Feature Description
pyqasm: 0.3.2
qiskit: 1.2.0
qiskit-qasm3-import: 0.5.1
Quantum programs with branching conditions containing classical registers should be considered for depth calculations. For example, the following circuit -
In [1]: from qiskit.qasm3 import loads
In [2]: qasm_str = """
...: OPENQASM 3.0;
...: include "stdgates.inc";
...: qreg q[3];
...: creg c[3];
...: measure q[0] -> c[0];
...: cx q[0], q[1];
...: measure q[0] -> c[1];
...: measure q[0] -> c[2];
...: if(c[2]) { h q[2]; }
...: """
In [3]: qc = loads(qasm_str)
In [4]: qc.draw(cregbundle = False)
Out[4]:
┌─┐ ┌─┐┌─┐
q_0: ┤M├──■──┤M├┤M├───────────────────────
└╥┘┌─┴─┐└╥┘└╥┘
q_1: ─╫─┤ X ├─╫──╫────────────────────────
║ └───┘ ║ ║ ┌────── ┌───┐ ───────┐
q_2: ─╫───────╫──╫─┤ If-0 ┤ H ├ End-0 ├─
║ ║ ║ └──╥─── └───┘ ───────┘
c_0: ═╩═══════╬══╬════╬═══════════════════
║ ║ ║
c_1: ═════════╩══╬════╬═══════════════════
║ ║
c_2: ════════════╩════■═══════════════════
In [5]: qc.depth()
Out[5]: 5has a depth of 5 whereas pyqasm gives a depth of 4 -
In [7]: import pyqasm
In [8]: qasm_str = """
...: OPENQASM 3.0;
...: include "stdgates.inc";
...: qreg q[3];
...: creg c[3];
...: measure q[0] -> c[0];
...: cx q[0], q[1];
...: measure q[0] -> c[1];
...: measure q[0] -> c[2];
...: if(c[2]) { h q[2]; }
...: """
In [9]: pyqasm.loads(qasm_str).depth()
Out[9]: 4This is because the classical register depth is not considered during the calculation of qubit depths inside the if block. The depth of c_2 is 4 when it is used to condition the H gate on q_2.
Implementation (Optional)
- While unrolling the branching conditions, we look at whether the condition contains a set of classical register bits or not. We need to update the depth of all these classical bits to the max depth of this set of bits, eg. -
In [75]: qc.draw(cregbundle=False)
Out[75]:
┌─┐ ┌─┐┌─┐
q_0: ┤M├──■──┤M├┤M├─────
└╥┘┌─┴─┐└╥┘└╥┘
q_1: ─╫─┤ X ├─╫──╫──────
║ └───┘ ║ ║ ┌───┐
q_2: ─╫───────╫──╫─┤ H ├
║ ║ ║ └─╥─┘
c_0: ═╩═══════╬══╬═══╬══
║ ║ ║
c_1: ═════════╩══╬═══╬══
║ ║
c_2: ════════════╩═══■══
c_3: ═══════════════════
In [76]: qc.depth()
Out[76]: 5Here, if c_0, c_1 and c_2 were used in a branching statement, the depth of each bit would be set to 4 as max depth of bits i.e. c_2 is 4. In above example, since c_2 is used for the classically conditioned H gate, the depth of the circuit is supposed to be 5 but is coming out as 4 with pyqasm -
In [82]: import pyqasm
In [83]: qasm_str = 'OPENQASM 3.0;\ninclude "stdgates.inc";\nbit[4] c;\nqubit[3] q;\nc[0] = m
...: easure q[0];\ncx q[0], q[1];\nc[1] = measure q[0];\nc[2] = measure q[0];\nif (c[2])
...: {\n h q[2];\n}\n'
In [84]: pyqasm.load(qasm_str).depth()
Out[84]: 4
- It is not entirely clear how this "max depth of involved classical bits" will be trickled down to the qubits of the if block. A brute force approach is used inside the qbraid sdk's depth function but might not work for a recursive approach used in
QasmVisitor.
Feature Description
pyqasm: 0.3.2
qiskit: 1.2.0
qiskit-qasm3-import: 0.5.1
Quantum programs with branching conditions containing classical registers should be considered for depth calculations. For example, the following circuit -
has a depth of 5 whereas
pyqasmgives a depth of 4 -This is because the classical register depth is not considered during the calculation of qubit depths inside the if block. The depth of
c_2is 4 when it is used to condition theHgate onq_2.Implementation (Optional)
Here, if
c_0,c_1andc_2were used in a branching statement, the depth of each bit would be set to 4 as max depth of bits i.e.c_2is 4. In above example, sincec_2is used for the classically conditionedHgate, the depth of the circuit is supposed to be 5 but is coming out as 4 withpyqasm-QasmVisitor.