Quantum mechanics involves events at a subatomic level invisible to the naked eye, where things happen contrary to our expectations and experience in the ‘real’ world. One example of this contrary behavior is superposition, where a quantum particle can be in several different states simultaneously.
Most people are familiar with the notion of Schrödinger’s Cat, albeit at a basic level similar to the ‘if a tree falls in a forest…’ conundrum. The cat is in a box, we don’t know if it is alive or dead, therefore it is both.
The theory is a bit more complex than that - Austrian physicist and 1933 Nobel Laureate Erwin Schrödinger was trying to explain the nature of quantum superposition, in which a particle can be in several states at once. The cat in his ‘thought-experiment’ is completely isolated from outside influences in a box with a bottle of cyanide that will be released when a radioactive atom decays. The decay is governed by the laws of quantum mechanics, according to which the radioactive material is in a superposition state of both having decayed and not yet decayed. Therefore the cat must also be in a superposition state of being both dead and alive. Quantum superposition is so sensitive to interaction with the environment that the slightest attempt to observe the cat would immediately ‘collapse’ the ‘cat-state’ to one of the two possible outcomes – dead or alive.
Serge Haroche and David Wineland, working independently but joint winners of the 2012 Nobel Prize for physics, found ways to sneak a peek inside the box although, instead of a cat, they trapped quantum particles and put them in cat-like superposition states.
In their Paris laboratory, Haroche and his research group set out to trap light itself. Microwave photons bounce between two mirrors, about three centimetres apart. The mirrors are cooled to a temperature just above absolute zero and are made of a superconducting material so reflective that a single photon can bounce back and forth inside the cavity for almost a tenth of a second before it is lost or absorbed. In this brief time the photon travels 40,000 kilometres, equivalent to about one trip around the Earth.
During this time, Haroche uses specially prepared Rydberg atoms, which are roughly 1,000 times larger than typical atoms to both control and measure the microwave photon. These atoms are passed through the cavity in a controlled manner, during which the interaction between the photon and the atom creates a change in the phase of quantum state of the atom. This phase shift can be measured when the atom exits the cavity, and thus a single photon can be measured without destroying it.
Serge Haroche was born in Casablanca in September 1944. In 1956, at the end of the French protectorate treaty, he left Morocco for France with his family. He attended the Ecole Normale Supérieure (ENS) where, under the supervision of Nobel laureate Claude Cohen-Tannoudji, he wrote a PhD dissertation on dressed atoms. He then spent a year as a visiting post-doc in Stanford University, California, working with another Nobel laureate, Arthur Schawlow.
During the 1970s , Haroche developed new laser spectroscopy methods based on the study of quantum beats and superradiance. He then studied Rydberg atoms, named after Swedish physicist Johannes Rydberg. These giant atoms are particularly sensitive to microwaves, which makes them well adapted for studying the interactions between light and matter.
He holds the Chair of Quantum Physics at College de France and he carried out his prize-winning research in the laboratoire Kastler Brossel at ENS, working with a team including two former students who have become long term collaborators, Jean-Michel Raimond and Michel Brune. Serge Haroche lives in Paris with his wife Claudine. They have two children.
Serge Haroche is a member of the Société Française de Physique (French Physical Society), the European Physical Society and a fellow and member of the American Physical Society. He is a member of the French Academy of Sciences and a foreign member of the National Academy of Sciences of the USA, the Brazilian Academy of Sciences and the Moroccan Academy of Sciences. He is also a commander of the Legion d’Honneur.
By Volker Steger
I have been waiting for this cat to show up on a sketch of a laureate since the start of the project. Here it is, Schrödinger's famous ambiguously dead and alive cat! Serge Haroche's work has led physics to a better undestanding of quantum states – from his drawing we can learn that even dead cats can be charming... (if we only knew for sure that it's dead!)
Exploring the Quantum World
by Adam Smith
The cats in Serge Haroche’s drawing are, of course, Erwin Schrödinger's famous felines, introduced in 1935 to illustrate a particularly puzzling aspect of quantum physics. Quantum rules clearly describe how a system can be in two different states at the same time, a situation known as superposition of states. But while this behaviour is observed at microscopic scales, with fundamental particles being, for instance, in two different places or having two different energies at once, such superposition never transfers to the larger scale of the macroscopic world we live in. Schrödinger’s thought experiment was to demonstrate this by linking the fate of a cat to that of a microscopic quantum system in superposition. Theoretically, then, the cat would be both dead and alive at the same moment!
Obviously, cats cannot exhibit superposition. But why? The cat is made up of atoms that individually exhibit this quantum behaviour, so why don’t the quantum rules transfer from the micro to the macro scale? That breakdown in following the rules is called ‘decoherence’, and Haroche set out to study this phenomenon by focusing on the boundary between the quantum and classical worlds.
At the bottom of the sketch is Haroche’s quantum cavity, with mirrored walls, in which he prepares a few photons in a superposition of two different states. The red and blue, dead and alive cats illustrate this superposition, and are also shown above in formal quantum notation, the plus sign between them showing that the two states exist simultaneously. Haroche and his group then examine these photons by sending specially prepared atoms into the cavity as probes, allowing observation of individual quantum states and an exploration of the coupling between the quantum systems and their environment.
“I don’t know who called these states Schrödinger cats in the first place,” Haroche remarks, “But when we tried to publish our first paper on that back about 20 years ago, the editor of the journal did not want to have the words ‘Schrödinger cat’ in the title. So we had to use some paraphrase to go around it; we called it ‘mesoscopic superposition of quantum states’, which is much less appealing for a general audience than ‘Schrödinger cats’. But that’s another story.”