The measurement of one photon of an entangled pair has an
instant influence on the measurement result of the other one. This was called by Einstein
"spooky action at a distance", and he was hoping for a physics without entanglement. The
question is: how is it decided which measurements are performed on the two photons?
It is evident that it would be desirable to have decisions made completely independently,
such that they cannot be influenced by a common cause. In the new experiment, the
fluctuations of the light from two quasars decided on each entangled photon separately
which polarisation is measured.
In the 1960s, the physicist John Bell calculated a theoretical limit beyond which such correlations must have a quantum, rather than a classical, explanation.
The quasars used for this experiment are about 12 and 8 billion light-years
away, on almost opposite directions in the sky. This is rather short after the Big Bang 13.8
billion years ago, and any possible influence on both quasars could have happened in only
4% of the known Universe.
The researchers ran their experiment twice, each for around 15 minutes and with two different pairs of quasars. For each run, they measured 17,663 and 12,420 pairs of entangled photons, respectively. Within hours of closing the telescope domes and looking through preliminary data, the team could tell there were strong correlations among the photon pairs, beyond the limit that Bell calculated, indicating that the photons were correlated in a quantum-mechanical manner.
"The crucial challenge in the experiment was to make sure that the choice of polarisation
measurements on each of the entangled photons was done completely independently from
us and from any environment, no matter how large", says Dominik Rauch, from the Austrian Academy of Sciences and the University of Vienna. "This light, that is completely independent from us and almost our entire
past, allowed us to use these distant quasars as cosmic random number generators."
The cosmic light used was in that sense ideally suited for the experiment. It also provides a
new way to obtain random numbers. That way, distant quasars where for the first time
applied as random number generators.
Anton Zeilinger explains: "This is the first time that light travelling to
us from nearly the edge of the known Universe has been used in a quantum experiment.
The results on the entangled photons confirmed the predictions of quantum mechanics.
This is also very important for quantum technologies, because uninfluenced measurements
on entangled states are important for a definitive proof of the security oyf various quantum
Dominik Rauch, Johannes Handsteiner, Armin Hochrainer, Jason Gallicchio, Andrew S.
Friedman, Calvin Leung, Bo Liu, Lukas Bulla, Sebastian Ecker, Fabian Steinlechner, Rupert
Ursin, Beili Hu, David Leon, Chris Benn, Adriano Ghedina, Massimo Cecconi, Alan H.
Guth, David I. Kaiser, Thomas Scheidl, and Anton Zeilinger, 2018, "Cosmic Bell test using random measurement settings from high-redshift quasars", Physical Review Letters, 121, 080403 [ADS].
"Light from Ancient Quasars Helps Confirm Quantum Entanglement: Results are Among the Strongets Evidence Yet for 'Spooky Action at a Distance'", MIT press release, 20th August 2018.
"Quantum Entanglement Confirmed with Light from Distant Quasars", OAW press release, 17th August 2018.
NOVA, Quantum Entanglement - Documentary HD, Jan 2019.
Dan Bejar, 2018, "Einstein was wrong: Why 'normal' physics can't explain reality", New Scientist, 3204y, 28 (17 November 2018).