April 2021

Abstracts of the QSIT/Quantum Center, ETH Zurich Lunch Seminar, Thursday, April 1, 2020

Scheduled Zoom meeting

A Multi-level IR for Quantum Program Optimization

David Ittah - Scalable Parallel Computing Laboratory (Hoefler group), Quantum Center, ETH Zurich

Intermediate representations (IR) have traditionally provided numerous benefits to compilation systems, in particular in the domain of static program analysis and optimization. However, the quantum programming landscape has yet to produce similarly powerful IRs, instead focusing on the development of embedded domain-specific languages (eDSL). As these only feature rudimentary IRs in the form of quantum circuit data structures, or simple quantum assembly (QASM) languages, they lack meaningful integration with their classical host compilation infrastructure. As a remedy, we propose a novel quantum IR design based on exposing SSA-like quantum dataflow in the IR alongside classical dataflow. Crucially, this allows for a host of optimizations that leverage dataflow analysis and maximizes the reuse of existing compilation infrastructure. A prototype implementation in MLIR shows that significant improvements in resource requirements are possible even through static optimization. In contrast to circuit optimization at runtime, this is achieved while incurring only a small constant overhead in compilation time, making this a compelling approach for quantum program optimization at application scale.

Towards an error corrected logical qubit with surface codes

Ants Remm - Quantum Device Lab (Wallraff group), ETH Zurich

The field of quantum computing with superconducting circuits has made tremendous progress over the last few years, with operation of systems consisting of over 50 qubits recently demonstrated. Current devices are operating in the noisy intermediate-scale quantum (NISQ) regime, where the size and fidelity of the circuits that can be executed is limited by the decoherence of the qubits. To move beyond the limitations of the NISQ regime, different quantum error correction schemes have been proposed, where the logical quantum information is encoded in a higher-dimensional Hilbert space, and physical errors can be detected and corrected without disturbing the logical state. In this talk, we present our progress towards implementing a logical qubit in the surface code architecture with superconducting qubits. First, we demonstrate error detection on a small surface code implemented on a 7-qubit device [1]. We prepare the logical |0〉, |1〉, |+〉 and |-〉 states with an average fidelity of 96.1%. Furthermore, we repeatedly measure the parity of the data qubits encoding the logical state to check for errors. We find that the quantum states are maintained with coherence times beyond that of any constituent physical qubit, when no errors are detected. Second, we present a 17-qubit device designed for operating a surface code capable of error correction. The measured error syndromes can be decoded using the minimum weight perfect matching algorithm to identify and track the physical errors that have occurred on the device.
[1] C. K. Andersen et al., Nature Physics 16, 875–880 (2020)