anton_zeilinger's picture
Nobel laureate (2022 - Physics); Physicist, University of Vienna; Scientific Director, Institute of Quantum Optics and Quantum Information; President, Austrian Academy of Sciences; Author, Dance of the Photons: From Einstein to Quantum Teleportation
Quantum Entanglement Is Independent Of Space And Time

The notion of quantum entanglement, famously called “spooky action at a distance” by Einstein emerges more and more as having deep implications for our understanding of the World. Recent experiments have perfectly verified the fact that quantum correlations between two entangled particles are stronger that any classical, local pre-quantum worldview allows. So, since quantum physics predicts these measurement results for at least eighty years, what’s the deal?

The point is that the predictions of quantum mechanics are independent of the relative arrangement in space and time of the individual measurements. Fully independent of their distance, independent of which is earlier or later etc. One has perfect correlations between all of an entangled system even as these correlations cannot be explained by properties carried by the system before measurement. So quantum mechanics transgresses space and time in a very deep sense. We would be well advised to reconsider the foundations of space and time in a conceptual way.

To be specific, consider an entangled ensemble of systems. This could be two photons, or any number of photons, electrons, atoms, and even of larger systems like atomic clouds at low temperature, or superconducting circuits. We now do measurements individually on those systems. The important point is that, for a maximally entangled state, quantum physics predicts random results for the individual entangled property.

To be specific, for photons this could the polarization. That is, for a maximally entangled state of two or more entangled photons, the polarization observed in the experiment could be anything, horizontal, vertical, any direction linear, right-handed circular, left-handed circular, any elliptical state, again, for the individual photon. Thus, if we do a measurement we observe a random polarization. And this for each individual photon of the entangled ensemble. But a maximally entangle state predicts perfect correlations between the polarizations of all photons making the entangled state up.

To me, the most important message is that the correlations between particles like photons, electrons, or atoms, or larger systems like superconducting circuits are independent of which of the systems are measured first and how large the spatial distance between them is.

At first sight, this might not be surprising. After all, if I measure the heights of peaks of the mountains around me, it also does not matter in which sequence I do the measurements and whether I measure the more distant ones first or the ones closer to each other. The same is true for measurements on entangled quantum systems. However, the important point is that the first measurement on any system entangled with others instantly changes the common quantum state describing all, the subsequent measurement on the next does that again and so on. Until, in the end, all measurement results on all systems entangled with each other, are perfectly correlated.

Moreover, as recent experiments finally prove, we now know definitely that all this cannot be explained by any communication limited by Einstein’s cosmic speed limit—the speed of light. Also, one might think that there is a difference if two measurements are done such that one if after the other in way that a signal could tell the second one what to do as a consequence of the first earlier measurements. Or whether they are arranged at such a distance and done sufficiently simultaneously such that no signal is fast enough.

Thus, it appears that on the level of measurements of properties of members of an entangled ensemble, quantum physics is oblivious to space and time.

It appears that an understanding is possible via the notion of information. Information seen as the possibility of obtaining knowledge. Then quantum entanglement describes a situation where information exists about possible correlations between possible future results of possible future measurements without any information existing for the individual measurements. The latter explains quantum randomness, the first quantum entanglement. And both have significant consequences for our customary notions of causality.

It remains to be seen what the consequences are for our notions of space and time, or space-time for that matter. Space-time itself cannot be above or beyond such considerations. I suggest we need a new deep analysis of space-time, a conceptual analysis maybe analogous to the one done by the Viennese physicist-philosopher Ernst Mach who kicked Newton’s absolute space and absolute time form their throne. The hope is that in the end we will have new physics analogous to Einstein’s new physics in the two theories of relativity.