Two photons are entangled, one in polarization and the other in orbital angular momentum — twisted light. By passing the polarization photon through the fiber and keeping the twisted light in air, multi-dimensional entanglement transport is possible even over single mode fiber. Credit: Wits University
New research done
at the University of the Witwatersrand in Johannesburg, South Africa, and
Huazhang University of Science and Technology in Wuhan, China, has exciting
implications for secure data transfer across optical fiber networks. The team
have demonstrated that multiple quantum patterns of twisted light can be
transmitted across a conventional fiber link that, paradoxically, supports only
one pattern. The implication is a new approach to realizing a future quantum
network, harnessing multiple dimensions of entangled quantum light.
Science Advances today, published online the research by a team led by Professor Andrew Forbes from the School of Physics at Wits University in collaboration with a team lead by Professor Jian Wang at HUST. In their paper titled: Multi-dimensional entanglement transport through single-mode fiber, the researchers demonstrate a new paradigm for realizing a future quantum network. The team showed that multiple patterns of light are accessible after a communication link of conventional optical fiber, that paradoxically can only support a single pattern. The team achieved this quantum trick by engineering entanglement in two degrees of freedom of light, polarization, and pattern, passing the polarized photon down the fiber and accessing the many patterns with the other photon.
“In essence, the
research introduces the concept of communicating across legacy fiber networks
with multi-dimensional entangled states, bringing together the benefits of
existing quantum communication with polarized photons with that of
high-dimension communication using patterns of light,†says Forbes.
A new twist, a new
paradigm
Present communication systems are very fast, but not fundamentally secure. To make them secure researchers use the laws of Nature for encoding by exploiting the quirky properties of the quantum world, such as in the case of the use of Quantum Key Distribution (QKD) for secure communication.
“Quantum†here
refers to the spooky action at a distance so abhorred by Einstein: quantum
entanglement. In the last few decades, quantum entanglement has been
extensively explored for a variety of quantum information protocols, notably
making communication more secure through QKD. Using so-called “qubits†(2D
quantum states) the information capacity is limited but it is easy to get such
states across fiber links using polarization as a degree of freedom for the
encoding. The spatial pattern of light, its pattern, is another degree of
freedom that has the benefit of high-dimensional encoding. There are many
patterns to use, but unfortunately this requires custom fiber optical cable and
so unsuitable to already existing networks. In the present work, the team have
found a new way to balance these two extremes, by combining polarization qubits
with high-dimensional spatial modes to create multi-dimensional hybrid quantum
states.
“The trick was to
twist the one photon in polarization and twist the other in pattern, forming
“spirally light†that is entangled in two degrees of freedom,†says Forbes.
“Since the polarization entangled photon has only one pattern it could be sent
down the long-distance single-mode fiber (SMF), while the twisted light photon
could be measured without the fiber, accessing multi-dimensional twisted patterns
in the free-space. These twists carry orbital angular momentum (OAM), a
promising candidate for encoding information.â€
Overcoming the
present challenges
Quantum
communication with high-dimensional spatial modes (for example OAM modes) is
promising but only possible in specially designed multi-mode fiber, which
however, is greatly limited by mode (pattern) coupling noise. Single-mode fiber
is free of this “pattern coupling†(which degrades entanglement) but can only
be used for two-dimensional polarization entanglement.
“The novelty in
the published work is the demonstration of multi-dimensional entanglement
transport in conventional single-mode fiber. The light is twisted in two
degrees of freedom: the polarization is twisted to form spirally light, and so
is the pattern. This is referred to as spin-orbit coupling, here exploited for
quantum communication,†says Forbes. “Each transmission is still only a qubit
(2D) but there are an infinite number of them because of the infinite number of
twisted patterns we could entangle in the other photon.â€
The team
demonstrated transfer of multi-dimensional entanglement states over 250 m of
single-mode fiber, showing that an infinite number of two-dimensional subspaces
could be realized. Each subspace could be used for sending information, or
multiplexing information to multiple receivers.
“A consequence of this new approach is that the entire high-dimensional OAM Hilbert space can be accessed, but two dimensions at a time. In some sense it is a compromise between simple 2D approaches and true high-dimensional approaches,†says Forbes. Importantly, high-dimensional states are unsuitable for transmission over conventional fiber networks, whereas this new approach allows legacy networks to be used.