# Cirq: Result of rotating qubit measurements never come [0 1] or [1 0 ], always come as [0,0] or [1,1]

I am creating a 2 qubit entangled state:

    def density_matrix_chsh(i, j, qs):
circuit = chsh_circuit(i, j, qs)
return cirq.final_density_matrix(circuit, qubit_order=qs)

def chsh_circuit(i, j, qs):
return cirq.Circuit(cirq.H(qs[i]), cirq.CNOT(qs[i], qs[j]))

qs = [a, b]

state = density_matrix_chsh(0, 1, qs)


Then I am applying rotation for maximum violation:

th = np.pi/(-2)
th1 = np.pi/(4)
th2 = 0
random_angles = []
newlist = []
b_big = cirq.kron(cirq.unitary(cirq.ry(th)),cirq.unitary(cirq.ry(th)))
results = []
for choosen in itertools.product([th2, th1], repeat=2):
random_angles.append(choosen)
for i in range (1000):
choosen_combination = random.choice(random_angles)
a_big = cirq.kron(cirq.unitary(cirq.rz(choosen_combination)),cirq.unitary(cirq.rz(choosen_combination)))
rotated_state = b_big @ a_big @ state @ b_big.conj().T
measurement_rotated = cirq.measure_density_matrix(rotated_state,indices=[0, 1])
print("measuring qubits ",measurement_rotated)
qubit_result = measurement_rotated
results.append(qubit_result)


For instance here I tried 1000 times and both qubits are either 0 or 1 at the sma time but I could never see one of them 1 and the other one is 0 after measurement. Probably I am doing something wrong and I can't see Thanks for helps

• You'll have a much easier time if you pass a circuit into cirq.Simulator().sample(...) rather than trying to do the matrix math yourself by assembling unitaries ad hoc. E.g. it looks like you applied a_big differently than b_big. Jan 23, 2021 at 15:17
• well sorry, I did not get your point. I am trying to have chsh measurements so I am rotating my state on bloch sphere with 45° angle and then I am measuring the state and saving the results. But my question is that: I could never see 0 1 or 1 0 for two qubits. I am always seeing 11 or 00. Jan 23, 2021 at 18:27
• b_big provides to rotate the state with -90 degree because I have measurement on computational basis Jan 23, 2021 at 18:40
• @Balint Pato maybe you have an idea? Jan 23, 2021 at 19:40

As Craig said, you applied b_big and a_big differently. The adjoint a_big is missing from your product. Try rotated_state = b_big @ a_big @ state @ a_big.conj().T @ b_big.conj().T. But Craig's right. Using Circuits would be much simpler:

import itertools
import random
import cirq
import numpy as np

def chsh_circuit(i, j, qs):
return cirq.Circuit(cirq.H(qs[i]), cirq.CNOT(qs[i], qs[j]))

qs = cirq.NamedQubit("a"), cirq.NamedQubit("b")

th = np.pi/(-2)
th1 = np.pi/(4)
th2 = 0
random_angles = []
newlist = []
b_big = cirq.Circuit(cirq.ry(th)(qs), cirq.ry(th)(qs))
results = []
for choosen in itertools.product([th2, th1], repeat=2):
random_angles.append(choosen)

print(random_angles)
for i in range (1000):
choosen_combination = random.choice(random_angles)
a_big = cirq.Circuit(cirq.rz(choosen_combination)(qs),
cirq.rz(choosen_combination)(qs))
rotated_state = cirq.Circuit(chsh_circuit(0,1, qs), a_big, b_big, cirq.measure(*qs))
res = cirq.Simulator().run(program=rotated_state, repetitions=1)
print("measuring qubits ",res.measurements)

• if th1 would be np.pi/(2) instead of np.pi/(4), would I need to write rotated_state = b_big @ a_big @ state @ a_big.conj().T @ b_big.conj().T instead of rotated_state = b_big @ a_big @ state @ b_big.conj().T? Jan 24, 2021 at 12:36
• No. Because you're working with a density matrix $|\psi\rangle \langle \psi |$ instead of a statevector ($|\psi\rangle$), unitary evolution (gate) $U$ is $U |\psi\rangle \langle \psi | U^\dagger$ instead of $U |\psi\rangle$. So you'll typically need the operator on both "sides". Jan 24, 2021 at 17:05
• so the answer is " yes I need to write for both sides"! and it is not related to angle. I thought mathematically they can cancelled, if they are equal but I saw your point. Thanks! Jan 24, 2021 at 18:15