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

# PhaseOracle

*class* `qiskit.circuit.library.PhaseOracle(expression, var_order=None)`

[GitHub](https://github.com/Qiskit/qiskit/tree/stable/2.5/qiskit/circuit/library/phase_oracle.py#L26-L144)

Bases: [`QuantumCircuit`](/docs/api/qiskit/qiskit.circuit.QuantumCircuit "qiskit.circuit.quantumcircuit.QuantumCircuit")

Phase Oracle.

The Phase Oracle object constructs circuits for any arbitrary input logical expressions. A logical expression is composed of logical operators & (logical AND), | (logical OR), \~ (logical NOT), and ^ (logical XOR). as well as symbols for literals (variables). For example, ‘a & b’, and (v0 | \~v1) & (\~v2 & v3) are both valid string representation of boolean logical expressions.

A phase oracle for a boolean function f(x) performs the following quantum operation:

$$
|x\rangle \mapsto (-1)^{f(x)}|x\rangle
$$

For convenience, this oracle, in addition to parsing arbitrary logical expressions, also supports input strings in the [DIMACS CNF format](https://web.archive.org/web/20190325181937/https://www.satcompetition.org/2009/format-benchmarks2009.html), which is the standard format for specifying SATisfiability (SAT) problem instances in [Conjunctive Normal Form (CNF)](https://en.wikipedia.org/wiki/Conjunctive_normal_form), which is a conjunction of one or more clauses, where a clause is a disjunction of one or more literals. See `qiskit.circuit.library.phase_oracle.PhaseOracle.from_dimacs_file()`.

From 16 variables on, possible performance issues should be expected when using the default synthesizer.

> **Deprecated since version 2.2**
>
> The class `qiskit.circuit.library.phase_oracle.PhaseOracle` is deprecated as of Qiskit 2.2. It will be removed in Qiskit 3.0. Use the class qiskit.circuit.library.PhaseOracleGate instead.

**Parameters**

- **expression** ([*str*](https://docs.python.org/3/library/stdtypes.html#str) *| BooleanExpression*) – A Python-like boolean expression string or a BooleanExpression object.
- **var\_order** ([*list*](https://docs.python.org/3/library/stdtypes.html#list)*\[*[*str*](https://docs.python.org/3/library/stdtypes.html#str)*] | None*) – A list with the order in which variables will be created. (default: by appearance)

## Attributes

### name

Type: `str`

A human-readable name for the circuit.

**Example**

```python
from qiskit import QuantumCircuit

qc = QuantumCircuit(2, 2, name="my_circuit")
print(qc.name)
```

```text
my_circuit
```

## Methods

### evaluate\_bitstring

`evaluate_bitstring(bitstring)`

[GitHub](https://github.com/Qiskit/qiskit/tree/stable/2.5/qiskit/circuit/library/phase_oracle.py#L83-L94)

Evaluate the oracle on a bitstring. This evaluation is done classically without any quantum circuit.

**Parameters**

**bitstring** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)) – The bitstring for which to evaluate. The input bitstring is expected to be in little-endian order.

**Returns**

True if the bitstring is a good state, False otherwise.

**Return type**

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

### from\_dimacs\_file

*classmethod* `from_dimacs_file(filename)`

[GitHub](https://github.com/Qiskit/qiskit/tree/stable/2.5/qiskit/circuit/library/phase_oracle.py#L96-L144)

Create a PhaseOracle from the string in the DIMACS format.

It is possible to build a PhaseOracle from a file in [DIMACS CNF format](https://web.archive.org/web/20190325181937/https://www.satcompetition.org/2009/format-benchmarks2009.html), which is the standard format for specifying SATisfiability (SAT) problem instances in [Conjunctive Normal Form (CNF)](https://en.wikipedia.org/wiki/Conjunctive_normal_form), which is a conjunction of one or more clauses, where a clause is a disjunction of one or more literals.

The following is an example of a CNF expressed in the DIMACS format:

```text
c DIMACS CNF file with 3 satisfying assignments: 1 -2 3, -1 -2 -3, 1 2 -3.
p cnf 3 5
-1 -2 -3 0
1 -2 3 0
1 2 -3 0
1 -2 -3 0
-1 2 3 0
```

The first line, following the c character, is a comment. The second line specifies that the CNF is over three boolean variables — let us call them $x_1, x_2, x_3$, and contains five clauses. The five clauses, listed afterwards, are implicitly joined by the logical AND operator, $\land$, while the variables in each clause, represented by their indices, are implicitly disjoined by the logical OR operator, $\lor$. The $-$ symbol preceding a boolean variable index corresponds to the logical NOT operator, $\lnot$. Character 0 (zero) marks the end of each clause. Essentially, the code above corresponds to the following CNF:

$(\lnot x_1 \lor \lnot x_2 \lor \lnot x_3) \land (x_1 \lor \lnot x_2 \lor x_3) \land (x_1 \lor x_2 \lor \lnot x_3) \land (x_1 \lor \lnot x_2 \lor \lnot x_3) \land (\lnot x_1 \lor x_2 \lor x_3)$.

**Parameters**

**filename** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)) – A file in DIMACS format.

**Returns**

A quantum circuit with a phase oracle.

**Return type**

[PhaseOracle](#qiskit.circuit.library.PhaseOracle "qiskit.circuit.library.PhaseOracle")