I'm trying to use sympy to compute symbolic results of quantum circuits using dirac notation. More specifically, I'd like to use the three rotation gates. I try to implement them in the following way, using sympy symbolic quantum physics package :

from IPython.display import display

from sympy import init_printing
from sympy import expand, Eq, Symbol, simplify, exp, sin,cos,I,srepr,Matrix
from sympy.physics.quantum import *
from sympy.physics.quantum.qubit import *
from sympy.physics.quantum.gate import *
from sympy.physics.quantum.circuitplot import circuit_plot

theta = Symbol('theta')

Ry_mat = Matrix([[cos(theta/2), -sin(theta/2)],[sin(theta/2),cos(theta/2)]]) #check
Rx_mat = Matrix([[cos(theta/2), -I*sin(theta/2)],[-I*sin(theta/2),cos(theta/2)]]) #check

Ry = UGate((0,), Ry_mat)
Rx= UGate((1,), Rx_mat)
c = Ry*Rx
circuit_plot(c, nqubits=2);


I get the error unhashable type: 'MutableDenseMatrix' when executing qapply. I tried to replace the line :

c = Rx*Ry


c = Rx

and then there is no more errors. Does someone know what's going on ? What am I doing wrong ?


1 Answer 1


I don't know why this doesn't work with Matrix, but a fix for this would be to use ImmutableMatrix instead

from sympy import Symbol, sin, cos, I, ImmutableMatrix
from sympy.physics.quantum import qapply
from sympy.physics.quantum.qubit import Qubit
from sympy.physics.quantum.gate import UGate

theta = Symbol('theta')

Ry_mat = ImmutableMatrix([[cos(theta/2), -sin(theta/2)],
Rx_mat = ImmutableMatrix([[cos(theta/2), -I*sin(theta/2)],
                        [-I*sin(theta/2) ,cos(theta/2)]])

Ry = UGate((0,), Ry_mat)
Rx= UGate((1,), Rx_mat)
c = Ry* Ry

print(qapply(c*Qubit('00'))) # -sin(theta/2)**2*|00> + 2*sin(theta/2)*cos(theta/2)*|01> + cos(theta/2)**2*|00>

I've filed an issue for your problem. In the meantime, use ImmutableMatrix.


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