Quantum computing has been making big promises for a number of years now, but there is still very little in the way of practical applications for the technology. Sweet physics will only take you so far, which is why a new discovery from John Morton at the University of Oxford is so interesting. The record time for maintaining quantum computing bits, or qubits, in a solid material has been broken in spectacular fashion. Researchers have pushed the limit from a few seconds to a full three minutes.
As engineers continue sparring with the physical limits of Moore’s law, quantum computing becomes more attractive. A traditional computer bit can represent either a 1 or a 0. A qubit in a quantum computer is capable of being both 1 and 0 at the same time. This is possible thanks to the property of superposition, which holds that a physical system can exist in all its theoretical states simultaneously. It also has the potential to make quantum computers incredibly fast, but if qubits can’t be maintained longer than a few seconds, you can’t do any meaningful work with them.
The trick used at the University of Oxford has to do with coaxing the computational particles into superposition and keeping them there in two different spin states simultaneously. Researchers used a form of extremely pure silicon-28 (which is non-magnetic and non-reactive) as the medium for the test. A number of phosphorus atoms were suspended in the silicon, and scientists found that the particles behaved as if they were in a vacuum. That is, there was very little interaction with the medium.
Reducing interference is essential in building solid-state quantum computers. Even the tiniest effect on the spin of a particle can disrupt its quantum state. The key to prolonging the qubits in this experiment was the use of radio frequency pulses. Researchers determined the necessary radio pulse intensity to flip the particles’ spin 180 degrees. The isolated phosphorus atoms were bombarded with pulses that were exactly half that intense. This left the phosphorus in a superposition of being both flipped and not flipped. Therefore, we have a qubit.
By pinging the system they devised at regular intervals with the right radio pulses, the researchers were able to successfully maintain the superposition of the atoms for 192 seconds, or a little over three minutes. The pulses prevent the phosphorus from interacting weakly with the silicon, and since there are no contaminants to exert some magnetic force on them, the qubits remain active.
What’s really fascinating about this, is that silicon is a material actually used in computers. Previous efforts at retaining qubits for this length of time required operating in a vacuum, or sometimes in liquid or gas phase mediums. These approaches are fine for research, but they don’t get us any closer to a practical quantum computer. Getting long-lived qubits to operate in a material that can be used outside a lab is a big step, and that’s just what Morton and his colleagues at Oxford have managed.
Read more at Science (paywalled)