A Step Toward Quantum Computing

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Prem Kumar (photo by Andrew Campbell)Prem Kumar (photo by Andrew Campbell)

For now, full-fledged quantum computers are the stuff of science fiction — as in the 2007 blockbuster movie Transformers, in which the bad guys use quantum computing to break into the U.S. Army’s secure files in just 10 seconds flat. But Prem Kumar and his research group are one step closer to realizing that technology, though for far better purposes. The group recently demonstrated one of the basic building blocks for distributed quantum computing using entangled photons generated in optical fibers.

“Because it is done with fiber and the technology is already globally deployed, we think that it is a significant step in harnessing the power of quantum computers,” says Kumar, the AT&T Professor of Information Technology in the Department of Electrical Engineering and Computer Science and director of the Center for Photonic Communication and Computing.

Quantum computing differs from classical computing in that a classical computer works by processing “bits” that exist in two states, either one or zero. Quantum computing uses quantum bits, or qubits, which in addition to being one or zero can also be in a “superposition” — or both one and zero simultaneously. This is possible because qubits are quantum units (like atoms, ions, or photons) that operate under the rules of quantum mechanics instead of classical mechanics. The “superposition” state allows a quantum computer to process significantly more information than a classical computer and in a much shorter time.

The field of quantum computing took off about 15 years ago, after mathematician and physicist Peter Shor created a quantum algorithm that could factor large integers much more efficiently than a classical computer. Though researchers are still many years away from creating a quantum computer capable of running the Shor algorithm, progress has been made. Kumar’s group, which uses photons as qubits, found that they can entangle two indistinguishable photons in an optical fiber very efficiently by using the fiber’s inherent nonlinear response to intense pulses of light. They also found that no matter how far you separate the two photons in standard transmission fibers, they remain entangled and are “mysteriously” connected to each other’s quantum state.

Kumar’s team used the fiber-generated indistinguishable photons to implement the most basic quantum computer task: a controlled-NOT gate, which allows two photonic qubits to interact. “This device that we demonstrated in the lab is a two-qubit device — nowhere near what’s needed for a quantum computer,” Kumar says. “So what can you do with it? It’s nice to demonstrate something useful to give a boost to the field, and there are some problems at hand that can be solved right now using what we have.”

Kumar has received funding for his group’s next effort — a study of how to implement a quantum network for physically demonstrating efficient public goods strategies. He says such a network could help out with high-stakes auctions: For example, if the Department of Defense wanted to build an expensive airplane, it would send out a request for bids. No one company could build the entire airplane, and there could be 15 companies that can build some part of the plane, whether it’s a navigation system or an engine. Instead of just giving the project to the lowest overall bidder, the government could save public dollars by allowing companies to make conditional bids. Maybe the engine company worked with the fuselage company before, and they could be more efficient and less expensive working together than other companies. The engine and fuselage companies could then send in conditional bids based on that possibility, along with bids covering other scenarios.

“Figuring out the best possible outcome is possible with quantum computers,” says Kumar. “Based on these fiber-type gates that utilize entanglement, the auctioneer has an efficient way of determining optimal outcomes when bidders make conditional bids. When the computation is done, it reveals only the winning strategy, and all other bids disappear — even the auctioneer does not know who bid what. Secrecy of conditional bids can be paramount in high-stakes scenarios, since unauthorized revelations could jeopardize future relationships among cooperating parties.”

Kumar says they hope to perform this experiment sometime in the next year. “The goal is to demonstrate the power of the computer,” he says. “With this experiment we can demonstrate a few things that show the promise of quantum computing.”

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