# 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 CX.**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.

Note

If `embodiments`

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

.

## Methods

### num_basis_gates

`num_basis_gates(unitary)`

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

NOTE: Used by ConsolidateBlocks.

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