# Changing one qubit causes changes in another independent qubit on ibmqx2 in IBM Q Experience

When I run this program on ibmqx2 (the backend matters) in IBM Q Experience, the results are interesting. Essentially, the program measures the error rate on $$|q_0\rangle$$ after fiddling with $$|q_1\rangle$$.

OPENQASM 2.0;
include "qelib1.inc";
gate nG0 ( param ) q  {
h q;
}

qreg q[2];
creg c[1];

ry(pi) q[1];
measure q[0] -> c[0];


Unexpectedly, the value of $$|q_0\rangle$$ depends on the angle of rotation around y-axis for $$|q_1\rangle$$. Here are typical runs of 8 192 shots on ibmqx2.

ry() q[1]   Error rate q[0]     Expected error rate q[0]
0            0.940%             ~1%
pi/4         9.351%             ~1%
pi/2        30.273%             ~1%
3pi/4       52.209%             ~1%
pi          60.742%             ~1%
5pi/4       51.941%             ~1%
3pi/2       30.518%             ~1%
7pi/4        9.509%             ~1%
2pi          0.684%             ~1%


A few notes:

• For $$0$$ and $$2\pi$$ the $$Ry$$ operations are optimized away in the transpiled code.
• Don't forget that we're operating on $$|q_1\rangle$$ and measuring $$|q_0\rangle$$. These qubits should be completely independent. Changes to $$|q_1\rangle$$ shouldn't impact $$|q_0\rangle$$.
• The error rate is above 50% from $$\frac{3}{4}\pi$$ to $$\frac{5}{4}\pi$$. That suggests that application of $$Ry$$ on $$|q_1\rangle$$ does not just destabilize the system, it causes that $$|q_0\rangle$$ favors the $$|1\rangle$$ state.
• Changing $$Ry$$ to $$Rx$$ also demonstrates the problem. Changing $$Ry$$ to $$Rz$$ makes the problem to go away.
• You mentioned that backend matters. What are error rates on other backends? Dec 28, 2019 at 10:37