weighted_bp_osd_example.py

'''
Example demonstrating how to run bp+osd with a non-uniform initial error channel.
Before running, install `bposd' and `ldpc`
`pip install -U ldpc bposd`
'''

import numpy as np
from bposd.hgp import hgp
from bposd import bposd_decoder
from ldpc.codes import rep_code

''''
Distance 2 surface code
'''
#build surface code via hypergraph product
h=rep_code(2) #repetition code seed
surface_code = hgp(h,h) #build surface code  
hz = surface_code.hz #z stabilisers parity check matrix
print("Distance-2 surface code hz parity check matrix")
print(hz)

'''
Example error
'''
m,n=hz.shape
error = np.zeros(n).astype(int)
error[[2]] = 1
syndrome = hz@error % 2

print("Error", error)
print("Syndrome", syndrome)

'''
Decoding with uniform initial error channel
'''

rho = 0.1*np.ones(n)
# rho[0]=0.0
bpd = bposd_decoder(hz, channel_probs=rho, bp_method="product_sum", osd_method="osd_e", osd_order=3, max_iter=5)

decoding = bpd.decode(syndrome)
print("Uniform error channel", rho)
print("Decoding (uniform initial error channel)", decoding)

'''
Decoding with non-uniform initial error channel
'''

rho[2]=0.05 #here I'm setting the probability on qubit 2 to be lower than the other qubits
bpd = bposd_decoder(hz, channel_probs=rho, bp_method="product_sum", osd_method="osd_e", osd_order=3, max_iter=5)

decoding = bpd.decode(syndrome)
print("Non-uniform error channel", rho)
print("Decoding (non-uniform initial error channel)", decoding)