Active Qubit Reset: Measurement Feedback vs. Passive Decay

Reset Protocol

Parameters

Relaxation time T₁ 60 μs

Readout fidelity F 96%

π-pulse error rate 1.0%

Playback speed 1.0×

Live Readouts

Idle — press Prepare |1⟩ & Reset to start a run.

P(|1⟩) population

10.0%

Elapsed time

0.00 μs

Feedback rounds / status

—

How it works

A superconducting qubit left alone relaxes to |0⟩ by spontaneous emission — the excited-state population decays exponentially with time constant T₁:

p1(t) = p1(0) · e^(−t/T1)

Waiting for this passively to reach 99.9% ground-state fidelity takes about 7×T1 — tens to hundreds of microseconds, dead time the qubit can't be used for a new circuit.

Active feedback reset shortens this by closing a real quantum-classical control loop: a dispersive readout pulse measures the qubit via its coupled resonator, a classical FPGA decides the outcome, and — if the qubit read out as |1⟩ — fires a conditional π-pulse to flip it back down. The measurement itself is imperfect, so the round is modelled with a symmetric readout-fidelity F:

P(measure 1) = p1·F + (1−p1)·(1−F)
each round ≈ readout + pulse time (here 0.4 μs)
loop repeats until 2 consecutive "0" reads confirm the ground state

Each conditional π-pulse also has a small residual error probability — it can occasionally fail to flip the state, which is why the loop re-checks rather than trusting a single shot.

Passive / Active — pick which reset strategy runs on Prepare |1⟩ & Reset.
T₁ slider — the qubit's natural relaxation time; longer T1 makes passive reset proportionally slower but barely touches active reset.
Readout fidelity — how reliably the resonator measurement tells |0⟩ from |1⟩; lower fidelity forces more feedback rounds.
π-pulse error — chance a conditional flip pulse silently fails, occasionally re-arming the loop.

Real-world relevance: this exact measure→decide→correct loop is the basic building block of active qubit reset on IBM/Google/Rigetti hardware, and the same primitive — mid-circuit measurement feeding a classical decision back into the quantum circuit — underlies quantum error correction and real-time feedback control more broadly.

Qubit Bloch vector Readout resonator Measurement photons

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.