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How Schrödinger could have his cat and see it too


Physicists in the US have devised a type of measurement interaction with an atom that could lead to new ways of controlling quantum systems. Andrew Masterson reports.


Erwin Schrödinger devised his famous cat thought experiment to demonstrate the bizarre implications of quantum theory.
MEHAU KULYK/SCIENCE PHOTO LIBRARY

Schrödinger’s cat need not die! In a small but significant way, quantum weirdness just got even weirder – and, as a result, quantum computing may have just moved one step nearer.

A team led by Kater Murch from Washington University in St Louis, US, has found a way to delay – in theory, indefinitely – radioactive atomic decay: the mechanism by which Erwin Schrödinger’s famous pet either survived or perished when observed.

To recap: Schrödinger posited a cat in a box with something potentially lethal and a radioactive atom. Quantum theory holds that as long as the system isn’t measured – observed, in other words – the atom remains in a superposition in which it is neither decayed nor complete, but both.

Similarly, the cat is neither dead nor alive, but both. Observing the system causes the quantum wave to collapse, resolving one way or the other. The atom decays or it doesn’t. The cat lives or it dies.

Murch’s team, however, has found a way in which the act of observation can be reconfigured, such that it resets the clock for the atom – starting the system again before it resolves. The method is not without risk: it is equally likely to accelerate the process, killing the cat more quickly than usual.

The research rests on something known as the Zeno effect. This arises when a quantum system is observed not once – which results in a 50% feline fatality rate – but many thousands of times every second.

It is named after an ancient Greek philosopher, whose own thought experiment modelled something broadly analogous. If you observe an arrow flying through the air, reasoned Zeno, at any given moment it is stationary. How then can it be simultaneously still and moving?

The same principle applies to atoms in a quantum superposition. If the first observation finds it in its non-decayed state, then repeated, ultra-fast observations will always find it in that state, potentially ad infinitum.

Of course, the reverse is equally possible: if the first observation finds the atom in its decayed state, so will every subsequent one. This is known, with pleasing binary logic, as the anti-Zeno effect.

There is, however, one important caveat to be applied. The Zeno effect only works as long as the information gained through the act of observation does not leave the system and flow into the outside world. If that happens, the quantum wave will still collapse, and the cat has a good chance of being permanently off its food.

Murch and his colleagues set out to do an end-run around this problem by building a system that took “quasi-measurements”: a way of measuring the electromagenetic state of a target atom every microsecond without allowing the information gained to leak out.

This act of measurement disturbed the quantum state but, critically, not enough to actually collapse it.

The team conducted their experiments using an artificially constructed atom known as a qubit, immersed in a bath of photons. If the bath possessed the same level of energy as the atom’s transition state, then quasi-measurements reduced the prospects of the atom decaying – effectively restarting the decay clock every microsecond.

If the energy level in the photon bath was raised to a higher level than that of the qubit, the reverse occurred and decay was accelerated.

“We observe both the suppression and acceleration of qubit decay as repeated measurements are used to modulate the qubit spectrum,” the team reports in the journal Physical Review Letters.

The results were surprising, Murch and colleagues report. The anti-Zeno effect was predicted ahead of the experiments, but the Zeno effect itself was not expected.

“To be honest, we were not completely sure what we would find,” says Murch.

“But days of data-taking conclusively showed that the quasi-measurements caused the Zeno effects in the same way as the usual measurements.

“This means it is really the disturbance of the measurement and not the collapse of the wave function that leads to these effects.”

Regulating the superpositions of qubits is a critical mechanism for quantum computing. Developing ways to use quasi-measurements as tools to do this may well be critical step along that path.

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Andrew Masterson is news editor of Cosmos.
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