qtune 0.1.0

Quantum dot auto tune framework

qtune Readme: Introduction

The qtune package contains tools for the setup of a general optimization program. It is originally designed for the automatic fine-tuning of semiconductor spin qubits based on gate defined quantum dots, but applicable to general optimization problems with dependent target parameters. An interface to the physical back-end must be provided. With this back-end, control parameters (here assumed to be voltages) are set and target parameters are measured.
Class names are written bold and functions cursive throughout the readme. UML class diagrams are inserted to show the heritage and dependencies, and UML activity diagrams visualize function calls. The package abbreviations are pd for pandas and np for numpy.

Installation

qtune is compatible with Python 3.5+. For development we recommend cloning the git repository https://github.com/qutech/qtune and installing by:

python setup.py develop

It can also be installed as pip package:

pip install qtune

Interface of the Physical Back-End

The core features of this program package do not require a specific structure of the measurement software. This section concerns only the required interface of the physical back-end. The Experiment class serves as abstraction of the physical experiment. It provides an interface to the control parameters with two functions called read_gate_voltages() and set_gate_voltages(new_voltages). The function set_gate_voltages(new_voltages) returns the voltages it has actually set to the experiment, which is useful if the hardware connected to the physical experiment uses a different floating point accuracy, or the Experiment is ordered to set voltages exceeding physical or safety limits.

The Evaluator class provides the function evaluate() which returns a fixed number of parameters and a measurement error, which is interpreted as the variance of the evaluation.

Proposed Measurement and Evaluation Structure

The implementation of a physical back-end, as contained in the qtune package, should be regarded as proposal.

The Experiment provides the function measure(Measurement), which receives an instance of the Measurement class and returns the raw data. The Measurement contains a dictionary of data of any type used to define the physical measurement. The Evaluator class calls the function Experiment.measure(Measurement) to initiate the physical measurements. It contains a list of Measurements and the analysis software required to extract the parameters from the raw data returned by the experiment. This could be for example a fitting function or an edge detection.

Parameter Tuning

This section describes how the dependency between parameters is taken into account. The parameters are grouped by instances of the ParameterTuner class. Each group is tuned simultaneously, i.e. depends on the same set of distinct parameters. The dependencies are assumed always one directional and static. The Autotuner structures the groups of parameters in an hierarchy, which is represented as list of ParameterTuners.

Consider the following example from the tuning of a quantum dot array. Imagine the following hierarchy consisting of three groups of parameters i.e. three ParameterTuners:

1. Contrast in the Sensing Dot Signal
2. Chemical Potentials / Positions of the Charge Stability Diagram
3. Tunnel Couplings

All scans require a good contrast in the sensing dot for an accurate evaluation of the parameters. Therefore the contrast in the sensing dot signal is the first element in the hierarchy. The measurement of tunnel couplings requires knowledge of the positions of transitions in the charge diagram. If the chemical potentials change, the charge diagram is shifted, therefore the position of the charge diagram i.e. the chemical potentials must be tuned before the tunnel couplings.

A ParameterTuner suggests voltages to tune the parameters in his group. It can be restricted to use any set of gates. It can also slice the voltage corrections to restrict the step size so that the algorithm is less vulnerable to the non-linearity of the target parameters. The tuning of a group of parameters does ideally not detune the parameters which the group depends on i.e. which are higher in the hierarchy.

The Autotuner class handles the coordination between the groups of parameters in the following sense. It decides which group of parameters must currently be evaluated or tuned and calls the ParameterTuner to evaluate the corresponding group of parameters or to suggest new voltages. It also sets the new voltages on the Experiment. It works as finite-state machine as described in the UML activity diagram below.

Optimization Algorithms

The voltage steps of each ParameterTuner are calculated by its member instance of the Solver class. This class can implement any optimization algorithm e.g. Nelder-Mead or Gauss-Newton algorithm. Gradient based Solvers like the Gauss-Newton algorithm use a instance of the GradientEstimator class for the calculation of the gradient of target parameter.

The GradientEstimator subclasses implement different methods for the gradient estimation. One example is the Kalman filter in the KalmanGradientEstimator. This is an algorithm which calculates updates on the gradient by interpreting each measurement as finite difference measurement with respect to the last voltages. The accuracy of the parameter evaluation is then compared to the uncertainty of the estimation of the gradient in order to find the most likely gradient estimation. Thereby, the gradient estimation is described as multidimensional normal distribution, defined by a mean and a covariance matrix. If the covariance becomes to large in a certain direction, the KalmanGradientEstimator suggests a tuning step in the direction of the maximal covariance. This tuning step does not optimize any parameter but should be understood as finite difference measurement.

The crucial point in the optimization of non orthogonal systems is the ability to tune certain parameters without changing the other ones. This requires communication between the Solver instances. Different Solvers can therefore share the same instances of the GradientEstimators so that they know the dependency of these parameters on the gate voltages.

Furthermore, the Autotuner communicates which parameters are already tuned to the ParameterTuners. A ParameterTuner can share this information with it's Solver, which then calculates update steps in the null space of the gradients belonging to parameters which are tuned by another ParameterTuners. A Solver also passes this information on to it's GradientEstimators, which calculate the gradients only in the mentioned null space.

Getting Started

The IPython notebook "setup_tutorial.ipynb" gives a detailed tutorial for the setup of an automated fine-tuning program. The physical back-end is replaced by a simulation to enable the tutorial to be executed before the connection to an experiment. In this simulated experiment, a double quantum dot and a sensing dot are tuned. The tuning hierarchy is given by

The ParameterTuners and Solvers which are used in the setup serve as an illustrative example. They are structured in the tuning hierarchy:

1. the sensing dot
2. the x and y position of the charge diagram
3. two parameters, being the inter dot tunnel coupling and the singlet reload time

The gates of the sensing dot are assumed to have only an negligible effect on the positions and parameters. Therefore the Solver of the sensing dot is independent of the others. The other gates are simultaneously tuning the positions and parameters. The positions and parameters are tuned by ParameterTuners restricted to the same gates and their Solver instances share all GradientEstimators. The GradientEstimators belonging to the parameters estimate the gradients only in the null space of the gradients belonging to the positions.

Features

Storage

After each evaluation of parameters, change in voltages or estimation of gradients, the full state of all classes except for the experiment is serialized and stored in an HDF5 file. The full state of the program can be reinitialized from any library file. This way, the program can be set back to any point during the tuning. The History class additionally saves all relevant information for the evaluation of the performance. The History class can plot the gradients, last fits, control and target parameters.

GUI

For real-time plotting of parameters and gradients, the user can couple the History and the Autotuner to the GUI. The GUI automatically stores the program data in the HDF5 library and lets the user start and stop the program conveniently. The program can also be ordered to execute only one step at a time. The program is logging its activity and the user can chose how detailed the logging describes the current activity by setting the log level.

Naming Convention

Voltages

are used in the Evaluator class to describe the voltages on the gates in the experiment.

Positions

are an abstraction of gate voltages in the Gradient and Solver classes. These classes could not only be used for the tuning algorithm but they could be reused in any gradient based solving algorithm.

Parameters

correspond to properties of the physical experiment. They are extracted from the measurement data by the Evaluator class and handed over to the ParameterTuner class.

Values

are the abstraction of parameters in the Gradient and Solver classes.

Options

describe the measurements in the Measurement class.

License

Copyright (c) 2017 and later, JARA-FIT Institute for Quantum Information,
Forschungszentrum Jülich GmbH and RWTH Aachen University

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.