Focus: Spooky at Any Speed

Published December 27, 2002  |  Phys. Rev. Focus 10, 29 (2002)  |  DOI: 10.1103/PhysRevFocus.10.29

Quantum Entanglement of Moving Bodies

Robert M. Gingrich and Christoph Adami

Published December 16, 2002
Figure 1
Jacques Devaud/NIST

Space clock. A theory predicting the creation of entanglement at near-light speeds is an early step toward enhancing the synchronization of atomic clocks aboard satellites.

Entanglement, the “spooky” effect in which far-distant particles appear to influence one another instantaneously, is a delicate property, hard to prepare and manipulate. But according to researchers publishing in the 30 December print issue of PRL, simply zipping along at close to the speed of light may create entanglement in particles that seemingly had none while at rest. The finding may help synchronize atomic clocks aboard satellites and lead to new ways of creating entanglement for futuristic applications.

If a pair of fundamental particles is entangled, measuring an attribute of one particle, such as spin, can affect the second particle, no matter how far away. Entanglement can even exist between two separate properties of a single particle, such as spin and momentum. In principle, single particles or pairs can be entangled via any combination of their quantum properties. And the strength of the quantum link can vary from partial to complete. Researchers are just beginning to understand how entanglement meshes with the theory of relativity. They have learned that the degree of entanglement between spin and momentum in a single particle can be affected by changing its speed (“boosting” it into a new reference frame) but weren’t sure what would happen with two particles.

Now Robert Gingrich and Christoph Adami of the Jet Propulsion Laboratory in Pasadena, California say that a boost can actually create spin or momentum entanglement, or both, between two particles that had neither to begin with. The speed change can also enhance entanglement in spin at the expense of momentum entanglement, or reduce them both. “If you can create [entanglement] just by moving with respect to what you’re measuring, then seemingly you’ve created something from nothing,” says Gingrich.

Of course, you don’t really start out with nothing. The strange effect occurs only if the particle pair starts out entangled in a set of traits that aren’t being measured. For example, the spin of one particle can be entangled with the momentum of the other, without having any entanglement between the two spins or the two momenta. A particle pair possesses a grand entanglement that encompasses all the possible combinations of spin and momentum entanglement and, crucially, stays constant when the system is boosted. But the change in reference frame alters the various components of the overall entanglement in a way that’s still poorly understood. Gingrich and Adami show that pre-existing overall entanglement can be concentrated in the spin, thus seemingly creating spin entanglement from thin air.

Gingrich says the effect may offer a simple way to create entangled particles for experiments on teleportation and superfast quantum computing. The theory should also help in the design of quantum techniques for synchronizing atomic clocks aboard satellites, which keep slightly different times because of their relative motion.

The new work is “a key contribution to the small but growing field of relativistic quantum information theory,” says Gerard Milburn of the University of Queensland in St. Lucia, Australia. Milburn eagerly anticipates studies that combine entanglement–the essential element of quantum information–with gravity, which is described by relativity theory. A strong gravitational field can actually create entangled particles; those that break free of the field are called Hawking radiation. “It would be very nice if this could be turned around and Hawking radiation derived as a consequence of quantum information in curved spacetime,” he says.

–JR Minkel

JR Minkel is a freelance science writer in New York City.


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