qrisp.QuantumCircuit.parity#

QuantumCircuit.parity(clbits: Clbit | Sequence[Clbit], expectation: int = 0, observable: bool = False) → ParityHandle[source]#

Append a parity (XOR) check over classical bits to the circuit.

Computes p = b_0 ⊕ b_1 ⊕ … ⊕ b_{n-1} ⊕ expectation, so p = 0 whenever the measured parity matches the expected value. This is useful for quantum error correction and when interfacing with Stim. When the circuit is converted via to_stim(), a parity instruction becomes either a DETECTOR (if observable=False) or an OBSERVABLE_INCLUDE (if observable=True) instruction.

Parameters:

clbitsClbit or Sequence[Clbit]: The classical bit(s) to compute parity over. A single Clbit measures the parity of one bit; any sequence (list, tuple, range, …) computes the XOR of all elements.
expectationint, optional: The expected parity value (0 or 1), XORed into the result so that p = 0 when the measured parity equals the expectation. Default is 0.
observablebool, optional: If True, this parity is treated as a Stim observable rather than a detector. Default is False.

Returns:

ParityHandle: A handle representing the parity result. Use it as a key to look up detector/observable indices in the maps returned by to_stim().

See also

qrisp.parity(): The gate function version for use in QuantumSessions
to_stim(): Convert to Stim circuit with detector/observable maps
qrisp.jasp.ParityHandle: Documentation of the ParityHandle class

Examples

Create a simple detector checking that two qubits have even parity:

>>> from qrisp import QuantumCircuit
>>> qc = QuantumCircuit(2, 2)
>>> qc.h(0)
>>> qc.cx(0, 1)
>>> qc.measure([0, 1], [0, 1])
>>> handle = qc.parity([qc.clbits[0], qc.clbits[1]], expectation=0)
>>> print(handle)
ParityHandle(Clbit(cb_2), Clbit(cb_3))

Convert to Stim and check the detector:

>>> stim_circuit, meas_map, det_map = qc.to_stim(
...     return_measurement_map=True,
...     return_detector_map=True
... )
>>> det_map[handle]  # Get the Stim detector index
0