As a child I caught lizards – brown anoles, as I later learned to call them. They were colored, as the name suggests, about the length of one of my hands, and annoyed my attention. But they were visiting our porch, and I had a butterfly net. So I caught lizards with my brother or a friend and watched them. They had throats that occasionally puffed up and exposed red skin, and tails that loosened and wriggled on their own to distract predators.

Some theorists may appreciate butterfly nets to catch experimenters. Some of us theorists will end a paper or talk with “… and these predictions are experimentally accessible”. There is pause after the paper or lecture is published in the hope that a reader or audience will accept the challenge. Usually nobody does this and the author or speaker retires to the Great Deck Chair of Theory on the Back Patio of Science.

I was shocked when, metaphorically speaking, an anole provided a superconducting qubit for an experiment I proposed.

The experimenter is one of the few people I can compare to a reptile without fear of complaining: Kater Murch, Associate Professor of Physics at Washington University in St. Louis. The most impressive description of Kater I can offer appeared in a previous blog post: “Kater exudes the sobriety of a full-time professor, but the disrespect of a Californian who wears his hair a little long and tattoos his wedding ring.”

Kater showed interest in an insecure relationship that I had proven with theoretical collaborators. According to some of the most well-known uncertainty relations, a quantum particle cannot have a precisely defined position and a precisely defined momentum at the same time. Measuring the position interferes with the impulse; Each subsequent pulse measurement gives a completely random or uncertain number. We measure uncertainties Entropies: The greater an entropy, the greater our uncertainty. We can represent uncertainty relations in the form of entropies.

Together with colleagues, I had proven an entropic uncertainty relation that describes the chaos in quantum systems with many particles. Other co-workers and I had shown that weak measurements that do not perturb a quantum system very much characterize chaos. So you can check our uncertainty relation with weak measurements as well as with strong measurements that strongly perturb quantum systems. One can simplify our uncertainty relation – remove the chaos from the problem and even remove most of the particles. There is an entropic uncertainty relation for weak and strong measurements.

Kater specializes in weak measurements and has therefore decided to test our uncertainty relation. Physical Examination Letters published the paper about our collaboration this month. Quantum measurements can not only create uncertainty, but also reduce it: Kater and his PhD student Jonathan Monroe used light to measure a superconducting qubit, a tiny electrical circuit in which electricity can flow forever. The qubit had properties analogous to position and momentum (the spins) z– and xComponents). If the atom began with a well-defined “position” (the zComponent) and the “impulse” (the xComponent) was measured, the result was very random; The overall uncertainty regarding the two measurements was great. But if the atom began with a well-defined “position” (zComponent) and another property (the spins yComponent) was added before the “impulse” (the xComponent) was measured strongly, the overall uncertainty was lower. The additional measurement should not seriously disturb the atom. But the nudge nudged the atom enough and provided the later “momentum” measurement (the x Measurement) more predictable. Quantum measurements can therefore not only create uncertainty, but can also be reduced through gentle quantum measurements.

I didn’t just learn physics from our experiment. Whenever I catch a lizard, I dump it into a tank with a lid that contains a magnifying lens and watch the lizard. I haven’t caught Tom and Jonathan under a magnifying glass, but I’ve watched their paths. Here’s what I learned about the species experimental quanticus.

1) You can conduct experiments remotely if a pandemic closes campus: A year ago, when the universities were closing and the cities were closing, I feared that our project would come to a standstill. But Jonathan turned the knobs and read the dials on his computer, and Kater showed up in the lab to occasionally fix the upper. Jonathan even continued his experiment from another state when he moved to Texas to join his parents. And here we theorists boast that our science can be practiced almost anywhere.

2) You speak with one level of abstraction less than me: For example, we have often talked about how the qubit is measured. I would call this thing “the detector”. Jonathan would call it “the cavity mode” and refer to the light that interacts with the qubit that is in a box, or cavity. I would say “poh-tay-Toe “; they would say” poh-tah-Toe “; but I’m glad we didn’t cancel the whole thing.

Fred Astaire: “Detector.”
Ginger Rogers: “cavity mode”.

3) Experiments take longer than expected – even if you expect them to take longer than expected: Kater and I worked out the plan for this project in June 2018. The experiment would take a few months, Kater estimated. It ended last summer.

4) How they explain their data: Usually related to decoherence, the qubit that loses quantum information into its surroundings. For example, to check that the setup was working properly, Jonathan ran a simple test that ended with a measurement. (Experts: He prepared a  sigma_z Eigenstatus, performed a Hadamard Gate and measured  sigma_z.) The measurement should have a 50% chance of return +1 and a 50% chance of income -1. But the -1 The results dominated the studies. Why? Decoherence pushed the qubit towards -1. (The amplitude attenuation dominated the noise.)

5) Seeing how a theoretical proposal becomes an experiment is satisfying: Partly because of point (3), experiments are not cheap. The laboratory’s willingness to invest in the idea I had developed with other theorists was encouraging. In addition, the experiment made us uncover more theory – for example, how tight the bound uncertainty might get.

After meeting an anole, I released her in our back yard and said goodbye.1 So Kater continued to experiment with topology, and Jonathan turned to graduation. But more and more visitors are wriggling in the butterfly network of theory-experiment collaboration. Stay tuned.

1Except for the anole, which I accidentally killed by keeping it in the tank for too long. But let’s not talk about it.


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