|An optical system for solving combinatorial optimisation problems has
been made available for use online, say its creators in Japan. Called
the Quantum Neural Network (QNN), the system has been developed by
Nippon Telegraph and Telephone (NTT), Japan's National Institute of
Informatics, and the University of Tokyo.
Combinatorial optimisation problems involve evaluating large numbers of
possible solutions to a problem and identifying the best one. A familiar
example is the 'travelling salesman problem' whereby a person wishes to
visit several different destinations by the shortest possible route.
Such problems can be found in a wide range of human endeavour from
scheduling medical procedures in a hospital to maximising the
performance of a complex system like an aircraft.
The QNN comprises a 1 km loop of optical fibre that incorporates a
phase-sensitive amplifier (PSA) and a field-programmable gate array
(FPGA). Information is encoded in 2000 optical parametric oscillators
(OPOs), which are light pulses that are each in a superposition of two
polarisation states (0 and p). These OPOs act as quantum bits, which are
injected into the loop. The FPGA makes measurements on sequential pairs
of OPOs and can then modify the OPOs to solve the desired combinatorial
optimisation problem. The process is repeated as the OPOs make as many
as 1000 trips around the loop, amplified at each pass by the PSA. This
process transforms the OPOs into a specific configuration of 0 and p
states, which is then read off as the solution to the problem.
Rather than being a universal quantum computer that can address a wide
range of problems, QNN is designed to optimise systems that can be
described by Ising models. These have constituents that can take one of
two values (0 or p) and only interact with their neighbours. While this
puts a limit on the usefulness of the QNN, Ising models can be used to
describe a wide range of phenomena. Access to the system is via
QNNcloud, where new users are invited to create an account.