Fault-tolerant Quantum Computing

Fifteen years ago, the quantum world was limited to the realm of atoms. Since then, quantum behavior has been achieved with solid-state systems at the micro- and millimeter scale. Several solid-state systems show promise for the manufacturability of quantum processors with hundreds or thousands or more qubits.

Realizing this promise, faces many exciting challenges on-chip, including materials, reliable fabrication, and connectivity between quantum elements.

Surface codes use a 2D array of data plus ancillary qubits. Data qubits carry the quantum information, ancillas detect errors (via parity checks) induced on data qubits by decoherence and faulty gates, and classical feedback electronics analyze the detected error signals and issue all corrective actions. The smallest circuits for demonstration of surface coding require 13 and 17 qubits.

We pursue their realization with three approaches : superconducting qubits, electron spin qubits in quantum dots, and spin qubits in diamond. Superconducting quantum processors with five qubits are already operational, with basic gate, readout and feedback operations demonstrated. Current emphasis is on maximizing the multiplexing of control technology to allow double-digit qubit numbers. The overall goal with quantum dot spin qubits is to demonstrate and exploit millisecond coherence and qubit control, initialization and readout at 99.9% fidelity in a surface-code compatible architecture.

In the third system, using electron and nuclear spins of atomic impurities in diamond, all basic gate, readout and feedback operations are in place. Increasing the speed of multi-qubit logic gates (via photonic coupling) is a major objective. This roadmap interacts with multiple industrial sectors. We collaborate with cryogenic equipment manufacturers (for example ‘Leiden Cryogenics’) to develop bigger and more powerful dilution refrigerators housing quantum processors and the classical electronics. Multiple connections are being established with manufacturers of digital electronics to explore cryogenic solutions. For the control of quantum processors, we discuss the tailoring of test and measurement equipment with leading manufacturers. Finally, FEI and ASM support the materials development within this roadmap.