Headline-grabbing quantum computing efforts by Google and IBM have mostly focused on building quantum bits, called qubits, out of loops of superconducting materials. But a recent breakthrough could enable a different technology based on spin-based silicon qubits to eventually dominate the rise of quantum computers.
The theoretical advantage of quantum computing rests upon qubits representing multiple states simultaneously, whereas classical computing bits can only represent information as either a 1 or 0. Toward that end, Australian and U.S. researchers have developed qubits based on either the nuclear or electron spin state of phosphorus atoms embedded in silicon. Their latest work has yielded the concept of a “flip-flop qubit” that combines both electron and nuclear spin states—an approach that enables neighboring qubits to remain coupled together despite being separated by larger physical distances. In turn, that makes it much easier to build control schemes for the large arrays of qubits necessary for full-fledged quantum computing.
“[T]he real challenge when trying to fabricate and operate 100, 1,000, or millions of qubits is how to lay out the classical components, such as interconnects and readout transistors,” says Andrea Morello, a quantum physicist at the University of New South Wales, in Australia. “So, having the qubits spaced out by 200 to 500 nanometers from each other means that we have all that space between them to intersperse all the classical control and readout infrastructure, while using fabrication technologies that are already commonplace in the semiconductor industry.”
Morello did not mince words in describing his team’s conceptual insight—spearheaded by lead author Guilherme Tosi and published in the Sept. 6 2017 online issue of the journal Nature Communications—as a “stroke of genius.” It seems counterintuitive at first glance, because it apparently gives up the potential for two qubits—one based on an electron spin and one based on nuclear spin—in exchange for just one qubit.
But the researchers based at the University of New South Wales and Purdue University in the United States soon realized that the advantage of the flip-flop qubit comes from inducing an electric dipole—separation of positive and negative charges—by pulling the electron a little bit away from the nucleus of the phosphorus atoms (which are themselves embedded in silicon). That electric dipole enables the spin-based silicon qubits to remain entangled together over longer distances and able to influence one another through quantum physics. The wider separations between individual qubits make it easier to squeeze in the classical computing circuitry necessary for controlling qubits.
This conceptual breakthrough in building larger arrays of spin-based silicon qubits could transform the future direction of quantum computing. With the notable exception of the Canadian company D-Wave, most companies are focused on building universal gate-model quantum computers that can tackle a wide range of problems. The largest universal quantum computing machines built so far have been based on superconducting qubit arrays—an approach embraced by tech giants such as Google and IBM. By the end of 2017, Google aims to build a 49-qubit chip based on superconducting qubits that can definitively prove quantum computing’s ability to outperform classical computers for the first time.
The Australian team pushing for spin-based silicon qubits has more modest goals by comparison: developing a 10-qubit array by 2022. But the new flip-flop qubit approach could make it “much more realistic and economical” to expand beyond 10 qubits and eventually scale up to thousands or millions of qubits, Morello says. (Both the spin-based silicon qubits and superconducting qubits can be manufactured relatively easily based on modern semiconductor industry techniques.)
Morello envisions spin-based silicon qubits potentially taking over the lead in the quantum computing race from superconducting qubits, possibly within a decade. That is because larger arrays of superconducting qubits could eventually run into scaling issues because of their relatively large individual qubit sizes. By comparison, researchers could theoretically place more than one million spin-based silicon qubits on a square millimeter of space. “[B]ased on what we know now, I do imagine that silicon could become the system of choice at the thousands or millions of qubits level,” Morello says.
The researchers also discovered that the new flip-flop qubits can be controlled with electric fields rather than magnetic fields. That’s a big deal because the electron and nuclear spins of phosphorus atoms respond only very weakly to magnetic fields. Previously, researchers tried creating stronger magnetic fields that could affect the spin states—but at the cost of also creating electrical fields that could interfere with components designed to readout the information from the qubits.
Morello and his colleagues eventually realized that flip-flop qubits respond strongly to resonant electric fields that are “tuned to the exact frequency at which the electron and nucleus flip-flop with each other,” Morello explains. As an added bonus, the flip-flop qubits do not respond to any other electric fields outside that particular frequency. That meant the researchers could use a relatively weak electric field to control the flip-flop qubits while being assured that no other electrical field interference would disturb the qubit operations.
There does not need to be only one winner between spin-based silicon qubits and superconducting qubits in the long-term race for quantum computing. Some superconducting qubit architectures, called transmon qubits, could naturally interface with flip-flop qubits based in silicon, Morello says. And besides, superconducting qubits are often manufactured as a layer on top of a silicon chip. It’s very possible that researchers may want to eventually “mix and match flip-flop qubits in silicon with superconducting qubits” in some future quantum computing applications—a way to leverage the best of both worlds.