June 2024

Abstracts of the Quantum Center Lunch Seminar

Date: Thursday, June 6, 2024
Place: ETH Zurich, Hönggerberg, HPF G 6
Time: 12:00 - 13:30

Enhancing Dispersive Readout of Superconducting Qubits Through Dynamic Control of the Dispersive Shift: Experiment and Theory

François Swiadek - Quantum Device Lab (Wallraff group), ETH Zurich

The performance of a wide range of quantum computing algorithms and protocols depends critically on the fidelity and speed of the employed qubit readout. Examples include gate sequences benefiting from mid-circuit, real-time, measurement-based feedback, such as qubit initialization, entanglement generation, teleportation, and perhaps most importantly, quantum error correction. A prominent and widely-used readout approach is based on the dispersive interaction of a superconducting qubit strongly coupled to a large-bandwidth readout resonator, frequently combined with a dedicated or shared Purcell filter protecting qubits from decay. By dynamically reducing the qubit-resonator detuning and thus increasing the dispersive shift, we demonstrate a beyond-state-of-the-art two-state-readout error of only 0.25% in 100 ns integration time. Maintaining low readout-drive strength,we nearly quadruple the signal-to-noise ratio of the readout by doubling the readout mode linewidth, which we quantify by considering the hybridization ofthe readout-resonator and its dedicated Purcell-filter. We find excellent agreement between our experimental data and our theoretical model. The presented results are expected to further boost the performance of new and existing algorithms and protocols critically depending on high-fidelity, fast, mid-circuit measurements.

Quantum delocalization of a levitated nanoparticle to atomic scales

Titta Carlon Zambon - Photonics Laboratory (Novotny group), ETH Zurich

Levitated objects in ultra-high vacuum offer a new paradigm for free-space optomechanics [1]. In the absence of clamping losses and of a background gas, an exceptional degree of isolation from the environment can be achieved, resulting in quantum optomechanical cooperativities exceeding one even at ambient temperature. In combination with near-Heisenberg limited position measurements, this allowed harnessing quantum-control techniques to prepare the motional groundstate of a levitated nanoparticle [2,3]. An important step forward would now be to demonstrate the ability to coherently manipulate the mechanical state of the nanoparticle. In this regard, generating a coherent expansion of the center-of-mass wavefunction is especially significant, as it represents a prerequisite for realizing matter-wave interferometry experiments with nano-scale objects [4].

I will present a series of recent experiments where we exploit rapid optical pulse sequences to manipulate the mechanical state of the nanoparticle. Our results indicate that we can delocalize the nanoparticle wavefunction to length scales comparable to the atomic radius of its elemental constituents -Silicon and Oxygen- with minimal added noise. Furthermore, coherent squashing of position implies squeezing of the conjugate momentum. From our measurements, we infer about 3dB squeezing of momentum below its zero-point fluctuation amplitude, a valuable resource for force-sensing applications [5].

[1] C. Gonzalez-Ballestero et al. - Science 374, 6564 (2021)
[2] L. Magrini et al. - Nature 595, 373-377 (2021)
[3] F. Tebbenjohanns et al. Nature 595, 378–382 (2021)
[4] O. Romero-Isart et al. PRL 107, 020405 (2011)
[5] S.C. Burd et al. - Science 364, 1163-1165 (2019)