Month: August 2020

# QCJC: Kannan 2020

I’m not sure if you can tell, but I’ve been recently starting to move away from reading “seminal” papers, and instead working through the new, hot papers of today. I suppose that one danger of doing so is that it’s harder to see if the individual papers that I read will pan out to be an actual game changer or not, whereas the papers from a decade ago have already (partially) withstood the test of time. However, it is life on the cutting edge that is more exciting – more area to explore, question, and discover!

This paper from the Oliver group at MIT explores a new topological way to use superconducting qubits for quantum simulations. Although we are still squarely in the superconducting space, the title, as well as much of this paper, takes inspiration from the atomic nature of these superconducting qubits. Primarily, the fact that superconducting qubits in a microwave waveguide resonator act very similarly to atomic qubits interacting with some wavelength mode. Typically, we see the atom as being much smaller than the wavelength that it is interacting with. However, for these “giant atoms” (which I cannot tell if they are actual things in AMO physics or not?), they are so large that they interact with multiple peaks of the wave at the same time. In this superconducting landscape, exactly where these qubits interact with the waveguide can then be carefully controlled, creating this beautiful braiding pattern shown in Figure 1c, d, e. Instead of the qubit being strictly interacting with the waveguide at a single junction, there are multiple ports to consider, which these authors claim can be used and tuned to protect the qubits even further.

The key idea that is brought up in this paper is that the photons in the waveguide cavity will experience a phase shift that depends on the resonant frequency of the qubit. Since this group is using tunable transmon qubits here, this can be a fairly easily adjustable parameter. The theory then supports that the relaxation rate of the qubit strongly depends on the phase of that photon, leading to the plot in Fig. 2b. The authors argue that when $\phi = \frac{\pi}{2}$, the qubits are maximally decoupled from relaxation into the waveguide. However, instead of the usual corresponding trade-off of having slower gates, there is still “strong physical couplings to the continuum of modes in a waveguide”.

The justification for this seems to primarily be in the braided design of these giant atoms. In a typical small atom, the interaction strength is proportional to $\sin(\theta / 2)$, where $\theta$ is the phase delay of a photon from one qubit’s coupling point to another qubit’s coupling point. However, in the braided configuration, there are multiple coupling points. Given that the two qubits have uniform coupling strengths to the waveguide, the expression simplifies to that of Eq. 5. At the minimal decoherence point of setting $\phi = \frac{\pi}{2}$, the interaction strength is directly proportional to $\gamma$.

This seems to pose a few interesting questions – namely, what would the interaction strength look like for braided giant atoms that are not symmetric in this way? Could you engineer a different interaction strength relationship in that case, and if so, could you actually amplify the signal in some way? However, the symmetry of the current system does seem to make it become much more simple to analyze, which has all of the benefits associated with that.

In fact, the authors go on to engineer two slightly detuned qubits, such that the decoherence-free frequency for each qubit was separated by 720 MHz, allowing the qubits to not only be protected from the environment, but also from each other. It’s a little less clear to me the purpose here – are they planning on tuning the qubits to then move closer to each other only during swapping gates, or just accept that there will be slower exchange rates? It does appear that they are able to achieve exchange excitation rates of 735 kHz, but I don’t really have a good intuition as to if this is good or bad.

In general, I think the bigger picture of this idea – of using giant atoms that are braided together – sounds like it has a vast amount of stuff to be explored. Right now, the two qubits are braided together in the most simple configuration. But what about three qubits? Or four? The number of knots that can be formed – especially if you use some kind of 2.5d geometry – seems incredibly vast. If there is a different error syndrome that can be prevented for each braiding pattern, then weaving together different braided atoms might create a fairly robust structure.

I think my main continued question at this point is how the qubits are different from normal transmon qubits, if there is any difference at all. It seemed to me from Figure 1 that they are just using normal transmons and hooked additional 50-Ohm terminated resonators on each side. Is there some limit to the number of points that a single transmon can be connected to as a giant atom? Is there more sensitivity to asymmetric flux noise from either of the two pathways?

# QCJC: Kono 2020

This was a fairly interesting paper that focuses on the introduction of what the authors call a Josephson Quantum Filter, which they claim to reduce qubit decoherence while not suffering from the conventional trade-off of requiring a stronger Rabi drive for longer gates. While I don’t think I really clearly follow along with their explanation of how this JQF is created and used, it sums like the basic ideas of this device would be a very interesting tool to be used in the future.

The basic working idea of the JQF looks very similar to a conventional flux qubit, where there is a small loop with Josephson junctions that is tunable to be resonant at some frequency, In this experiment, the authors set the JQF frequency to be very close to that of the qubit (transmon) that they are using , and separated from the qubit by a half wavelength length of a resonator/transmission line. Control signals will be sent out such that they must pass through the JQF before they reach the qubit. The primary purpose of the JQF, however, does not seem to exactly filter out noise that comes in from the signal line, but to prevent the qubit from spontaneously decaying. I think that the mental picture that I had was that the JQF was a piece of one way glass, where only when the “light” is turned on from the outside is there some kind of transmitted signal. To me, this almost sounds like a saturated band stop filter, so perhaps there is some way to use conventional electrical engineering concepts here as well? In any case, Fig. 1 is quite useful for seeing some of the basic illustrations of how this JQF works.

The authors here show quite a bit of theory explaining how the excited states of the qubit are coupled to dark states of the JQF, but to be honest, I was almost completely unable to follow their formalism in their theory section, especially when they discussed the “strongly asymmetric external coupling to the control lines”. This seemed like a really significant part about why the JQF works, but I was not able to follow how “maximizing the correlated decay” leads to this behavior.

However, taking everything at face value, it appears that the JQF is able to act as a reflecting barrier to prevent the qubit from spontaneously decaying, yet becomes transparent when there’s a pulse of energy sent through at its resonant frequency. This allows for control signals to go through and execute faithful Rabi flops on the qubit, making it an advantageous tool to use. However, what does that mean if you want to drive a signal off resonance on the qubit? This … might be an embarrassing shortcoming in my own knowledge of gate drives for transmon qubits, but I thought slightly off-resonant transitions (decoupled by a few MHz?) are needed for various complex operations. Is that not true? Am I mixing trapped ion knowledge with superconducting knowledge here? Is there some bandwidth of region where the JQF still allows signals to pass through? The authors do note that when the JQF is far-detuned from the qubit, the qubit acts as if the JQF does not exist, which also doesn’t quite make sense.

Intriguingly, one of the principal drawbacks of the JQF is that it might do its job too effectively. The authors note that using the JQF leads to the thermal population of the qubit to be increased by almost a factor of 8. This is not because the JQF introduces noise, but instead prevents the cooler temperature of the dilution refrigerator to cooldown the qubit. Despite this higher thermal population, the authors here report a relaxation and coherence time improvement by about a factor of 4.

One of the more confusing sections of this paper is where the authors describe replacing the “transmon JQF with a two-level JQF with the same parameters”. I have no clue what they mean here. What are the same parameters? Did they swap out the JQF loop for something else? Does it just not have Josephson Junctions in the JQF? What’s the point of doing this? I am quite unclear about the purpose of this, and also how it happens either.

Overall, this paper does seem to introduce a new tool that might be useful in the future, but I am a bit doubtful of the scalability of this JQF. Is it then necessary to have a JQF, with half wavelength resonator, attached to each qubit that you want to produce? That sounds like there is a lot of room for error, especially when you are trying to drive multi-qubit systems.