# XXDecomposer

*class *`qiskit.synthesis.XXDecomposer(basis_fidelity=1.0, euler_basis='U', embodiments=None, backup_optimizer=None)`

Bases: `object`

A class for optimal decomposition of 2-qubit unitaries into 2-qubit basis gates of `XX`

type (i.e., each locally equivalent to $CAN(\alpha, 0, 0)$ for a possibly varying $alpha$).

**Parameters**

**basis_fidelity**(*dict**|**float*) – available strengths and fidelity of each. Can be either (1) a dictionary mapping`XX`

angle values to fidelity at that angle; or (2) a single float`f`

, interpreted as`{pi: f, pi/2: f/2, pi/3: f/3}`

.**euler_basis**(*str*) – Basis string provided to`OneQubitEulerDecomposer`

for 1Q synthesis. Defaults to`"U"`

.**embodiments**(*dict**[**float**,**QuantumCircuit**] | None*) – A dictionary mapping interaction strengths alpha to native circuits which embody the gate $CAN(\alpha, 0, 0)$. Strengths are taken so that $\pi/2$ represents the class of a full`CXGate`

.**backup_optimizer**(*Callable[...,**QuantumCircuit**] | None*) – If supplied, defers synthesis to this callable when`XXDecomposer`

has no efficient decomposition of its own. Useful for special cases involving 2 or 3 applications of $XX(\pi/2)$, in which case standard synthesis methods provide lower 1Q gate count.

If `embodiments`

is not passed, or if an entry is missing, it will be populated as needed using the method `_default_embodiment`

.

### __call__

`__call__(unitary, basis_fidelity=None, approximate=True, use_dag=False)`

Fashions a circuit which (perhaps approximately) models the special unitary operation `unitary`

, using the circuit templates supplied at initialization as `embodiments`

. The routine uses `basis_fidelity`

to select the optimal circuit template, including when performing exact synthesis; the contents of `basis_fidelity`

is a dictionary mapping interaction strengths (scaled so that $CX = RZX(\pi/2)$ corresponds to $\pi/2$) to circuit fidelities.

**Parameters**

**unitary**(*Operator**or ndarray*) – $4 \times 4$ unitary to synthesize.**basis_fidelity**(*dict**or**float*) – Fidelity of basis gates. Can be either (1) a dictionary mapping`XX`

angle values to fidelity at that angle; or (2) a single float`f`

, interpreted as`{pi: f, pi/2: f/2, pi/3: f/3}`

. If given, overrides the basis_fidelity given at init.**approximate**(*bool*) – Approximates if basis fidelities are less than 1.0 .**use_dag**(*bool*) – If true a`DAGCircuit`

is returned instead of a`QuantumCircuit`

when this class is called.

**Returns**

Synthesized circuit.

**Return type**

## Methods

### num_basis_gates

`num_basis_gates(unitary)`

Counts the number of gates that would be emitted during re-synthesis.

This method is used by `ConsolidateBlocks`

.