Circuit semantics changed when applying GlobalisePhasedX

[Bennybenassius]

Ilan and I have found this bug when testing random circuits on the GlobalisePhasedX pass:

Running this code:

from pytket import Circuit, Qubit
from pytket.circuit import CircBox, OpType, MultiplexorBox, Op
import numpy as np
from pytket.passes import *

subcirc_0 = Circuit(0, "subcirc_0")
qbit_0 = Qubit("qbit_0", 0)
subcirc_0.add_qubit(qbit_0)
qbit_1 = Qubit("qbit_1", 0)
subcirc_0.add_qubit(qbit_1)

subcirc_0.Z(qbit_0)
circbox_subcirc_0 = CircBox(subcirc_0)

main_circ = Circuit(4, 2, "main_circ")

"""
# works
main_circ.add_gate(circbox_subcirc_0,[3, 1]) 
main_circ.X(0)
"""

# doesn't work
main_circ.add_gate(circbox_subcirc_0,[0, 1]) 
main_circ.X(0)

"""
# works
main_circ.X(0)
main_circ.add_gate(circbox_subcirc_0,[0, 1]) 
"""

sv1 = main_circ.get_statevector()

print(sv1)

GlobalisePhasedX().apply(main_circ)

sv2 = main_circ.get_statevector()

print(sv2)

print(abs(np.vdot(sv1, sv2)))

And when checking out the state-vector of the Circuit after the pass, we noticed that the state-vector seems to imply that the state |0000> has 100% probability of occurring, which was confirmed when we checked the circuit:

This seems to only happen when a multi-qubit Circbox is added before a single-qubit gate on the same qubit.