Researchers Invent New Architecture for Quantum Computing

A new architecture, based on so-called ‘flip-flop’ qubits, allows for a silicon quantum processor that can be scaled up without the precise placement of atoms required in other approaches. Importantly, it allows qubits to be placed hundreds of nanometers (nm) apart and still remain coupled.

Schematic view of a large-scale quantum processor based upon phosphorus (31P) donors in silicon, operated and coupled through the use of an induced electric dipole. Idle qubits have electrons at the interface, leaving the 31P nucleus in the ultra-coherent ionized state. Electrons are partially shifted toward the donor for quantum operations. The sketch shows a possible architecture where a cluster of qubits is locally coupled via the electric dipole, and a subgroup thereof is further coupled to another cluster through interaction with a shared microwave cavity (aqua). Image credit: Guilherme Tosi, University of New South Wales.

Schematic view of a large-scale quantum processor based upon phosphorus (31P) donors in silicon, operated and coupled through the use of an induced electric dipole. Idle qubits have electrons at the interface, leaving the 31P nucleus in the ultra-coherent ionized state. Electrons are partially shifted toward the donor for quantum operations. The sketch shows a possible architecture where a cluster of qubits is locally coupled via the electric dipole, and a subgroup thereof is further coupled to another cluster through interaction with a shared microwave cavity (aqua). Image credit: Guilherme Tosi, University of New South Wales.

“It’s a brilliant design, and like many such conceptual leaps, it’s amazing no-one had thought of it before,” said Andrea Morello, professor of quantum engineering at the University of New South Wales in Australia and senior author of a paper published in the journal Nature Communications.

“What we have invented is a new way to define a ‘spin qubit’ that uses both the electron and the nucleus of the atom.”

“Crucially, this new qubit can be controlled using electric signals, instead of magnetic ones. Electric signals are significantly easier to distribute and localize within an electronic chip.”

“The design sidesteps a challenge that all spin-based silicon qubits were expected to face as teams begin building larger and larger arrays of qubits: the need to space them at a distance of only 10-20 nm, or just 50 atoms apart,” added lead author Dr. Guilherme Tosi, also from the University of New South Wales.

“If they’re too close, or too far apart, the ‘entanglement’ between quantum bits — which is what makes quantum computers so special — doesn’t occur.”

“If we want to make an array of thousands or millions of qubits so close together, it means that all the control lines, the control electronics and the readout devices must also be fabricated at that nanometric scale, and with that pitch and that density of electrodes. This new concept suggests another pathway,” Professor Morello said.

“At the other end of the spectrum are superconducting circuits — pursued for instance by IBM and Google — and ion traps. These systems are large and easier to fabricate, and are currently leading the way in the number of qubits that can be operated. However, due to their larger dimensions, in the long run they may face challenges when trying to assemble and operate millions of qubits, as required by the most useful quantum algorithms.”

“Our new silicon-based approach sits right at the sweet spot. It’s easier to fabricate than atomic-scale devices, but still allows us to place a million qubits on a square millimeter.”

 

In the single-atom qubit used by the researchers, a silicon chip is covered with a layer of insulating silicon oxide, on top of which rests a pattern of metallic electrodes that operate at temperatures near absolute zero and in the presence of a very strong magnetic field.

At the core is a phosphorus atom, from which the team has previously built two functional qubits using an electron and the nucleus of the atom. These qubits, taken individually, have demonstrated world-record coherence times.

The team’s conceptual breakthrough is the creation of an entirely new type of qubit, using both the nucleus and the electron.

In this approach, a qubit ‘0’ state is defined when the spin of the electron is down and the nucleus spin is up, while the ‘1’ state is when the electron spin is up, and the nuclear spin is down.

“We call it the ‘flip-flop’ qubit. To operate this qubit, you need to pull the electron a little bit away from the nucleus, using the electrodes at the top. By doing so, you also create an electric dipole,” Dr. Tosi said.

“This is the crucial point. These electric dipoles interact with each other over fairly large distances, a good fraction of a micron, or 1,000 nm,” Professor Morello said.

“This means we can now place the single-atom qubits much further apart than previously thought possible. So there is plenty of space to intersperse the key classical components such as interconnects, control electrodes and readout devices, while retaining the precise atom-like nature of the quantum bit.”

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Guilherme Tosi et al. 2017. Silicon quantum processor with robust long-distance qubit couplings. Nature Communications 8, article number: 450; doi: 10.1038/s41467-017-00378-x

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