Ghostly entanglement holds great promise (report of the KNAW symposium 'The importance of quantum entanglement')


Four quantum physicists from Leiden and Delft addressed a full room of 200 people on entanglement at a mini symposium of the Royal Dutch Academy of Sciences (KNAW). Carlo Beenakker and Dirk Bouwmeester took the stage on behalf of Leiden University and Ronald Hanson and Stephanie Wehner represented TU Delft. Together they explained the ‘ghost-like’ existence of entanglement between elementary particles, and how we can make use of this bizarre phenomenon.

Erwin Schrödinger’s famous thought experiment shows how strange the laws of quantum mechanics are. Beenakker opens the symposium with the most talked about cat in history. Elementary particles are never in one place at a time. They occupy multiple positions simultaneously, according to a probability distribution that reflects the odds of where the particle will report at the moment we look at it. Because only when we look, we force the particle to take a seat. As if the music stops at musical chairs. This uncertainty goes not only for position, but also for properties like spin, polarization and decay. In his mind, Schrödinger put his cat in a box, together with a mechanism that either breaks a bottle of poison or leaves it intact, depending on the quantum state of one atom. As long as he doesn’t look inside the box, the quantum state is ambiguous, and the cat is both dead and alive!

In an absurdist way, the fate of Schrödinger’s cat is intertwined with the particle’s state. In this example we still see a logical link between poison and death, but we could also connect two elementary light particles so that the one state depends on the other. If you would measure a vertical polarization for one photon, it is automatically determined that the other is horizontally polarized. Scientists call this entanglement. Or in the cynical words of Albert Einstein: ‘spooky action at a distance’. The weird thing is that the second photon’s fate is instantly determined by measurement of the first photon. Information would then travel faster than light!

Bell test
Last year, Einstein paid a price for his historic statement when Ronald Hanson made the front pages by conclusively proving that entanglement indeed exists. Hanson explains his so-called Bell test as a game played by Alice and Bob. Two questions are possible—‘Which glass of wine?’ and ‘Which bottle of wine?’—and two answers—‘red wine’ and ‘white wine’. If both are asked about the bottle, they will score points if they answer the same. In any other case they should answer differently. Mathematicians know that the best possible joint strategy will lead to a score of no more than 75%. Hanson had two entangled particles play a similar game, and those managed to obtain a miraculous score of more than 75%. Einstein suspected that quantum states are predetermined in some way, even though we don’t see it. Hanson proved this sumption wrong. ‘Two tiny particles beat the most powerful supercomputers,’ says Hanson. ‘That truly shows the power of quantum.’

Another example of quantum entanglement’s promise is uncrackable encryption, on which Dirk Bouwmeester elaborates with many formulas. Those are, no matter how tedious, indispensable when talking quantum mechanics. Beenakker agrees: ‘In my classes I never talk about interpretations, but I use formulas. That is the modern way.’ In the end, mathematics is the only language in which the theory can be truly explained. Sometimes this leads to misunderstandings, as Bouwmeester experienced while working in London. ‘We were working on quantum encryption when three men from the secret service MI5 paid us a visit, in long black coats. I explained that it truly is perfectly safe. They responded almost simultaneously: “that is for us to decide”.’

The promise that perhaps speaks most to the imagination is teleportation. In theory you could teleport a human body to Australia in literally zero seconds; modern faxing in a sense. Matter itself doesn’t travel, but the information on quantum states is copied into different atoms. Whether consciousness is copied along is still fuel for philosophical debate. Yet, quantum mechanics has evolved from a philosophical problem in the previous century to a technological challenge in this century, according to Beenakker. We are on the threshold of an era of quantum technology.