"We never experiment with just one electron or atom or (small) molecule. In thought-experiments we sometimes assume that we do; this invariably entails ridiculous consequences", wrote1 Erwin Schrödinger in 1952.

And yet, sixty years later, the Nobel Prize in Physics has been awarded for experimental developments that have made the measurement and control of individual atoms and photons possible. Far from being thought ridiculous, precision experiments using a single photon or a single atom are the basis of today's time and frequency standards – and of tomorrow's quantum technologies.

The precise control of individual quantum systems has been built on several other developments already recognized by the Nobel committee: trapping of ions in electromagnetic fields (Dehmelt and Paul in 1989); laser cooling of atoms (Chu, Cohen-Tannoudji and Phillips in 1997); the theory of optical coherence (Glauber in 2005); and other theoretical and experimental breakthroughs in the field of atomic physics. The body of research honoured by this year’s prize includes techniques used in two quite different, but conceptually similar, physical systems: ions in a harmonic trap and photons in a cavity. Charged particles oscillating in an electromagnetic field and chargeless light quanta bouncing back and forth between mirrors can be modelled as two-level systems coupled to a quantized harmonic oscillator – a textbook problem described by the well known Jaynes–Cummings Hamiltonian. This elegant model is the starting point for the investigation of light–matter interactions at the quantum level.

Trapped ions interact with laser light, and photons in a cavity interact with excited atoms — atoms probed by light and light probed by atoms — in the experimental techniques developed by Wineland and Haroche in the late 1990s and early 2000s to achieve fine control of quantum systems. Wineland and his group at NIST, Colorado, pioneered ion-trapping and laser-cooling techniques and developed ways of controlling the internal energy states and motional states of the atoms using laser light; hence, they were able to create complex quantum superpositions and entangled states. Serge Haroche at École Normale Supérieure in Paris devised methods of probing and controlling the quantum states of photons in a cavity through their interaction with atoms passing though the cavity.

As for the "ridiculous consequences" of playing around with single atoms; Haroche, Wineland and their colleagues have been able to perform 'Schrödinger's cat' experiments with photons and ions, creating superpositions of two states (ion-motional states or photon-number states) corresponding to the 'dead' and 'alive' feline. These experiments have proved influential in both understanding quantum mechanics and in developing new technologies that exploit the surprising features of the quantum world.

 FURTHER READING

The Nobel Prize in Physics 2012

 From Nature Physics:

Realization of a programmable two-qubit quantum processor.  D. Hanneke, J. P. Homei, J. D. Jost, J. M. Amini, D. Leibfried & D. J. Wineland. Nature Phys. 6, 13–16 (2010). doi:10.1038/nphys1453 FREE

Stylus ion trap for enhanced access and sensing.  Robert Maiwald, Dietrich Leibfried, Joe Britton, James C. Bergquist, Gerd Leuchs & David J. Wineland. Nature Phys. 5, 551–554 (2009). doi:10.1038/nphys1311 FREE

Quantum simulations with trapped ions.  R. Blatt & C. F. Roos Nature Phys. 8, 277–284 (2012). doi:10.1038/nphys2252 FREE

 From Nature:

Real-time quantum feedback prepares and stabilizes photon number states.  Sayrin, C. et al. Nature 477, 73–77 (2011). doi:10.1038/nature10376

Reconstruction of non-classical cavity field states with snapshots of their decoherence.  Deléglise, S. et al. Nature 455, 510–514 (2008). doi:10.1038/nature07288

Entangled states of trapped atomic atoms.  Blatt, R. & Wineland, D. Nature 453, 1008–1015(2008). doi:10.1038/nature07125

Progressive field-state collapse and quantum non-demolition photon counting.  Guerlin, C. et al. Nature 448, 889–893 (2007). doi:10.1038/nature06057

Quantum jumps of light recording the birth and death of a photon in a cavity.  Gleyzes, S. et al. Nature 446, 297–300 (2007). doi:10.1038/nature05589

Realization of the Cirac–Zoller controlled-NOT quantum gate.  Schmidt-Kaler, F. et al. Nature 422, 408–411 (2003). doi:10.1038/nature01494

Decoherence of quantum superpositions through coupling to engineered reservoirs.  Myatt, C. J. et al. Nature 403, 269–273 (2000). doi:10.1038/35002001

Seeing a single photon without destroying it.  Nogues, G. et al. Nature 400, 239–242 (1999). doi:10.1038/22275