June 2019

Abstracts of the QSIT Lunch Seminar, Thursday, June 6, 2019

Quantum Error Correction via Hamiltonian Learning

Eliska Greplova – Condensed Matter Theory and Metamaterials (Huber group), ETH Zurich

Successful implementation of error correction is imperative for fault-tolerant quantum computing. At present, the toric code, surface code and related stabilizer codes are state of the art techniques in error correction.
Standard decoders for these codes usually assume uncorrelated single qubit noise, which can prove problematic in a general setting.
In this work, we use the knowledge of topological phases of modified toric codes to identify the underlying Hamiltonians for certain types of imperfections. The Hamiltonian learning is employed to adiabatically remove the underlying noise and approach the ideal toric code Hamiltonian. This approach can be used regardless of correlations. Our method relies on a neural network reconstructing the Hamiltonian given as input a linear amount of expectation values. The knowledge of the Hamiltonian offers significant improvement of standard decoding techniques.

The THz Quantum Cascade Laser: past and future challenges

Lorenzo Bosco – Quantum Optoelectronics Group (Faist group), ETH Zurich

THz radiation is subject to a wide range of research and technological efforts, from solid state fundamental physics to biomedicine and astrophysics. However, widespread application are often hindered by the lack of compact and powerful THz sources. A promising candidate for this role is the quantum cascade laser (QCL), although it currently requires cryogenic cooling since they only operate below 200 K. After seven years from the last improvement in temperature performance, we present the first THz QCL operating on a thermoelectric cooler, up to a record-high temperature of 210 K. The design achieves high-temperature operation thanks to a systematic optimization by means of a non-equilibrium Green's function model, which also reliably reproduces the experimental results. Thanks to the relatively high peak power measured at 206 K (>1 mW), the laser spectra were acquired with a commercial room-temperature detector, making the whole setup cryogenic free and paving the way to possible applications.