OpenQASM 3 exporter outputs invalid integer/float expressions

[eginez]

Hi there while using the parser we notice the following circuit excerpt fails to parse

OPENQASM 3;
include "stdgates.inc";
input float[64] _x_0_;
gate ZZFeatureMap(_x_0_) _gate_q_0 {
h _gate_q_0;
p(2*(3 - _x_0_)*(3 -_x_0_)) _gate_q_0;
}

It seems like there are couple of unspecified cases in the logic to handle binary expressions?

[jakelishman]

That particular failure is slightly different; that's because the input OQ3 (that looks like Qiskit generated it) is not valid - it attempts to interact integers with floats (3 - _x_0_), which isn't allowed. I'll move this over to the Qiskit repository because strictly it's a bug against the exporter.

That said, we currently don't have a path to fixing this with the current ParameterExpression model of Qiskit; those are symbolic parameters, and our current method of exporting them is pretty much "call str and hope for the best", because it's very difficult to work with the sympy/symengine tree in a coherent manner.

The way we will be moving with this is likely a pivot away from the ParameterExpression as the "default" type of parameter in favour of the explicit runtime expressions and variables (where appropriate) - see #10922 - and ParameterExpression will be more for places where a symbolic expression is required, with them expected to be resolved before attempting to interoperate with OpenQASM 3, etc. The story on this isn't fully fleshed out, though.

[Cryoris]

Is this the same reason why loading a gate definition where a float is multiplied by a parameter does not work?

E.g. the following snippet cannot be imported if the RZ angle is float * parameter but it works correctly for int * parameter. I'm a bit confused about this, since it is only an issue in a gate definition. Using rz(float * parameter) outside works fine.

Here's a snippet:

OPENQASM 3;
include "stdgates.inc";
input float[64] a;
gate rzz(a) _gate_q_0, _gate_q_1 {
  cx _gate_q_0, _gate_q_1;
  rz(2.0*a) _gate_q_1;              // <---- works if it is defined as rz(2*a)
  cx _gate_q_0, _gate_q_1;
}
qubit[2] q;
rzz(2.0*a) q[0], q[1];