# Strange circuit with multi-qubit 'unitary' gates after transpiler with optimization level 3

Recently, I used the transpile() function with the level 3 preset pass manager to compile a very long and complex circuit on a real device coupling map. I was curious to see how much the depth of the circuit would increase due to the inevitable swaps. The result was quite strange, the depth of the transpiled circuit was an order of magnitude lower than the original circuit.

When I drew the transpiled circuit I found that it was composed of some strange multi-qubit gates labeled as 'unitary' (see picture below), while most of the two-qubits gates from the original circuit were gone.

I was wondering what these unitaries could mean.

A unitary in this case is quantum gate which can be represented by a matrix. You could try calling to_matrix() on this gate so you can see what action it is performing. It may have been formed by combining several gates into one. The 0 and 1 labels show which qubits it is acting on.