Inside a Quantum Computer
The qubit is a circuit you could see with the naked eye, the gates are microwave pulses, and the famous golden chandelier is not the computer — it is the refrigerator.
Before this: Gates & Circuits
In this lesson you’ll learn
- Explain why a plain harmonic LC oscillator cannot be a qubit, and how the Josephson junction's anharmonicity fixes it
- Describe how a shaped resonant microwave pulse implements the rotation gates, and why RZ costs nothing
- Compute why a 5 GHz qubit demands millikelvin operation: hf/k_B ≈ 240 mK, and e^{−16} ≈ 10⁻⁷ at 15 mK
- Describe dispersive readout through a coupled resonator, and contrast the transmon with a trapped-ion qubit honestly
The qubit is a circuit you could see
Seven lessons in, a qubit has been an abstraction: two amplitudes, a point on a sphere, a wire in a diagram. Time to open the box. In the machines built by IBM, Google and a good fraction of the field, the object playing the role of the qubit is a transmon — a small superconducting circuit, patterned in aluminium on a chip, that you could point to with a finger. Its main capacitor is a few hundred micrometres across: visible to the naked eye, millions of times larger than an atom, and made of billions of paired electrons all doing the same thing.
The starting point is the humblest circuit in the electronics curriculum: a capacitor and an inductor in a loop, an LC oscillator, with charge sloshing back and forth at a resonant frequency set by L and C. Build it from superconducting metal, cold enough that resistance is exactly zero, and the oscillation stops being damped classical sloshing and becomes a quantum system: its energy is quantized into discrete levels, a ladder of states you can hope to use. The demonstration that an electrical circuit — a macroscopic, manufactured object with soldered leads — really does show quantized energy levels and tunnelling was made by John Clarke, Michel Devoret and John Martinis in the mid-1980s, and it earned them the 2025 Nobel Prize in Physics. Every transmon is a working consequence of that result.
It is worth pausing on what this is not. The qubit is not an electron, and nothing here is small — "quantum" is not a synonym for "tiny". What makes the circuit quantum is not its size but its isolation: superconductivity removes the resistance that would leak its energy, and the rest of this lesson is about removing everything else.
But a plain LC oscillator, quantized or not, fails as a qubit — and the way it fails is the most instructive fact in this lesson. Before reading on, commit to a prediction.
Predict before you read on
A superconducting LC circuit is a harmonic oscillator: its energy levels form a ladder with exactly equal spacing, every rung hf apart. You drive it with microwaves at frequency f, intending to use the bottom two rungs as |0⟩ and |1⟩. What happens?
Commit to an answer — the next section is the payoff.