Not the math. The metal. Scroll down.
A silicon chip. 7 millimeters across.
Cooled to 15 millikelvin — colder than outer space.
Cold means quiet.
At room temperature, thermal energy is much larger than the qubit's energy gap. Random photons constantly kick the qubit between |0⟩ and |1⟩ — you can't even tell which state it's in. At 15 mK, thermal energy is negligible. The qubit naturally sits in |0⟩, waiting. You have a clean starting point.
Same reason atoms have sharp spectral lines at low temperature. A transmon is an artificial atom.
An artificial atom.
A real atom has discrete energy levels — it absorbs light only at specific frequencies. This cross-shaped circuit does the same thing, but with microwaves instead of light, and at frequencies engineers can choose. The energy levels come from the Josephson junction (the pink dot) — we'll zoom in later.
300 μm of niobium on silicon. Four arms: drive, flux, readout, coupling.
The microwave arrives.
A room-temperature signal generator produces microwaves at the qubit's exact frequency — 5.55 GHz for this qubit. How do we know the frequency? Spectroscopy: we sweep across frequencies and watch for absorption. Same technique astronomers use to identify elements in stars.
This is the key insight: gates are precisely timed pulses. A half-cycle (π pulse) is an X gate. A quarter-cycle (π/2 pulse) creates superposition. The pulse duration, amplitude, and phase determine which gate you apply.
Every gate is a microwave pulse.
Different duration, different gate.
All single-qubit gates are just microwave pulses with different durations, amplitudes, and phases. The pulse shape is a Gaussian envelope modulated at the qubit frequency. Gate times: ~20–40 nanoseconds.
Building entanglement.
Step through the circuit.
Both qubits in |0⟩. Cold. Quiet. Separate.
Single-qubit gates (H, X, Z) are microwave pulses on one qubit. They can create superposition but never entanglement. You need a two-qubit gate — which requires a physical coupling element between the qubits on the chip.
Coherence is phase memory.
When a qubit is in superposition, the relative phase between |0⟩ and |1⟩ carries quantum information. Decoherence means losing that phase relationship. The qubit “forgets” where it is in the oscillation cycle. The environment — thermal photons, magnetic noise, defects in the oxide — randomly shifts the phase until it's meaningless.
T2 ≈ 10–20 µs on Tuna-9. Gates take ~20 ns. So: a few hundred operations before the phase is lost.
The nonlinear element.
This oxide layer makes the energy levels anharmonic — unequally spaced. Without it, the circuit would be a harmonic oscillator: equally spaced levels, no way to address just one transition. With it, you get an artificial atom with addressable levels. Ten atoms of oxide is the difference between a resonator and a qubit.
Same idea as real atoms: hydrogen absorbs 121.6 nm light because that's one specific transition. A transmon absorbs 5.55 GHz microwaves for the same reason — engineered instead of natural.
Six orders of magnitude.
The barrier that makes quantum computing possible is about ten atoms thick.