Researchers from HP Laboratories
in Bristol, UK, have proposed an approach to distributed
optical quantum computing with a technique that
is highly efficient, flexible and scalable.
Quantum computing is expected to be much more powerful than conventional
information processing. It should be able to search faster and simulate
better, factor large numbers efficiently and virtually guarantee secure
Optical quantum computing – using photons instead of electrons for computation – is
one possible approach to quantum computing. The technology might still
be several decades away from practical implementation.
The researchers – Dr Bill Munro and Dr Tim Spiller, of HP Labs Bristol,
with Professor Kae Nemoto, of the National Institute of Informatics (NII),
Tokyo – have proposed an approach that generates interactions between
photons by using so-called weak optical nonlinearities and intense laser
fields. The result is the creation of two-photon gates, the basic building
blocks of a quantum computer. They have published their results in the
New Journal of Physics: (http://stacks.iop.org/1367-2630/7/137)
Normally, photons, the basic components of light, do not easily
interact or ‘talk' to each other. That is why multiple light signals carrying different
information can be sent along a thin optical fibre without interfering
with each other. But for quantum information processing and communication,
it is vital that photons do interact when called upon to do so. The photons
are the qubits – the basic information bits – in this model of a quantum
Why use an optical quantum system rather than solid state? Dr Spiller
points out that light can be used for both quantum computing and
quantum communication at the same time, which would not be the
case with a solid-state system, where “static” quantum information would have to be mapped onto light to
communicate it. This means that the approach is suitable for distributed
quantum computing, so that small but useful clusters of qubits can be physically
separated – even at different sites – but linked together for computation.
The new approach uses weak nonlinearities and strong laser pulses
to generate the interaction between the two individual photons.
The laser pulse acts as an intermediary between the photons, first ‘talking'
to one, then the other, so that the two photons become entangled.
In quantum processing, generally attempting to check on the state
of entangled qubits leads to the collapse of the information they
carry. But with the HP-NII team's approach, only the information
in the laser pulse collapses; the qubit photons become entangled
through this collapse.
Dr Spiller describes single photons – in fact any kind of qubit – as “precious” and
points out that optical quantum computing systems that have previously
been proposed would need hundreds of them to operate at all. And most
of those photons would be wasted. The HP-NII system operates with
single photons and wastes none. This makes it much more practical
and efficient for quantum information and communication because
today, single photons are hard to generate.
At the heart of the system is a single-photon detector – an innovation
proposed by the HP Labs team – that is also used as a single-photon source.
This is used to generate photons on demand.
The scheme is reliable because the communication between separated quantum
processing sites can be mediated by robust laser pulses rather than fragile
single photon qubits.
Dr Munro said: “Our approach provides the fundamental building blocks for
quantum computation, including highly efficient non-absorbing single-photon
detectors, two-qubit parity detectors, near deterministic CNOT gates and
more. All these elements are essential quantum information processing devices.” The
approach is open for experimentalists to test.
HP Labs is one of the leading corporate research institutions with activities
in the field of quantum science. As a global IT company, it is important
for HP to be involved in such far-reaching research in quantum information
processing, which could have a significant impact on information and communication
technology in the future.
For Further information contact Julian Richards
(PR Manager, HP Laboratories Bristol) on +44
117 312 7625; firstname.lastname@example.org
HP is a technology solutions provider to consumers, businesses and institutions
globally. The company's offerings span IT infrastructure, global services, business
and home computing, and imaging and printing. For the four fiscal quarters ended
April 30, 2005, HP revenue totaled $83.3 billion. More information about HP (NYSE,
Nasdaq: HPQ) is available at www.hp.com.