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17 August, 2018
Quantum Entanglement Confirmed with Light from Distant Quasars
A team of scientists led by quantum physicist Anton Zeilinger from the Austrian Academy of Sciences and the University of Vienna has made a new test of quantum entanglement this time using photons from distant astronomical objects as collected by the William Herschel Telescope (WHT) and the Telescopio Nazionale Galileo (TNG).
In the experiment, entangled pairs of photons were created and sent to receiving stations the researchers set up right next to the two telescopes. The telescopes looked at two different, almost opposite locations in the sky and observed quasars. Variations of the colour in the quasar light were then used to control which kind of measurement was performed on the two photons from an entangled pair created in a mobile laboratory located near the Nordic Optical Telescope (NOT).
Of these photons, one was sent to a receiving station near the WHT, the other one to another receiver next to the TNG. There, the individual polarisation of each entangled photon was measured as decided by the fluctuations of the light from its respective quasar.
A source of entangled photons sends light particles to receiver stations from a mobile quantum physical laboratory located near NOT telescope. The measurement of the entangled photons was controlled by the light of distant quasars which was captured by the WHT and the TNG. Credit: Massimo Cecconi. Large format: [ JPEG ].
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 procedures."
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].
"Quantum Entanglement Confirmed with Light from Distant Quasars", OAW press release, 17th August 2018.
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