Galileo's Pendulum

The 2012 Nobel Prize in physics was announced this morning; the awards went to Serge Haroche (Collège de France and Ecole Normale Supérieure) and David J. Wineland (National Institute of Standards and Technology (NIST) and University of Colorado Boulder). The researchers work in the realm of quantum measurement and optics, studying the limits of manipulating and measuring the quantum states of ions (in the case of Wineland) and photons (for Haroche). I’m sure the web will be full of other explanations, but here I go….

Quantum measurement in boxers brief

In quantum physics, particles of a given type are identical: every electron is indistinguishable from every other, and so forth. Similarly, every hydrogen atom, which consists of one proton and one electron, is identical to every other, in terms of their fundamental properties. On the other hand, the quantum state of an electron or atom or whatever need not be identical. The state encompasses properties like energy level, spin orientation, and in more complex atoms, fun stuff like nuclear configuration and “filling” of available electron states. Since this isn’t a full explanation of quantum physics, the details aren’t important at the moment; just know that there are various ways particles and atoms can be configured, and that’s what we mean by the quantum state.

A major challenge is measuring the state of a quantum system without modifying it. On the macroscopic scale, we can generally measure mass, size, and the like without worrying about destroying anything, but quantum mechanics is more like medicine. The most reliable way to determine if something is wrong with a person is to cut right in, hack things apart, and extract the bits that are causing problems—but for obvious reasons, that’s a bad idea under most circumstances if you want the patient to live. Just as the treatments that kill cancer cells often can kill healthy cells as well, measurement of a quantum state can alter or even destroy the system under study.

For photons, measurement is often impossible, since unlike matter particles, they can be created and destroyed easily. Usually we say absorbed and emitted instead, but it amounts to the same thing: first there was a photon, then there was no photon, then there is. No conservations laws are violated, since the energy to make a photon comes from something else (transitions between energy levels in an atom, for example). In addition, it’s trickier to trap photons than you might think: they don’t have electric charge and they (obviously) move very fast. The best way is the obvious one: put the photons in boxes with mirrors on both ends. However, unless the mirrors are very specially designed, they will still absorb some of the photons or scatter them around, altering their quantum states.

To make matters worse, quantum systems don’t exist in isolation: even without us taking invasive measurements, they interact with atoms, photons, and other particles in the environment, which also can affect the quantum state. Those interaction are known as decoherence, which is the bugbear of quantum computing. (Quantum computing is the study of using quantum states as bits, which can process complex information far faster than their conventional digital cousins. However, quantum computing is still in its infancy, with no true computer existing yet.) So even if we can measure a system non-destructively in a particular state, decoherence means it may not stay in that state for very long.

It’s a trap!

For his Nobel Prize-winning contribution, Wineland (and his collaborators) used electromagnetic fields to confine very cold ions—atoms with a missing electron, so they are positively charged—with lasers to slow them down until they could be trapped. Other researchers who helped develop laser cooling, as it is called, were awarded the 1997 Nobel Prize in physics. (One of these laureates is Steven Chu, currently Secretary of Energy in the United States, and one of a handful Nobel winners I’ve actually met.) The advantage of such cooling and trapping is it helps isolate them from their environment, limiting decoherence. Wineland and colleagues not only used the laser to help trap the ions, they used it to manipulate and measure the ions’ quantum state.

A few details are in order. The Wineland-style ion trap uses oscillating electric fields to make a wavelike pattern. The ion sits at the intersection of the fields, but moves up and down as they vary in time, so it exhibits harmonic motion, like a pendulum swinging back and forth. Like the ammonia molecule I described in an earlier post, however, the oscillations obey the rules of quantum physics, so they themselves constitute a quantum state. The internal state of the ion (electronic energy levels) and the external quantum state created by the trap can be connected using the control laser—providing the means to measure all aspects of the quantum system in detail in a non-destructive way.

Haroche and colleagues performed research that is almost complementary, trapping photons. They used microwave photons, which are much lower energy than visible light, and as with the ion traps, cooled everything nearly to absolute zero to minimize thermal problems. Additionally, the cavity was bounded by superconducting mirrors, which (among other things) exclude magnetic fields—meaning they turn back the photons perfectly, since photons are made of oscillating electric and magnetic fields.

To perform non-destructive measurements on the photons, Haroche and collaborators sent Rydberg atoms through the chamber. In Rydberg atoms, the electrons are pumped into an excited state, so they are barely connected to their nuclei. Rydberg atoms are pretty huge by atomic standards (about 125 nanometers across, as opposed to ordinary atoms which are in the range of 0.01 nanometers across). These Rydberg atoms are sent across the cavity one at a time, where they interact with the microwaves without absorbing them, like a resonator. By comparing the atoms’ states before and after passing through the photon trap, the researchers are able to determine the quantum states of the photons in a non-destructive way.

Both the ion and photon traps are important for quantum information applications: being able to read the state of an atom, ion, or photon without altering it is essential for basic computing tasks like reading, writing, and mathematical operations. I hope I’ve been able to convey some of why this work is very important, and worthy of wider recognition.

References

• The Nobel website has a lot of good information for non-scientists and for those with a bit more technical background.
• Chad Orzel (himself involved with quantum optics research) has a good blog post on the Nobel research, with links to even more about Haroche’s work.
• The Institute of Physics (IoP) has a very good set of technical research papers from both laureates, available for free download. Be warned, though: most of these are not aimed at non-physicists, and even physicists who aren’t trained in quantum optics might find them hard going (I certainly do).