Scientific Method / Science & Exploration

Physics Nobels go to cat herders (Schrödinger’s cat, that is)

Nobels reward "measuring and manipulating single quantum systems."

Serge Haroche (let) and David Wineland.

This year's Nobel Prize in Physics goes to a pair of physicists that pioneered experimental approaches to the quantum world. The two, University of Colorado-Boulder's David Wineland and Serge Haroche from the Collège de France, used coupled systems of single atoms and photons to provide ways of monitoring a quantum object without destroying its quantum nature. Like any good quantum experiment, there are multiple ways of looking at what they've done, but the Prize announcement highlights the fact that their work is already having practical applications, as it's being put to use in ultra-high-precision atomic clocks and attempts to build quantum computers.

Classical physics, which governs the everyday world of the objects around us, is deterministic; an object is either in a given state, or it's not. But quantum systems are probabilistic, with their objects only having certain odds of being in different states. And until that state is determined, the objects actually behave as if they are in all possible states. That's why a single photon can behave as if it travels through two paths at once, creating an interference pattern with itself.

This is most famously thought of in terms of Schrödinger's cat, in which the cat's life is dependent upon the state of a quantum system. As long as the cat is in the box, we can't know whether it's alive or dead. From a quantum perspective, the cat is in a superposition of both alive and dead. The cat also demonstrates other aspects of the quantum system. For example, by measuring its state—opening the box and looking at the cat—we typically destroy its quantum nature. And it shows that there's some poorly defined boundary between the quantum and classical worlds. No matter how we set up the experiment, the cat will always be classical, either alive or dead.

But for decades after Schrödinger first described it, cat-like states were purely hypothetical. We actually couldn't create and measure a well-defined quantum system. Wineland and Haroche are being honored for developing the systems that changed that.

Both of them worked with a kind of trap that held an individual object. In Wineland's case, his work took advantage of the development of ion traps, which held individual ions at temperatures near absolute zero, where they could be probed with lasers. Haroche did things the opposite way around, creating a trap for a single photon using highly reflective mirrors that are spaced at distances shorter than the photon's wavelength. To probe the state of the photon, researchers can slide specially prepared atoms (called Rydberg atoms) through the trap, which change state based on the presence or absence of the photon.

(The Nobel announcement highlights an impressive fact about these optical traps. Even though the mirrors are only 2.7cm apart, a typical photon will manage to travel about 40,000km between them before being lost).

The resulting systems have allowed the researchers to study quantum entanglement, observe the system as measurements destroy its quantum state, and study how its interactions with its environment gradually cause the quantum system to return to a classical state.

As is often the case, these systems weren't developed in isolation. Ion traps had been around for roughly a decade before Wineland figured out how to set the ions in one into the lowest energy ground state. In another case, other researchers proposed a theoretical method for transferring quantum states within an ion trap; Wineland's group published their implementation of it later that same year. Haroche's group did much the same thing the year afterwards, when his own group first proposed, then generated a system for creating and observing the decay of a cat-like system—all in under a year.

Neither group has sat still since these accomplishments. Haroche has helped develop systems where equipment can provide real-time feedback to a quantum state, delaying its decay into a classical system. Depending on the route eventually taken to quantum computing, this sort of system may be essential for stabilizing the qubits. Meanwhile, Wineland has been using his systems to develop atomic clocks that enable staggeringly precise time measurements. "The accuracy recently achieved by the optical clocks has allowed Wineland and coworkers to measure relativistic effects," the Nobel's press material states "such as time dilation at speeds of a few kilometers per hour or the difference in gravitational potential between two points with a height difference of only about 30cm."

Perhaps the greatest tribute to the work of the researchers, however, is how commonplace this has all become. Skimming through the latest papers in physics and optics shows that the study of single quantum objects goes on in hundreds of labs around the globe now. It makes it hard to imagine that, only a few decades before, it was an important breakthrough.

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