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Practical Approach for Characterizing Large-Scale Quantum Systems

Quantum devices are growing more complicated and powerful all the time. Scientists from the University of Innsbruck, in partnership with the Johannes Kepler University Linz and the University of Technology Sydney, have developed a method for characterizing even huge quantum computers using a single measurement setting.

View inside an ion trap, the heart of an ion trap quantum computer. Image Credit: C. Lackner/University of Innsbruck.

Quantum tomography, which, like medical tomography, can create a comprehensive picture of a quantum system from a sequence of snapshots, is the gold standard for the characterization of quantum devices. While tomography provides numerous insights, the number of measurements necessary rapidly increases, with three times as many measurements required for each extra qubit.

Tomography has only been feasible on devices with a few qubits due to the time required to do all of these tests. However, recent advances in quantum computing have effectively scaled up system sizes far beyond tomography's capabilities, making characterization a challenging constraint.

Pictured From a Single Measurement Setting

A group of physicists guided by Martin Ringbauer from the University of Innsbruck’s Department of Experimental Physics, in collaboration with theoretical physicists from Linz and Sydney, has now constructed and demonstrated a practical approach for characterizing even large-scale quantum systems, depending on a single measurement setting that is independent of system size.

This is accomplished by deviating from the binary computing inherent in both quantum computers and their classical predecessors. In actuality, the atomic ions employed for quantum information processing contain far more than the two qubit levels that are artificially limited. By involving more levels, it is possible to store substantially more information per particle.

Extending qubits to four-level ququarts thus enables us to store and measure the entire information necessary for tomography in one go.

Roman Stricker, Physicist, University of Innsbruck

The researchers demonstrated an extremely effective characterization approach by combining this method of measurement with a data analysis approach known as "classical shadows," which was created originally by Richard Küng of Johannes Kepler University Linz and colleagues.

They were able to fully characterize an eight-qubit system in real-time for the first time using integrated techniques. Küng stresses that their methodology has the ability to enable real-time characterizations of huge future devices, which is an important step toward quantum computer scalability.

Widely Applicable Technique

The key technological issue was properly transferring the qubit information into the four states of the ququart and extracting it in a single experiment run.

Our readout capabilities so far could only distinguish between two levels per detection, so we adapted our setup such that we can detect three times in a row to identify all four levels.

Michael Meth, University of Innsbruck

Thomas Monz adds, “We managed to overcome this problem by programming a fast camera readout and counteract detection-induced heating of the ions by employing an extra laser cooling step.”

To avoid the loss of quantum information throughout the extended detection process, these adaptations are essential. The Innsbruck team’s leader, Martin Ringbauer, stresses that “all the building blocks developed in this work are readily applicable to other quantum computer architectures that have access to higher-dimensional information carriers”.

The study was financially supported by the Austrian Science Fund FWF, through the SFB BeyondC, the Austrian Research Promotion Agency FFG, and the European Union, among others.

Journal Reference

Stricker, R., et al. (2022) Experimental Single-Setting Quantum State Tomography. PRX Quantum.


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