ENERGY LEVELS

Quantum things have energy levels

Only certain energies Atoms and circuits have fixed levels, like rungs on a ladder.

E = hf A photon is absorbed only if its energy matches a gap.

Selectivity You can address one transition and leave the others alone.

Quantised levels are what make precise qubit control possible.

PICK TWO LEVELS

Why a qubit needs uneven steps

Even ladder A plain circuit's levels are evenly spaced, so one pulse drives them all.

Uneven ladder A transmon's steps shrink a little each time.

The qubit Drive exactly the 0→1 frequency and only those two levels respond.

The Josephson junction makes the ladder uneven; you'll build one in Part 3.

MICROWAVES

Superconducting qubits About 4–8 GHz, near Wi-Fi.

Spin qubits About 10–40 GHz.

Ions and atoms Lasers, plus microwave clock transitions.

Quantum computing borrows heavily from RF and microwave engineering.

COLDER THAN SPACE

A 5 GHz qubit Its energy matches heat at only about 0.24 kelvin.

Room temperature Heat would flip it about half the time.

10 millikelvin The chance drops to about one in a hundred billion.

Dilution refrigerators make superconducting qubits possible.

DRIVING A QUBIT

A resonant drive Rotates the qubit back and forth between 0 and 1.

A π pulse Half a cycle: flips 0 into 1, the quantum NOT gate.

Detuning Drive at the wrong frequency and it never fully flips.

Every gate on a superconducting chip is a carefully timed microwave pulse.

UNIT 10 STUDY COMPLETE

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You've seen why a qubit needs uneven energy levels, why it lives at microwave frequencies and why it must be so cold.