---
title: checked_circuit (latest version)
description: API reference for qiskit_paulice.checked_circuit in the latest version of qiskit-addon-paulice
source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit-addon-paulice/qiskit-paulice-checked-circuit
---

# Checked circuit

`qiskit_paulice.checked_circuit`

A class for specifying a circuit containing coherent spacetime Pauli checks.

### CheckedCircuit

*class* `CheckedCircuit(circuit, target_qubits=(), check_qubits=(), check_support=(), cost=None, cost_metric=None)`

[GitHub](https://github.com/Qiskit/qiskit-paulice/tree/main/qiskit_paulice/checked_circuit.py#L45-L176)

Bases: [`object`](https://docs.python.org/3/library/functions.html#object)

A quantum circuit and information about spacetime Pauli checks it contains.

**Parameters**

- **circuit** ([*QuantumCircuit*](/docs/api/qiskit/qiskit.circuit.QuantumCircuit))
- **target\_qubits** ([*tuple*](https://docs.python.org/3/library/stdtypes.html#tuple)*\[*[*int*](https://docs.python.org/3/library/functions.html#int)*, ...]*)
- **check\_qubits** ([*tuple*](https://docs.python.org/3/library/stdtypes.html#tuple)*\[*[*int*](https://docs.python.org/3/library/functions.html#int)*, ...]*)
- **check\_support** ([*tuple*](https://docs.python.org/3/library/stdtypes.html#tuple)*\[*[*tuple*](https://docs.python.org/3/library/stdtypes.html#tuple)*\[*[*int*](https://docs.python.org/3/library/functions.html#int)*, ...], ...]*)
- **cost** ([*float*](https://docs.python.org/3/library/functions.html#float) *| None*)
- **cost\_metric** ([*str*](https://docs.python.org/3/library/stdtypes.html#str) *| None*)

#### circuit

A quantum circuit containing `0` or more spacetime Pauli checks.

**Type**

[qiskit.circuit.quantumcircuit.QuantumCircuit](/docs/api/qiskit/qiskit.circuit.QuantumCircuit)

#### target\_qubits

Qubit indices of `circuit` which were used to entangle the check qubits to the payload. `None` if `circuit` contains no checks.

**Type**

[tuple](https://docs.python.org/3/library/stdtypes.html#tuple)\[[int](https://docs.python.org/3/library/functions.html#int), …]

#### check\_qubits

Qubit indices of the ancilla qubits in `circuit`. The `i``th check uses ``check_qubits[i]` to detect errors on `target_qubits[i]` and other qubits in `check_support[i]`.

**Type**

[tuple](https://docs.python.org/3/library/stdtypes.html#tuple)\[[int](https://docs.python.org/3/library/functions.html#int), …]

#### check\_support

For each check, the qubit indices whose measurement outcomes XOR together to give that check’s syndrome bit.

**Type**

[tuple](https://docs.python.org/3/library/stdtypes.html#tuple)\[[tuple](https://docs.python.org/3/library/stdtypes.html#tuple)\[[int](https://docs.python.org/3/library/functions.html#int), …], …]

#### cost

The value of the cost function with respect to the checks in `circuit`

**Type**

[float](https://docs.python.org/3/library/functions.html#float) | None

#### cost\_metric

The metric used to evaluate check quality (`gamma` or `LER`)

**Type**

[str](https://docs.python.org/3/library/stdtypes.html#str) | None

#### get\_postselection\_method

`get_postselection_method()`

[GitHub](https://github.com/Qiskit/qiskit-paulice/tree/main/qiskit_paulice/checked_circuit.py#L139-L176)

Return a function that maps a single shot’s outcome to a syndrome vector.

No errors were detected iff every entry of the returned vector is zero. The returned function accepts either bitstrings or bit arrays.

**Return type**

[*Callable*](https://docs.python.org/3/library/collections.abc.html#collections.abc.Callable)\[\[[str](https://docs.python.org/3/library/stdtypes.html#str) | [*ndarray*](https://numpy.org/doc/stable/reference/generated/numpy.ndarray.html#numpy.ndarray)], [*ndarray*](https://numpy.org/doc/stable/reference/generated/numpy.ndarray.html#numpy.ndarray)]

#### uncovered\_paulis

Type: [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple)\[[`UncoveredPauli`](#qiskit_paulice.checked_circuit.UncoveredPauli "qiskit_paulice.checked_circuit.UncoveredPauli"), ...]

Locations where a single qubit Pauli error is undetectable by some checks.

Each entry is an `UncoveredPauli(qubit, after_instruction, pauli)` triple, where `qubit` is the qubit of the single-qubit error, `after_instruction` is the `circuit.data` index of the instruction which immediately precedes the error, and `pauli` is the type of error (`"X"`, `"Y"`, or `"Z"`).

Only locations on input wires and immediately after 2-qubit gates are enumerated; errors after single qubit gates are folded into the next 2-qubit-gate wire.

### UncoveredPauli

*class* `UncoveredPauli(qubit, after_instruction, pauli)`

[GitHub](https://github.com/Qiskit/qiskit-paulice/tree/main/qiskit_paulice/checked_circuit.py#L30-L42)

Bases: [`NamedTuple`](https://docs.python.org/3/library/typing.html#typing.NamedTuple)

A spacetime location at which a single qubit Pauli error is undetectable by a set of checks.

**Parameters**

- **qubit** ([*int*](https://docs.python.org/3/library/functions.html#int))
- **after\_instruction** ([*int*](https://docs.python.org/3/library/functions.html#int) *| None*)
- **pauli** ([*Literal*](https://docs.python.org/3/library/typing.html#typing.Literal)*\['X', 'Y', 'Z']*)

#### qubit

Index of the qubit where the undetected error sits

**Type**

[int](https://docs.python.org/3/library/functions.html#int)

#### after\_instruction

Index (into `circuit.data`) of the instruction the error occurs after; `None` means the error sits on the qubit’s input wire.

**Type**

[int](https://docs.python.org/3/library/functions.html#int) | None

#### pauli

The undetected Pauli error (`"X"`, `"Y"`, or `"Z"`)

**Type**

Literal\[‘X’, ‘Y’, ‘Z’]

Create new instance of UncoveredPauli(qubit, after\_instruction, pauli)

#### count

`count(value, /)`

Return number of occurrences of value.

#### index

`index(value, start=0, stop=9223372036854775807, /)`

Return first index of value.

Raises ValueError if the value is not present.