---
title: AncillaQubit (v0.46)
description: API reference for qiskit.circuit.AncillaQubit in qiskit v0.46
source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit/0.46/qiskit.circuit.AncillaQubit
---

# AncillaQubit

*class* `qiskit.circuit.AncillaQubit(register=None, index=None)`

[GitHub](https://github.com/qiskit/qiskit/tree/stable/0.46/qiskit/circuit/quantumregister.py)

Bases: [`Qubit`](/docs/api/qiskit/0.46/qiskit.circuit.Qubit "qiskit.circuit.quantumregister.Qubit")

A qubit used as ancillary qubit.

Creates a qubit.

**Parameters**

- **register** ([*QuantumRegister*](/docs/api/qiskit/0.46/qiskit.circuit.QuantumRegister "qiskit.circuit.QuantumRegister")) – Optional. A quantum register containing the bit.
- **index** ([*int*](https://docs.python.org/3/library/functions.html#int)) – Optional. The index of the bit in its containing register.

**Raises**

[**CircuitError**](/docs/api/qiskit/0.46/circuit#qiskit.circuit.CircuitError "qiskit.circuit.CircuitError") – if the provided register is not a valid [`QuantumRegister`](/docs/api/qiskit/0.46/qiskit.circuit.QuantumRegister "qiskit.circuit.QuantumRegister")

## Attributes

### index

Get the index of an old-style bit in the register that owns it.

In modern Qiskit Terra (version 0.17+), bits are the fundamental object and registers are aliases to collections of bits. A bit can be in many registers depending on the circuit, so a single containing register is no longer a property of a bit. It is an error to access this attribute on bits that were not constructed as “owned” by a register.

> **Deprecated since version 0.17**
>
> The property `qiskit.circuit.bit.Bit.index` is deprecated as of qiskit-terra 0.17. It will be removed in the Qiskit 1.0 release. Instead, use `find_bit()` to find all the containing registers within a circuit and the index of the bit within the circuit.

### register

Get the register of an old-style bit.

In modern Qiskit Terra (version 0.17+), bits are the fundamental object and registers are aliases to collections of bits. A bit can be in many registers depending on the circuit, so a single containing register is no longer a property of a bit. It is an error to access this attribute on bits that were not constructed as “owned” by a register.

> **Deprecated since version 0.17**
>
> The property `qiskit.circuit.bit.Bit.register` is deprecated as of qiskit-terra 0.17. It will be removed in the Qiskit 1.0 release. Instead, use `find_bit()` to find all the containing registers within a circuit and the index of the bit within the circuit.