My pick for this week:
Quantum Biotechnology
Seeing this title, my first reaction was “Oh no”. BUT, this review turns out to be pretty dope. The first few sections are essentially about quantum sensing and imaging for biological applications. NV centers are already widely used as sensitive, non-toxic magnetometers that can be deployed IN the sample, unlocking regimes of sensing that were previously unaccessible by NMR.
SQUID based magnetometers are also widely in use, but the next generation of atomic magnetometers (cold gas magnetometers, I think?) basically work at room temperature with the same/better spatial resolution as SQUIDs without the cryo overhead.
Squeezed light microscopy is also of great interest for imaging small molecules that would otherwise be obscured by shot-noise.
The section on Quantum Control in Biotechnology was new to me, but apparently a fairly mature field, at least in the realm of coherent control of electron excitation pathways. The new hotness is “molecular polaritonics”. I’ll just quote straight from the paper:
In the field of molecular polaritonics laser driving is combined with a nanoscale optical or plasmonic resonator (see Fig. 5 a)). The resonance can be used to modify the molecular density of states, changing the states that molecules are able to transition in to, and thereby providing a new control handle. The intensity build-up in the resonator, combined with nanoscale confinement of light, offers the prospect of quantum control at the single molecule level, evading the heterogeneity and ensemble averaging that often obscures underlying single-molecule effects. Furthermore, the time delay associated with the build-up of intensity introduces a time lag in the response to molecular dynamics. This provides the possibility of dynamical backaction between field and molecule, which could be used to cool molecular degrees of freedom, or to control them in a variety of other ways.
This is further extended by adding in some cavity optomechanics, in which the biomolecule acts as the mechanical resonator in the optical cavity. Where molecular polaritonics focuses mostly on electronic states of the molecule, the optomechanical approach probes the vibrational/rotational modes of the molecule!
The final section touches on quantum effects in biology, which is still a controversial subject. It seems… not implausible that there might be some quantum effects of interest in biology, since many of the scales of interest are quite small. On the other hand, biological environments are usually extremely hot compared to the energy scales of quantum mechanical interest. I suspect there’s something to be found in this area, given results from quantum comms research that suggests entanglement can persist even through extremely shitty, noisy channels.
Other papers of interest:
Solving the sampling problem of the Sycamore quantum supremacy circuits
What once was expected to take TEN THOUSAND YEARS, can be done with 512 GPUs in 15 hours. A lot of details here about tensor networks and other shit that is meaningless to me, but might intrigue you. I just like the continued rivalry.
A nice result out of Q-CTRL, featuring some machine learning for better gates. I’m kind of curious how these would look with changing time-step resolution in whatever instrument synthesizes the pulse waveforms.
Simulations of Quantum Circuits with Approximate Noise using qsim and Cirq
Simulating noise in the “““NISQ Era””” seems like it would be important. Haven’t read this one, but included mostly as a reminder to myself to revisit this when necessary.
Helium surface fluctuations investigated with superconducting coplanar waveguide resonator
I’ve got a weak spot for helium work, since I was ‘In Helium’ back in a previous life. This paper in particular has a few points of interest for me. The first is that this paper has authors affiliated with EeroQ and I like to keep an eye on other approaches to qubit tech. The second is that they include a few noise power spectral density plots, some of which compare NPSD with dilution refrigerator pulse tubes ON to pulse tubes OFF. Undoubtedly, everyone who has worked with dil fridges has made such a comparison, but I’ve never seen PSD data actually published. It’s a nice to be able to reference. Finally, quoting from the conclusions:
Our results on the frequency shift of the resonator are consistent with FEM calculations and previous measurements and point to vibrations induced by the PT as the most serious impediment to achieving strong coupling between a microwave resonator and the in-plane motion of a trapped single electron.
Dil fridges have some real downsides, some of which turn into challenges for qubit tech. I wonder if EeroQ will end up spurring the evolution of the dil fridge in a direction more compatible with their proposed technology?
NISQ: Error Correction, Mitigation, and Noise Simulation
This one seems like it should be a useful paper for those of us interested in errors, noise, etc. Unfortunately my eyes start to glaze over when I see words like “Proposition”, “Theorem”, “Lemma”. When I wake up from my blacked out state, I’m at the end of the paper, but with no recollection of the intervening time.
One- and two-qubit gate infidelities due to motional errors in trapped ions and electrons
In the spirit of keeping tabs on those wily ion trappers:
We show that, for small errors, each of our expressions for infidelity converges to its respective numerical simulation; this shows that our formulae are sufficient for determining error budgets for high-fidelity gates, obviating numerical simulations in future projects. All of the derivations are general to any internal qubit state, and any mixed state of the ion crystal’s motion. Finally, we note that, while this manuscript focuses on laser-free systems, static motional frequency shifts, trap anharmonicities, heating, and motional dephasing are also important error mechanisms in laser-based gates, and our expressions apply.
Seems like nice work.
Magnetic-field resilience of 3D transmons with thin-film Al/AlOx/Al Josephson junctions approaching 1 T
https://arxiv.org/abs/2111.01115
Some absolute mad lads/lasses successfully measure a 3D transmon up to 1 T magnetic field (in plane). Dope. Here’s a quote from their concluding paragraphs:
With thinner films and possibly shifting to a JJ fabrication that minimizes spurious JJs, such as Manhattan style JJs [43] or JJs that are made with two lithography steps [44], it would be possible to make an Al-AlOxAl JJ transmon that can work above 1 T. If the target magnetic field is known in advance and the film properties are largely characterized, one can account for the reduction in EJ due to suppression of the superconducting gap. Then, the Al-AlOx-Al JJ advantages of high quality, decent yield and targeting will remain available even in experiments that require high magnetic fields.
They speculate one might be able to probe quasiparticle dynamics at the high fields, etc etc. Honestly I’m surprised these qubits still worked, so this is pretty cool.
Improved superconducting qubit state readout by path interference
A neat, short paper on improving readout efficiency by utilizing both reflected and transmitted readout signals. Seems like a decent way to get better readout fidelity without having to necessarily increase probe tone power.
NISQ-HHL: Portfolio Optimization for Near-Term Quantum Hardware
Financial applications of quantum computing fail to excite me, but you gotta give the folks at JP Morgan’s FLARE team credit for trying (succeeding?) to shoe-horn the HHL algorithm into a more NISQ friendly state. While I’m sure that portfolio optimization for 2 assets is trivial, I am not sure how many assets becomes ‘business relevant’. 100? 1000?
Non-equilibrium quantum thermodynamics of a particle trapped in a controllable time-varying potential
This one is sort of in the weeds, but I found it interesting because time-varying potentials of the kind the authors discuss (one well → two wells) have been used in superconducting qubits for a while. This was D-Wave’s whole schtick when they were still a purely quantum annealing company. I am not sure I saw an obvious connection between their use case, a levitated nano-particle, and a superconducting qubit undergoing the same change. I’ll need to re-read and think about it more.
On the complexity of quantum partition functions
The complexity of computing the partition function for n-qubit partition functions. I usually try not to touch this kind of stuff, but seems like it should be important to someone.
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