zx.qasm does not recognize gates imported from qelib1.inc

[mhlr]

qelib1.inc defines the controlled Hadamard gate ch, but zx.qasm does not recognize ch even when qelib1.inc is imported

import pyzx
g = zx.qasm('''
OPENQASM 2.0;
include "qelib1.inc";

qreg a[2];
ch a[0], a[1];
''')

---

TypeError Traceback (most recent call last)
in
6 qreg a[2];
7 ch a[0], a[1];
----> 8 ''')

/opt/conda/lib/python3.7/site-packages/pyzx/circuit/qasmparser.py in qasm(s)
188 """Parses a string representing a program in QASM, and outputs a Circuit."""
189 p = QASMParser()
--> 190 return p.parse(s, strict=False)
191

/opt/conda/lib/python3.7/site-packages/pyzx/circuit/qasmparser.py in parse(self, s, strict)
64 gates: List[Gate] = []
65 for c in commands:
---> 66 self.gates.extend(self.parse_command(c, self.registers))
67
68 circ = Circuit(self.qubit_count)

/opt/conda/lib/python3.7/site-packages/pyzx/circuit/qasmparser.py in parse_command(self, c, registers)
182 gates.append(g)
183 continue
--> 184 raise TypeError("Unknown gate name: {}".format(c))
185 return gates
186

TypeError: Unknown gate name: ch a[0], a[1]

[jvdwetering]

Okay, I can add a controlled-Hadamard gate to PyZX. Are there any other gates from the standard QASM library that are missing?

[mhlr]

Controlled Hadamard is the only one I tried. I am new to both ZX and QASM. PyZX does not read qelib1.inc to get the definitions?

[gwwatkin]

Hi @jvdwetering , it looks like the crz gates are also missing:

[...]
    pyzx_circ = zx.Circuit.load(fname)
  File "/home/george/.local/lib/python3.9/site-packages/pyzx/circuit/__init__.py", line 276, in load
    return Circuit.from_qasm_file(circuitfile)
  File "/home/george/.local/lib/python3.9/site-packages/pyzx/circuit/__init__.py", line 360, in from_qasm_file
    c = p.parse(f.read())
  File "/home/george/.local/lib/python3.9/site-packages/pyzx/circuit/qasmparser.py", line 66, in parse
    self.gates.extend(self.parse_command(c, self.registers))
  File "/home/george/.local/lib/python3.9/site-packages/pyzx/circuit/qasmparser.py", line 206, in parse_command
    raise TypeError("Unknown gate name: {}".format(c))
TypeError: Unknown gate name: crz(pi/2) q[1],q[0]

It would be very helpful to our project.

[jvdwetering]

So, for the CRZ gate there is a bit of a difficulty in which phase convention to adopt. The qiskit library (https://qiskit.org/documentation/stubs/qiskit.circuit.library.CRZGate.html) apparently sees an RZ(alpha) gate as diag(e^{-i alpha/2},e^{i alpha/2}). This leads to a different controlled phase gate than I would expect (namely, a matrix with 2 diagonal elements not equal to 1, instead of just 1 element not equal to 1). For instance, a CRZ(pi) gate does not match the CZ gate.
This does seem to be the way it is defined in the QASM spec however (https://arxiv.org/pdf/1707.03429.pdf, p.12), so I guess I'll just follow that.

[dlyongemallo]

In case anyone comes across this issue later, support for all the gates mentioned above have all been added as of PR #156.