Ethan Kleinbaum

Gap Reversal at Filling Factors 3 + 1 / 3 and 3 + 1 / 5 : Towards Novel Topological Order in the Fractional Quantum Hall Regime

In this work we report the opening of an energy gap at the filling factor ν = 3 + 1/3, firmly establishing the ground state as a fractional quantum Hall state. This and other odd-denominator states unexpectedly break particle-hole symmetry. Specifically, we find that the relative magnitudes of the energy gaps of the ν = 3 + 1/3 and 3 + 1/5 states from the upper spin branch are reversed when compared to the ν = 2 + 1/3 and 2 + 1/5 counterpart states in the lower spin branch. Our findings raise the possibility that the former states have a non-conventional origin.

Note: a transimpedance amplifier for remotely located quartz tuning forks.

The cable capacitance in cryogenic and high vacuum applications of quartz tuning forks imposes severe constraints on the bandwidth and noise performance of the measurement. We present a single stage low noise transimpedance amplifier with a bandwidth exceeding 1 MHz and provide an in-depth analysis of the dependence of the amplifier parameters on the cable capacitance.

SQUID-based current sensing noise thermometry for quantum resistors at dilution refrigerator temperatures

We present a dc Superconducting QUantum Interference Device (SQUID)-based current amplifier with an estimated input referred noise of only 2.3 fA/Hz. Because of such a low amplifier noise, the circuit is useful for Johnson noise thermometry of quantum resistors in the kΩ range down to mK temperatures. In particular, we demonstrate that our circuit does not contribute appreciable noise to the Johnson noise of a 3.25 kΩ resistor down to 16 mK. Our circuit is a useful alternative to the commonly used High Electron Mobility Transistor-based amplifiers, but in contrast to the latter, it offers a much reduced 1/f noise. In comparison to SQUIDs interfaced with cryogenic current comparators, our circuit has similar low noise levels, but it is easier to build and to shield from magnetic pickup.