Events

Characterizing states and measurements: principles and approaches

IQC Special Seminar, Jahan Claes, Yale Department of Applied Physics

Large-scale quantum computers will require error correction in order to reliably perform computations. However, the hardware overhead for error correction remains dauntingly large, with each logical qubit potentially requiring thousands of physical qubits for reliable operation.

Brad Ramshaw

Abstract: Strange metals have linear-in-temperature (T-linear) down to low temperature. Strange metals are found in many families of correlated electron materials, leading to the conjecture that a universal bound - the "Planckian" bound - limits the scattering rate of electrons to a value set by fundamental constants. If the Planckian bound exists, it would provide a natural explanation for why a host of seemingly disparate systems, including high-temperature superconductors and twisted bilayer graphene, all have T-linear resistivity. Perhaps more dramatically, T-linear resistivity suggests that electron-electron interactions are so strong that conventional concepts such as quasiparticles and Boltzmann transport do not apply in strange metals. I will present our work on the cuprate Nd-LSCO and the 5-layer superconducting nickelate that shows that conventional quasiparticle transport is alive and well, even in the strange metal regime where the Planckian bound is saturated. This suggests that we may not need to abandon the quasiparticle picture entirely, but that we need to better understand the source of scattering in these materials. 

Landau-Zener tunneling: from weak to strong environment coupling

Debanjan Chowdhury: The good, the bad and the strange: Unconventional metallic behaviour in the vicinity of Mott insulators

Abstract: In recent years, we have witnessed remarkable experimental breakthroughs in uncovering the intriguing properties of correlated metals in the vicinity of Mott transitions. Describing these phenomena theoretically remains an open challenge. This talk will focus on three recent examples of puzzling electronic behavior near Mott insulating phases and address the various conundrums. In the first part of the talk, I will discuss the microscopic origin of an unconventional T-linear resistivity with Planckian scattering in a quasi-two-dimensional “good” metal with long mean-free path, consisting of highly conducting metallic and Mott insulating layers, respectively. In the second part, I will address the origin of a low-temperature “bad” metallic behavior in the vicinity of a continuous bandwidth-tuned metal-insulator transition in a moiré semiconductor. I will end by presenting some new theoretical insights into the experimental observation of an anomalous particle-hole continuum and overdamped plasmon in the density response of cuprate “strange” metals. 

Quantum mechanics helps us understand what happens below what a microscope can see, describing the world of atoms, electrons, photons, and more. In celebration of World Quantum Day on April 14th, Dr. John Donohue from the Institute for Quantum Computing will sit down with Exploring by the Seat of Your Pants to explore quantum science and its applications, from light particles and electron waves to superconductors and quantum computers.

AEQuO: A Comprehensive Measurement Allocation Protocol

IQC Colloquium on ZOOM - Mark Zhandry, NTT Research

Public verification of quantum money has been one of the central objects in quantum cryptography ever since Wiesner's pioneering idea of using quantum mechanics to construct banknotes against counterfeiting. In this talk, I will discuss some recent work giving both attacks and new approaches to building publicly verifiable quantum money.

Development of InSb Surface Quantum Wells for hybrid superconducting device applications. 

Abstract: Surface quantum well (QW) heterostructures in III-V semiconductors are compatible with proximitized superconductivity and offer a scalable planar platform for superconductor-semiconductor systems, such as those suggested for topological quantum computation and those suitable for topological phase transitions involving Majorana zero modes. Amongst III-V binary semiconductors, Indium Antimonide (InSb) has the smallest electron effective mass, highest spin orbit coupling and largest Land´e g-factor. Such material properties makes the pursuit of InSb QWs desirable for a number of quantum device applications, including quantum sensing, quantum metrology, and quantum computing.

Unfortunately, high quality two-dimensional electron gases (2DEGs) in InSb QWs have so far been difficult to realize. InSb QWs have generally relied on the use of modulation doping for 2DEG formation, but these structures have frequently reported issues with parasitic parallel conduction and unstable carrier densities. We report on the transport characteristics of field effect 2DEGs in surface InSb quantum wells which overcome these challenges and are suitable for future hybrid superconducting device applications.

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No, you have not discovered a Majorana Fermion

Abstract: Is what I tell myself. There was a time when I thought I may have discovered it, others did too. Around 2012 several groups including ours found evidence of these quantum excitations in electrical circuits containing nanowires of semiconductor covered by a superconductor. The dramatic signatures were peaks in conductance that appeared under conditions expected from theory for Majorana modes, which are their own anti-modes and may possess non-Abelian properties. But a few years later, similar features in the data were identified due to an interesting, but a more mundane effect - which we call trivial states such as Andreev bound states. Over time more and more data pointed at the trivial and not at the exotic explanation. But because Majorana claims kept coming, this led to some digging and even retractions. What we learned after 10 years is that we have a much better handle on what effects show up in these nanowires, which positions us well for the ultimate Majorana discovery which we should be able to tell apart from all the non-Majorana things we saw. The second lesson we learned is that materials quality of device constituents, superconductors and semiconductors, as well as how samples are fabricated - are the make-or-break factors for making this happen. So while  I cannot report an exciting physics discovery, I can walk you through the scientific process that took place, a 10-year event of independent value which taught me how to do science better.