Explicitly introducing gate times affects setup modelling?

[diogovalada]

Hello

For my research, I compile some kernels in OpenQL, which then I'll simulate in quantumsim.
However, I need to make sure that the gate scheduling provided by OpenQL and the one fed to quantumsim are the same.

To make sure of this, I tried, for each quantum operation, to specify the timestamp using the time argument in
b.add_gate(gate, time = ... )
I made sure to introduce the gates in the middle of their duration, and measurements at 1/4 of their duration.

However, I noticed that, for very simple examples (where scheduling shouldn't be a problem), I still get different results, depending on whether I provide the gate timestamp or not:

b = Builder(setup)
b.add_gate('Measure', ["2"], output_bit = "2_out")
b.finalize()
b.circuit.gates

yields:

[<quantumsim.circuit.ButterflyGate at 0x218d1a744e0>,
 <quantumsim.circuit.AmpPhDamp at 0x218d1a74a90>,
 <quantumsim.circuit.Measurement at 0x218d1a743c8>,
 <quantumsim.circuit.AmpPhDamp at 0x218d1a74be0>,
 <quantumsim.circuit.ButterflyGate at 0x218d1a74c88>,
 <quantumsim.circuit.AmpPhDamp at 0x218d1a74cc0>,
 <quantumsim.circuit.AmpPhDamp at 0x218d1a746d8>]

While if I put the pass the time argument time = 150 (which is the time that one could get on the example above by checking b.circuit.gates[2].time )

b = Builder(setup)
b.add_gate('Measure', ["2"], output_bit = "2_out", time = 150)
b.finalize()
b.circuit.gates

Then this yields:

[<quantumsim.circuit.ButterflyGate at 0x218d1a399b0>,
 <quantumsim.circuit.Measurement at 0x218d1a98a90>,
 <quantumsim.circuit.ButterflyGate at 0x218d1a39c88>]

Why is this behaviour different?
I'm afraid that if I don't provide the time argument for all the gates, that the scheduling that qsoverlay calculates might be different than the one calculated by OpenQL.
How can I make sure that the timestamps are the same, while still benefiting from the more complete model provided by the 1st example (where I did not provide the timestamp)?

[obriente]

If you specify the time at which a gate occurs, qsoverlay doesn't account for it when calculating its estimate of the length of the circuit (namely the Builder.times dictionary). This data is used in Builder.finalize() to call the Circuit.add_waiting_gates() function - so in the example given, the waiting gates were calculated for a circuit of zero length.

If you want to fix the times of each gate, it's probably easiest to manually calculate the length of the circuit and update the Builder.times dictionary before calling Builder.finalize().