A major obstacle holding back useful large-scale quantum computing is the difficulty in detecting and correcting errors that arise during operations due to noise, imperfect control signals, and other sources. When an error happens, it can flip the value of a sensitive quantum bit, or qubit, from 1 to 0 or vice versa. More qubits flipping leads to accumulating errors that can ultimately result in miscalculations.
Now, researchers in Australia say they have devised a method of encoding quantum information that makes the accumulation of errors less impactful and opens the door to flagging errors and correcting them when they do occur.
To demonstrate this, quantum engineers at the University of New South Wales (UNSW) and Melbourne University in Australia embedded an antimony atom into a silicon device to create a qubit. While standard qubits have two spin states—an intrinsic property of particles that can be likened to tiny spinning magnets—the heavy antimony atom has eight possible nuclear spin states. This allowed the researchers to encode the spin state at one end of the antimony atom’s spin sequence as a 0 and the spin state at the other end as a 1, with the six intermediate states serving as ancillary states.
As a way of explaining the advantage of having multiple ancillary states, Andrea Morello, professor of quantum engineering at UNSW and leader of the research group, refers to the old myth of cats having nine lives.
“The antimony atom has eight quantum levels, and it takes seven steps to go from the first to the last level, like a cat with seven [additional] lives,” says Morello. “When an error occurs, it kicks the zero up a spin level, which is not used to encode quantum information, so it retains its value and the cat survives.” This continues as errors occur until the eighth level is reached “and the 0 is flipped into a 1, and the cat dies.”
This tolerance for errors before a qubit’s value changes presents a new way to perform quantum computations, says Morello. Information is still encoded in 0s and 1s, but errors can now occur without scrambling the information.
Harnessing Antimony’s Eight “Lives”
The experimental device consists of a silicon chip upon which alignment marks are made. The chip is coated with a resist (in this case, a protective polymer resin layer to shield against ion implantation)—except for a tiny window of space that is left bare. The chip is placed into an ion implanter and then bombarded with antimony atoms until one makes its way through the bare patch and becomes embedded in the silicon.
Researchers turned to an atom of antimony, a relatively heavy element, to create error-tolerant qubits.Björn Wylezich/Alamy
“This is the standard way all semiconductors are doped,” Morello says. “What we have done is to push the process to the single atom level.” And while single atom doping in this situation is fine for research purposes, to make the process practical for quantum computing, scaling is also possible. Morello’s group, together with collaborators at Melbourne University, have also devised and demonstrated several scaling methods for embedding arrays of antimony atoms into silicon. The results of that work were published in Advanced Materials last August.
After embedding the single antimony atom, the resist is removed, and using the alignment marks for reference, the engineers fabricated a transistor and a micro antenna on top of the antimony atom. This enables them to use the antenna to deliver radio frequency and microwave signals to the atom to change and control its quantum states. As for the transistor, the researchers designed it to switch on and off as the antimony atom’s quantum states change, so that they could observe those changes.
Morello chose silicon for the chip as it is not only “the semiconductor industry workhorse,” but also because it can be processed to remove almost all atoms that could create unwanted quantum interference through their magnetic properties.
“This means we have created a very quiet environment for the antimony atom to work in,” says Morello. “And we can control its quantum states with high fidelity.” The research results were published in Nature Physics on 14 January.
Looking ahead, when more antimony atoms will be needed to build a quantum computer, the atoms will be embedded so that their locations will enable them to interact with each other and can be controlled using electrodes layered on top of the chip. “This is how we will perform logic operations,” Morello says.
But the next—and most important—step is to detect errors and correct them by measuring their locations in the intermediary quantum spin levels of the antimony atom. “This is the Holy Grail in quantum computing,” says Morello. “And it is something we hope to accomplish soon.”
