31 research outputs found
Erasing the orbital angular momentum information of a photon
Quantum erasers with paths in the form of physical slits have been studied
extensively and proven instrumental in probing wave-particle duality in quantum
mechanics. Here we replace physical paths (slits) with abstract paths of
orbital angular momentum (OAM). Using spin-orbit hybrid entanglement of photons
we show that the OAM content of a photon can be erased with a complimentary
polarization projection of one of the entangled pair. The result is the
(dis)appearance of azimuthal fringes based on whether the \which-OAM"
information was erased. We extend this concept to a delayed measurement scheme
and show that the OAM information and fringe visibility are complimentary
Hybrid entanglement for quantum communication
A dissertation submitted to the Faculty of Science
in partial fulfillment of the requirements for the Degree of
Master of Science
School of Physics
University of Witwatersrand
November 1, 2017The generation and detection of entangled photons is a topic of interest in quantum
communication. With current state-of-the-art methods it is possible to manipulate
any degree of freedom (DoF) of photons, e.g, polarisation, transverse momentum,
orbital angular momentum and energy. Furthermore, it is possible to combine these
DoF to realise hybrid entanglement { entanglement between the DoF of photons. In
this dissertation we focus on hybrid entanglement between photon states of coupled
orbital angular momentum and polarisation.
We engineer hybrid-entanglement using geometric phase control between spatially
separated photons produced from spontaneous parametric down conversion.
We present a new type of quantum eraser that does not rely on physical path interference.
We show that in principle any other degree of freedom can be used and
demonstrate this e ectively through polarisation control.
The use of high dimensional hybrid photon states in quantum communication,
particularly in quantum cryptography, is still in its infancy. Here we tailor photon
states that are coupled in their polarisation and spatial DoF (orbital angular momentum)
to realise high dimensional encoding alphabets. We show how photons entangled
in their internal DoF can be generated and deterministically detected. We exploit
them in a demonstration of a high dimensional quantum key distribution protocol
and show that our scheme generates secure keys at high rates.MT 201
Creation and characterization of vector vortex modes for classical and quantum communication
Vector vortex beams are structured states of light that are non-separable in
their polarisation and spatial mode, they are eigenmodes of free-space and many
fibre systems, and have the capacity to be used as a modal basis for both
classical and quantum communication. Here we outline recent progress in our
understanding of these modes, from their creation to their characterization and
detection. We then use these tools to study the propagation behaviour of such
modes in free-space and optical fibre and show that modal cross-talk results in
a decay of vector states into separable scalar modes, with a concomitant loss
of information. We present a comparison between probabilistic and deterministic
detection schemes showing that the former, while ubiquitous, negates the very
benefit of increased dimensionality in quantum communication while reducing
signal in classical communication links. This work provides a useful
introduction to the field as well as presenting new findings and perspectives
to advance it further
Self-healing high-dimensional quantum key distribution using hybrid spin-orbit Bessel states
Using spatial modes for quantum key distribution (QKD) has become highly
topical due to their infinite dimensionality, promising high information
capacity per photon. However, spatial distortions reduce the feasible secret
key rates and compromise the security of a quantum channel. In an extreme form
such a distortion might be a physical obstacle, impeding line-of-sight for
free-space channels. Here, by controlling the radial degree of freedom of a
photon's spatial mode, we are able to demonstrate hybrid high-dimensional QKD
through obstacles with self-reconstructing single photons. We construct
high-dimensional mutually unbiased bases using spin-orbit hybrid states that
are radially modulated with a non-diffracting Bessel-Gaussian (BG) profile, and
show secure transmission through partially obstructed quantum links. Using a
prepare-measure protocol we report higher quantum state self-reconstruction and
information retention for the non-diffracting BG modes as compared to
Laguerre-Gaussian modes, obtaining a quantum bit error rate (QBER) that is up
to 3 times lower. This work highlights the importance of controlling the radial
mode of single photons in quantum information processing and communication as
well as the advantages of QKD with hybrid states.Comment: Published version, 15 pages, 6 figures, 2 table
A robust basis for multi-bit optical communication with vectorial light
Increasing the information capacity of communication channels is a pressing
need, driven by growing data demands and the consequent impending data crunch
with existing modulation schemes. In this regard, mode division multiplexing
(MDM), where the spatial modes of light form the encoding basis, has enormous
potential and appeal, but is impeded by modal noise due to imperfect channels.
Here we overcome this challenge by breaking the existing MDM paradigm of using
the modes themselves as a discrete basis, instead exploiting the polarization
inhomogeneity (vectorness) of vectorial light as our information carrier. We
show that this encoding basis can be partitioned and detected almost at will,
and measured in a channel independent fashion, a fact we confirm experimentally
using atmospheric turbulence as a highly perturbing channel example. Our
approach replaces conventional amplitude modulation with a novel modal
alternative for potentially orders of magnitude channel information
enhancement, yet is robust to fading even through noisy channels, offering a
new paradigm to exploiting the spatial mode basis for optical communication.Comment: 15 pages, 8 figure