Quantum dot-based quantum simulation

Contact Winston Pouse, Connie Hsueh, or Praveen Sriram for more information.

We live in a world full of materials exhibiting a wealth of exciting properties. Superconductivity, quantum criticality and ferromagnetism are just a few of the amazing emergent properties under active study. Both fundamental understanding and technological applications benefit from a solid understanding of the fundamental quantum many-body physics underlying these phenomena. However, such systems are often characterized by the presence of many such interactions at the same time. This richness is both a blessing and a curse: while the competition of different effects can lead to new and surprising behavior, it can also be an impediment to a more detailed understanding.

Modern nanofabrication techniques have enabled the creation of mesoscopic systems, tailored for the study of a specific effect. These systems provide exquisite control and tunability of the different parameters allowing for a bottom-up approach to study how macroscopic properties emerge from microscopic parameters. This approach is known as quantum simulation.

Our lab focuses on using semiconductor quantum dot-based devices to simulate the phenomena that emerges from competing many-body interactions. A quantum dot is a small, confined region of electrons, for which adding or removing electrons requires an additional energy Ec, known as the charging energy. When tuned to an odd number of electrons, the dot can be described as a spin ½ site. We can then generate a many-body interaction known as the Kondo effect, in which a quantum dot acts as a spin which is screened by a reservoir of surrounding conduction electrons to form a singlet. Additional interactions which also try to screen the spin compete with the Kondo interaction, leading to frustration and quantum critical behavior [1]. With complete control over the Hamiltonian through simple electrical voltage knobs, we can experimentally measure how the system behaves as it is tuned through a quantum phase transition.

A long-term goal is to scale these devices to chains and lattices of sites, where we hope to simulate the behavior of bulk materials. Semiconductor dots are limited in that their energy spectra are discrete and cannot always be tuned to behave identically. Recent work demonstrated a workaround by combining the GaAs semiconductor with a metal, turning discrete quantum levels a continuum. Using these metal-semiconductor islands, we demonstrated a quantum critical point involving two coupled islands [2]. However, limitations on how small such metal-semiconductor islands can be in GaAs prompts research in other material systems. Current and future work is thus also aimed towards building similar devices in InAs, which has no such limitation [3].

Two-channel Kondo device — Quantum dot device exhibiting a two-channel Kondo effect [1]. Fabricated by Andrew Keller in the Submicron Center at the Weizmann Institute of Science.

Quantum point contact steps in InAs — Quantum point contact steps in an InAs device [3]. Fabricated by Connie Hsueh in the Stanford Nano Shared Facilities

Coupled metallic-semiconductor islands — Double charge Kondo device using coupled hybrid metal-semiconductor islands [2]. Fabricated by Lucas Peeters in the Stanford Nano Shared Facilities

Selected Publications:

[1] A. J. Keller et al., Nature 526, 237-240 (2015).
[2] Pouse, W., et al., arXiv:2108.12691. (2022)
[3] Hsueh, C.L., Sriram, P., et al. Phys. Rev. B 105, 195303 (2022).
Goldhaber-Gordon, D., et al., Phys. Rev. Lett. 81, 5225 (1998).
S. Sasaki, et al., Nature 405, 764-7 (2000).
Leo Kouwenhoven and Leonid Glazman Phys. World 14 (1) 33 (2001).