OAM) for creating multiple channels within free-space optical
communication systems. One limiting issue is that, for a given beam size at the
transmitter, the beam divergence angle increases with increasing OAM, thus
requiring a larger aperture at the receiving optical system if the efficiency
of detection is to be maintained. Confusion exists as to whether this
divergence scales linarly with, or with the square root of, the beam's OAM. We
clarify how both these scaling laws are valid, depending upon whether it is the
radius of the Gaussian beam waist or the rms intensity which is kept constant
while varying the OAM.Comment: 4 pages, 2 figure
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