Abstract:
In this seminar, I will briefly touch upon three different quantum platforms and materials systems that, each in their own way, exemplify different aspects of the overarching theme of my research work thus far: quantum control of solid-state quantum emitters.

In the first half of my talk, I will discuss a series of cryogenic experiments performed on single spins at high magnetic fields in III-V quantum dots. At the single-qubit level, using ultrafast optical pulses, we demonstrated full optical control of both single electron and hole spin qubits, and we identified the dominant dephasing mechanisms in each case [1-2]. We then moved on to two-qubit operation, where we demonstrated, through a novel, ultrafast non-linear measurement technique, the highest to-date solid-state spin-photon entanglement fidelity, as confirmed by full state tomography [3-4]. I will then briefly analyze the fundamental limitations of this materials system in terms of scalability, and potential solutions thereof.

One such solution we recently explored, involves the creation of a fully novel type of quantum device, combining the advantages of an electrostatically controlled quantum dot with the versatile optical access provided by optical quantum dots. For this, we harness the strong optical response and tight excitonic binding energy of monolayer transition metal dichalcogenides (TMDs) [5]. I will show new results demonstrating both the observation of Coulomb blockade in quantum dot transport devices, as well as electrostatic control of the optical emission of rudimentary nanowire-quantum dots [6].

Time permitting, I will also briefly discuss molecular defects in wide-bandgap semiconductors, particularly the nitrogen-vacancy (NV) center in diamond. In a first series of experiments, we used a scanning-NV magnetometer with a scanning magnetic gradient to map out, with sub-nanometer and single-spin resolution, the magnetic and spin environment of shallow NV centers. Such shallow NVs are commonly used for high-resolution magnetometry experiments. We observed a pronounced and dominant spin noise contribution which could be attributed to spin-1/2 defects at the diamond surface [7]. We then developed and studied in detail a novel oxygenation procedure that reduced the surface spin noise by over an order of magnitude – allowing for the observation of the NMR signal of a single, denatured ubiquitin protein [8].

[1]. D. Press, K. De Greve et al., Nature Photonics 4, 367 (2010)
[2] K. De Greve, P. McMahon et al, Nature Physics 7, 872 (2011)
[3] K. De Greve, L. Yu et al, Nature 491, 421 (2012)
[4] K. De Greve*, P. McMahon* et al, Nature Comms. 4, 2228 (2013).
[5] G. Scuri, Y. Zhou et al., PRL 120, 037402 (2018)
[6] K. Wang, K. De Greve et al, Nature Nano. 13, 128 (2018)
[7] M. Grinolds, M. Warner, K. De Greve et al., Nature Nano. 9, 279 (2014)
[8] I. Lovchinsky,.., K. De Greve et al., Science 351, 836 (2016)

 
Bio:
Kristiaan De Greve is currently a research associate and HQOC prize postdoctoral fellow in the Physics department at Harvard University, where he studies experimental and theoretical quantum science under the supervision of Prof. Mikhail Lukin, with strong collaborations with Profs. Philip Kim and Hongkun Park. His current work involves theoretical applications of NISQ quantum networks, experimental studies of quantum effects in two-dimensional semiconductors, and the quantum control of molecular defects near noisy solid-state surfaces. Kristiaan obtained his PhD in Electrical Engineering at Stanford University under Prof. Yoshi Yamamoto. His graduate work involved the single- and multi-qubit control of spins and photons in semiconductor quantum dots, as well as optical studies of pnictide high-Tc superconductors – work for which he was awarded a Springer Thesis Prize. Kristiaan is a graduate of KU Leuven, Belgium, where he was valedictorian of the engineering school.