Entanglement Swapping: A New Quantum Trick

October 11, 2007 by Laura Mgrdichian feature

In an important step for the infant field of quantum communications, researchers from the University of Geneva in Switzerland have, for the first time, realized an “entanglement swapping” experiment with photon pairs emitted continuously by two different sources. This experiment is a key facet of quantum entanglement, the strange phenomenon in which two photons or other quantum bodies behave as one unit, even if spatially separate. Entanglement is at the heart of many proposed quantum information and communications schemes, including quantum computing and encryption.

University of Geneva physicist Matthäus Halder, the experiment's corresponding scientist, explained to PhysOrg.com, “Normally, entanglement between two photons is obtained by emitting them simultaneously by the same source. We've shown that entanglement can be transferred, or swapped, onto two particles that originated from different sources and were formerly completely independent. This is the first time that two autonomous photons from continuous sources have been entangled.”

In their scheme, two independent pairs of entangled photons, A1-A2 and B1-B2, are emitted by autonomous sources. By taking a joint measurement on one photon in each pair (A1 and B1), these photons fall into an entangled state (later verified using detectors), one of the four so-called “Bell states,” named for physicist John Bell, a key contributor to quantum physics. The joint measurement is thus known as a Bell-state measurement (BSM), and it is the foundation of the experiment.

As a result of the BSM, the two remaining photons (A2 and B2) are projected on an entangled state despite being unaware of the other's presence and never having previously interacted. Hence the entanglement of the initial pairs has been “swapped.”

The key element of a successful BSM is the precise timing of the two photon pairs. This has been obtained, up until now, by using pulsed sources, which send out photons in discrete bunches. But pulsed sources must be synchronized to emit the photon bunches at an exact time, a very difficult task.
Alternatively, Halder and his colleagues show that continuous photon sources can be used. These sources do not require any synchronization, and are therefore likely to be much easier to incorporate into future real-world quantum communications systems.

In this case, photons with the proper timing is obtained not when they are emitted, but when they are later detected by separate detectors. The detectors' temporal resolution (the precision of its measurements with respect to time) allowed Halder and his group to “post-select” only those photons that were emitted at the right time.

This research is described in the August 19, 2007, online edition of Nature Physics.

Citation: Matthäus Halder, Alexios Beveratos, Nicolas Gisin, Valerio Scarani, Christoph Simon, and Hugo Zbinden, “Entangling independent photons by time measurement” Nature Phys, 3, 692-659 (2007)

Copyright 2007 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

Explore further: Silicon-based metamaterials could bring photonic circuits

Related Stories

Silicon-based metamaterials could bring photonic circuits

January 29, 2016

New transparent metamaterials under development could make possible computer chips and interconnecting circuits that use light instead of electrons to process and transmit data, representing a potential leap in performance.

Organic crystals allow creating flexible electronic devices

February 4, 2016

Scientists from the faculty of physics of the Moscow State University have grown organic semiconductor crystals that can reduce the cost of creating light, flexible and transparent light-emitting electronic devices. The researchers, ...

Recommended for you

Scientists glimpse Einstein's gravitational waves (Update)

February 11, 2016

In a landmark discovery for physics and astronomy, scientists said Thursday they have glimpsed the first direct evidence of gravitational waves, ripples in the fabric of space-time that Albert Einstein predicted a century ...

Superconductors could detect superlight dark matter

February 9, 2016

(Phys.org)—Many experiments are currently searching for dark matter—the invisible substance that scientists know exists only from its gravitational effect on stars, galaxies, and other objects made of ordinary matter. ...

A 'magical' space-time ripple that wasn't believed, at first

February 11, 2016

The wave that made history snuck up on them. David Shoemaker will never forget the date—September 14, 2015—when he woke up to a message alerting him that an underground detector had spotted a 1.3-billion-year-old ripple ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

maynard
not rated yet Oct 14, 2007
Does this mean that every photon across the entire universe that is emitted at the same instant becomes entangled? And if not, does spacial distance form the constraint, or is it something else?
holoman
5 / 5 (1) Oct 17, 2007
How It Works: Quantum computing: qubits

http://www.trnmag...905.html

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.