RegRefError: Trying to act on the same subsystem more than once

Photonic Software Strawberry Fields
1

I’m new to StrawberryFields. I’ve completed the X8 tutorial and am now working the Squeezer Characterization tutorial. Here is the code, should look familiar:

import numpy as np

from numpy.core.fromnumeric import swapaxes

import strawberryfields as sf

def run_job(

    job_name="default_name",

    n_spatial_modes=4,

    unitary=None,

    squeezing_amplitudes=None,

    n_samples=500000, ):

    prog = sf.Program(n_spatial_modes*2, name=job_name)

    # If no unitary is provided, default to the identity

    if unitary is None:

        unitary = np.identity(n_spatial_modes)

    

    # If no squeezing amplitudes are provided, default to all of them being off

    if squeezing_amplitudes is None:

        squeezing_amplitudes = [0]*n_spatial_modes

    with prog.context as q:

        for i in range(n_spatial_modes):

            sf.ops.S2gate(squeezing_amplitudes[i]) | (q[i], q[i*n_spatial_modes])

        for qumodes in (q[:n_spatial_modes], q[n_spatial_modes:]):

            sf.ops.Interferometer(unitary) | qumodes

        sf.ops.MeasureFock() | q

    eng = sf.RemoteEngine("X8_01")

    return eng.run(prog, shots=n_samples, disable_port_permutation=True)

# collect sample that allow us to characterize the squeezer

squeezer0 = run_job(job_name="squeezer0_char", squeezing_amplitudes=[1, 0, 0, 0])

# Analysis

# https://strawberryfields.ai/photonics/demos/squeezer_tests.html#analysis

# The samples span 4 spatial x 2 frequency = 8 detector channels.

# we calculate the mean photon number of each

n_means = np.mean(squeezer0.samples, axis=0)

print(f"The mean photon numbers <n> of each channel are: \n{n_means}")

Upon executing the code I get this Traceback:

(qenv) C:\Users\cfernandez37\Documents\Quantum\Xanadu>c:/Users/cfernandez37/Documents/Quantum/Xanadu/qenv/Scripts/python.exe c:/Users/cfernandez37/Documents/Quantum/Xanadu/SqueezerCharacterization/squeezer_char.py
Traceback (most recent call last):
  File "c:/Users/cfernandez37/Documents/Quantum/Xanadu/SqueezerCharacterization/squeezer_char.py", line 34, in <module>
    squeezer0 = run_job(job_name="squeezer0_char", squeezing_amplitudes=[1, 0, 0, 0])
  File "c:/Users/cfernandez37/Documents/Quantum/Xanadu/SqueezerCharacterization/squeezer_char.py", line 24, in run_job
    sf.ops.S2gate(squeezing_amplitudes[i]) | (q[i], q[i*n_spatial_modes])
  File "c:\Users\cfernandez37\Documents\Quantum\Xanadu\qenv\lib\site-packages\strawberryfields\ops.py", line 142, in __or__
    reg = pu.Program_current_context.append(self, reg)
  File "c:\Users\cfernandez37\Documents\Quantum\Xanadu\qenv\lib\site-packages\strawberryfields\program.py", line 427, in append
    reg = self._test_regrefs(reg)
  File "c:\Users\cfernandez37\Documents\Quantum\Xanadu\qenv\lib\site-packages\strawberryfields\program.py", line 410, in _test_regrefs
    raise RegRefError("Trying to act on the same subsystem more than once.")
strawberryfields.program_utils.RegRefError: Trying to act on the same subsystem more than once.

I’ve tried searching the internet, the Slack channel, and this forum, and all I can find is the Docs page forRegRefError.

Can someone help me debug what I did wrong? I was able to activate my API key and work through the X8 tutorial with minimal issues.

2

Case closed. I mis-typed line 24.

sf.ops.S2gate(squeezing_amplitudes[i]) | (q[i], q[i*n_spatial_modes])

should be
sf.ops.S2gate(squeezing_amplitudes[i]) | (q[i], q[i + n_spatial_modes])
Where the * should be a +.

1 Like
3

Glad you solved it @cfernandez. Don’t hesitate to open another thread if you have any questions!

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