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How It Works

qgate implements quantum trajectory filtering — a technique that uses mid-circuit measurements to monitor subsystem fidelity and applies decision rules to accept or reject quantum computation shots.

The Pipeline

 ┌────────────┐    ┌─────────┐    ┌───────────┐    ┌───────────┐
 │   Adapter   │───▶│  Score   │───▶│ Threshold │───▶│  Accept/  │
 │ build + run │    │  Fusion  │    │   Gate    │    │  Reject   │
 └────────────┘    └─────────┘    └───────────┘    └───────────┘
       │                                                   │
       │              TrajectoryFilter                      │
       └───────────────────────────────────────────────────┘
  1. Adapter builds a circuit with Bell-pair subsystems and mid-circuit parity measurements, then executes it on a backend.
  2. Scoring computes per-shot LF/HF scores and fuses them with α-weighting.
  3. Thresholding applies a static or dynamic threshold to determine accept/reject.
  4. The result is a FilterResult with acceptance statistics, scores, and full config provenance.

Bell-Pair Subsystems

Each subsystem is a 2-qubit Bell pair \((|00\rangle + |11\rangle)/\sqrt{2}\). Under noise, the pair's parity may flip. Mid-circuit Z-parity measurements detect these flips without collapsing the computational state.

Multi-Rate Monitoring

  • HF (high-frequency): Parity measured every cycle
  • LF (low-frequency): Parity measured every 2nd cycle

The two rates provide complementary signal: LF captures slow drift, HF catches fast errors.