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.