The Myths of Quantum Optics
|One irony here is that the double slit experiment is often used to illustrate the mysteries of quantum theory, when in fact it was performed a century before quantum theory was developed - the mystery is how an something can behave like a wave and at the same time like a particle (as shown by the photoelectric effect for example). Sometimes people forget that both aspects are required to demonstrate the quantum nature of a system, and when I hear about a device which illustrates some weirdness of quantum theory, my first thought is generally 'can this in fact be explained using just wave optics?'|
Every year, we expect our computers to get faster and faster. For the last few decades this expectation has been satisfied via electronic components, but there's only so far that such devices can be pushed. So, naturally, computer makers are always on the lookout for alternative technology which can be used for faster computing. One possibility is optical computation, where instead of electrons being used to control the flow of other electrons, light beams are used to control the flow of other light beams. The high speed of light means that such a device has the promise of operating much faster than electronic devices. However there's a catch, which is that light beams don't normally influence other light beams in this way - normally two light beams would just pass straight through each other. Now there are devices that can make light beams do this, but we need to be sure that we're getting what we think we're getting. But first a bit of history...
Different kinds of optics
The behaviour of light can be studied in several different ways. The simplest is geometrical optics. Here we consider the behaviour of beams of light. Normally they travel in straight lines, but they can be bounced of mirrors and refracted by different types of glass. For many purposes this is all we need to know. However, geometrical optics doesn't look into the underlying nature of light - it worked when it was thought that light was made of corpuscles, but it also works as an approximation for light as an electromagnetic wave. This brings us on to the second way of studying light - wave optics. The wave nature of light was demonstrated by Thomas Young at the start of the nineteenth century, in particular with his well known double slit experiment. Following on from this came Maxwell's equations, which indicated that light was in fact a wave of electromagnetic disturbance. However, in neither geometrical nor wave optics does one light beam influence the passage of another. For that you need quantum optics, which relies on the interaction of light and matter on the quantum scale. Now this is where things can get a bit confusing...
A short history of quantum theory
Quantum theory originated with Max Planck's idea that when a hot object radiated electromagnetic waves, then it would only do so in packets of energy known as quanta. Planck didn't think that everything was necessarily quantized, rather that this was a property of the interaction of electromagnetic waves and matter. Einstein went further than this, to explain the photoelectric effect he proposed that light was made up of tiny particles called photons, but somehow kept its wave nature. In the 1920's this idea of wave/particle duality was applied to matter and so quantum mechanics was created. Note, however, that for matter the particles are the classical things and the waves the mysterious quantum objects, whereas for light its the other way round - Maxwell's equations are not the electromagnetic counterpart of the Schrödinger equation. Hence quantum mechanics did not provide a satisfactory theory of the interaction of light and matter. That required Quantum Field Theory to be developed. Work on QFT was started in the late 1920's but it was plagued by infinities and it took about 20 years for a satisfactory result - Quantum Electrodynamics - to be achieved. This theory makes impressively accurate predictions, but has never really entered the general consciousness as the way that light interacts with matter. Rather we tend to get a muddled version of the ideas of the 1920's. For instance we often hear that when an atom absorbs a photon, sending an electron into a higher orbital, it does so as a 'quantum leap' - that is to say instantaneously, without going through any intermediate states. On the other hand time-dependent quantum theory can be used to calculate the path it takes through those intermediate states which it's not supposed to go through, and the length of time it takes. But that's (nearly) enough of quantum muddles - let's get on to some more interesting stuff.
Quantum optical technology
|What about quantum computers? Well these often use quantum optical effects, but their speedup arises in a different way from that of optical computers. For optical computers it is because of the high speed of light, whereas for quantum computers it is due to the ability of a quantum system to perform many calculations at once. And another difference is that while I think one day we'll have optical processors in our computers, I have to say that I don't think quantum computers will ever work|
The first device which could really be called quantum optical was the laser, invented in 1960. (I would note here that some people came up with arguments on why masers and lasers couldn't possibly work). The laser has lead to many interesting applications, and in particular has led to the demonstration of quantum entanglement and quantum teleportation, that is transferring the quantum state of a photon from one location to another. Squeezed light is another specifically quantum optical phenomenon. So there are plenty of things that demonstrate the power of quantum optics. However, I feel that many devices are made to sound more advanced than they really are, using words such as Photonics, which sounds like it should involve photons - that is quantum optics - at a fundamental level. For instance photonic switches, as used in fibre optical devices. Do we need quantum theory to understand these? Well no, typically they are small, moveable mirrors - impressive technology, but wholly understandable using geometrical optics. What about photonic crystals, which manipulate photons in the same way that a semiconductor manipulates electrons - surely they count as true quantum optical devices. Again, these are impressive objects, but fully understandable in terms of Maxwell's equations - that is wave optics. What is needed to build an optical computer is a device in which light behaves in a non-linear way. Such devices do exist and are used in fibre optical networks, but so far not to build a commercial computer. So in conclusion: we'll get optical computers one day, but until then don't be fooled by fancy words