Martin Gärttner

Multiplexed qubit readout quality metric beyond assignment fidelity

  1. Andras Di Giovanni,
  2. Adrian Skasberg Aasen,
  3. Jürgen Lisenfeld,
  4. Martin Gärttner,
  5. Hannes Rotzinger,
  6. and Alexey V. Ustinov
The accurate measurement of quantum two-level objects (qubits) is crucial for developing quantum computing hardware. Over the last decade, the measure of choice for benchmarking readout
routines for superconducting qubits has been assignment fidelity. However, this method only focuses on the preparation of computational basis states and therefore does not provide a complete characterization of the readout. Here, we expand the focus to the use of detector tomography to fully characterize multi-qubit readout of superconducting transmon qubits. The impact of different readout parameters on the rate of information extraction is studied using quantum state reconstruction infidelity as a proxy. The results are then compared with assignment fidelities, showing good agreement for separable two-qubit states. We therefore propose the rate of infidelity convergence as an alternative and more comprehensive benchmark for single- and multi-qubit readout optimization. We find the most effective allocation of a fixed shot budget between detector tomography and state reconstruction in single- and two-qubit experiments. To address the growing interest in three-qubit gates, we perform three-qubit quantum state tomography that goes beyond conventional readout error mitigation methods and find a factor of 30 reduction in quantum infidelity. Our results demonstrate that neither quantum nor classical qubit readout correlations are induced even by very high levels of readout noise. Consequently, correlation coefficients can serve as a valuable tool in qubit readout optimization.
arxiv pdf

Universal readout error mitigation scheme characterized on superconducting qubits

  1. Adrian Skasberg Aasen,
  2. Andras Di Giovanni,
  3. Hannes Rotzinger,
  4. Alexey V. Ustinov,
  5. and Martin Gärttner
Quantum technologies rely heavily on accurate control and reliable readout of quantum systems. Current experiments are limited by numerous sources of noise that can only be partially
captured by simple analytical models and additional characterization of the noise sources is required. We test the ability of readout error mitigation to correct realistic noise found in systems composed of quantum two-level objects (qubits). To probe the limit of such methods, we designed a universal readout error mitigation protocol based on quantum state tomography (QST), which estimates the density matrix of a quantum system, and quantum detector tomography (QDT), which characterizes the measurement procedure. By treating readout error mitigation in the context of state tomography the method becomes largely device-, architecture-, noise source-, and quantum state-independent. We implement this method on a superconducting qubit and benchmark the increase in reconstruction fidelity for QST. We characterize the performance of the method by varying important noise sources, such as suboptimal readout signal amplification, insufficient resonator photon population, off-resonant qubit drive, and effectively shortened T1 and T2 decay times. As a result, we identified noise sources for which readout error mitigation worked well, and observed decreases in readout infidelity by a factor of up to 30.
arxiv pdf