Simulator API

The top-level surface:

from qliff import Simulator, Circuit, PauliString, expectation

Name	What it is
Simulator	The ergonomic, stim-style facade you will use most.
Circuit	A stim-like IR with noise, detectors and observables -- see Noise and Error Correction.
PauliString, expectation	Pauli observables.
noise, qec	The Noise and Error Correction subpackages.
__version__	The package version (sourced from the Rust crate).

This page documents Simulator.

Simulator

Simulator(num_qubits, seed=None)

A Clifford stabilizer simulator. Starts in $|0\dots 0⟩$. Pass an integer seed for reproducible measurement sampling.

Gate methods return self, so operations chain:

from qliff import Simulator

print(Simulator(2).H(0).CX(0, 1).canon())
# ['+XX', '+ZZ']

Target conventions

• Single-qubit gates accept one or more targets. sim.X(0, 1, 2) applies the gate to each listed qubit.
• Two-qubit gates take flattened (control, target) pairs. sim.CX(0, 1) is one CX; sim.CX(0, 1, 2, 3) applies CX(0->1) then CX(2->3). An odd number of targets raises ValueError.
• Iterables are flattened, so sim.H([0, 1]) works too.

Single-qubit gates

Each returns self. Accepts one or more targets.

Method	Gate
H(*q)	Hadamard
S(*q)	Phase ($S$), maps $X\to Y$
S_DAG(*q)	Inverse phase ($S^{†}$)
X(*q)	Pauli $X$
Y(*q)	Pauli $Y$
Z(*q)	Pauli $Z$
SX(*q)	$\sqrt{X}$, applied as H S H
SX_DAG(*q)	${\sqrt{X}}^{†}$, applied as H S_DAG H

from qliff import Simulator

print(Simulator(1).H(0).S(0).canon())
# ['+Y']

Two-qubit gates

Each returns self. Takes flattened (control, target) pairs.

Method	Gate
CX(*t)	Controlled-$X$ (CNOT)
CNOT(*t)	Alias for CX
CZ(*t)	Controlled-$Z$
SWAP(*t)	Swap two qubits

from qliff import Simulator

ghz = Simulator(3).H(0).CX(0, 1, 1, 2)
print(ghz.canon())
# ['+XXX', '+ZZI', '+IZZ']

Measurement and reset

Method	Returns	Description
M(*q)	int or list[int]	Measure in the $Z$ basis, collapsing the state. Single target -> int; several -> list. Appends to record.
MR(*q)	int or list[int]	Measure, then reset the measured qubit(s) to $
R(*q)	self	Reset qubit(s) to $
MX(*q), MY(*q)	int or list[int]	Measure in the $X$ / $Y$ basis (basis change, M, change back).
MZ(*q)	int or list[int]	Alias for M.

from qliff import Simulator

s = Simulator(2).X(0)
print(s.M(0), s.M(1))   # 1 0
print(s.record) # [1, 0]

record

sim.record  # -> list[int]

Measurement outcomes so far, in order. clear() empties it and returns self.

Inspection

num_qubits

sim.num_qubits  # -> int

The number of qubits in the register.

copy()

sim.copy()  # -> Simulator

A deep copy with an independent tableau. Useful for probing whether a measurement is deterministic without disturbing the original.

fidelity(other)

sim.fidelity(other)  # -> float

The overlap $|⟨\text{sim}|\text{other}⟩{|}^2$ of two stabilizer states; see State fidelity.

stabilizers()

sim.stabilizers()  # -> list[str]

The raw (non-canonical) signed-Pauli generators, e.g. '+XX', '-Z'. These depend on the gate history.

canon()

sim.canon()  # -> list[str]

The canonical signed-Pauli generators: a unique reduced form of the stabilizer group. Two states are equal iff their canonical stabilizers match -- so this is the tool for equality checks.

from qliff import Simulator

a = Simulator(2).H(0).CX(0, 1)
b = Simulator(2).H(1).CX(1, 0)
assert a.canon() == b.canon()

Observables

Two methods read or measure a Pauli operator $P$. Both accept a PauliString or a signed string like "ZZ" or "-X".

Method	Returns	Description
peek(P)	$-1\mid 0\mid +1$	$⟨P⟩$ without collapsing the state (0 if not an eigenstate)
measure(P, force=None)	(value, random)	measures $P$ in place; value in $\{+1,-1\}$, random flags a coin flip

bell = Simulator(2).H(0).CX(0, 1)
bell.peek("ZZ")        # +1
bell.measure("XX")     # (1, False)

measure collapses the state and works on multi-qubit stabilizers -- the primitive behind syndrome extraction. Pin a random outcome with force=+1/-1.

See Observables & Metrics for PauliString, the expectation free function, and fidelity.