Dr Charles Bennett

Doctor of Science

Friday 9 July 2010 - Orator: Professor Sandu Popescu

Madam Pro Vice-Chancellor,

Studying demons is not generally thought to be one of the ways to make a brilliant career in physics. Yet it is precisely such a study that is one of the main reasons we honour Dr Charles Bennett today. Studying demons is not supposed to be what a physicist does, unless, that is, if you are interested in the particular demon discovered by the famous nineteenth century physicist James Clerk Maxwell. It is a demon that wreaks havoc on the laws of physics as we know them. But don’t worry; the demon will do us no harm. Please don’t look at Dr Bennett and imagine him, or a younger version of him, as a warrior who slaughtered the demon. He did nothing like that. The demon was in fact harmless from the very beginning. But for over a century, nobody knew why Dr Bennett discovered its secret.

Maxwell’s Demon is in fact not nasty at all. It’s not even a demon; it is only called so because it is the most powerful machine ever imagined: it’s the demon which makes order out of disorder. But as any parent of a teenager will know, putting order in their room is not an easy task. In fact there is always a price to pay. At the end of the day by ordering a part of the world, the world as a whole always becomes more disordered. And no, I’m not just thinking of the cupboard you shouldn’t open. Reorganising the room is hard work and we need to eat to keep us going. We break down the molecules of food, and the beautiful way in which their atoms were organised is now lost. Order thus lost at the molecular level far outweighs the order gained in the room. Everything always gets more disorganised.

That disorder is ever increasing in the universe is one of the basic laws of nature. Arguably, it is more basic and all encompassing than the laws discovered by Newton, and Einstein, and those of quantum mechanics. Newton’s laws were found to be only approximate and corrected by Einstein. Einstein’s laws had to be modified in the light of quantum mechanics. Yet, the second law of thermodynamics, as the increasing-disorder law is called, remains unchallenged. Except, that is, by Maxwell’s Demon.

If not for this law, we would live in an unusual and wonderful world.

You are all aware that we consume energy and we increasingly need more and more of it. Yet, paradoxically, as many of you might have heard at school, energy is always conserved. It is never created or consumed. So what is going on? Indeed energy is never lost, but it gets degraded. When I drop a ball, and it eventually stops bouncing and rests on the floor, energy is not lost. But rather than all the molecules in the ball moving in an organised way, up and down, now the molecules in the ball and in the floor wiggle around in a disordered manner. The energy is there, but disordered. But if the energy is there, why can’t all the molecules in the floor just wiggle together in a co-ordinated fashion and kick the ball up once again?

Pushing molecules in the right direction is exactly Maxwell’s Demon’s task. Maxwell’s Demon was supposed to work at the molecular level and restore order. It should look at each molecule and put it in its right place. It all seemed doable but every specific attempt to design it failed for one reason or another.

Maxwell’s Demon simply doesn’t work. Dr Charles Bennett understood why. For over a century people had tried to understand why the demon can’t put the molecules in their place, and they all found that it should be able to do so. What Dr Bennett discovered is that it’s not putting the molecules in their place that the demon cannot do, but that finding out where the molecules came from in the first place fills up the demon’s memory. This is the resource that gets used; when its memory becomes full, it stops working: resetting the memory by erasing the information stored there produces even more disorder elsewhere.

And it is precisely this, the interaction of physics and information that constitutes the core of Dr Bennett’s work: the physics of information.

When we think of mathematical statements, such as 1+1 = 2, or Pythagoras’ theorem, we think of them as being abstract logical constructions. Yet, every time an actual computation takes place there is an underlying physical process going on; a bead is moved on a string in an abacus, an electrochemical process takes place in our brain, current flows in a computer. So what, one could ask, are the essential properties of these physical processes required to do the computation? For example, do we need a given amount of energy per step of computation? Is there a minimum amount of heat that will be dissipated? And so on.

Dr Bennett was one of a handful of researchers who, in the early seventies, started to raise these questions. Building on earlier ideas of Landauer, he invented reversible computation, and proved that although present day computers use substantial amounts of energy, in principle computation doesn’t require any energy at all.

By the early eighties, Dr Bennett had already accomplished enough to account for a very distinguished career. But then he took the major step that changed the course of his research. He embraced quantum mechanics. That Bristol University has a building dedicated to Quantum Information, and that all the major universities in the world have groups working in this area, is, to a large extent, due to his work. To start with, he, together with Gilles Brassard, building on ideas of Stephen Wiesner, invented Quantum Cryptography. Quantum Cryptography is a method of sending secret messages between two remote parties, in a way in which absolute secrecy is guaranteed by the laws of nature. This is in contrast to all previous methods which rely on unproven mathematical assumptions, as, for example, in present day internet communication protocols. Not only this, but together with a student, he built the first quantum cryptographic device. It was the first time in the history of mankind that absolute secrecy was achieved… as long as one didn’t listen too carefully! Indeed, the primitive electronics used at that time made a characteristic noise from which one could immediately decipher the message.

In 1994, Dr Bennett shocked the world again. He co-invented what is arguably the most important procedure in what was to become Quantum Information. He discovered a method of communication in which part of the information just jumps, instantaneously, from the transmitter to the receiver. Having co-invented this procedure would have been enough. But Dr Bennett also gave it a name. He named it teleportation. As a matter of fact, naming it so was not an easy task. One of his colleagues, a linguistic purist, noted that teleportation combines the Greek word tele with the Latin portation, and suggested the pure Greek telephoresis. It is safe to assume that the impact on the general public of telephoresis was bound to be far less than that of teleportation – no more “Beam me up Scotty”! But Dr Bennett stood his ground, and history was made.

Dr Bennett received his PhD at Harvard, and worked for most of his career at IBM’s Watson Research Laboratory in New York, where he still is. I don’t know how much he contributed to the bottom line of IBM, but he increased their intellectual clout enormously. In recognition of his results, he was elected a member of the National Academy of Sciences, awarded the 2008 Harvey Prize and the 2006 Rank Prize in opto-electronics.

Dr Bennett started as a chemist, worked as a computer scientist, and is a top physicist. And last but not least, he is an excellent linguist. Most of us have used sentences with double meanings. Some of us may have even accomplished sentences with more. But Dr Bennett has an unsurpassable record; a sentence with an infinite number of meanings.

Madam Pro Vice-Chancellor, I present to you Charles H Bennett, as eminently worthy of the degree of Doctor of Science, honoris causa.