Engineering light-matter interactions at the quantum mechanical level enables new types of linear and nonlinear behavior not possible with natural materials which can be used for device applications such as single microwave photon detection, quantum limited amplification, and non-reciprocal devices. We use the concepts of metamaterials [1-4] and circuit quantum electrodynamics (cQED) to advance **microwave quantum optics** for computing applications. Superconducting metamaterials, made by for example by embedding Josephson junctions in a transmission line [1,3], are naturally well-suited to applications requiring large bandwidth and high dynamic range due to their lack of a bandwidth restricting cavity and ability to utilize weakly nonlinear elements.

- C. Macklin, K. O'Brien, D. Hover, M. E. Schwartz, V. Bolkhovsky, X. Zhang, W. D. Oliver, I. Siddiqi. "A near-quantum-limited Josephson traveling-wave parametric amplifier."
*Science***350**, 307-310 (2015). doi:10.1126/science.aaa8525 - K. O'Brien*, H. Suchowski*, J. Rho, A. Salandrino, B. Kanté, X. Yin, X. Zhang. "Predicting nonlinear properties of metamaterials from the linear response."
*Nat. Mat.***14**, 379-383 (2015). doi:10.1038/nmat4214 - K. O'Brien, C. Macklin, I. Siddiqi, X. Zhang. "Resonant Phase Matching of Josephson Junction Traveling Wave Parametric Amplifiers."
*Phys. Rev. Lett.***113**, 157001 (2014). doi:10.1103/PhysRevLett.113.157001 - H. Suchowski*, K. O'Brien*, Z. J. Wong*, A. Salandrino, X. Yin, X. Zhang. "Phase Mismatch-Free Nonlinear Propagation in Optical Zero-Index Materials."
*Science***342**, 1223-1226 (2013). doi:10.1126/science.1244303

The design and fabrication of superconducting qubits and superconducting quantum processors requires solving hard problems in materials science, electromagnetics, cryogenics, and quantum mechanics. These challenges motivate the burgeoning discipline of quantum engineering. Quantum computers capable of solving practical problems will require orders of magnitude lower error rates than current processors and order of a million qubits [3]. Reaching this requires your clever ideas and novel solutions!

- M. S. Blok, V. V. Ramasesh, T. Schuster, K. O'Brien, J.M. Kreikebaum, D. Dahlen, A. Morvan, B. Yoshida, N. Y. Yao, I. Siddiqi. "Quantum Information Scrambling in a Superconducting Qutrit Processor" arXiv:2003.03307 (2020)
- J. M. Kreikebaum, K. P. O'Brien, I. Siddiqi. "Improving wafer-scale Josephson junction resistance variation in superconducting quantum coherent circuits." arXiv:1909.09165 (2019)
- National Academies of Sciences, Engineering, and Medicine. 2019. Quantum Computing: Progress and Prospects. Washington, DC: The National Academies Press. doi:10.17226/25196.

LPS, MIT SoE, DOE

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