Superconducting Qubits: The Silicon of Quantum Tech

Introduction

Superconducting qubits are electronic circuits fabricated on silicon or sapphire chips, operating at microkelvin temperatures. They are currently the leading technology used by major quantum computing developers, including IBM, Google, and Rigetti.

The Josephson Junction

A simple LC resonator circuit behaves like a harmonic oscillator, having evenly spaced energy levels. This makes it unsuitable as a qubit, as a photon pulse would drive transitions to higher states ($|1\rangle \to |2\rangle$) instead of remaining in the $|0\rangle \leftrightarrow |1\rangle$ subspace.

To solve this, superconducting qubits use a Josephson Junction (a thin insulating barrier between two superconductors). The junction acts as a non-linear inductor, introducing anharmonicity. This spreads the energy spacing, isolating the lowest two energy levels ($|0\rangle$ and $|1\rangle$) for use as a qubit.

The Transmon Qubit

The most common design is the Transmon (Transmission line shunted plasma oscillation qubit). It shunts the Josephson junction with a large capacitor to drastically reduce the qubit's sensitivity to charge noise, extending coherence times from nanoseconds to hundreds of microseconds.