R. G. Herb Condensed Matter Seminars

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Events During the Week of September 24th through October 1st, 2017

Monday, September 25th, 2017

Spin-orbit interaction at the level of single electrons
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Dr. Andrea Hofmann, ETH Zurich
We measure the anisotropy of spin-orbit interaction (SOI) using real-time charge detection of single electrons tunneling between different states of GaAs/AlGaAs-based double quantum dots (DQDs). The strength of the SOI depends on the crystallographic direction of the electron tunneling, and on the relative alignment between the tunneling direction and the spin quantization axis. In the DQD, the tunneling direction is defined by the main axis of the device, and the spin quantization axis is chosen by the direction of an external in-plane magnetic field. This set-up allows us to control the strength of the spin-orbit interaction and leads to spin lifetimes of 10 s.

We fabricate two DQDs on a GaAs heterostructure, one with its main axis along the [110] crystal axis, and another one with the main axis rotated by 90 degrees, i.e. along [-110]. By applying suitable gate voltages to metallic top-gates, each DQD is brought into a configuration where two electrons reside in the device, and tunneling to the source and drain is suppressed. Using a charge detector, we distinguish between two resonant charge states: one state where both electrons reside in the right quantum dot, (0,2), and one state where each dot is occupied by a single electron, (1,1). We argue that in this configuration, the Pauli spin blockade can be used to measure the strength of the spin--orbit interaction experienced by tunneling electrons.

We use the two DQDs for measuring the different strengths of the SOI experienced by electrons moving along distinct crystallographic axes. We find that the SOI induces spin-flips for electrons moving along [110], and that the SOI vanishes for an electron moving along [-110]. For a given tunneling direction, we vary the strength of the experienced SOI by changing the alignment between the tunneling direction and spin-quantization axis by means of rotating the direction of the applied in-plane field. We find a sinusoidal dependence on the relative angle between the two directions.

A high magnetic field facilitates suppression of incoherent spin-relaxation processes within single dots. We measure the anisotropy of spin-flip tunneling rates between two energetically resonant quantum states in this setting and find that the spin--orbit interaction can be turned from on to almost completely off.
Host: Eriksson
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Tuesday, September 26th, 2017

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Wednesday, September 27th, 2017

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Thursday, September 28th, 2017

Dynamical control techniques with superconducting qubits
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Dr. Simon Gustavsson , MIT
Abstract: Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation (T1). In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons - quasiparticles - in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability [1].
In a separate experiment, we investigate qubit dephasing (T2) due to photon shot noise in a flux qubit transversally coupled to a coplanar microwave resonator. Due to the AC Stark effect, photon fluctuations in the resonator cause frequency shifts of the qubit, which in turn lead to dephasing. While this is universally understood, we have made the first quantitative spectroscopy of this noise for both thermal (i.e., residual photons from higher temperature stages) and coherent photons (residual photons from the readout and control pulses). By mapping out the noise power spectral density seen by the qubit, we uniquely identify thermal shot noise as the dominant source of dephasing. When implementing the CPMG dynamical-decoupling protocol, we are able mitigate to the adverse influence of the photon shot noise, and improve T2 Echo ~ 40 us to reach T2 CPMG ~ 80 us ~ 2*T1. Furthermore, by improving the filtering for thermal noise in a subsequent cooldown, we are able to reduce the residual photon population to 0.0004, resulting in T2 echo times approaching 100 us [2].

[1] Science 354, 1573 (2016)
[2] Nature Communications 7, 12964 (2016)
Host: McDermott
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Friday, September 29th, 2017

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