Stanford physicists take first step toward quantum cryptography

November 19, 2012

To achieve the desired result, the group sends the laser through a series of lenses and other instruments (credit: Kristiaan De Greve et al./Stanford University)

Quantum mechanics promises the potential to create absolutely secure telecommunications networks by harnessing a fundamental phenomenon of quantum particles.

Now, a team of Stanford physicists has demonstrated a crucial first step in creating a quantum telecommunications device that could be built and implemented using existing infrastructure.

The work done by the Stanford research group of Yoshihisa Yamamoto, a professor of applied physics and of electrical engineering, in collaboration with the group of applied physics Professor Martin Fejer, provides a solution to one of the key wrinkles of quantum telecommunications.

If electrons, for example, are thousands of miles apart, say San Francisco and New York, they can’t be directly entangled; instead, they become entangled by sending photons between the two over fiber optic cables.

The San Francisco-based sender would coax its electron to emit a photon, which would itself be entangled to the electron. Then the  New York-based recipient’s electron would also emit an entangled photon, and when the two photons interact, the electrons on either end would then become entangled and share the same quantum spin.

A challenge is that light “leaks” from even the best fiber optic cables; current systems employ multiple devices called “repeaters” that receive a fading light signal and then amplify and reproduce it to send to the end recipient. In order to transmit quantum messages, scientists need to build “quantum repeater” systems that enable entanglement between electrons to be created over many repeater stations between the start and end locations.

Quantum telecommunication networks such as this are still years away, but the new Stanford research — published this week in Nature, with recent PhD graduate Kristiaan De Greve as lead author — demonstrates a critical first step in that chain.

Other researchers have entangled an electron and photon and created a memory using the electron, but those efforts required complex systems that are bulky and difficult to duplicate. The new approach, however, is potentially easier to scale up to systems that have many entangled pairs of electrons, said one of the study co-authors, Peter McMahon, an electrical engineering PhD candidate at Stanford.

The Stanford group used a quantum dot array — provided by collaborators at the University of Würzburg in Germany — that sits on a postage stamp-size semiconductor chip that contains a layer that is covered with millions of tiny bubbles, or dots. Each of these “quantum dots” contains a single electron; in essence, each quantum dot is an artificial atom. By focusing a laser on a single dot, the researchers excited the electron inside and caused it to emit a photon. Tests showed that the emitted photon’s orientation corresponded to the electron’s spin direction, confirming that the photon and electron were quantum entangled.

An advantage of quantum dot arrays is that they could be produced using standard semiconductor-manufacturing technology. A single chip could contain millions of dots, arranged in a regular grid layout that would allow for the generation of long keys at high speeds.

Another experimental first achieved by the Stanford group is that the photon entangled with the electron is produced at the optimal frequency for transmission along fiber optic cables. The ability to slide into existing telecommunications networks and the ease of manufacture make Stanford’s approach particularly appealing for future research, McMahon said.

The next step will involve building a receiver and confirming that electrons on both ends become entangled. And though that work could take several years, McMahon and his colleagues are optimistic. “We have demonstrated a fundamental building block of a quantum repeater node,” he said. “It should not be impossible to create the second link.”

The work was supported by the Japan Society for the Promotion of Science, the National Science Foundation, the National Institute of Information and Communications Technology, the National Institute of Standards and Technology, Special Coordination Funds for Promoting Science and Technology, and the State of Bavaria. Funding also came from the Herb and Jane Dwight Stanford Graduate Fellowship, a David Cheriton Stanford Graduate Fellowship and a Robert N. Noyce Stanford Graduate Fellowship.