Natalia Ares
Royal Society University Research Fellow
I started in the Materials Department at Oxford in 2013 and I was awarded a Royal Society University Research Fellowship in 2019. I focus on radio-frequency reflectometry for fast and sensitive readout of spin qubits and carbon nanotube electromechanics. I now aim at realising thermodynamics experiments and I work on machine learning for qubit scalability.
New Postgraduate Research Projects Available
Selected Publications
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Machine learning enables completely automatic tuning of a quantum device faster than human experts.
2020|Journal article|Nature communicationsVariability is a problem for the scalability of semiconductor quantum devices. The parameter space is large, and the operating range is small. Our statistical tuning algorithm searches for specific electron transport features in gate-defined quantum dot devices with a gate voltage space of up to eight dimensions. Starting from the full range of each gate voltage, our machine learning algorithm can tune each device to optimal performance in a median time of under 70 minutes. This performance surpassed our best human benchmark (although both human and machine performance can be improved). The algorithm is approximately 180 times faster than an automated random search of the parameter space, and is suitable for different material systems and device architectures. Our results yield a quantitative measurement of device variability, from one device to another and after thermal cycling. Our machine learning algorithm can be extended to higher dimensions and other technologies. -
Sensitive radiofrequency readout of quantum dots using an ultra-low-noise SQUID amplifier
2020|Journal article|JOURNAL OF APPLIED PHYSICS -
Radio-frequency optomechanical characterization of a silicon nitride drum.
2020|Journal article|Sci RepOn-chip actuation and readout of mechanical motion is key to characterize mechanical resonators and exploit them for new applications. We capacitively couple a silicon nitride membrane to an off resonant radio-frequency cavity formed by a lumped element circuit. Despite a low cavity quality factor (QE ≈ 7.4) and off resonant, room temperature operation, we are able to parametrize several mechanical modes and estimate their optomechanical coupling strengths. This enables real-time measurements of the membrane's driven motion and fast characterization without requiring a superconducting cavity, thereby eliminating the need for cryogenic cooling. Finally, we observe optomechanically induced transparency and absorption, crucial for a number of applications including sensitive metrology, ground state cooling of mechanical motion and slowing of light. -
A coherent nanomechanical oscillator driven by single-electron tunnelling
2020|Journal article|Nature PhysicsA single-electron transistor incorporated as part of a nanomechanical resonator represents an extreme limit of electron-phonon coupling. While it allows for fast and sensitive electromechanical measurements, it also introduces backaction forces from electron tunnelling which randomly perturb the mechanical state. Despite the stochastic nature of this backaction, under conditions of strong coupling it is predicted to create self-sustaining coherent mechanical oscillations. Here, we verify this prediction using time-resolved measurements of a vibrating carbon nanotube transistor. This electromechanical oscillator has intriguing similarities with a laser. The single-electron transistor, pumped by an electrical bias, acts as a gain medium while the resonator acts as a phonon cavity. Despite the unconventional operating principle, which does not involve stimulated emission, we confirm that the output is coherent, and demonstrate other laser behaviour including injection locking and frequency narrowing through feedback.cond-mat.mes-hall, cond-mat.mes-hall -
Efficiently measuring a quantum device using machine learning (vol 5, 79, 2019)
2019|Journal article|NPJ QUANTUM INFORMATION -
Efficiently measuring a quantum device using machine learning
2019|Journal article|njp Quantum InformationScalable quantum technologies will present challenges for characterizing and tuning quantum devices. This is a time-consuming activity, and as the size of quantum systems increases, this task will become intractable without the aid of automation. We present measurements on a quantum dot device performed by a machine learning algorithm. The algorithm selects the most informative measurements to perform next using information theory and a probabilistic deep-generative model, the latter capable of generating multiple full-resolution reconstructions from scattered partial measurements. We demonstrate, for two different measurement configurations, that the algorithm outperforms standard grid scan techniques, reducing the number of measurements required by up to 4 times and the measurement time by 3.7 times. Our contribution goes beyond the use of machine learning for data search and analysis, and instead presents the use of algorithms to automate measurement. This work lays the foundation for automated control of large quantum circuits.quant-ph, quant-ph, cond-mat.mes-hall, cs.LG -
Measuring carbon nanotube vibrations using a single-electron transistor as a fast linear amplifier
2018|Journal article|APPLIED PHYSICS LETTERS -
Displacemon Electromechanics: How to Detect Quantum Interference in a Nanomechanical Resonator
2018|Journal article|PHYSICAL REVIEW X -
Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet.
2017|Journal article|Physical review lettersCoupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (circuit QED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature. -
Hyperfine and Spin-Orbit Coupling Effects on Decay of Spin-Valley States in a Carbon Nanotube.
2017|Journal article|Physical review lettersThe decay of spin-valley states is studied in a suspended carbon nanotube double quantum dot via the leakage current in Pauli blockade and via dephasing and decoherence of a qubit. From the magnetic field dependence of the leakage current, hyperfine and spin-orbit contributions to relaxation from blocked to unblocked states are identified and explained quantitatively by means of a simple model. The observed qubit dephasing rate is consistent with the hyperfine coupling strength extracted from this model and inconsistent with dephasing from charge noise. However, the qubit coherence time, although longer than previously achieved, is probably still limited by charge noise in the device.
