Two research teams have independently added an extra layer of quantum oddity – the property of entanglement – to a test of wave-particle duality, a real-life demonstration of the ideas captured by physicist Erwin Schrödinger’s famous thought experiment involving a box and a precarious puss.
This extra layer of entanglement lets the researchers delay measuring the results of the test for an indefinite amount of time, even though the measurement itself is supposed to have determined earlier on whether a photon is behaving as a particle or a wave at a particular point in the experiment. It’s the equivalent of putting off the decision to check whether Schrödinger’s cat is alive, dead or something in between, for as long as you like.
Understanding this doubly quantum effect could be useful when building quantum computers and communication networks, which depend on entanglement to function.
Both research groups achieved the effect by creating a new version of the “delayed choice” experiment. Devised by physicist John Wheeler around 30 years ago, it involves an interferometer that contains two beam splitters. The first splits the incoming beam of light, and the second recombines them, producing an interference pattern.
Such a device demonstrates wave-particle duality in the following way. If light is sent into the interferometer a single photon at a time, the result is still an interference pattern – even though a single photon cannot be split. The explanation is that the photon is behaving as a wave, which is capable of being split.
What’s more, if you remove the device that recombines the two beams, interference is no longer possible, and the photon emerges from the interferometer as a particle. So you can control whether the beam acts as a particle or a wave by the presence or absence of the second beam splitter.
This ability to be both a particle and a wave is reminiscent of Schrödinger’s cat, an imaginary puss in a box whose fate depends on a radioactive atom. Because the atom’s decay is governed by quantum mechanics – and so only takes a definite value when it is measured – the cat is, somehow, both dead and alive until the box is opened. Choosing to measure the photon as a particle is akin to opening the box and discovering whether the cat is dead or alive, while measuring it as a wave leaves the box closed with the cat in a superposition, both dead and alive at once.
The “delayed choice” comes in because, bizarrely, this ability to control the photon’s character simply by measurement works even if you decide whether or not to remove the second beam splitter after the photon has passed through the first one. “The question that Wheeler posed was whether the photon knows in advance how to behave,” says Alberto Peruzzo at the University of Bristol, UK.
The answer was no: the light remains in an undecided state of both unsplittable particle and splittable wave, even after it has passed through the very device that would split it.
Now Peruzzo and colleagues have taken Wheeler’s idea one step further by replacing the second beam splitter with a quantum version that is simultaneously operational and non-operational.
This quantum beam splitter can be in this dual state because it is intimately linked to a second photon outside the interferometer via a process called entanglement. This ensures that the second beam splitter’s state – whether it is operational or not – depends on the second photon, and can only be determined by measuring the state of that second photon.
The researchers found that this allowed them to delay the photon’s wave or particle quality until after it has passed through all the experimental equipment, including the second beam splitter tasked with determining that very thing. “We can delay by a few nanoseconds, but in principle it’s equivalent to delaying as much as you want,” explains Peruzzo.
Meanwhile, Sébastien Tanzilli at the University of Nice Sophia-Antipolis in France and colleagues have shown exactly the same thing using a slightly different set-up.
The upshot of both experiments can be cast in the language of Schrödinger’s cat. “Long after the cat has supposedly been killed or not, one can choose to determine if it is dead or alive or determine if it is dead and alive,” says Seth Lloyd at the Massachusetts Institute of Technology, who was not involved in either experiment.
“There aren’t so many experimentally accessible demonstrations of quantum weirdness available, and this is one of the coolest,” he adds.
Peruzzo also reckons the effect could have practical applications. Because the bits in a quantum computer are entangled, they could be affected by the same bizarre effects. “Every technology that will use quantum information will have to take this into account,” he says.