Since last year’s American Physical Society (APS) March Meeting was cancelled due to the pandemic, I’ve been looking forward to my yearly fill of meeting old friends and former students, and hearing the latest results in areas that I haven’t been following closely. It won’t be quite the same in this year’s digital format, but I’m still looking forward to it.
The March Meeting isn’t just about learning new results; it’s also a forum to present your own results, get questions and inspiration for next steps. Our team will be discussing our progress in more than 50 talks at this year’s meeting. I won’t try to summarize all of them here, but I want to highlight a few. These include our ground-breaking work in novel materials combinations, complex device simulations, and new measurement and data analysis techniques.
Complex device simulations
Microsoft’s Roman Lutchyn will be speaking about our progress in simulating super-semi nanowires and experimental methods for detecting topological superconductivity. Our understanding of superconductor-semiconductor heterostructures has advanced greatly in recent years, as has our understanding of measurement protocols that can connect simulations to experiments on engineered devices. One important development that Roman will be presenting is a protocol for detecting a topological superconducting phase and for determining the bulk topological gap. This protocol avoids some of the challenges associated with trying to detect a topological superconducting state through local properties. Moreover, the protocol returns a measurement of the topological gap, which is a key parameter for any application of a topological phase. Of course, this protocol is only useful in a device that has a reasonable chance of showing a topological phase. Roman will be presenting simulations that address this problem.
Direct observation of anyons
One of the biggest highlights of the last year in physics was the direct observation of anyons in a fractional quantum Hall state by a team from Purdue and Microsoft, including Mike Manfra and Geoff Gardner. Mike’s student, James Nakamura, will be giving a talk explaining this work.
The defining property of anyons is that when one anyon moves around another (braids with other anyons) the quantum mechanical wavefunction acquires a phase that only depends on the topology of the path. However, the phase that results from braiding anyons is difficult to directly measure because there are many reasons why a quantum state can acquire a phase.
In order to isolate the braiding phase, an interferometer can be constructed to cancel the other phases. However, even once this is done, the braiding phase can remain elusive because it can only be extracted by comparing the phase change with and without the second anyon enclosed in the interferometer loop. But changing the number of anyons within the loop is not entirely straightforward. In a quantum Hall interferometer, the experimenter has direct control over gate voltages and the magnetic field. But none of these necessarily translates directly into the number of anyons within a loop. The number of anyons contained in the loop, the area enclosed in the loop, and the total charge enclosed within the loop can all change as a function of the two knobs that are under the experimenter’s control.
The Purdue and Microsoft team engineered a material stack and a device fabrication process that ensured that charge can freely move into or out of the loop so that the area of the loop remains constant as the magnetic field is varied, provided that the gate voltage is held constant. If the gate voltage is increased, thereby increasing the area of the loop, then a decrease in the magnetic field will keep the interference phase constant. The number of anyons enclosed within the loop also remains constant—most of the time. However, an anyon occasionally enters the loop at certain reproducible values of the magnetic field and gate voltage. When this occurs, the phase shifts by the predicted amount. This demonstrates that anyons are real and observable, not merely hypothetical particles, which is truly a landmark event.
Novel materials combinations
In his talk, Hybrid Epitaxial Materials at the Heart of the Quantum Computing Revolution, Charlie Marcus will describe novel superconductor-semiconductor-magnetic insulator epitaxial heterostructures, made possible by the compatibility between the growth and fabrication conditions for aluminum (superconductor), indium arsenide (semiconductor), and europium sulfide (ferromagnet). They have fabricated devices that combine superconductivity and ferromagnetism in a gate-tunable semiconductor, as I discussed in a previous blog post. These remarkable hybrid structures have potential applications for topological quantum computation.
A landmark result about the power of Quantum Annealing
Annealing is an elegant method for solving optimization problems: we construct a simulation of a physical system whose energy is the cost function of interest. By slowly lowering the temperature of the system, we allow the system to settle into its lowest energy state, thereby finding the minimum of the cost function. But one limitation is that the system may find itself in a local minimum from which it takes a very long time to escape by thermal excitation. In such a situation, a quantum simulation could be advantageous since quantum mechanics allows the system to tunnel out of the local minimum. Rather than lowering the temperature, one could, instead, vary the Hamiltonian from one with a known ground state to one with the ground of interest that minimizes the cost function; this is called quantum annealing. But it’s not a slam dunk in all cases. Some quantum systems are more readily simulated classically because they have no “sign problem.” Some people have called such systems “stoquastic.” Intuitively, one might expect that quantum annealing would have no advantage over classical annealing, but Matt Hastings proved that this isn’t the case: there is a computational advantage for quantum computers even in for such optimization problems, as he will explain in his talk.
Join us at the APS March Meeting
This is just a sampling of the cutting-edge work and scientific discoveries that we’ll be presenting at the APS March Meeting, which is far more than I can discuss in this blog post. For my part, I’m looking forward to hearing about the advances made during this pandemic year, to talks from prospective job applicants, and to the presentations from our team on our latest results. We look forward to a vigorous discourse in the community.