ENERGY DECAY

Qubits don't last forever: T₁

Relaxation An excited qubit gives its energy away at a random moment.

T₁ After one T₁, about 37% are still excited.

Today Superconducting qubits reach hundreds of microseconds.

T₁ is set largely by how lossy the chip's materials are.

LOSING THE PHASE

Dephasing: T₂

Frequency jitter Noise makes the qubit tick at slightly different rates.

The result The phase blurs out, even if no energy is lost.

The limit T₂ is never more than 2T₁.

Phase is fragile: it's the first thing noise destroys.

THE ECHO

Halfway A π pulse flips every arrow.

At the end Fast and slow arrows meet again.

Limits Works for slow noise, not for fast flicker.

Clever pulses can recover coherence that seemed lost.

THE ENEMIES

Materials Defects in surface oxides and interfaces soak up energy.

Environment Heat, stray magnetic fields and even cosmic rays.

Wiring Noise travelling down the control lines.

Part 5 is about fighting these losses in the cleanroom.

COMPARING PLATFORMS

Superconducting Short-lived but very fast.

Ions and atoms Very long-lived, slower gates.

Silicon spins Long-lived in pure silicon-28.

Each platform trades lifetime against speed differently.

UNIT 11 STUDY COMPLETE

⏳

You can explain T₁ and T₂, how a spin echo works and what steals coherence.