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| An array of microdisks and nanobeam photonic cavities formed from a single crystal diamond membrane. The scale bar indicates 2 microns. |
In the race to design the world's first universal quantum computer, a
special kind of diamond defect called a nitrogen vacancy (NV) center is
playing a big role. NV centers consist of a nitrogen atom and a vacant
site that together replace two adjacent carbon atoms in diamond crystal.
The defects can record or store quantum information and transmit it in
the form of light, but the weak signal is hard to identify, extract and
transmit unless it is intensified.
Now a team of researchers at Harvard, the University of California,
Santa Barbara and the University of Chicago has taken a major step
forward in effectively enhancing the fluorescent light emission of
diamond nitrogen vacancy centers -- a key step to using the atom-sized
defects in future quantum computers. The technique, described in the
journal Applied Physics Letters, from AIP Publishing, hinges on
the very precise positioning of NV centers within a structure called a
photonic cavity that can boost the light signal from the defect.
A Potential Qubit Power Couple
NV centers contain an unpaired electron that can store information in
a property known as spin. Researchers can "read" the spin state of the
electron by observing the intensity of particular frequencies of the
light that the NV center emits when illuminated by a laser.
At room temperatures, this pattern of light emission couples to
multiple "sideband" frequencies, making it difficult to interpret. To
amplify the most important element of the signal researchers can use a
structure called a photonic cavity, which consists of a pattern of
nanoscale holes that serve to enhance the NV center's light emission at
its main frequency.
"A photonic cavity that is properly matched to the NVs can
substantially augment their capabilities," said Evelyn Hu, a researcher
at Harvard whose group studies the optical and electronic behavior of
materials that have been carefully sculpted at the nanoscale.
NV centers whose signal is enhanced by photonic cavities could act as
qubits, the fundamental units of quantum information in a quantum
computer.
Matchmaker, Matchmaker, Make Me a Match
Photonic cavities best enhance the signal of NV centers located in a
"hot spot" where the cavities' resonant fields are strongest, but making
sure an atom-sized defect's location matches up with this spot is
extremely tricky.
"Strong spatial overlap is the hardest [task] to achieve in designing
and fabricating a photonic cavity for NV centers," Hu said.
She compared the task to turning on a fixed small light beam in a
dark room containing ultra-small transmitters that send out information
once they are illuminated by the 'right' beam. If the match is right,
the signal from the transmitter is returned strongly, but the challenge
is that the chances of the light hitting the transmitter are very small.
Hu and her colleagues ultimately aim to make sure the beam (or field
of the photonic cavity) will always hit the transmitter (or NV center),
so that information will always be read out. They can do this by knowing
the exact position of the tiny NV centers.
The team took an important first step toward this goal by controlling
the depth of the diamond defects using a technique called delta doping.
"Integrating a plane of spins into these structures enables us to
engineering the spin-photon interaction and exploit quantum effects for
future technologies," said David Awschalom, a researcher at the
University of Chicago whose group grows and characterizes these systems.
The technique confines the possible location of NV centers to a layer
approximately 6 nanometers thick sandwiched inside a diamond membrane
approximately 200 nanometers thick. The researchers then etched holes
into the membrane to create the photonic cavities.
Using this method the researchers were able to increase the intensity
of the light emitted by the NV centers by a factor of about 30 times.
The team believes they can further enhance the emission by also
controlling the position of the defects in the horizontal plane and are
currently working on possible ways to achieve full 3-D control.
Computers, Sensors and More
Nitrogen vacancy centers aren't the only candidate for qubits, but
they have attracted a lot of interest because their electrons have long
spin lifetimes at room temperature, meaning they can maintain quantum
information for a relatively long time.
And the promise of NV centers doesn't stop at ultrafast computers. NV
centers can also be used in non-computing applications, for examples as
molecular-scale magnetic and temperature sensors that could measure the
properties within single cells.
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