Qubits turn into time travellers

Think of it as a quantum Tardis

Dr Who Tardis

A group of scientists from Spain’s Institute of Fundamental Physics has made the world just that little bit more weird, proposing a form of quantum entanglement that spans not just space, but time.

Physics followers will already be familiar with entanglement across space: two quanta (a pair of photons is a handy example) that have interacted in a way that gives them the same quantum state will remain entangled even though they’re in different places. Measure the quantum state of one photon, and the other photon will acquire that state apparently instantly – the “spooky action at a distance” that was one of Einstein’s objections to quantum mechanics.

Since entanglement has been repeatedly demonstrated, the world has become accustomed to the idea, even if it still seems strange, but only in the space domain. The latest research, by lead researcher Carlos Sabin (now at the University of Nottingham), Borja Peropadre, Marco del Rey, and Eduardo Martín-Martínez (now at the University of Waterloo in Canada), moves entanglement to the time domain – the entangled quanta are sharing a quantum state at different points in time.

Their work looks at the behavior of quantum fluctuations – which I have previously described in a very compressed shorthand as being the way the Universe preserves uncertainty even in a vacuum. If a bit of space is truly a complete vacuum, it’s possible to get a precise description of its state; so pairs of particles spontaneously come into existence and annihilate.

Again: quantum fluctuation seems like a too-strange-to-be-true theoretical construct, but it’s been both detected and manipulated. Moreover, the “vacuum field” is thought to contain entanglement even when it's empty (no, I don't understand how that bit works).

Sabin’s research describes how quantum electrodynamics allows a time-like entanglement. In their proposed experiment – which the researchers say can be performed with current technologies – two superconducting qubits are connected to a quantum field vacuum. In sequence, the two qubits (P for past and F for future) interact with the field – without both doing so at the same time.

(I’ve telegraphed the punch, haven’t I?) After the two interactions, P and F can become entangled without ever interacting with each other – forming a correlation that’s called “past-future entanglement”.

As the paper states, once the correlation exists, “past-future quantum correlations will have to be transferred to the qubits, even if the qubits do not coexist at the same time.”

“Qubit F interacts with the vacuum quantum fluctuations that are correlated with the vacuum quantum fluctuations that qubit P interacted with in the past,” Sabín said. “It's like if the qubits were exchanging ‘virtual’ – as opposed to real – photons, these undetectable particles propagating faster than light that are usually employed in quantum field theory to illuminate the computations.”

Published in Physical Review Letters, the paper can also be read in full at Arxiv. ®

Bootnote: As always, I don't mind being told I've slipped up in something as strange as this. ®

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