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Articles / Physics

Eighty years on, French physicists say they have created Einstein's box: a device just 1.1 inches big that snares a photon, enabling it to be monitored from birth to death.

Photons are arguably the ultimate existential particle in physics. By switching on a light bulb, you release a million billion photons every second.

But as soon as you see a photon, it dies, for its contact with the retina expends the energy that made it exist.

"Photons are easy to detect. You do it yourself, every second for instance when you are looking at a computer screen," Jean-Michel Raimond of France's National Center for Scientific Research (CNRS), who is one of the innovators, said.

"But you do this only once. It's post-mortem analysis. We, though, can now analyze it in real time, while the photon is still alive."

The box comprises a cavity with walls made from ultra-reflective, superconducting mirrors able to trap a photon for about a seventh of a second.

That may not seem much but it is worth considering that, in the same time, a free photon would travel about a tenth of the distance from the Earth to the Moon.

The conventional way of counting photons is by a light detector which works by absorbing the energy by impacting particles. But the collision destroys the photons, so what is needed is a "transparent" counter.

The French team, reporting their work in the British journal Nature on Thursday, say they found the answer in a stream of rubidium atoms, which cross the box in which the photon is trapped.

Photons have an electrical field that slightly changes the energy levels of the atom, but in this case, not enough to let the atom absorb energy from the field.

When an atom crosses the photon's electrical field, this causes a tiny delay in the electrons that orbit the atom's nucleus. The delay is measurable, using the technique of modern atomic clocks, which use electrons' orbit as a "pendulum" to provide a precise time.

In a commentary also published by Nature, Ferdinand Schmidt-Kaler, a quantum physicist at Germany's University of Ulm, described the French achievement as an "experimental masterwork" with major implications for quantum computing, a field that proponents claim will make today's supercomputers look like an abacus.

Instead of using the binary digits 0 and 1 to hold information, quantum computing is based on a principle of quantum mechanics changes of state, called superposition, that occur at atomic level.

Quantum information, or a qubit, can be a 0 or 1 or simultaneously as both 0 and 1, amounting to a fanastic potential boost in data storage, but only useful so long as it can be controlled and accessed.

Photons, atoms and ions have been used as qubit carriers in this still fledgling area of research.

The experiment demonstrates that "a stream of atomic qubits can be fully controlled by the qubit state of a trapped photon," said Schmidt-Kaler.

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