Background

The current Qulacs backend is effectively noise-free (ideal simulation).
We want to introduce a practical noise model so simulation can reflect device calibration/topology data.

Current state (before implementation)

• Execution uses ideal state simulation only
• No gate noise is applied
• No readout error is applied
• Device topology calibration fields are not used for noise behavior

Proposal

Add a configurable noise model for the Qulacs backend with the following scope:

1. Gate noise

• Single-qubit depolarizing noise
• Two-qubit depolarizing noise
• Option to derive probabilities from topology fidelity:
  • 1Q: p = 1 - qubits[].fidelity
  • 2Q: p = 1 - couplings[].fidelity
• Optional fixed-probability override via config

2. Readout error

• Support asymmetric readout error using topology fields:
  • prob_meas1_prep0
  • prob_meas0_prep1
• Apply readout error to measurement outcomes in a clear, testable way

3. Config interface (proposed)
   Under plugin.noise_model:

• enabled
• use_topology_fidelity
• single_qubit_depolarizing (optional override)
• two_qubit_depolarizing (optional override)
• readout_error

Review points

• Is asymmetric readout error required for expected use cases?
• Should readout be modeled exactly (custom post-processing) or approximated with standard-only behavior?
• Should topology-derived noise be default, with fixed values as optional override?
• Do we need directional handling policy for coupling lookup (control->target only vs bidirectional)?

Suggested acceptance criteria

• Team agrees on model scope and default behavior
• Config schema and docs are explicit about supported behavior
• Tests cover:
  • backward compatibility when noise is disabled
  • topology-derived gate noise
  • readout error behavior
  • probability bounds validation ([0, 1])

Affected areas (expected)

• src/device_gateway/plugins/qulacs/backend.py
• src/device_gateway/plugins/qulacs/circuit.py
• tests/device_gateway/plugins/qulacs/test_backend.py
• config/config.yaml
• config/config.yaml.qulacs
• config/example/config.yaml