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The Entropic Price of Building the Perfect Clock: Q&A with Natalia Ares
Experiments investigating the thermodynamics of clocks can teach us about the origin of time's arrow.

Schrödinger’s A.I. Could Test the Foundations of Reality
Physicists lay out blueprints for running a 'Wigner's Friend' experiment using an artificial intelligence, built on a quantum computer, as an 'observer.'

Expanding the Mind (Literally): Q&A with Karim Jerbi and Jordan O'Byrne
Using a brain-computer interface to create a consciousness 'add-on' to help test Integrated Information Theory.

Quanthoven's Fifth
A quantum computer composes chart-topping music, programmed by physicists striving to understand consciousness.

The Math of Consciousness: Q&A with Kobi Kremnitzer
A meditating mathematician is developing a theory of conscious experience to help understand the boundary between the quantum and classical world.

February 6, 2023

Crawling at the Speed of Light
To reveal what happens at the quantum realm—and to aid the quest to build quantum computers—physicists are striving to halt light in its tracks.
by Bob Swarup
November 10, 2010
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Hobbling Light
Students at the University of Rochester place obstacles in light’s path.
We live in a frenzied fast-paced world. But for physicists hoping to reveal exactly what happens in the quantum realm, finding ways to slow down is not just a luxury, it’s essential—at least in terms of freezing and storing quantum information. With this in mind, some physicists are now taking on the Herculean task of stopping the universe’s fastest mover: light itself. Their work could help bring quantum computers a step closer to reality.

Einstein famously taught us that the speed of light—in a vacuum, at least—is a constant, and nothing can outrun it. So it may seem like a thankless task to try and halt light. But over 50 research groups around the world now work on controlling and manipulating the speed of light.

Quantum Non-Demolition

The efforts of these groups have direct benefits for studying the foundations of quantum mechanics experimentally—which calls for supreme control over photons, says FQXi member Jeff Tollaksen at Chapman University, California. When physicists measure a photon, they disturb it—"collapsing" it from being in a fuzzy superposition with multiple contradictory properties, into a definite state with set properties.

In theory, having performed this measurement, it should be possible to repeat the same measurements on the same photon again and again and get the same set results. But, in practice, most real measurements carried out in the lab disturb the system too much, preventing physicists from performing multiple measurements on the same photon. But technology that slows down light can be used to produce single photons that can be precisely and repeatedly measured in "quantum non-demolition" experiments, says Tollaksen.

John Howell, an optical physicist at the University of Rochester in New York, and his colleagues, are leading the charge to pause light. Think back to school experiments investigating how light is refracted: A straw appears to be bent when looking at it in a glass of water because the speed of the light is affected when it moves through different materials. Howell’s group, and others, are exploiting this by shooting nanosecond pulses of light through hot gases to slow it down.

This represents a whole new
paradigm for quantum control.
- Lene Hau
By carefully adjusting the interaction between the light pulses and the atoms in the gas, Howell’s group have slowed light pulses down from their usual 300,000 kilometers per second to just 0.2 millimeters per second. That means that light, which at full speed can cover the distance from the Earth to the Sun in less than eight and half minutes, can be reduced to traveling no more than roughly the breadth of a single hair in a second.

"John Howell and his whole group at Rochester are first rate, really exceptional physicists," says Tollaksen.

Tollaksen has a particular interest in Howell’s efforts to improve quantum experimental techniques: He and his colleagues are currently using a $70,000 grant from FQXi to investigate a radical reformulation of quantum mechanics—one that suggests that future effects can influence the past in quantum experiments—and derive testable predictions of the idea. Howell’s group has already independently performed some precise photon experiments that Tollaksen believes support the reformulation. (See “The Destiny of the Universe.”)

Quantum Computing Leap

But Howell has another major motive for trying to persuade light to dally. If quantum computers are ever to make the leap from fantasy to reality, physicists will need to find ways to slow down light so that it can be stored directly, says Howell.

John Howell’s group are first
rate, really exceptional physicists.
- Jeff Tollaksen
Quantum computers, in principle, would outperform standard classical computers by utilizing the full information capacity within quantum particles. In practice, however, they are hard to build. The most efficient way to transport quantum information is to encode it in light, but the problem is how to store and manipulate this quantum information effectively. This is already an issue for current classical data-transmission methods, such as fibre-optic networks, which convert light into electronic signals and then back into light again when required. This back-and-forth conversion rapidly degrades the quality of information, which is far from ideal, says Howell.

Break-neck Speed

The field of slowing light is moving forward at break-neck speed. Lene Hau and colleagues at Harvard University are using Bose-Einstein condensates—dense, ultra-cold clouds of atoms that are locked into the same quantum state—to compress light and bring it to a complete halt. Meanwhile physicist Nir Davidson at the Weizmann Institute in Israel and his colleagues have now managed to store and retrieve images in atoms. The race is now on to push storage times beyond their current limits of a few seconds and turn a nifty quantum trick into something meaningful.

Ifan Hughes, an optical physicist at Durham University, UK, explains that the ultimate prize would be creating a quantum hard drive, in which the information from light could be stored in atoms. "This would be an ideal medium for quantum computing," he says.

Lene Hau, a physicist at Harvard University, speaks for many of these researchers when she says: "This represents a whole new paradigm for quantum control."

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Recent Comments

For the John Howell group & other experimenters along this line.

When you slow down light, can you conduct at the same time the double slit experiment. I would like to confirm that the "slow" version of light will act as a particle, while the fast version of light, a wave.

CIG Theory (posted on Out Of The Darkness & Real-Time Physics this same FQXi site - see June 15, 2011 posting) suggests that mass becomes spatial when it travels at or near "c" rates. My guess is, that if you...

Ok, I just came across an article that stated that you get two different answers depending on whether you consider the photon as a wave or as a particle.

There is a sort of resolution claimed in the article (that both are correct depending on what you are asking) but for me I want to know this

If you have a spaceship in deep space emitting a 1 KW beam in a certain direction (or a laser to make it simple) you get a very very...

I have two questions.

1) Is the light really just being absorbed and re-emitted and in between that going at c in the vacuum? So the average velocity is slowed.

2) If the speed of light is slowed in a meduim, does that mean that the momentum of the light is a lot higher if n were really that high?

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