Characterize Large Quantum Computers

Quantum computers are constantly becoming more complex and powerful. Researchers at the University of Innsbruck, in cooperation with the Johannes Kepler University Linz and the University of Technology Sydney, are now presenting a method with which even large quantum computers can be characterized with just a single measurement setting.

The gold standard for the characterization of quantum computers is the so-called quantum tomography, which, based on medical tomography, can draw a complete picture of the quantum calculation from a series of snapshots of the system. While the method provides deep insights, the number of measurements required for tomography is increasing rapidly, with three times as many measurements required for each additional quantum bit (qubit). Due to the large amount of time it takes to perform all these measurements, tomography has so far been limited to devices with just a few qubits. However, recent technological developments enable much larger quantum computers today, which makes their characterization much more difficult.

A single measurement setting is required
A team of physicists led by Martin Ringbauer from the Institute for Experimental Physicsat the University of Innsbruck, in cooperation with theoretical physicists from Linz and Sydney, has now developed a practical approach to characterizing large quantum systems in just one measurement setting and independent of the system size, and has successfully tested it in experiments. This is achieved by partially moving away from the binary math used by both quantum computers and their classical predecessors. In fact, the atomic ions used for quantum information processing have much more than just the two qubit states to which they are artificially constrained in the quantum computer. If more states are included, significantly more information can be stored per particle. "The extension of qubits to ququarts, the latter consisting of four states, allows us to to store and measure all the information required for tomography at once,” says Innsbruck physicist Roman Stricker. By combining this measurement method with a new data analysis approach called "classical shadows", originally developed by Richard Küng of Johannes Kepler University Linz and colleagues, the team has demonstrated a highly efficient characterization approach. This made it possible for the first time to analyze an eight-qubit system in real time. Küng emphasizes that their technique has the potential to enable real-time characterizations of future large devices, which is a significant next step towards scalability of quantum computers. By combining this measurement method with a new data analysis approach called "classical shadows", originally developed by Richard Küng of Johannes Kepler University Linz and colleagues, the team has demonstrated a highly efficient characterization approach. This made it possible for the first time to analyze an eight-qubit system in real time. Küng emphasizes that their technique has the potential to enable real-time characterizations of future large devices, which is a significant next step towards scalability of quantum computers. By combining this measurement method with a new data analysis approach called "classical shadows", originally developed by Richard Küng of Johannes Kepler University Linz and colleagues, the team has demonstrated a highly efficient characterization approach. This made it possible for the first time to analyze an eight-qubit system in real time. Küng emphasizes that their technique has the potential to enable real-time characterizations of future large devices, which is a significant next step towards scalability of quantum computers. This made it possible for the first time to analyze an eight-qubit system in real time. Küng emphasizes that their technique has the potential to enable real-time characterizations of future large devices, which is a significant next step towards scalability of quantum computers. This made it possible for the first time to analyze an eight-qubit system in real time. Küng emphasizes that their technique has the potential to enable real-time characterizations of future large devices, which is a significant next step towards scalability of quantum computers.

Versatile technology
The biggest technological challenge for the physicists was to reliably transfer the qubit information into the four states of the ququart and then read it out in a single experiment run. “Until now, our readout options could only differentiate between two states per detection. So we adapted our setup so that we can now detect three times in a row to identify all four states," explains Michael Meth from the University of Innsbruck. "We solved this problem by programming a fast camera readout and counteracting the detection-related heating of the ions with an additional laser cooling step," explains Thomas Monz. These adjustments are crucial to prevent the loss of quantum information during the additional detections.

The research work was published in the journal PRX Quantum and funded by the Austrian Science Fund FWF, the special research area BeyondC, the Austrian research funding agency FFG and the European Union, among others.