Quantum optics strikes again

Here’s a piece I wrote for the Prospect blog on the physics Nobel. For my Prospect article on the renaissance of interest in the foundations of quantum theory, see here.

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There’s never been a better time to be a quantum physicist. The foundations of quantum theory were laid about a hundred years ago, but the subject is currently enjoying a renaissance as modern experimental techniques make it possible to probe fundamental questions that were left hanging by the subject’s originators, such as Albert Einstein, Niels Bohr, Erwin Schrödinger and Werner Heisenberg. We are now not only getting to grapple with the alleged weirdness of the quantum world, but also putting its paradoxical principles to practical use.

This is reflected in the fact that three physics Nobel prizes have been awarded since 1997 in the field of quantum optics, the most recent going this year to Serge Haroche of the Collège de France in Paris and David Wineland of the National Institute of Standards and Technology in Boulder, Colorado. It’s ‘quantum’ because the work of these two scientists is concerned with examining the way atoms and other small particles are governed by quantum rules. And it’s ‘optics’ because they use light to do it. Indeed, light is itself described by quantum physics, being composed (as Einstein’s Nobel-winning work of 1905 showed) of packets of energy called photons. The word ‘quantum’ was coined by Max Planck in 1900 to describe this discrete ‘graininess’ of the world at the scale of atoms.

The basic principle of a quantum particle is that its energy is constrained to certain discrete amounts, rather than being changeable gradually. Whereas a bicycle wheel can spin at any speed (faster speeds corresponding to more energy), a quantum wheel may rotate only at several distinct speeds. And it may jump between them only if supplied with the right amount of energy. Atoms make these ‘quantum jumps’ between energy states when they absorb photons with the right energy – this in turn being determined by the photon’s wavelength (light of different colours has different wavelengths).

Scientists since Planck’s time have been using light to study these quantum states of atoms. The trouble is that this entails changing the state in order to observe it. Haroche and Wineland have pioneered methods of probing quantum states without destroying them. That’s important not just to examine the fundamentals of quantum theory but for some applications of quantum behaviour, such as high-precision atomic clocks (central to GPS systems) and superfast quantum computers.

Wineland uses ‘atom traps’ to capture individual electrically charged atoms (ions) in electric fields. One counter-intuitive conclusion of quantum theory is that atoms can exist in two or more different quantum states simultaneously, called superpositions. These are generally very delicate, and destroyed when we try to look at them. But Wineland had mastered ways to probe superpositions of trapped ions with laser light without unravelling them. Haroche does the opposite: he traps individual photons of light between two mirrors, and fires atoms through the trap that detect the photon’s quantum state without disturbing it.

‘Reading out’ quantum states non-destructively is a trick needed in quantum computers, in which information is encoded in quantum superpositions so that many different states can be examined at once – a property that would allow some problems to be solved extremely fast. Such a ‘quantum information technology’ is steadily becoming reality, and it is doubtless this combination of fundamental insight and practical application that has made quantum optics so popular with Stockholm. Quantum physics might still seem other-worldly, but we’ll all be making ever more use of it.