Serge Haroche and David Wineland share this year’s Nobel Prize in Physics

Quantum mechanics shows up in small enclaves within the classical macroworld. Observing nonlocality, entanglement, and other weird quantum effects in action entails not only keeping a quantum system isolated for long enough to make a measurement, but also probing it so delicately that the measurement itself leaves the system’s quantum nature intact. For their respective successes in pulling off that difficult feat, Serge Haroche of the Collège de France in Paris and David Wineland of NIST’s campus in Boulder, Colorado, have each been awarded half of this year’s Nobel Prize in Physics.

Haroche’s and Wineland’s contributions build on techniques pioneered in the 1970s and 1980s for cooling and trapping single atoms and ions. Although both of their respective experimental approaches rely on the interaction between light and matter, they’re different and complementary: Haroche uses atoms to probe the quantum state of trapped photons; Wineland uses light to probe the quantum state of trapped ions.

The complementariness extends to the significance of the Nobelists’ work. Haroche and his group have investigated some of the quantum world’s most puzzling features, including quantum jumps and Schrödinger cat states. Wineland and his group have exploited their quantum control of ions to build increasingly sophisticated quantum logic gates and increasingly precise atomic clocks.

Between two mirrors

Haroche and his principal collaborators, Michel Brune and Jean-Michel Raimond, devised their basic approach in 1990. Then, as now, their quantum system consists of one or more microwave photons bouncing back and forth between two spherical mirrors. To probe the system, they send single rubidium atoms through the gap between the mirrors.

In their ground state or lowest excited states, Rb atoms would fly through the cavity unperturbed by the microwave photons. To make the atoms more sensitive to the photons’ electromagnetic field, Haroche and his team first excite them into a so-called Rydberg state, in which the single outer electron is pushed out so far from the nucleus that a modest EM field suffices to kick it up to the next state.

Using a laser, Haroche and his team put the atoms into an equal superposition of two Rydberg states. In passing through the cavity, the atoms don’t acquire enough energy from the photon field to abandon their superposition and jump up to the higher Rydberg state. They do, however, acquire enough of a phase change to alter the proportion of the two superposed states—provided a photon is present.

By zapping the atoms with a carefully tuned laser pulse as they emerge from the cavity, Haroche and his team can determine whether or not the atoms encountered a microwave photon. In a 1999 experiment , the researchers demonstrated they could sense the presence of a microwave photon without demolishing the delicate quantum state that occupies the cavity. A followup experiment , conducted in 2007 with an improved pair of mirrors, revealed occasional quantum jumps between the cavity’s zero- and one-photon states. More recently, Haroche and his team created what amounted to a movie of the cavity as its state jumped between states of definite numbers of photons (so-called Fock states) and arbitrary superpositions (‘Schrödinger cat’ states).

Ions in traps

In the early 1980s physicists succeeded in trapping single ions by using a laser-based technique called Doppler cooling. Although the first trapped atoms and ions lacked the energy to escape the static and oscillating electric fields that held them, they still had enough vibrational energy to bounce vigorously back and forth.

Probing and manipulating the ions’ quantum states requires cooling them further, to the point that they occupy the trap’s lowest vibrational states. To reach that goal, Wineland’s team devised a technique called side-band tuning that makes use of the coupling between the ions’ internal electronic states and the trap’s vibrational states.

An ion that resides in, say, the trap’s electronic ground state and its second, ν = 2 vibrational state can be excited by a narrow-band laser to the trap’s first excited electronic state and its first, ν = 1 vibrational state. When the ion de-excites by emitting a photon, it prefers to land in the same vibrational state—which is lower in energy than its original vibrational state. In that way, photons emitted during de-excitation carry away vibrational energy. After successive doses of side-band tuning, ions settle into their vibrational ground state.

The trick of manipulating ions through their electronic and vibrational states can be extended to pairs or larger groups of ions. In 1995 Ignacio Cirac and Peter Zoller made a theoretical proposal for building quantum logic gates out of trapped ions: The ions’ electronic states would constitute the qubits, while the ions’ coupled vibrational states would provide the entanglement.

A key element of Cirac and Zoller’s scheme is the ability to transfer superpositions (that is, the weighting factors of each of the superposition’s components) between ions. The transfer works because ions in the same trap share the same vibrational states and because some transitions that have the same energy as others are off-limits, thanks to the nonexistence of the ν = −1 state.

Wineland and his team have used that technique not only to implement quantum logic gates along the lines that Cirac and Zoller envisioned, but also to create an ultraprecise atomic clock . Building the clock entailed coupling an aluminum ion to a beryllium ion. The Al ion provides a superbly narrow transition for timekeeping, while the Be ion provides what the Al ion lacks: a convenient, laser-accessible transition. Thanks to their mutual coupling, the two ions can be cooled together, thereby narrowing the clock transition still further. At one part in 10¹⁷, the precision of Wineland’s clock surpasses that of cesium clocks by two orders of magnitude.

Articles by Haroche and Wineland in Physics Today

• J. C. Bergquist, S. R. Jefferts, D. J. Wineland, ‘Time measurement in the millennium,’ Physics Today, March 2001, p. 37.
• S. Haroche, ‘Entanglement, decoherence and the quantum/classical boundary,’ Physics Today, July 1998, p. 36.
• S. Haroche, J.-M. Raimond, ‘Quantum computing: dream or nightmare?,’ Physics Today, August 1996, p. 51.
• S. Haroche, D. Kleppner, ‘Cavity quantum electrodynamics,’ Physics Today, January 1989, p. 24.
• D. J. Wineland, W. M. Itano, ‘Laser cooling,’ Physics Today, June 1987, p. 34.
• H. Hellwig, K. M. Evenson, D. J. Wineland, ‘Time, frequency, and physical measurement,’ Physics Today, December 1978, p. 23.