Stable Synchronization over 52 Meters and 14 Days

August 23, 2023 by Andreas Messner

Introduction

Multi-qubit experiments require reliable and precise timing synchronization between instruments and their in- and output channels. The Zurich Instruments Quantum Computing Control System provides such synchronization out of the box. But if you are like us, you might ask: What does reliable synchronization really mean? For us, it means that you don’t have to worry about it, and that it is measured against your experimental challenges: quantum experiments can run over days. The synchronization must span one or more racks comfortably, and some experiments, such as in quantum communications or fundamental physics [1], may even require synchronization over tens of meters.

Here, we show results from two experiments. First, we assess the long-term stability of our ZSync synchronization link across varying lengths: 3 meters and 26 meters, each for a 14-day period. The results show minimal day-night-cycle drift and minimal jitter, with no instance surpassing ~50 picoseconds. Second, employing the 26-meter links, we synchronize to instruments to drive the emission of a microwave photon from a qutrit and receive it with another qutrit. This intricate procedure, sensitive to delays at the nanosecond scale, was executed as part of the SuperQuLAN project in the Quantum Device Lab at ETH Zurich.

Figure 1: The PQSC System Controller connected to two SHFQC Qubit Controllers. With a 26-meter link between the instruments, we can reach a 52-meter span, close to the wing span of an A330-300 long-range aircraft.

Long-term Timing Stability

First, we tested the 14-day long-term stability of our ZSync synchronization in our own lab. To this end, we connected three HDAWG Arbitrary Waveform Generators with our PQSC Programmable Quantum System Controller. Two of the connections were made with the conventional 3-meter ZSync cable, while the third connection was made with a 26-meter link stemming from the SuperQuLAN project (refer to Fig 2c for the setup schematic). Throughout the ensuing 14 days, the instruments were triggered every 10 minutes, and the arrival times of the played pulses were measured on an oscilloscope.

No drift could be discerned in the measurement data for the 3-meter links. The evaluation of the measured timing jitter underscored its confinement within a 50 picosecond range and a standard deviation of 20 picoseconds. This only slightly exceeds the 14-picosecond standard deviation imposed by the signal sampling with our 20-GSa/s oscilloscope. The out-of-the-box inter-channel skew is well below the nanosecond level, namely below 50 picoseconds between channels within the same instrument, and only 150 picoseconds between channels in separate instruments.

The HDAWG connected via the 26-meter link exhibited a subtle drift of 50 picoseconds, mirroring the day-night cycle, likely attributed to ambient temperature fluctuations. Interestingly, the drift hardly impacted the inter-channel skew, which remained constant at around 50 picoseconds.

In both 3-meter and 26-meter links, the absence of jitter related to the HDAWG clock cycle (300 MHz; 3.3 ns) or the sampling time (2.4 GSa/s; 0.4 ns) is noteworthy.

Figure 2: Experimental setup and results for the in-house 14 days long-term measurements. Minimal drift and jitter have been observed. In particular, no event indicated a jitter of the order of the clock rate (3.3 ns) or sampling time (0.4 ns). (a) Two HDAWGs were connected via 3-meter ZSync links. The skew between channels on different instruments is 150 ps, the skew between channels on the same instrument 50 ps. (b) A third HDAWG was connected via a 26-meter ZSync link. The inter-channel skew is again ~50 ps. A 24-hour periodic day-night drift of ~50 ps is visible and impacts both channels, so that their relative timing remains constant. (c) Setup schematic.

Quantum Experiment: Sending and Catching Photons

Together with the Quantum Device Lab at ETH Zurich, one of our project partners in the SuperQuLAN project, we set up an experiment to test the 26-meter ZSync links on actual quantum hardware. Our aim was to transmit a microwave photon emitted from a transmon qutrit through a coaxial cable to another qutrit on a second chip. The control setup comprised a PQSC and two SHFQC Qubit Controllers (one per chip) controlled via our LabOne Q software framework.

In the experiment, the first qutrit is prepared in its second excited state (f-state) by a series of pi-pulses before the pitch pulse is played to send off the microwave photon. A short time later, the catch pulse is played on the second qutrit and the second qutrit is measured. After a successful transmission of the microwave photon, we’d find the second qutrit in its f-state. A diagram of the pulse sequence is shown in Figure 3. The figure also shows the code snippet that specifies the pulse sequence in LabOne Q for three instruments as if they were a single one.

Figure 3: Pulse sequence for the experiment. The first qutrit is prepared in its f-state by two sequential pi pulses at the frequencies of the g-e and e-f transitions, respectively. Then, the pitch pulse is played. On the second qutrit, a catch pulse is sent with a delay. The code to define the pulse sequence in LabOne Q is shown on the right side. Figure adapted from [2]

A precise temporal alignment between the pitch and catch pulses is necessary. An experimental sweep of the delay time reveals that a 28-nanosecond delay is optimal and leads to an overall transmission efficiency of 35%.

Figure 4: Measurement data demonstrating the pitching and catching of microwave photons. The plot shows the probability of finding the second qutrit in the f state relative to the total number of measurement outcomes. The maximum of f-state probability of 35% at a delay of 28 nanoseconds indicates that the highest photon transfer success rate is reached at this delay.

Summary

In summary, the findings presented within this blog post underscore the capabilities of the Zurich Instruments Quantum Computing Control System. It adeptly demonstrates the attainment of stability and precision in timing synchronization across various channels and instruments. The PQSC Programmable Quantum System Controller effortlessly synchronizes up to 144 AWG channels. The synchronization capability is further facilitated by Zurich Instruments' LabOne Q software framework, allowing the programming of these channels with a single-instrument feeling. Say goodbye to concerns about timing, synchronization, and jitter. The Zurich Instruments QCCS liberates you to fully concentrate on delving into your next experiment with unwavering focus and confidence.

References

S. Storz, J. Schär, A. Kulikov, P. Magnard, P. Kurpiers, J. Lütolf, T. Walter, A. Copetudo, K. Reuer, A. Akin, J-C. Besse, M. Gabureac, G. J. Norris, A. Rosario, F. Martin, J. Martinez, W. Amaya, M. W. Mitchell, C. Abellán, J-D. Bancal, N. Sangouard, B. Royer, A. Blais, and A. Wallraff, “Loophole-free Bell inequality violation with superconducting circuits”, Nature 617, 265-270 (2023)
Philipp Kurpiers, “Quantum Networks with Superconducting Circuits”, PhD thesis, ETH Zurich (2019)