The component the team has built uses particles of light, photons, to encode quantum mechanical information Image: J.C.F. Matthews |
An international research group led by scientists from the
Univ. of Bristol, UK, and the Universities of Osaka
and Hokkaido, Japan, has demonstrated a
fundamental building block for quantum computing that could soon be employed in
a range of quantum technologies.
Professor Jeremy
O’Brien, Director of the Univ.
of Bristol’s Center for
Quantum Photonics, and his Japanese colleagues have demonstrated a quantum
logic gate acting on four photons. The researchers believe their device could
provide important routes to new quantum technologies, including secure
communication, precision measurement, and ultimately a quantum computer.
Unlike
conventional bits or transistors, which can be in one of only two states at any
one time (1 or 0), a qubit can be in several states at the same time and can
therefore be used to hold and process a much larger amount of information at a
greater rate.
“We have realized
a fundamental element for processing quantum information—a controlled-NOT or
CNOT gate—based on a recipe that was theoretically proposed 10 years ago,” said
Professor O’Brien. “The reason it has taken so long to achieve this milestone
is that even for such a relatively simple circuit we require complete control
over four single photons whizzing around at the speed of light!”
The approach
taken by Professor O’Brien and his colleagues combined several methods for
making optical circuits that must be stable to within a fraction of the
wavelength of light, that is, nanometers. In 2001 optical quantum computing
became possible when a theoretical recipe for realizing this CNOT gate, as well
as the other necessary components, was developed. However, the technological
challenges associated with making the optical circuits have prevented its realization
until now. The implications for this new approach are far-reaching.
“The ability to
implement such a logic gate on photons is critical for building up larger scale
circuits and even algorithms,” said Professor O’Brien. “Using an integrated
optics on a chip approach that we have pioneered here at Bristol over the last
several years will enable this to proceed far more rapidly, paving the way to
quantum technologies that will help us understand the most complex scientific
problems.”
In the short
term, the team expect to apply their new results immediately for developing new
approaches to quantum communication and measurement and then for simulation
tools in their lab. In the longer term, a small-scale quantum simulator based
on a multi-photon optical circuit could be used to simulate processes which
themselves are governed by quantum mechanics, such as superconductivity and
photosynthesis. “Our technique could improve our understanding of such
important processes and help, for example, in the development of more efficient
solar cells,” said Professor O’Brien. Other applications include the development
of ultra-fast and efficient search engines, designing high-tech materials and
new pharmaceuticals.
The leap from
using one photon to two photons is not trivial because the two particles need
to be identical in every way and because of the way these particles interfere,
or interact, with each other. There is no direct analogue of this interaction
outside of quantum physics.
“Now that we can
implement the fundamental building blocks for quantum circuits, the move to a
larger scale devices will become our focus. Because of the increasingly
complexity the results will be just as exciting,” said Professor O’Brien. “Each
time we add a photon, the complexity of the problem we are able to investigate
increases exponentially, so if a one-photon quantum circuit has 10 outcomes, a
two-photon system can give 100 outcomes and a three-photon system 1000
solutions and so on.”
The Center for
Quantum Photonics now plans to use their chip-based approach to increase the complexity
of their experiment not only by adding more photons but also by using larger
circuits.
