For more than 30 years, Dr. Robert Schoelkopf has been working on one of quantum computing's biggest challenges: how to build a better qubit. His answer is the dual-rail qubit, a first-of-its-kind superconducting qubit that embeds error detection directly at the hardware level. In this episode of Quantum Matters, host Murray Thom sits down with Rob, a pioneer of gate-model quantum computing and now Chief Scientist at D-Wave. Rob traces the evolution of qubit design, from the transmon to the dual-rail, and explains why the first qubit you build isn't necessarily the one that scales, and how a design that can flag its own errors offers a faster, more efficient path to fault-tolerant gate quantum computing. Along the way, Rob shares how gate-model systems differ from annealing quantum computers, his take on quantum hype, and why fault-tolerant gate systems may make their first big impact in scientific discovery. Learn More: https://www.dwavequantum.com/solutions-and-products/systems/gate-model-quantum-computing/ Glossary Cat qubit A superconducting qubit that encodes information in special resonator states designed to suppress certain types of errors. One of several qubit designs Dr. Schoelkopf helped develop. Coherence / coherence time How long a qubit maintains its fragile quantum state before noise degrades it. Longer coherence means more operations can be completed before errors accumulate. In the ice sculpture analogy, it's how long the blocks last before melting. Cooper pair box An early superconducting qubit design based on pairs of electrons (Cooper pairs) on a tiny superconducting island. A precursor to the transmon, and part of the story of Dr. Schoelkopf's early work. Dual-rail qubit (DRQ) A first-of-its-kind superconducting cavity-based qubit architecture, invented by Dr. Schoelkopf and colleagues, that embeds error detection directly in the device design. Two cavities encode a quantum bit of information in a single, shared photon. The dominant error mode, photon loss, produces an invalid state that the qubit itself can detect and flag, enabling highly efficient error correction. Entanglement A quantum phenomenon in which two or more qubits become correlated so strongly that the state of one cannot be described independently of the others; measuring or operating on one affects its partners. Entangling gates are the operations that create this connection, and they're a core building block of gate-model computation. Error correction Methods for protecting quantum information so a computation can continue reliably despite errors. Error correction requires redundancy, typically many physical qubits working together to protect each logical qubit, and this overhead is one of the biggest costs in quantum computing. Because the dual-rail qubit detects errors on its own at the hardware level, it is designed to reduce that overhead by a factor of 10. In the ice sculpture analogy, it's refreezing the blocks as you build. Error detection The dual-rail qubit's built-in ability to recognize when it has experienced an error. When the qubit's photon is lost, the result is an invalid state the hardware itself identifies and flags. Conventional qubits fail silently, and errors must be inferred indirectly by measuring many additional qubits; the dual-rail qubit identifies the error itself, at the individual qubit, as it happens. Error awareness What error detection makes possible for programmers: knowing when and where errors occur during a computation and being able to use that information, in real time, within an algorithm. Error awareness turns errors from silent failures into usable data. Fault tolerance The ability of a quantum system to keep computing reliably despite errors, achieved in gate quantum computing through error correction. The key requirement for commercial-scale gate-model applications. Noise Unwanted disturbance from the environment, such as heat, vibration, or stray electromagnetic fields, that corrupts fragile quantum states and causes errors. In the ice sculpture analogy, noise is what melts the blocks. Photon A single particle of light. In the dual-rail qubit, one photon shared between two cavities carries the quantum information. Physical qubit vs. logical qubit A physical qubit is an actual hardware device. A logical qubit is an error-protected unit of information built from many physical qubits working together. Conventional error-correction approaches can require roughly 1,000 physical qubits per logical qubit; D-Wave's dual-rail approach is designed to reduce that to roughly 100. Quantum gate A basic operation applied to one or more qubits, such as a bit flip or an entangling gate. The building blocks of gate-model programs, analogous to logic gates in classical computing. Quantum simulation Using a quantum computer to model quantum-mechanical systems such as molecules and materials, which are hard for classical computers precisely because they are intrinsically quantum. As Dr. Schoelkopf puts it in the episode, it's "fighting quantum with quantum." Shor's algorithm A quantum algorithm, discovered by Peter Shor in 1994, for factoring large numbers exponentially faster than known classical methods. A landmark result that sparked serious interest in building quantum computers, and a turning point in Dr. Schoelkopf's own career. Superconductivity The property of certain materials, when cooled to extremely low temperatures, to conduct electricity with zero resistance. The physical foundation of D-Wave's qubits, both annealing and dual-rail. Superposition A qubit's ability to exist in a combination of 0 and 1 at the same time, rather than one or the other. Part of what gives quantum computers their power. Transmon A widely used type of superconducting qubit developed by Dr. Schoelkopf and colleagues at Yale. It greatly improved qubit stability and reproducibility and became the dominant design across the industry, and the starting point of the arc that led to the dual-rail qubit. Highlights: 02:33 - The Early Days of Superconducting Qubits 06:20 - Lessons from the Quantum Computing Industry 23:09 - The Future Impact of Quantum Computing
