Topological Quantum Computing

For quantum computing to take its next big step forward, we need a stable qubit. 

The qubits used in today’s quantum computers are inherently unstable. The slightest external stimulus — whether a change in temperature, a shift in electromagnetic field or an errant particle — can cause a qubit to lose stability and therefore its computational capabilities. The lifespan of these qubit often lasts mere milliseconds. Using this first generation of qubits requires building massive redundancy into a quantum computer, in essence using brute force to correct the chain reaction of errors these unstable qubits generate.

Nokia Bell Labs is researching a new generation of qubit that is intrinsically stable. This topological approach to quantum computing uses electromagnetic fields to maneuver charges around a supercooled electron liquid. This manipulation of charges acts as a switch between topological states in the qubits. The resulting braided structure is extremely stable as the quantum states of qubits are locked in place for days or weeks at a time. 

This topological qubit could be revolutionary for the nascent quantum computing industry, enabling computers of incredible power, while minimizing their size and cost as well as the resources necessary to maintain them.

Bell Labs’ topological quantum computing research is based on decades of experience in ultra-pure semiconductor materials, enabled by the invention of molecular beam epitaxy (MBE), and fundamental advances in ultra-cold physics, specifically our Nobel Prize winning discovery of the Fractional Quantum Hall Effect. Since our first demonstration of a topological state over a decade ago, we have advanced our understanding and technical capabilities, which has led to significantly higher quality devices and lifetimes of hours to days.

Our topological system consists of electrons confined in a gallium arsenide quantum well. At low temperatures (<100mK) and high magnetic field (>1T), these electrons condense into a liquid that can host electrical charges that are a fraction of the charge of an electron. The liquid can be shaped by applying fields from surface electrodes and individual fractional charges can be maneuvered around one another through properly choreographed voltages applied to these surface electrodes. The braiding of charges acts as a switch between topological states in the electron liquid. It is incredibly unlikely for two charges to accidentally move around each other in exactly the correct way to change the topological state, which makes these states extremely stable.

A full qubit is a device that can perform a universal set of computational gates that change its state, gates that include Hadamard and Phase gates. To perform these operations, only three types of charge maneuvers are required: (1) moving a charge from one side of a device to a central capacitor called a “charging dot”, (2) moving a charge from one side of a device to the other side, and (3) exchanging the charge on one charging dot with the charge from another charging dot. All these maneuvers are performed through a coordinated set of voltages applied to the different electrodes in a device.

We have already perfected the side-to-charging dot maneuver and plan to demonstrate the remaining two maneuvers in the next year. Once this is complete, all three will be combined to perform a full set of single-qubit gates and therefore demonstrate full topological qubit. We hope to achieve this milestone in 2026.
 

Scanning electron microscope images (top row) and schematics (bottom row) of the three fundamental devices required to demonstrate universal charge manipulations in a topological qubit.