Background Noise Processes

Each register slot can also declare which background processes act on it. These are long-lived physical effects such as decay, dephasing, or other noise that is present whether or not someone is actively touching that subsystem.

reg = Register([Qubit(), Qubit()], [T2Dephasing(10.0), nothing])

This is a declarative noise model. You state what process is present, once, at model construction time. You do not manually weave noise updates through every gate, wait, and measurement in the protocol code.

Why This Matters

Real protocol behavior often depends on waiting time. Memory lifetimes, classical round trips, retries, and queueing delays all change the quantum state even when no gate is being applied.

If noise were modeled by hand, every protocol would need custom bookkeeping for "advance the state, then apply the operation, then advance it again." That is error-prone and it makes protocol code much harder to read.

QuantumSavory keeps that bookkeeping in the framework instead.

Time Evolution Is Demand Driven

Each subsystem carries its own local simulation time. When a protocol applies a gate, requests an observable, or otherwise touches part of the state, QuantumSavory advances the relevant subsystem to the requested time before continuing.

This means:

untouched subsystems do not consume work yet,
protocol code stays focused on protocol logic, and
different parts of a model can advance at different rates until an interaction forces synchronization.

Backend Lowering Is Automatic

The same declared noise process may need a different mathematical treatment in different numerical backends. One backend may use Kraus operators, another a Lindblad generator, and another an approximation such as twirling.

QuantumSavory handles that lowering for you. You do not need to manually derive backend-specific versions of the same physical process each time you change representation.

Available Background Types

Currently QuantumSavory implements:

AmplitudeDamping	A depolarization background.
Depolarization	A depolarization background. The `τ` parameter specifies the average time between depolarization events (assuming a Poisson point process). I.e. after time `t` the probability for an depolarization event is `1-exp(-t/τ)`.
PauliNoise	A Pauli noise background.
KrausAltWrapper	Alternative Krauss operator for testing
T1Decay	A background describing the T₁ decay of a two-level system.
T1T2Noise	A background combining both T₁ decay and T₂ dephasing.
T2Dephasing	A background describing the T₂ dephasing of a two-level system.

If you want to inspect how a declared background process is represented, use paulinoise, krausops, and lindbladop.

Where To Go Next

Read Modeling Registers, Factorization, and Time for the larger explanation of registers and framework-managed time.
Read Properties for the subsystem side of the same model description.