In the sequence diagram of PlantUML, the message lines are not aligned with the lifelines (bars). For example, qubit_1

The code is:

activate qubit_0

activate qubit_1

activate qubit_2

qubit_0 -> qubit_0 : H <<Quantum Gate>>

qubit_2 -> qubit_2 : H <<Quantum Gate>>

qubit_1 -> qubit_1 : H <<Quantum Gate>>

deactivate qubit_0

deactivate qubit_1

deactivate qubit_2

See also discussion here: