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Production of high-fidelity
entangled photons exceeds 1 million per second
E. Kloeppel, Physical Sciences Editor
CHAMPAIGN, Ill. —
Like virtuosos tuning their violins, researchers at the University of
Illinois at Urbana-Champaign have tuned their instruments and harmonized
the production of entangled photons, pushing rates to more than 1 million
pairs per second.
The brighter and purer entangled states could assist researchers in
applications involving quantum information processing – such as
quantum computation, teleportation and cryptography – and help
scientists better understand the mysterious transition from quantum
mechanics to classical physics.
“Entangled states are the quintessential feature of quantum mechanics,”
said Paul Kwiat, a John Bardeen Professor of Electrical
and Computer Engineering and Physics at Illinois. “All the manifestations of quantum mechanics in the
world around us arise from the basic but bizarre coupling that exists
between entangled particles.”
For example, the properties of entangled photons are inextricably linked
to each other, even if the photons are located on opposite sides of
the galaxy. To study this “correlation at a distance,” Kwiat
and graduate students Joseph Altepeter and Evan Jeffrey produce pairs
of polarization-entangled photons by passing a laser pulse through two
adjacent nonlinear crystals.
“You can think of polarization as the ‘wiggle’ direction
of the photon – either horizontal, vertical or diagonal,”
Kwiat said. “As soon as you determine the wiggle direction of
one photon in an entangled pair, you immediately know the wiggle direction
of the other photon, no matter how far apart they are.”
A major production problem, however, is that entangled photons are emitted
in many directions and with a wide range of polarization phase relationships,
each acting like an individual singer in a large choir.
“Instead of hearing a soloist hit one note, we were hearing many
choir members, some of whom were singing off-key,” Kwiat said.
The trick was to come up with a way of tuning the system. “We
found that we could pass the photons through another crystal –
one that has a different phase profile – to compensate for the
different phase relationships,” Kwiat said. “The dissonance
is corrected and the system becomes harmonized.”
In the same manner as a corrector lens in a telescope removes chromatic
aberration and improves image quality, the researchers’ special
birefringent crystal removes distortions in the quality of the entanglement.
“After the compensator crystal, the photons are all entangled
in exactly the same way,” Altepeter said. “We can open the
iris and get more than 1 million useful pairs per second.”
Ultrabright, ultrapure sources of entangled photons are essential for
pursuing quantum computing and quantum networks, as a resource for teleportation
in quantum communication, and for sending more information faster by
means of quantum cryptography. High fidelity quantum states can also
provide researchers with a clearer picture of how the universe works
on a very fundamental level.
“Using a low-brightness source is like looking into the quantum
world through a foggy window,” Altepeter said. “With a bright,
pure source, we have a very clear window that allows us to see phenomena
we couldn’t see before.”
The ultimate goal is to understand and develop an intuition for the
quantum nature of reality, said Kwiat, who will report the team’s
findings at the International Conference on Quantum Communication, Measurement
and Computing, to be held July 25–29 in Glasgow, United Kingdom.
“Higher production rates of nearly perfectly entangled photons
will help us better understand the rules of the quantum universe, how
to navigate that universe, and how to characterize it in a very precise
The work was funded by the National Science Foundation, the Army Research
Office, and the Advanced Research and Development Activity.