Ville Maisi

Research:

My research interests lies in single-electron devices where the movement of individual electrons is controlled. On one hand, our research of these devices aims to develop applications such as thermometers, quantized electrical current sources and qubits for quantum computing. On the other hand, these devices suit ideally for addressing fundamental physics questions because their operation is based on quantum mechanics. In addition, these nanometer-sized devices are so small that fluctuations in them play an important role. This opens up an excellent opportunity to study fluctuations and non-equilibrium thermodynamics experimentally. The nanothermodynamics of the small systems has become an active field of paramount importance in recent years as the size of electronic devices continue to shrink towards the atomic scale.

Our experience covers both metallic and semiconducting single-electron devices. One of the main experimental techniques utilized is real-time charge detection that is illustrated in Fig. 1 and 2 below. Figure 1 shows the a real-time detection of spin flips in semiconductor quantum dots. The device, shown in panel (a), consists of two dots holding two electrons that are indicated with the arrows. We utilize a nearby quantum point contact (QPC) to measure 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).

Figure 2 shows another example on the field of nanothermodynamics with a metallic device. We demonstrated the fundamental relation between information and physical work. Our device holds one bit of information in form of an electron that can reside either on left or right metallic island. Panel (a) shows an energy diagram and (b) the actual device with blue circles denoting the possible locations of the electron. By applying a fast pulse locking the electron to one island followed by a slow adiabatic drive back to the original state with equal probability for the electron to reside in either of the islands, we converted information into useful work W. Our system performed this conversion close to the fundamental limit of W = kT ln(2), which is the inverse cycle of the renown Landauer's principle. Fig. 2 (c) shows typical example traces of the information-to-energy conversion cycles.

Fig. 2. Information-to-energy conversion close to the Landauer limit. Figure adapted from PNAS 111, 13786 (2014).

Currently my research focuses on studying quantum dots in InAs nanowires. As one-dimensional structures with strong spin-orbit interactions, they form an interesting host material for example for studying spin physics, electron-phonon interactions in reduced dimensionality as well as Majorana states that are foreseen as the building blocks of topologically protected quantum computing.