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Spying on a Photon Without Harming It

16 July 1999 5:00 pm
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In this week's Nature, a team of physicists reports a groundbreaking quantum manipulation experiment: They have managed to detect a single photon repeatedly without destroying it. The experiment is a unique demonstration of a phenomenon known as quantum nondemolition.

"I think it's marvelous," says Wojciech Zurek, a quantum measurement guru at the Los Alamos National Laboratory in New Mexico. "They have implemented one of the goals, one of the mileposts, which has defined the field of quantum measurement for close to 20 years."

It's a fact of life in quantum mechanics that observing or measuring an object alters or destroys it. But over the past decade several teams have managed to flout this rule and observe a quantum system without affecting it--although never with anything as fragile as a single photon. Now a team at the Ecole Normale Supérieure in Paris has used a clever trick to observe a single photon, and then observed it again, confirming that they had achieved quantum nondemolition.

"The basic idea is that we can trap a single photon in a box ... and monitor and make repeated measurements on it," says lead researcher Serge Haroche. The "box" was an open-sided cavity 3 centimeters long and 5 centimeters in diameter bounded at either end with spherical niobium mirrors, which reflect photons of the correct microwave wavelength. To detect the photon, the researchers shot a rubidium atom through the cavity. The atom had been pumped up with energy first, so that its outermost electrons were boosted into orbits far from the nucleus. This guaranteed the strongest possible interaction with any microwave photons lurking in the cavity, enabling the atom to absorb photons and then emit them before leaving the cavity.

At first sight, the exiting atom appeared unchanged from when it entered. But the cycle of absorption and emission did leave an imprint on the atom wave by altering its phase. A separate system compared the phases before and after, revealing a half-wave phase shift--the signature of an encounter with a photon. Sending a second atom through the cavity produced the same result. "The first atom has made a measurement and left the photon behind for the second atom to read it," concludes Haroche.

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