Last Word is New Scientist’s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
Quantum physicists talk a lot about entanglement. How exactly does one entangle two photons? And can I try it at home?
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Mike Follows
Sutton Coldfield, West Midlands, UK
One method of entangling two photons involves using a laser and a process known as spontaneous parametric down-conversion. When a high-energy photon from a laser interacts with a specially designed nonlinear crystal, it splits into two lower-energy photons. These are generally created in an entangled state in terms of their polarisation, momentum or other properties. For instance, momentum is conserved, meaning the total momentum of the entangled photons remains equal to that of the original photon.
There is more than one philosophical interpretation of quantum mechanics, partly due to the fuzziness imposed by Werner Heisenberg’s uncertainty principle, which says we can’t precisely measure both the momentum and the position of a particle. One interpretation is that a particle only has clear-cut properties when it is observed. Albert Einstein challenged this: along with Boris Podolsky and Nathan Rosen, he proposed the so-called EPR thought experiment in 1935. The idea was that a particle could split into two, with the resulting halves moving in opposite directions.
This isn't something you could set up in your living room, but entanglement is happening all around us
Based on our understanding of the quantum mechanics of entangled particles, by measuring the properties of one particle, conservation laws could be used to infer the properties of the second without directly observing it. Furthermore, changing the properties of one particle would instantly change those of the other particle – no matter how far away it is. This would mean that information passing between the particles might need to travel faster than the speed of light. That appeared to contradict Einstein’s theories of relativity, leading him to reject what he termed “spooky action at a distance” and what we call quantum entanglement.
Alain Aspect used a laser to perform the first definitive test of what became known as the EPR paradox in 1982 to prove that particles can indeed be entangled. Scientists reconcile it with relativity by saying that it is information, not matter, that is being communicated between the particles. This isn’t something you could set up in your living room, but quantum entanglement is happening spontaneously all around us. The phenomenon will be exploited in quantum computers and may one day facilitate Star Trek-style teleportation.
Sam Edge
Ringwood, Hampshire, UK
Entangling photons is a matter of getting them to share some state before being separated, as a result of them interacting with either each other or a shared apparatus, such as a diffraction grating – a structure that diffracts light into several beams.
You are entangling photons all the time at home with every biochemical and electrical activity your body and belongings perform. However, these photons remain entangled for such a short period of time that you don’t notice.
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