Invalid serialization of DiagonalBox?

[aborgna-q]

The following pytket/tket2 script (by @johnchildren) shows a mismatched encoding/decoding of diagonal boxes. The bug seems to originate from this crate, but we should write a reduced test case.

import json
from pytket.circuit import DiagonalBox, QControlBox
from pytket import Circuit
import numpy as np
from tket2.circuit import Tk2Circuit
from dictdiffer import diff

u_diagonal = np.array([1, 1, np.e ** (1j * np.pi / 4), np.e ** (1j * np.pi / 8)])


d_box = DiagonalBox(u_diagonal)

controlled_d = QControlBox(d_box, n_controls=1)

circ = Circuit(3).add_gate(controlled_d, [0, 1, 2])

circ.get_unitary()  # This works, circuit is unitary

tket2_circ = Tk2Circuit.from_tket1_json(json.dumps(circ.to_dict()))

# Causes a different dictionary ot be output.
postpostcirc = json.loads(tket2_circ.to_tket1_json())

# Dictionaries are different
print(circ.to_dict())
print(postpostcirc)

print(list(diff(circ.to_dict(), postpostcirc)))

# ValueError: The input diagonal passed to DiagonalBox is not unitary.
Circuit.from_dict(postpostcirc)

pytket circuit json:

{"bits": [], "commands": [{"args": [["q", [0]], ["q", [1]], ["q", [2]]], "op": {"box": {"control_state": 1, "id": "0dadbfea-5391-41c2-ad71-fb01fdaecddd", "n_controls": 1, "op": {"box": {"diagonal": [[[1.0, 0.0]], [[1.0, 0.0]], [[0.7071067811865476, 0.7071067811865475]], [[0.9238795325112867, 0.3826834323650898]]], "id": "919b6c13-b9c4-4ffc-ad1a-660088402d02", "type": "DiagonalBox", "upper_triangle": true}, "type": "DiagonalBox"}, "type": "QControlBox"}, "type": "QControlBox"}}], "created_qubits": [], "discarded_qubits": [], "implicit_permutation": [[["q", [0]], ["q", [0]]], [["q", [1]], ["q", [1]]], [["q", [2]], ["q", [2]]]], "phase": "0.0", "qubits": [["q", [0]], ["q", [1]], ["q", [2]]]}