Invalid Blackbird circuit is generated from a Program

[Mandrenkov]

Issue description

Running a Program loaded from a Blackbird circuit which was generated by another Program does not always work. For example, consider the "Teleportatation" example from Strawberry Fields Interactive:

import math
import strawberryfields as sf
from strawberryfields.ops import *
​
sf_prog = sf.Program(3)
​
# See the "Teleportation" algorithm in https://strawberryfields.ai/interactive/.
with sf_prog.context as q:
    Coherent(math.sqrt(5/4), math.atan(1/2)) | q[0]
    Sgate(2, 0) | q[2]
    Sgate(-2, 0) | q[1]
    BSgate(0.7854, 0) | (q[1], q[2])
    BSgate(0.7854, 0) | (q[0], q[1])
    MeasureP | q[1]
    MeasureX | q[0]
    Xgate(q[0].par*1.41421356237000) | q[2]
    Zgate(q[1].par*1.41421356237000) | q[2]

# Convert the SF program ​into a Blackbird program.
bb_prog = sf.io.to_blackbird(sf_prog)
bb_prog._name = "Teleportation"
bb_prog._version = 1.0
bb_prog._target["name"] = "gaussian"

# Recover the SF program from the Blackbird program.
bb_circuit = bb_prog.serialize()​
re_prog  = sf.io.loads(bb_circuit)

# Run the recovered SF program with the Gaussian engine.
eng = sf.LocalEngine("gaussian")
results = eng.run(re_prog)

• Expected behavior: A Results object is returned by the final line of the script.

• Actual behavior: A TypeError exception is raised.

• Reproduces how often: Every time.

• System information:

  
  Strawberry Fields: a Python library for continuous-variable quantum circuits.
  Copyright 2018-2020 Xanadu Quantum Technologies Inc.

Python version: 3.9.5 Platform info: Linux-5.4.72-microsoft-standard-WSL2-x86_64-with-glibc2.31 Installation path: /home/mikhail/.local/lib/python3.9/site-packages/strawberryfields Strawberry Fields version: 0.18.0 Numpy version: 1.20.3 Scipy version: 1.6.3 SymPy version: 1.8 NetworkX version: 2.5.1 The Walrus version: 0.15.1 Blackbird version: 0.3.0 TensorFlow version: None

#### Source code and tracebacks

Running the script above yields the following error:

```python
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-16-da60180d5e6d> in <module>
     28 # Run the recovered SF program using the Gaussian backend.
     29 eng = sf.LocalEngine("gaussian")
---> 30 results = eng.run(re_prog)

~/.local/lib/python3.9/site-packages/strawberryfields/engine.py in run(self, program, args, compile_options, **kwargs)
    492                     )
    493 
--> 494         result = super()._run(
    495             program, args=args, compile_options=compile_options, **eng_run_options
    496         )

~/.local/lib/python3.9/site-packages/strawberryfields/engine.py in _run(self, program, args, compile_options, **kwargs)
    272             target = self.backend.compiler
    273             if target is not None:
--> 274                 p = p.compile(compiler=target, **compile_options)
    275             p.lock()
    276 

~/.local/lib/python3.9/site-packages/strawberryfields/program.py in compile(self, device, compiler, **kwargs)
    610             target = compiler.short_name
    611 
--> 612         seq = compiler.decompose(self.circuit)
    613 
    614         if kwargs.get("warn_connected", True):

~/.local/lib/python3.9/site-packages/strawberryfields/compilers/compiler.py in decompose(self, seq)
    222                 try:
    223                     kwargs = self.decompositions[op_name]
--> 224                     temp = cmd.op.decompose(cmd.reg, **kwargs)
    225                     # now compile the decomposition
    226                     temp = self.decompose(temp)

~/.local/lib/python3.9/site-packages/strawberryfields/ops.py in decompose(self, reg, **kwargs)
    450         Like :func:`Operation.decompose`, but applies self.dagger.
    451         """
--> 452         seq = self._decompose(reg, **kwargs)
    453         if self.dagger:
    454             # apply daggers, reverse the Command sequence

~/.local/lib/python3.9/site-packages/strawberryfields/ops.py in _decompose(self, reg, **kwargs)
   1500     def _decompose(self, reg, **kwargs):
   1501         # into a displacement
-> 1502         r = self.p[0] / np.sqrt(2 * sf.hbar)
   1503         return [Command(Dgate(r, 0), reg)]
   1504 

TypeError: ufunc 'true_divide' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

Additional information

The issue appears to have something to do with the final two lines of the generated Blackbird circuit:

name Teleportation
version 1.0
target gaussian

Coherent(1.118033988749895, 0.4636476090008061) | 0
Sgate(2, 0) | 2
Sgate(-2, 0) | 1
BSgate(0.7854, 0) | [1, 2]
BSgate(0.7854, 0) | [0, 1]
MeasureHomodyne(phi=1.5707963267948966) | 1
MeasureHomodyne(phi=0) | 0
Xgate("1.41421356237*q0") | 2
Zgate("1.41421356237*q1") | 2

[Mandrenkov]

Do you mind looking into this, @thisac ?