Erwin Schr?dinger dreamed up the famous thought experiment about a cat that is both dead and alive to demonstrate the absurdity of applying quantum mechanics to ordinary objects. Now two teams have made the closest thing yet to a Schr?dinger's cat in the lab ? by connecting hundreds of millions of photons via the strange quantum property of entanglement.
"It's not the entanglement of something as big as a cat, but it's at least a kitten," says Seth Lloyd of the Massachusetts Institute of Technology, a quantum physicist who was not involved in the work.
The results, which were presented on 23 July at the Second International Conference on Quantum Technologies, in Moscow, Russia, suggest that the rules of quantum mechanics may extend to much larger objects than we thought ? and that this could have practical uses.
We know that quantum properties such as entanglement ? the linking of the states of two objects ? and superposition ? the ability of something to be in two states at once ? describe the behaviour of very small objects, but our experience tells us that these properties don't apply to large ones. Schr?dinger's thought experiment emphasises this discrepancy.
If a cat is in a box with a radioactive atom that can decay to trigger the release of a poison from a flask, then the state of the cat and the state of the atom are entangled: if the radioactive atom decays, the cat dies. But according to quantum mechanics, the atom, a quantum object, can be in a superposition of decayed and not decayed at the same time. And that means that the cat is also both alive and dead ? though in the real world, this seems absurd.
No superposed cats
An apparent lack of cats ? and other large objects ? in such a superposition has led physicists to wonder where exactly the quantum realm ends, and why. "Is there a border between micro and macro, or does quantum mechanics apply on all scales?" asks Alexander Lvovsky, who works at the University of Calgary in Alberta, Canada, and the Russian Quantum Center in Moscow, which organised the conference.
Previous experiments aimed to answer this question this by getting ever bigger objects to display quantum properties. For example, two 3-millimetre diamonds have been entangled, and a drum the size of a grain of sand has been caught obeying the uncertainty principle, which says you cannot simultaneously determine a quantum particle's exact position and momentum.
Lvovsky and colleagues wanted to mimic the Schr?dinger's cat scenario more faithfully. They used a semi-transparent mirror to put a single photon into a mixture of two quantum states ? one corresponding to the photon passing through the mirror, and the other corresponding to reflection. They then entangled the two states.
Next, the team used lasers to amplify one of the states, so that it was spread over hundreds of millions of photons. This beam was big enough to see in principle, though the frequency of the light was not in the visible range.
They then restored the light to its original one-photon state. Measurements confirmed that the entanglement had remained throughout the experiment ? even though one state had been applied to a macroscopic system for a time.
Macro-micro link
The researchers say this represents the first entanglement between a microscopic and macroscopic object. Just as the atom in Schr?dinger's thought experiment is linked with the cat, in Lvovsky's experiment, the state of a single photon is linked to the hundred-million-strong array.
"Our breakthrough has been that, so far, people have been able to build these superposition states containing only a few photons, and we've been able to do it with 160 million photons," says Lvovsky.
Meanwhile, Nicolas Gisin and his colleagues from the University of Geneva in Switzerland got similar results, but with a slightly different experimental set-up.
Lloyd suggests that macro-micro entanglement could be used to dramatically increase the accuracy of interferometers ? devices that use entanglement to measure very small differences in length.
Both teams say that they are still far away from reproducing the experiment with a real cat. As Lvovsky points out, that would be "inhumane" anyway.
Journal references: Lvovsky: Nature Physics, DOI: 10.1038/nphys2682; Gisin: Nature Physics, DOI: 10.1038/nphys2681
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