May 2023
Abstracts of the Quantum Center Lunch Seminar
Date: Thursday, May 4, 2023
Place: ETH Zurich, Hönggerberg, HPF G 6
Time: 12:00 - 13:30
Quantum Simulations beyond Electronic-Structure Methods: from Tensor Networks to Quantum Computing
Alberto Baiardi - Laboratory of Physical Chemistry (Reiher group), ETH Zurich
The simulation of quantum many-body systems is emerging as one of the most promising targets for quantum computing.1 In fact, quantum hardware promises to exponentially reduce the computational cost of these simulations compared to classical computers. In chemistry, quantumcomputing algorithms have been mostly designed for time-independent electronic-structure calculations. However, other classes of many-body problems are relevant for chemical simulations. This includes vibrational-structure,2–4 coupled vibrational-electronic,5,6 and time-dependent calculations.7 In this contribution, we will first describe methods that we developed, based on the density matrix renormalization group (DMRG) theory, for studying quantum-chemical manybody systems beyond electronic-structure problems. We will show how these methods can target systems that are hard challenges for alternative state-of-the-art quantum-chemical many-body methods. They set, therefore, the bar that quantum-computing algorithms must overcome to yield a practical quantum advantage in quantum-chemical simulations, as we will critically discuss for the case of molecular vibrational-structure calculations.
[1] Baiardi, A.; Christandl, M.; Reiher, M. arXiv 2023, 2212.12220. [2] Baiardi, A.; Stein, C. J.; Barone, V.; Reiher, M. J. Chem. Theory Comput. 2017, 13, 3764–3777. [3] Baiardi, A.; Stein, C. J.; Barone, V.; Reiher, M. J. Chem. Phys. 2019, 150, 094113. [4] Baiardi, A.; Kelemen, A. K.; Reiher, M. J. Chem. Theory Comput. 2021, 18, 430. [5] Muolo, A.; Baiardi, A.; Feldmann, R.; Reiher, M. J. Chem. Phys. 2020, 152, 204103. [6] Feldmann, R.; Muolo, A.; Baiardi, A.; Reiher, M. J. Chem. Theory Comput. 2022, 18, 250. [7] Baiardi, A.; Reiher, M. J. Chem. Phys. 2020, 152, 040903.
Josephson-like tunnel resonance in GaAs-based electron-hole bilayers
Miranda Davis - Advanced Semiconductor Quantum Materials (Wegscheider group), ETH Zurich
When a layer occupied by electrons and a layer occupied by holes are brought into close proximity, electron-hole pairing into excitons is predicted to lead to interesting many-body states. Namely, the bosonic nature of excitons would permit the formation of a Bose-Einstein condensate (BEC) at sufficiently low temperatures. We present promising signs of excitonic interactions in GaAs-based systems. Modified p-i-n diodes with a 10 nm AlGaAs barrier separating the intrinsic layers are biased to form stable 2D electron-hole bilayers. A small AC excitation voltage is added to the DC bias and the resulting AC interlayer current measured with a lock-in amplifier, yielding the capacitance and differential conductance of the device. A prominent peak is revealed in the differential tunnel conductance at temperatures in the millikelvin range, which is suppressed as the temperature of the sample is increased. This occurs in a density range where Josephson-like tunneling due to the formation of an excitonic BEC has been predicted. An accompanying enhancement of the interlayer capacitance provides corroborating evidence. The tunnel peak displays interesting behavior in the presence of a magnetic field applied perpendicular to the 2D charge layers, shifting to higher biases/densities and growing in magnitude as the field is increased. Further study is required to determine if this aligns with the excitonic-BEC interpretation.