Engineers have invented a new architecture for quantum computing based on novel “flip-flop” qubits. The new chip design allows for a silicon quantum processor that can be scaled up without the precise placement of atoms required in other approaches. It allows quantum bits (qubits) — the basic unit of information in a quantum computer — to be placed hundreds of nanometers apart and still remain coupled.
The technology is a “spin qubit” that uses both the electron and the nucleus of the atom. The new qubit can be controlled using electric signals instead of magnetic signals. 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 engineers began building larger and larger arrays of qubits: the need to space them at a distance of only 10-20 nanometers, or just 50 atoms apart. If the qubits are too close or too far apart, the entanglement between quantum bits does not occur.
In order to make an array of thousands or millions of qubits so close together, the control lines, control electronics, and readout devices also must be fabricated at the nanometer scale, and with that pitch and that density of electrodes. The new concept suggests an alternative pathway.
In the single-atom qubit, a silicon chip is covered with a layer of insulating silicon oxide, on top of which rests a pattern of metallic electrodes that operates at temperature near absolute zero, and in the presence of a very strong magnetic field. At the core is a phosphorus atom, from which the engineers previously built two functional qubits using an electron and the nucleus of the atom. In the new 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 — this is called a flip-flop qubit.
To operate the qubit, the electron must be pulled away from the nucleus using the electrodes at the top. By doing so, an electric dipole also is created. These dipoles interact with each other over fairly large distances — a fraction of a micron, or 1,000 nanometers. Thus, the single-atom qubits can be placed much further apart than previously thought possible, enabling the interspersing of key components such as interconnects, control electrodes, and readout devices, while retaining the precise atom-like nature of the quantum bit.