A Quantum Communications Switch

The device could one day let superfast quantum computers talk to each other.

The Internet is made of photons that zip through fiber-optic cables and flow through devices like switches, modulators, and amplifiers. But those standard devices would be inadequate for superfast quantum computing or communications—experimental approaches that exploit the peculiar properties of particles at the quantum scale to carry out complex calculations incredibly quickly or to prevent anyone from eavesdropping on messages. 

Switch it up: The components shown here can reroute entangled photons.

Commercial switches have various problems that make them unsuitable for rerouting entangled photons. Those that are made of micro-electromechanical components keep entangled states in tact, but operate too slowly. Other opto-electronic switches either add too much noise so that single photons are difficult to detect, or they completely destroy the quantum information.

Prem Kumar, professor of electrical engineering and computer science at Northwestern University, has developed a quantum routing switch that can shuttle entangled photons along various paths while keeping the quantum information intact.

The device could be particularly useful for quantum computing, says James Franson, professor of physics at the University of Maryland, Baltimore County. “To build a quantum computer using photons, we need the ability to switch [entangled] photons,” says Franson. A quantum switch could also someday allow entangled photons from different quantum computers to be shared over long distances—like cloud computing, but with quantum information.

Kumar says the switch will also make ultra-secure quantum networks a reality. Today’s information is typically secured using what’s called public key encryption, which relies on the practical impossibility of performing certain mathematical tasks, like factoring extremely large numbers. Quantum networks would offer an even more secure alternative to public key encryption. Using entangled photons to communicate ensures security because any attempt to intercept a message would disturb the particles’ quantum state.

To build the new quantum switch, the researchers used commercial fiber-optic cable and other standard optical components, says Kumar. “My goal is to do things in the quantum information space that are very compatible with existing fiber infrastructures,” he says.

The first step is to prepare the photons. Entangled photons have properties, such as polarization, that are fundamentally linked. If two photons are entangled, then the measured polarization of one reveals the corresponding state of the other. The researchers used a technique in which they mixed together multiple wavelengths of light within a standard fiber to create entangled photon pairs.

The next step is to send one photon down the optical fiber to the switch, which changes the photon’s course. The researchers’ switch is made of only optical components, including a spool of 100 meters of optical fiber arranged in a loop. One photon of an entangled pair is sent through one end of the loop, and through a multiplexer, while a powerful laser sends pulses of light into the spool. The photon is shifted in such a way that at the other end of the loop it separates out along a separate path, while remaining entangled with its partner.

The end result is a switch that’s very fast, has low background noise, and most importantly, preserves the quantum information. Single photon detectors at the end of the fibers confirm that both photons maintained their entangled state, showing that the quantum information was preserved. The work is described in a recent issue of the journal Physical Review Letters.

“It’s an important development, because switching photons is really the main difference in going ahead in further progress in quantum computing using photons,” says Franson.

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