Ville Maisi

Ville Maisi

Senior lecturer

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The expertise and interests of our research group forms around individual electrons and photons interacting with each other in electronic quantum devices. On one hand, we aim to develop new devices such as microwave photon counters which find use in quantum technology and sensitive astronomy measurements. On the other hand, these devices suit ideally for addressing fundamental physics questions, because their operation is based on quantum mechanics. One of the most intriguing question for me is how quantum measurements work in these systems. For example, often physicists describe measurements as an instantaneous collapse of the system state. With our continuous measurement scheme, we can study the dynamics beyond this wavefunction collapse picture.


Following the motion of individual electrons:

One of the main experimental techniques we use is real-time detection of electron tunnelling. Figure 1 presents an example of the technique where the statistics of the electron tunnelling events reveal spin flips in a semiconductor quantum dot: The device, shown in panel (a), consists of two dots holding two electrons that are indicated with the arrows. A nearby quantum point contact (QPC) measures whether the electrons are residing on separate dots, in state (1,1), or in the same dot, in state (2,0). Panel (b) shows the measured current thought the QPC as a function of time. Electron tunneling results in switching between the two states. If the electrons have spins in parallel direction, the system stays a long time in the (1,1) state and no tunneling occurs due to Pauli exclusion principle. Once the spin flips, the tunneling becomes possible resulting in a burst of tunneling events until blockade occurs again. The beginning and the end of such burst pinpoints the spin flipping events in our system.

Fig. 1. Real-time detection of spin blockade in a double quantum dot. Figure adapted from Phys. Rev. Lett. 116, 136803 (2016).


Counting microwave photons:

Currently our group focuses on building microwave photocounters. The operation principle is based on inducing an electron tunnelling event for each photon via high efficiency absorption process, and then using charge detection to gain access to the photon counting statistics. Our research group demonstrated the continuous conversion of photons into electron tunneling events with quantum dots, see Fig. 2 and Nature Comm. 12, 5130 (2021). Now we are integrating the charge detection to the device to demonstrate photon counting. We also use and study high-impedance resonator systems which yield coherent interactions between the electrons and photons.