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LOv rewriting rules in Perceval

The aim of this notebook is to rewrite a circuit using rewriting rules based on the article: LOv-Calculus: A Graphical Language for Linear Optical Quantum Circuits.

We show how to use these rewriting rules to generate unique triangular normal forms.

import perceval as pcvl
from perceval.utils.algorithms.optimize import optimize
from perceval.utils.algorithms.norm import frobenius
import random

from perceval.rendering.circuit import DisplayConfig, SymbSkin
DisplayConfig.select_skin(SymbSkin)

This is the first rewrite rule used in this noteobok. It is the rule 37 in the article.

pattern1=pcvl.Circuit(3, name="pattern1")//(0,pcvl.PS(pcvl.P("phi0")))//(0,pcvl.BS(theta=pcvl.P("theta1")))//(0,pcvl.PS(pcvl.P("phi2")))//(1,pcvl.PS(pcvl.P("phi1")))//(1,pcvl.BS(theta=pcvl.P("theta2")))//(0,pcvl.BS(theta=pcvl.P("theta3")))
pattern1._color = "lightgreen"
pcvl.pdisplay(pcvl.Circuit(3).add(0,pattern1,False), recursive=True)

rewrite1=pcvl.Circuit(3, name="rewrite")//(0,pcvl.PS(pcvl.P("beta2")))//(1,pcvl.PS(pcvl.P("beta1")))//(1,pcvl.BS(theta=pcvl.P("alpha1")))//(0,pcvl.BS(theta=pcvl.P("alpha2")))//(1,pcvl.PS(pcvl.P("beta3")))//(1,pcvl.BS(theta=pcvl.P("alpha3")))//(0,pcvl.PS(pcvl.P("beta4")))//(1,pcvl.PS(pcvl.P("beta5")))//(2,pcvl.PS(pcvl.P("beta6")))
rewrite1._color = "lightgreen"
pcvl.pdisplay(pcvl.Circuit(3).add(0,rewrite1,False), recursive=True)

Let us implement now the rule number 1.

pattern2=pcvl.Circuit(1, name="pattern2")//pcvl.PS(pcvl.P("phi1"))//pcvl.PS(pcvl.P("phi2"))
rewrite2=pcvl.Circuit(1, name="rewrite2")//pcvl.PS(pcvl.P("phi"))

pcvl.pdisplay(pcvl.Circuit(1).add(0,pattern2,False), recursive=True)

pcvl.pdisplay(pcvl.Circuit(1).add(0,rewrite2,False), recursive=True)

In fact, this rule has been directly implemented in Perceval, with the call of simplify.

from perceval.utils.algorithms.simplification import simplify

The third rule used in this notebook is the following one:

pattern3=pcvl.Circuit(2, name="pattern3")//(1,pcvl.PS(pcvl.P("phip")))//(0,pcvl.BS(theta=pcvl.P("theta")))
pattern3._color = "pink"
pcvl.pdisplay(pcvl.Circuit(2).add(0,pattern3,False), recursive=True)

rewrite3=pcvl.Circuit(2, name="rewrite3")//(0,pcvl.PS(pcvl.P("phi1")))//(0,pcvl.BS(theta=pcvl.P("theta")))//(0,pcvl.PS(pcvl.P("phi2")))//(1,pcvl.PS(pcvl.P("phi3")))
rewrite3._color = "pink"
pcvl.pdisplay(pcvl.Circuit(2).add(0,rewrite3,False), recursive=True)

And the fourth rule is the rule 38 in the article.

pattern4=pcvl.Circuit(2, name="pattern4")//(0,pcvl.BS(theta=pcvl.P("theta1")))//(0,pcvl.PS(pcvl.P("phi1")))//(0,pcvl.BS(theta=pcvl.P("theta2")))
pattern4._color = "orange"
pcvl.pdisplay(pcvl.Circuit(2).add(0,pattern4,False), recursive=True)

rewrite4=pcvl.Circuit(2, name="rewrite4")//(0,pcvl.PS(pcvl.P("beta1")))//(0,pcvl.BS(theta=pcvl.P("alpha1")))//(0,pcvl.PS(pcvl.P("beta2")))//(1,pcvl.PS(pcvl.P("beta3")))
rewrite4._color = "orange"
pcvl.pdisplay(pcvl.Circuit(2).add(0,rewrite4,False), recursive=True)

a=pcvl.Circuit.generic_interferometer(4, lambda idx:pcvl.Circuit(2)//pcvl.PS(phi=random.random())//pcvl.BS(theta=random.random()), depth=8, shape="rectangle")
pcvl.pdisplay(a, recursive=True, render_size=0.7)

Normalizing Circuit

import drawSvg as draw
import copy

reverse = []
direct=[]
def draw_frame(a):
    if isinstance(a, pcvl.Circuit):
        d = pcvl.pdisplay(a, recursive=True, render_size=0.6)
        reverse.insert(0, d)
        direct.append(d)
        return d
    return a

rules = [(pattern1, rewrite1, "lightgreen"), # (pattern2, rewrite2, "lightblue"),
         (pattern3, rewrite3, "pink"), (pattern4, rewrite4, "orange")]

with draw.animate_jupyter(draw_frame, delay=0.1) as anim:
    anim.draw_frame(a)
    while True:
        found = False
        for pattern, rewrite, color in rules:
            start_pos = 0
            while True:
                print("matching pattern", pattern.name)
                matched = a.match(pattern, browse=True, pos=start_pos)
                if matched is None:
                    break
                print("matching ok", matched.v_map)
                idx = a.isolate(list(matched.pos_map.keys()), color=color)
                anim.draw_frame(a)
                for k, v in matched.v_map.items():
                    pattern.param(k).set_value(v)
                v = pattern.compute_unitary(False)
                print("optimizing rewrite",rewrite.name)
                res = optimize(rewrite, v, frobenius, sign=-1)
                print("found params with distance", res.fun)
                subc = rewrite.copy()
                found = True
                a.replace(idx, subc, merge=False)
                anim.draw_frame(a)
                a.replace(idx, subc, merge=True)
                pattern.reset_parameters()
                rewrite.reset_parameters()
                a = simplify(a)
                anim.draw_frame(a)
                start_pos = idx
        if not found:
            break

matching pattern pattern3
matching pattern pattern4
matching pattern pattern1
matching pattern pattern3
matching pattern pattern4

This representation is exactly the normal form that we wanted to obtain !

Reference

1. Clément, N. Heurtel, S. Mansfield, S. Perdrix, B. Valiron. LOv-Calculus: A Graphical Language for Linear Optical Quantum Circuits, arXiv:2204.11787 (2022).