A DARPA program is claiming a major breakthrough in quantum computing. The Optimization with Noisy Intermediate-Scale Quantum devices (ONISQ) program has created the world's first quantum circuit with logical quantum bits (qubits).
Based on concepts that almost seem like magic or lunacy, quantum computing has the potential to revolutionize computers as we know them. By exploiting quantum effects and some relatively complex mathematics, quantum computing could speed up information processing by several orders of magnitude over classical computing and push the frontiers of artificial intelligence, biochemistry, cryptography, and many more fields.
That's all very impressive but it all stumbles at the gate because getting quantum computing beyond the experimental phase has proven rather difficult. Part of the reason is that quantum computing has a very high error rate, which isn't surprising when the principle is based on the fact that instead of the binary of ones and zeros of classical computing, something can be one or zero or both at the same time.
The trick is to find a way to turn these error-prone or "noisy" processors into something more practical by combining them with classical systems. In the case of DARPA, this involved focusing on solving optimization problems of the kind that occur in defense and industry by developing logical qubits, which are a higher-level abstraction that act like quantum algorithms and are based on Rydberg qubits that are physical components that act like a two-state quantum system.
"Rydberg qubits have the beneficial characteristic of being homogeneous in their properties – meaning each qubit is indistinguishable from the next in how they behave," said Dr. Mukund Vengalattore, ONISQ program manager in DARPA’s Defense Sciences Office. “That’s not the case for other platforms such as superconducting qubits where each qubit is unique and therefore not interchangeable.
"The homogeneity of Rydberg qubits allows them to scale rapidly and also allows them to be manipulated and moved around easily using lasers on a quantum circuit. This overcomes the current error-prone methods of performing qubit operations by having to connect them sequentially, which propagates errors throughout the chip. It’s now possible to imagine dynamic reconfiguration of qubits on a quantum chip, where you’re no longer limited to a sequential process of running quantum circuits.
"Now, you can bring entire collections of qubits, all of them, from one place in the circuit to another place on the circuit using laser tweezers, run an operation, and then put them back where they were originally. Dynamically reconfigurable and transportable Rydberg logical qubits open up completely new concepts and paradigms for designing and building scalable quantum computing processors."
Currently, DARPA has connected 48 logical qubits, but many more will be needed to approach the level of complexity needed for practical quantum computers. However, this will be far less than the millions originally imagined would be required for a fault-tolerant quantum computer.
"If anyone had predicted three years ago when the ONISQ program began that Rydberg [an excited atom with one or more electrons that have a very high principal quantum number] neutral atoms could function as logical qubits, no one would have believed it," said Dr Guido Zuccarello, a DARPA technical adviser. "It’s the DARPA way to bet on the potential of these less-studied qubits along with the more well-studied ions and superconducting circuits. As an exploratory program, ONISQ gave researchers the leeway to explore unique and new applications beyond just the optimization focus. As a result, the Harvard-led team was able to leverage much more of the potential of these Rydberg qubits and turn them into logical qubits, which is a very significant discovery."
Source: DARPA
