Researchers Demonstrate New Building Block in Quantum Computing

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Researchers with the Department of Energy’s Oak Ridge National Laboratory have demonstrated a new level of control over photons encoded with quantum information. 

Joseph Lukens, Brian Williams, Nicholas Peters, and Pavel Lougovski, research scientists with ORNL’s Quantum Information Science Group, performed distinct, independent operations simultaneously on two qubits encoded on photons of different frequencies, a key capability in linear optical quantum computing. Qubits are the smallest unit of quantum information.

Quantum scientists working with frequency-encoded qubits have been able to perform a single operation on two qubits in parallel, but that falls short for quantum computing.

“To realize universal quantum computing, you need to be able to do different operations on different qubits at the same time, and that’s what we’ve done here,” Lougovski said.

According to Lougovski, the team’s experimental system—two entangled photons contained in a single strand of fiber-optic cable—is the “smallest quantum computer you can imagine. This paper marks the first demonstration of our frequency-based approach to universal quantum computing.”

“A lot of researchers are talking about quantum information processing with photons, and even using frequency,” said Lukens. “But no one had thought about sending multiple photons through the same fiber-optic strand, in the same space, and operating on them differently.”

The team’s quantum frequency processor allowed them to manipulate the frequency of photons to bring about superposition, a state that enables quantum operations and computing.

Unlike data bits encoded for classical computing, superposed qubits encoded in a photon’s frequency have a value of 0 and 1, rather than 0 or 1. This capability allows quantum computers to concurrently perform operations on larger datasets than today’s supercomputers.

Using their processor, the researchers demonstrated 97 percent interference visibility—a measure of how alike two photons are—compared with the 70 percent visibility rate returned in similar research. Their result indicated that the photons’ quantum states were virtually identical.

The researchers also applied a statistical method associated with machine learning to prove that the operations were done with very high fidelity and in a completely controlled fashion.

“We were able to extract more information about the quantum state of our experimental system using Bayesian inference than if we had used more common statistical methods,” Williams said.

“This work represents the first time our team’s process has returned an actual quantum outcome.”

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