Speaker: Morten Kjaergaard
Time: October 8, 2020 : 2:00PM - 3:00PM
Meeting ID: 657 8412 2083
Abstract: A quantum algorithm consists of a sequence of operations and measurements applied to a quantum processor. To date, the instructions defining this sequence have been provided by a classical computer and passed via control hardware to the quantum system. I will discuss our recent results showing the first use of quantum instructions: a fixed sequence of classically-defined gates performs an operation that uniquely depends only on the setting of an auxiliary quantum instruction state  (the Density Matrix Exponentiation algorithm ). The ability to directly use quantum instructions, without tomographically reconstructing and recompiling the quantum input into an equivalent set of classical instructions opens the door to exponential speedups in many algorithms, including principal component analysis of large quantum states , quantum semi-definite programming , and efficient measurement of entanglement spectra . The algorithm is executed on two superconducting transmon qubits, with a 99.7% fidelity controlled-phase gate (measured with randomized benchmarking), and we demonstrate circuits to depth 70 with algorithm fidelities close to 90%. To achieve this performance we developed several novel strategies for tuning up quantum gates specifically in the context of structured quantum algorithms. Finally, to demonstrate the algorithm we developed a stochastic quantum operation which approximately resets a known quantum state without using classical feedback, with applications to open-loop error mitigation in logical states .
 M. Kjaergaard, M. Schwartz et al, arXiv:2001.08838 (2020)
 S. Lloyd et al, Nat. Phys, 10 (2014)
 F. Brandao et al arXiv:1710.02581 (2017)
 H. Pichler et al, PRX, 6, (2016)
 A. Greene, M. Kjaergaard et al, in preparation (2020)
Bio: Morten Kjaergaard is currently a postdoc in the Engineering Quantum Systems group with Will Oliver at MIT. His research focuses on both experimental and theoretical aspects of implementing small scale quantum algorithms and error correction, as well as extensible, practical approaches to quantum verification and validation techniques. From November 2020 he will take up position as Assistant Professor at the Center for Quantum Devices in University of Copenhagen where his group will continue research in both foundational and applied quantum information processing using superconducting qubits.