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UCSB Physicists Examine Quantum Entanglement

Physicists at the Kavli Institute for Theoretical Physics recently designed a computer system that details characteristics of quantum entanglement, a state in which a pair of electron spins are entangled with one another.

Entanglement occurs when particles, such as photons or electrons, interact with one another and then separate. Even if separated over a large distance, the action of one affects the other, which may open up a new means of secure and quick communication.

Electrons in a pair have what is called spin, in which one electron points up and the other points down. Similar to tiny magnets, the pairs have a north pole and a south pole. With these two electrons, a nonclassical ‘entangled state’ can be prepared. Although it is not known whether the electron spin points up or down, measuring one gives knowledge of the spin of the other.

Leon Balents, a physics professor in the KITP and author of a paper detailing quantum entanglement published in the journal Nature Physics, said that quantum entanglement offers certainty of the state of electrons in a pair, regardless of the distance between the separated pairs.

“You can form an entangled spin and you can send one to the Moon and one to Mars. Someone measures the first particle on the Earth and if it is up, they know for sure that the person measuring on Mars will definitely measure it down,” Balents said. “Somehow there is some kind of action that happens in quantum entanglement that can be used to correlate information in different places.”

Balents’ group is using a large scale, up to 1023, of electrons entangled with one another into a state called quantum spin liquid, the holy grail of quantum physics for its possibilities in communication.

Zhenghan Wang, a researcher with Microsoft Station Q at UCSB who worked on the mathematics of the project and co-authored Balents’ paper, said that the new computer was designed to analyze the quantum spin liquid.

“It is very abstract mathematics. In terms of those quantum phases of physics, it is very difficult to compute. It is just too complicated for classical computers to compute. So we have basically found this one that can do it,” Wang said. “You make assumptions. From a theoretical question, I did not realize how big this problem is for condensed physics.”

According to Balents, their research on quantum spin liquid may lead to more secure and large-scale communication.

“Photons pairs can be used to carry out perfectly secure communication. In ordinary communication, someone can always tap into the signal pull of a little current and, in principle, listen in [on] what you are doing,” Balents said. “Secure cryptic communication is hard to break but not impossible. As computers get better and better, the cryptics have to be better. With quantum entanglement, it is possible to create an algorithm which in principle is impossible to break.”


A version of this article appeared on page 5 of January 15th, 2013’s print edition of the Nexus.

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