RESEARCH OVERVIEW
The group is concentrating on measurements that reveal characteristics of coherent mesoscopic systems that are not easily measured by ubiquitous conductance measurements. Conductance measuremnets are directly related to the transmission coefficient of the system, hiding phase and temporal behavior. Hence, examples of experiments are: Phase evolution of electrons is measured via novel electron interferometers; Sources of decoherence are studied by ‘which path’ type experiments, where an artificial environment is coupled to the interferometer; Charge and statistics are deduced from delicate measurements of quantum shot noise. One would expect different statistics from that of electrons (fermions) and photons (bosons), such as fractional statistics. Efforts are spent in producing the highest quality semiconducting layers produed by molecular beam epitaxy. Research in home-grown semiconductor nano-wires, via MBE, is being initiated.
Current flows along the sample edges and is carried by charged excitations (quasiparticles) whose charge is a fraction of the electron charge. Although earlier research concentrated on odd denominator fractional values of v, the observation of the even denominator v=5/2 state sparked much interest. This state is conjectured to be characterized by quasiparticles of charge e/4, whose statistics are 'non-abelian' in other words, interchanging two quasiparticles may modify the state of the system into a different one, rather than just adding a phase as is the case for fermions or bosons. As such, these quasiparticles may be useful for the construction of a topological quantum computer. Here we report data on shot noise generated by partitioning edge currents in the v=5/2 state, consistent with the charge of the quasiparticle being e/4, and inconsistent with other possible values, such as e/2 and e. Although this finding does not prove the non-abelian nature of the v=5/2 state, it is the first step towards a full understanding of these new fractional charges.
For a macroscopic object this happens due to unavoidable interactions with the environment. A thorough understanding of such quantum to classical transition is of fundamental importance, especially with the emergence of quantum information research. Experiments, where path detection was not accurate enough, leading thus only to weak dephasing, was already performed before in mesoscopic systems. Here, however, we employ a sensitive quantum ‘which path’ detector to perform an accurate path determination in a two-path electron-interferometer, leading to full suppression of the interference. We then demonstrate that phase information is not lost, but can be recovered from the combined system: interferometer & detector.
While each alone is phase insensitive, measuring the combined shot noise of both currents exhibits the original interference oscillations. This is a direct result of the cross-correlation term of the two current fluctuations (being part of the shot noise). Our present work resembles previous experiments where the two quantum systems were two parametrically - down converted - entangled photons. Moreover, in our experiment, approximately a single electron in the detector fully dephases an electron in the interferometer, leading to entanglement of single pairs of electrons.
Electrons that travel the two paths via edge channels, feel only the edge potential and the strong magnetic field; both typical in the IQHE regime. Yet, the interference of these electrons via the Aharonov-Bohm (AB) effect, behaves surprisingly in a most uncommon way. We found, at filling factors 1 and 2, high visibility interference oscillations, which were strongly modulated by a lobe-type structure as we increased the electron injection voltage.
The visibility went through a few maxima and zeros in between, with the phase of the AB oscillations staying constant throughout each lobe and slipping abruptly by pi at each zero. The lobe pattern and the 'stick-slip' behavior of the phase were insensitive to details of the interferometer structure; but highly sensitive to magnetic field. The observed periodicity defines a ‘new energy scale’ with an unclear origin. The phase rigidity, on the other hand, is surprising since Onsager relations are not relevant here.
Expected mesoscopic features in the phase such as spin degeneracy or exchange effects were never observed. Presently, there is no satisfactory explanation for this unexpected behavior where the QD seems always to return to the same ground state phase! Here we report on a new set of phase measurements conducted on dots with occupation varying from a single to twenty electrons. The measurements are performed on a QD embedded in one arm of a two path electron interferometer. An electron counter is placed in close proximity to the QD.
Unlike the universal phase behavior of larger dots the phase evolution in these small dots exhibits a mesoscopic type behavior when they are occupied with less then some ten electrons. Features in the phase indicate spin degeneracy and exchange effects that depend on the exact shape and occupation of the dots. As the number of electrons increases from around ten to some fifteen electrons the phase evolves through a transition region, later (for N>15) collapsing to the familiar universal behavior with its characteristic repetitive behavior.
This provides clear evidence that the behavior of QDs can not be fully accounted by the ubiquitous single particle model. As in previous phase measurements, the phase proves again to be far more sensitive than the conductance to intricate physical effects.