Device tuner

Tuner

The base class implements the most common tuning methods such as updating a device’s normalization constants, measuring a pinchoff or taking a charge diagram. These methods use the corresponding Tuningstages to take and analyse data. In general, a tuner acts on an instance of Device abstraction to perform the tuning. It has methods to combine default measurement settings defined in the tuner’s data and setpoint settings with device-related ones.

Measurement settings

Measurement related settings are grouped in two dataclasses, DataSettings and SetpointSettings. A tuner class keeps an instance of each with general settings used for tuning of all devices. Examples of setpoint setting used for all measurements are setpoint_method, optionally voltage_precision and high resolution precisions. As for data settings, data base location or size of charge diagram segments used for classification also stay unchanged in most cases.

Beside the methods listed above, the base class also has a method to determine the initial ranges of a helper gate, such as a top barrier of a 2D layout or bottom gate underneath the entire 1D system.

Tuning results are saved whenever a TuningResult is returned by a tuning stage, i.e. when measuring a pinchoff or charge diagram. They are saved to the tuner’s tuning_history using the device name as key. Saving results in these low lying methods used in every other tuning procedure ensures that all of them are retained.

Characterizer

The Characterizer sub-class implements the device characterization step by applying the base class’ gate characterization step to all gates. Optionally, gates can be left out, e.g. in the case of a helper gate which has been set beforehand.

Dot tuner

The DotTuner contains methods required to implement the tuning procedure shown in Fig. 11. If desired, the entire sequence can by run using the tune method, however all methods should also be seen as building blocks for other sequences.

Fig. 11 Dot tuning algorithm implemented by the DotTuner class.

The steps outlined in the diagram have a corresponding method within the class.

step	method
Set top barrier	set_helper_gate
Characterize central barrier Set central barrier	set_central_barrier
Characterize outer barriers Set outer barriers	set_outer_barriers
Characterize plungers	characterize_plunger, used in set_valid_plunger_ranges
Characterize charge stability diagram	get_charge_diagram
Classify charge state	in get_charge_diagram
Change central barrier	update_gate_configuration

Note that get_charge_diagram adjusts plunger ranges if the resulting diagram shows a signal too low or too high for a dot regime. If the signal is too high, the plunger ranges are decreased and increased if too low. If the plunger ranges hit their safety voltage ranges during this update, outer barriers are adjusted. This is the small inner loop labelled no success on the left. Similarly, if no plunger ranges within the respective safety limits are found to begin with, the outer barriers are changed.

When the desired dot regime is not found, update_gate_configuration which implements the loop going left and right from “Classify state”, is called. It executes adjust_all_barriers_loop, which in turn calls adjust_all_barriers, but with different inputs depending on the classification outcome. adjust_all_barriers_loop sets adjusts voltage of one gate, specified as input, before launching adjust_all_barriers. If required, i.e. adjusting outer or central barriers was not successful because they reached their safety limit, the helper gate is changed too.

Specifically, these two outer loops in Fig. 11 do teh following:

Large left loop - no good dot found:

• Uses termination reasons to update outer barriers first. The decisive information is whether the device was too pinched off or open.

• Update top barrier/helper gate in adjust_all_barriers, called in adjust_all_barriers_loop

Right loop - no success changing central barrier:

• Change central barrier with initial_voltage_update in adjust_all_barriers_loop.

• If central barrier is set successfully, the loop in adjust_all_barriers_loop is not executed and thus tuning resumes with plunger range characterization.

• If after setting the central barrier other changes are needed, the loop in adjust_all_barriers_loop will set the top barrier, then central and also outer barriers.

The class also has methods to take high resolution charge diagram, with voltage precisions specified in setpoint_settings.high_res_precisions. There are two types of high res diagram: the entire one and only diagram segments which have been classified to be the target regime, e.g. good double. This serves to confirm the tuning result and get data for publications or presentations.

Demonstration

The autonomous tuning paper showed that this tuning procedure is able to tune several devices without manual input. For good devices, only one pass through the algorithm is required, meaning only a few 1D and a one 2D measurement is taken. The loops adjusting barriers is not executed.