Northwestern University researchers have developed a new switching device that takes quantum communication to a new level. The device is a practical step toward creating a network that takes advantage of the mysterious and powerful world of quantum mechanics. The research demonstrates the first all-optical switch suitable for single-photon quantum communications.
The researchers can route quantum bits, or entangled particles of light, at very high speeds along a shared network of fiber-optic cable without losing the entanglement information embedded in the quantum bits. The switch could be used toward achieving two goals of the information technology world: a quantum Internet, where encrypted information would be completely secure, and networking superfast quantum computers.
The device would enable a common transport mechanism, such as the ubiquitous fiber-optic infrastructure, to be shared among many users of quantum information. Such a system could route a quantum bit, such as a photon, to its final destination just like an e-mail is routed across the Internet today.
The bits we all know through standard, or classical, communications only exist in one of two states, either “1” or “0.” All classical information is encoded using these ones and zeros. What makes a quantum bit, or qubit, so attractive is it can be both one and zero simultaneously as well as being one or zero. Additionally, two or more qubits at different locations can be entangled — a mysterious connection that is not possible with ordinary bits.
Researchers need to build an infrastructure that can transport this “superposition and entanglement” (being one and zero simultaneously) for quantum communications and computing to succeed.
The qubit Kumar works with is the photon, a particle of light. A photonic quantum network will require switches that don’t disturb the physical characteristics (superposition and entanglement properties) of the photons being transmitted, Kumar says. He and his team built an all-optical, fiber-based switch that does just that while operating at very high speeds.
To demonstrate their switch, the researchers first produced pairs of entangled photons using another device developed by Kumar, called an Entangled Photon Source. “Entangled” means that some physical characteristic (such as polarization as used in 3-D TV) of each pair of photons emitted by this device are inextricably linked. If one photon assumes one state, its mate assumes a corresponding state; this holds even if the two photons are hundreds of kilometers apart.
The researchers used pairs of polarization-entangled photons emitted into standard telecom-grade fiber. One photon of the pair was transmitted through the all-optical switch. Using single-photon detectors, the researchers found that the quantum state of the pair of photons was not disturbed; the encoded entanglement information was intact.
“Quantum communication can achieve things that are not possible with classical communication,” said Kumar, director of Northwestern’s Center for Photonic Communication and Computing. “This switch opens new doors for many applications, including distributed quantum processing where nodes of small-scale quantum processors are connected via quantum communication links.”
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. Entangled photons are sent through one end, and at the other end, a powerful laser sends pulses of light into the spool. The photons are shifted in such a way that one of them separates out along a separate path.
The end result is a switch that’s very fast, has low background noise, and most importantly, preserves the quantum information