Published online 27 May 1999 | Nature | doi:10.1038/news990527-4

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More ways to skin Schrödinger's cat

Hearing of Erwin Schrödinger’s famous thought-experiment to illustrate the weirdness of quantum mechanics, an impassioned reader of the UK newspaper the Guardian was once moved to write “[D]isregarding the metaphysical aspects of Schrödinger’s cats, I must protest at the use of (possibly live) animals for experiments such as these. I urge readers to boycott whatever product this research is leading to.”

Cat-lovers can now relax: there is no need to put a cat in the uncomfortable superposition of live and dead states in order to demonstrate this particular quirk of the quantum world. Physicists S. Bose and colleagues, from Imperial College in London, UK, have devised a means of realising Schrödinger’s experiment without a moggy in sight. Instead, it can all be done with mirrors.

Schrödinger’s intention was to illustrate a paradox that seems to arise when we try to reconcile the rules of the quantum world with those of the everyday, ‘macroscopic’ world. Quantum mechanics allows a particle to be in two different states at once. For example, whereas a coin must always fall as heads or tails, the quantum equivalent - an atom, say, that can exist in one of two quantum states - can also exist as a ‘superposition’ of the two states, a bit of one and a bit of the other. In the macroscopic world, a coin that can lie half heads and half tails is nonsensical; in the quantum world, such superpositions are commonplace.

But, said Schrödinger, what if a macroscopic event were to be controlled by a quantum event? That’s to say, supposing we rigged up a system in which a particle in one state would trigger a device that kills a cat, whereas a particle in the other state leaves the device inactive. What, then, does a particle in a superposition of the two states do? Does it leave the cat in a superposition of live and dead states?

Schrödinger’s experiment is paradoxical only because of our preconceptions, shaped by the macroscopic world we live in. Says physicist Wojciech Zurek, “The only ‘failure’ of quantum theory is its inability to provide a natural framework that can accommodate our prejudices about the workings of the universe”.

In the case of Schrödinger’s cat, the solution to the puzzle has long been known. To make the fate of a cat truly dependent on the state of a quantum particle, the particle and the cat-killing macroscopic device it controls have to remain acting in concert: they have to be ‘coherent’. But this is possible only if both items are entirely isolated from the world outside. In practice, the device always sits in some kind of environment with which it interacts by, for example, exchanging heat. As a result, the quantum coherence between particle and device leaks out into the environment - there is ‘decoherence’, which destroys the precise relationship between the state of the particle and the behaviour of the device. The cat may or may not be saved - but it is never a superposition of live and dead cats.

But how does decoherence take place, and in particular, how rapidly? This is a critical question, not least for our hapless cat. Experimenters have previously been able to study decoherence in systems of interacting quantum particles, but have not yet managed to look at how it takes place in a macroscopic object that is controlled by a quantum event - the case of Schrödinger’s cat. In the May issue of the journal Physical Review A, Bose and colleagues describe a way of doing so.

Schrödinger’s quantum particle was a radioactive atom, whose decay was determined by quantum-mechanical laws. But Bose and colleagues suggest instead using an electromagnetic field - basically, a light field - bouncing around in a cavity between two mirrors, one fixed and the other attached to a spring. The light field is a ‘quantum object’ - it can be prepared in a ‘pure’ quantum state, or in a superposition of states. The movable mirror is the ‘cat’ - it is a macroscopic object that can vibrate back and forth in response to the light field.

What are the mirror’s ‘live’ and ‘dead’ states? Well, it doesn’t really have these pure states - it is just vibrating messily owing to thermal motions. But, say the physicists, this doesn’t matter. For the purposes of watching the decoherence, all that’s important is that the quantum system is coupled to the macroscopic system. In other words, we don’t have to ‘kill the cat’ to see decoherence - we just have to let the ‘cat’ be affected by the quantum event. So the mirror simply has to be small enough to be affected by a change in the light field bouncing off it.

That’s the catch. A spring-bound mirror that changes its state of vibration in response to the change in precise quantum state of the light field (which, say the researchers, might be induced by firing a single atom through the cavity) has to be very sensitive indeed - either extremely small and light, or extremely reflective. The kind of instrumentation available at present just isn’t up to the challenge. But the researchers argue that only moderately optimistic technical advances are needed to make the scheme possible. “The experimental challenge”, they say, “is in either of the two directions: to improve the reflectivity of existing macroscopic mirrors or to decrease the mass of the mirrors without decreasing the existing reflectivity… We do not see any real obstacle in progress directed at the possible realization of our proposal”. And the cat would be thankful for that.