Researchers at Oak Ridge National Laboratory (ORNL) have demonstrated a frequency-based approach to quantum computing. The researchers performed two distinct, independent operations simultaneously on two qubits encoded on photons of different frequencies. 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 never two distinct operations, said the ORNL team. According to the researchers, coherent quantum frequency operations are challenging because it is difficult to mix frequencies arbitrarily and with low noise. “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,” Pavel Lougovski, a research scientist, said. Brian Williams, Joseph Lukens, Pavel Lougovski, and Nicholas Peters (from left), research scientists with ORNL’s Quantum Information Science Group, have demonstrated two simultaneous operations on two qubits, a new capability that represents a building block toward quantum computing. Courtesy of Carlos Jones/Oak Ridge National Laboratory, U.S. Department of Energy. For their experiment, the team used two entangled photons contained in a single strand of fiber optic cable. Because the photons were traveling through the same device, stability and control over the photons were maintained. “When the photons are taking different paths in the equipment, they experience different phase changes, and that leads to instability,” said Brian Williams, a researcher on the team. The researchers implemented distinct quantum gates in parallel on two entangled frequency-bin qubits in the optical fiber. The team’s quantum frequency processor allowed it to manipulate the frequency of photons to bring about superposition, the state that allows quantum computers to perform operations concurrently. Through this quantum operation the researchers were able to control the spectral overlap between adjacent spectral bins, observe frequency-bin interference, and demonstrate 97 percent interference visibility (i.e., a measure of how alike two photons are). These results indicate that the photons’ quantum states were virtually identical. By integrating this tunability with frequency parallelization, the researchers were able to synthesize independent gates on entangled qubits. The researchers applied Bayesian inference — a statistical method associated with machine learning — to confirm that the operations on the quantum processor were done with high fidelity and with absolute control. “A lot of researchers are talking about quantum information processing with photons, and even using frequency,” researcher Joseph Lukens said. “But no one had thought about sending multiple photons through the same fiber optic strand, in the same space, and operating on them differently.” Lukens said the team’s results show that “we can control qubits’ quantum states, change their correlations, and modify them using standard telecommunications technology in ways that are applicable to advancing quantum computing.” Once the building blocks of quantum computers are in place, he said, “We can start connecting quantum devices to build the quantum internet, which is the next exciting step.” The team believes that leveraging the existing fiber optic network infrastructure — which cost billions of dollars — is practical. Its realization of closed, user-defined gates on frequency-bin qubits in parallel could be used to develop fiber-compatible quantum information processing and quantum networks. The research was published in Optica, a publication of OSA, The Optical Society (https://doi.org/10.1364/OPTICA.5.001455).