Superconducting Quantum Computing » Runs

Looked more into fundamental mode at 2.6 GHz. It is definitely very sensitive to power, however shift in frequency with power reduction is not observed. Lorenzian curve suddenly disappears when power level goes below -32-35 dBm. It is very similar to what we observed last Run in 5 GHz cavity.

Many other high order modes are found also. I will now check if there is sensitivity to power level in those modes.

I see shift +1.430 KHz at the 4.737712 mode, when reduce power from -30 dBm to -60 dBm. Doesn't seem to move further after that. Signal is very weak, unfortunately we don't have quantum amp on this cavity, it would help a lot.

Finished scan around 4.933 GHz peak, can see it clearly. I will now measure s21 of the cavity around this frequency, to investigate if it is maybe just cavity mode. Although our isolators operate only up to 4.8 GHz, so this measurement may be problematic.

Last night we run a long qubit scan from 4.8GHz to 5.8GHz.

5.13,5.21, 5.37 are known to be cavity modes.

There is a shift a little above 5.4GHz. This is interesting because is going in the opposite direction of the bright state.
I am running a finer scan in this range.

We are observing some interesting results. Earlier we found 4.933 GHz peak. We measured s21 of the cavity with qubit around this frequency and found same resonance at 4.933 GHz and also different peak at 4.934881 GHz.

The we connected 2.6 GHz cavity without qubit to network analyzer to see if there are similar modes in it. And we didn't find anything, at least within span of 100 MHz. We clearly see other higher order modes like 4.7, 5.3 etc. But these two modes at 4.933 and 4.934881 we observe only in the cavity with qubit.

Q loded of these modes is also high, 1e8 for 4.934881 GHz and 1e7 for 4.933 GHz.

4.933 peak in more details...

... and finished scan of 4.93488 GHz peak. Now I will switch to pulsed mode spectroscopy.

I am looking into the best frequency to readout in pulsed mode. Here is yesterday's scan of dispersive shift at 4.7 GHz.

Finished scan qubit power frequency vs. qubit frequency around 4.933 GHz peak.
This scan was done in pulsed mode. Qubit pulse = 300 us, readout pulse = 30 us.
Optimal readout frequency was 4.737710981 GHz, -22 dBm power.
Previous CW measurements were done at -52 dBm, unfortunately this power level is too low to use in pulsed mode with reasonable averaging.
I will now set it to scan around 4.93488 GHz peak.

Finished scan for 4.737710981 GHz mode. We will stop for today with qubit to do NIST amplifier measurements.

A summary of the qubit spectroscopy. I used the R&S signal generator, up to 40dBm.

Spectroscopy shows some lines that seem not to correspond to cavity modes at:
-- 5.199 GHz
-- 4.905 GHz
-- 4.353 GHz

-- I have a "feeling" for 5.4GHz, there are very tiny lines, at same distance and repeated at 30dBm and 40dBm, but more data are needed to make a statistics.

See the attached presentation.

Daniil is going to continue to work on these frequencies in the afternoon.

Some update. We found frequency= 3.657 GHz that may be the qubit candidate.
Here is plot from the VNA with three conditions. We used low-Q 4.7 GHz mode for readout.

Green - VNA is at low power, no additional signal is applied (dispersive shift regime).
Blue - VNA is at high power, observing normal cavity resonance.
Orange - VNA is at low power and additional signal 3.657 GHz, +35 dB is applied. In this case resonance peak shifts even futher away from normal cavity resonance ~300 Hz to the left, which was not observed before with other frequecies.

Found good evidence of the Qubit at 3.657 GHz.

The picture show several peaks and broad bandwidth.

Best parameters found:
Qubit: f=3.65678GHz, P=-10dBm
Cavity readout: f=4.73771177 GHz, P=-17dBm (VNA) -40dBm(attenuator) - 3dBm (splitter)