In 1927, Albert Einstein conceived of a box in which light was trapped and a single light particle, or photon, was released in a theoretical experiment to measure the relationship between mass and energy.Eighty years on, French physicists say they have created Einstein's box: a device just 2.7 centimetres big that snares a photon, enabling it to be monitored from birth to death.
They publish their work today in the journal Nature.
Photons are arguably the ultimate existential particle in physics. By switching on a light bulb, you release a million billion of them every second.
But as soon as you see a photon, it dies, as 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," says co-author Professor Jean-Michel Raimond of France's National Centre for Scientific Research (CNRS).
"But you do this only once. It's post-mortem analysis. We, though, can now analyse it in real time, while the photon is still alive."
The box is 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.
A new way to count photons
The conventional way of counting photons is by a light detector that 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 says it has 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.
Quantum 'masterpiece'
In a commentary also published by Nature, Professor Ferdinand Schmidt-Kaler, a quantum physicist at Germany's University of Ulm, describes the achievement as an "experimental masterwork".
He says it has major implications for quantum computing, a field that proponents claim will make one of 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 the atomic level.
Quantum information, or a qubit, can be a 0 or 1 or simultaneously as both 0 and 1, amounting to a 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 area of research.
The experiment demonstrates that "a stream of atomic qubits can be fully controlled by the qubit state of a trapped photon", says Schmidt-Kaler.
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