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Triphoton Squeezed to Limit

Photonics.com
Jan 2009
TORONTO, Jan. 7, 2009 -- A new technique squeezes light to the fundamental quantum limit, a finding that has potential applications for high-precision measurement, next-generation atomic clocks, novel quantum computing and our most fundamental understanding of the universe.

Krister Shalm, Rob Adamson and Aephraim Steinberg of the University of Toronto's department of physics and Centre for Quantum Information and Quantum Control demonstrated the technique, which uses the photon, the smallest particle of light. The photon is so small that an ordinary light bulb emits billions of photons in a trillionth of a second.
Unsqueezedphoton.jpg
An unsqueezed triphoton. The quantum uncertainty in this state is represented by a circular blob on the north pole of the sphere. (Image: Krister Shalm)
Light is one of the most precise measuring tools in physics and has been used to probe fundamental questions in science ranging from special relativity to questions concerning quantum gravity. But light has its limits in the world of modern quantum technology.
"Precise measurement lies at the heart of all experimental science: The more accurately we can measure something, the more information we can obtain. In the quantum world, where things get ever-smaller, accuracy of measurement becomes more and more elusive," said PhD graduate student Shalm.

"Despite the unimaginably effervescent nature of these tiny particles, modern quantum technologies rely on single photons to store and manipulate information. But uncertainty, also known as quantum noise, gets in the way of the information," said professor Steinberg.
Squeezedtriphoton.jpg
 A maximally squeezed triphoton. The quantum uncertainty in this state has been transformed from a circular blob and has now  been smeared around the surface of the sphere. (Image: Krister Shalm)
Squeezing is a way to increase certainty in one quantity, such as position or speed, but it does so at a cost. "If you squeeze the certainty of one property that is of particular interest, the uncertainty of another complementary property invevitably grows," he said.

In their experiment, the physicists combined three separate photons of light together inside an optical fiber to create a triphoton.

"A strange feature of quantum physics is that when several identical photons are combined, as they are in optical fibers such as those used to carry the Internet to our homes, they undergo an 'identity crisis,' and one can no longer tell what an individual photon is doing," Steinberg said.

The authors then squeezed the triphotonic state to glean the quantum information that was encoded in the triphoton´s polarization. (Polarization is a property of light which is at the basis of 3-D movies, glare-reducing sunglasses, and a coming wave of advanced technologies such as quantum cryptography.)
TriphotonState.jpg
A progression of squeezed triphoton states spiraling outwards. The quantum uncertainty in the triphotons can be represented as a blob on a sphere that becomes progressively "squeezed". (Image: Victoria Feistner)
In all previous work, it was assumed that one could squeeze indefinitely, simply tolerating the growth of uncertainty in the uninteresting direction. "But the world of polarization, like the Earth, is not flat," said Steinberg.

"A state of polarization can be thought of as a small continent floating on a sphere. When we squeezed our triphoton continent, at first all proceeded as in earlier experiments. But when we squeezed sufficiently hard, the continent lengthened so much that it began to 'wrap around' the surface of the sphere," he said.

"To take the metaphor further, all previous experiments were confined to such small areas that the sphere, like your hometown, looked as though it was flat. This work needed to map the triphoton on a globe, which we represented on a sphere providing an intuitive and easily applicable visualization. In so doing, we showed for the first time that the spherical nature of polarization creates qualitatively different states and places a limit on how much squeezing is possible," Steinberg said.

"Creating this special combined state allows the limits to squeezing to be properly studied," Adamson said. "For the first time, we have demonstrated a technique for generating any desired triphoton state and shown that the spherical nature of polarization states of light has unavoidable consequences. Simply put: to properly visualize quantum states of light, one should draw them on a sphere."

The findings were published in the Jan. 1 issue of the journal Nature.

For more information, visit: www.physics.utoronto.ca


GLOSSARY
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
photon
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
polarization
With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam...
quantum noise
Noise generated within an optical communications system link that has both internal (dark current) and external (background noise, or noise in signal) components.
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