7
$\begingroup$

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.

Error rate of q[0] vs ry() rotation of q[1]

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.
$\endgroup$
  • $\begingroup$ You mentioned that backend matters. What are error rates on other backends? $\endgroup$ – Martin Vesely Dec 28 '19 at 10:37
12
$\begingroup$

Thanks for pointing this out! It turns out that this device was mis-calibrated in a way that was leading to that behavior. We just fixed the calibrations, so the problem should be gone now. I apologize for the trouble, and we will try to update our routine calibrations to detect and prevent this problem from coming up in the future :-).

|improve this answer|||||
$\endgroup$
6
$\begingroup$

This was a readout crosstalk error that has now been resolved.

|improve this answer|||||
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.