In week seven of the Of Schemes and Memes blog series, which features weekly interviews with the art team at Nature, Art Director Kelly Krause and Cover Designer Brad Baxley explain the decisions behind this week’s futuristic front cover on quantum droplets. Also, in this week’s blog, Mackillo Kira, one of the paper’s authors, explains the scientific process behind the research.
Excitons, plasmons and phonons are some of the better known quasiparticles — exotic entities that act in some respects like ordinary particles. New types do not come along all that often but here is one — a fundamentally new many-body particle named the ‘dropleton‘. Mackillo Kira and colleagues have identified this new quantum entity, a quantum droplet created when four or more electrons and holes (electronic vacancies) form a tiny correlation bubble via the Coulomb attraction, in direct-gap semiconductors such as gallium arsenide. The cover illustrates the pair-correlation function g(r) of quantum droplets — the central peak of the correlation function shows that electrons and holes are likely to be co-located and the ripples show that otherwise they form regularly spaced shells. (Cover art: Brad Baxley.)
From the Art Desk:
Art Director, Kelly Krause, explains:
“The cover image struck me as gorgeous from the first moment, but I initially struggled to understand what was being represented. So I reached out to author Mack Kira for an explanation, and he sent along an elegant explanation of both the cover art and the science behind it.”
From the Author:
Kira said: “The understanding of complicated many-particle systems critically depends on finding stable configurations known as quasiparticles. In semiconductors, the light absorption generates multiple electrons and electronic vacancies called holes. The cover picture illustrates our first glimpse at the quasiparticle we discovered – the dropleton that consists of a tiny liquid-like bubble of electrons and holes. The step-like structure shown in red and blue shading stems from the dropleton’s quantized energy levels, each having a different number of rings in the quantum-mechanical distribution (overlaid).”
From the Cover Designer:
Brad Baxley explains his creative process:
“Quite often, deciding which aspect of quantum research to visualize is the toughest part. The choice lies between depicting reality and representing reality. I have found each taps a different mode of creativity. In trying to depict reality, there might be an effort to imagine what it would be like to be very small and right in the middle of what is going on down there. In representations, we develop conventions to communicate certain ideas.
This cover design is a representation that was generated using a rather vast workflow (custom method of working involving multiple software and the mitigation between them) in order to use key data outputs, directly from the investigators, and arrange them in a way that attempts to reveal certain relationships. Ultimately, I think we arrived at a rather elegant solution.”
Mackillo Kira talks Nature through the scientific process:
Quantum mechanical objects live in a world that is alien to our everyday experiences. The research effort to find new quantum quasiparticles does resemble sending a probe to an alien world, with the constraint that the probe’s capabilities would be strongly limited by the rules of our world. Suppose that the probe could transmit only a set of old-fashioned photos to us, while our task was to determine the dimensions and general behaviour of the alien object on the basis of the photo set. It clearly is a massive and time-consuming task to find the elusive object, take enough high-quality photos, and then conclusively construct the behaviour of the alien object. These are the basic challenges every quantum physicist encounters in their day-to-day work.
Experimental part: The experiments were performed at JILA (U. Colorado/NIST, USA) by Andrew Almand-Hunter, Hebin Lee, and Steven Cundiff.
In connection with semiconductors, ultrafast laser spectroscopy allows us to take a systematic set of “photos” of the alien objects, i.e. quasiparticles. For consistent results, our semiconductor samples are state-of-the-art quantum wells; carefully constructed structures built one layer of atoms at a time in a high-quality vacuum, where no stray particles are around to interfere with the process. We start by taking a series of old-fashioned “photos” which are later processed to reveal the quantum aspects of the alien objects. The processing requires very good photos which are, in reality, carefully calibrated and low-noise absorption spectra of a pulse from an ultrafast laser.
To get a complete picture of the alien objects, we first prepare the sample by hitting it with a “pump” pulse. The properties of the pump pulse can be adjusted to, in effect, see the object from all sides. For each photo, the properties of the laser are carefully adjusted and locked, so that “long-exposure” photos can be taken to improve the accuracy of our data. Many hours, even days, of data collection are required to capture a full picture for processing using the rules of the alien quantum world.
Theory part: The theory was developed at Univ. Marburg (Germany) by Martin Mootz, Mackillo Kira, and Stephan Koch.
The analysis and planning of these experiments requires a deep understanding of quantum theory, i.e., the rules of the alien world. With this insight, we could pinpoint the potentially interesting conditions and quantum objects for the experiments in order to maximize the success of the experimental mission. In our work, the theoretic part was critically important also in the analysis of the collected data because we needed to construct the full quantum imagery of the alien objects, i.e. quasiparticles. To realize all this, we really had to push present-day laser spectroscopy to a new regime of quantum-optical spectroscopy where the quasiparticle characterization is based on the rules of the quantum world. In our realization of quantum-optical spectroscopy, the quantum aspects were projected from the connections between a large set of classical measurements, as paradoxical as it may sound. A major portion of our work involved finding systematic connection between classical measurements, quantum-optical spectroscopy, and quasiparticles.
Synthesis: The synthesis of quantum-optical spectroscopy was performed jointly by the whole research team.
It took us roughly two years before all aspects of experiment and theory were developed to their peak performance, and we were ready for the final stage, exposing new quasiparticles. It was a truly glorious moment when the first quantum-optical spectra were distilled from the measured data. We still remember the day when the discrete dropleton energy steps emerged for the first time; it was completely unexpected and breathtaking.
As we continued deeper in the analysis, we could observe oscillation dynamics between the discrete energy levels, which is characteristic of quantum-coherent transitions. Once we knew what the dropletons looked like, we could even detect a mixture of several dropleton quantum oscillations in a classical measurement. This means even a classical probe can detect dropletons if one knows what one is looking for. But still, the quantum characterization is needed to isolate individual dropletons.
With these firm observations, we continued to rule out the most relevant, already known quasiparticels. We performed measurement that could reject molecular electron-hole states as an explanation for energy quantization. We also demonstrated that dropletons follow from quantum evolution, not thermodynamics. And, last but not least, we could associate the quantized energy levels to a liquid-like behavior of electron-hole quantum droplets, which provided the last piece of conclusive evidence that we had found a new type of quasiparticle.