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The ambition of harnessing the quantum for computation is at odds with the fundamental phenomenon of decoherence. The purpose of quantum error correction (QEC) is to counteract the natural tendency of a complex system to decohere. This cooperative process, which requires participation of multiple quantum and classical components, creates a special type of dissipation that removes the entropy caused by the errors faster than the rate at which these errors corrupt the stored quantum information. Previous experimental attempts to engineer such a process faced an excessive generation of errors that overwhelmed the error-correcting capability of the process itself. Whether it is practically possible to utilize QEC for extending quantum coherence thus remains an open question. We answer it by demonstrating a fully stabilized and error-corrected logical qubit whose quantum coherence is significantly longer than that of all the imperfect quantum components involved in the QEC process, beating the best of them with a coherence gain of $G = 2.27 \pm 0.07$. We achieve this performance by combining innovations in several domains including the fabrication of superconducting quantum circuits and model-free reinforcement learning.

We present the design and characterization of a large-area Cryogenic PhotoDetector (CPD) designed for active particle identification in rare event searches, such as neutrinoless double beta decay and dark matter experiments. The detector consists of a $45.6$ $\mathrm{cm}^2$ surface area by 1-mm-thick $10.6$ $\mathrm{g}$ Si wafer. It is instrumented with a distributed network of Quasiparticle-trap-assisted Electrothermal feedback Transition-edge sensors (QETs) with superconducting critical temperature $T_c=41.5$ $\mathrm{mK}$ to measure athermal phonons released from interactions with photons. The detector is characterized and calibrated with a collimated $^{55}$Fe X-ray source incident on the center of the detector. The noise equivalent power is measured to be $1\times 10^{-17}$ $\mathrm{W}/\sqrt{\mathrm{Hz}}$ in a bandwidth of $2.7$ $\mathrm{kHz}$. The baseline energy resolution is measured to be $\sigma_E = 3.86 \pm 0.04$ $(\mathrm{stat.})^{+0.23}_{-0.00}$ $(\mathrm{syst.})$ $\mathrm{eV}$ (RMS). The detector also has an expected timing resolution of $\sigma_t = 2.3$ $\mu\mathrm{s}$ for $5$ $\sigma_E$ events., Comment: 6 pages, 5 figures

We present the design and laboratory evaluation of a cryogenic continuously rotating half-wave plate (CHWP) for the POLARBEAR-2b (PB-2b) cosmic microwave background (CMB) receiver, the second installment of the Simons Array. PB-2b will observe at 5,200 m elevation in the Atacama Desert of Chile in two frequency bands centered at 90 and 150 GHz. In order to suppress atmospheric 1/f noise and mitigate systematic effects that arise when differencing orthogonal detectors, PB-2b modulates linear sky polarization using a CHWP rotating at 2 Hz. The CHWP has a 440 mm clear aperture diameter and is cooled to $\approx$ 50 K in the PB-2b receiver cryostat. It consists of a low-friction superconducting magnetic bearing (SMB) and a low-torque synchronous electromagnetic motor, which together dissipate < 2 W. During cooldown, a grip-and-release mechanism centers the rotor to < 0.5 mm, and during continuous rotation, an incremental optical encoder measures the rotor angle with a noise level of 0.1 $\mathrm{��rad / \sqrt{Hz}}$. We discuss the experimental requirements for the PB-2b CHWP, the designs of its various subsystems, and the results of its evaluation in the laboratory. The presented CHWP has been deployed to Chile and is expected to see first light on PB-2b in 2020 or 2021., PREPRINT. Submitted to Review of Scientific Instruments, September 2020. v2 updates refs 41-42

In this letter, we present the performance of a $100~\mu\mathrm{m}\times 400~\mu\mathrm{m} \times 40~\mathrm{nm}$ tungsten (W) Transition-Edge Sensor (TES) with a critical temperature of 40 mK. This device has a measured noise equivalent power (NEP) of $1.5\times 10^{-18}\ \mathrm{W}/\sqrt{\mathrm{Hz}}$, in a bandwidth of $2.6$ kHz, indicating a resolution for Dirac delta energy depositions of $40\pm 5~\mathrm{meV}$ (rms). The performance demonstrated by this device is a critical step towards developing a $\mathcal{O}(100)~\mathrm{meV}$ threshold athermal phonon detectors for low-mass dark matter searches., Comment: 5 pages, 6 figures. Accepted for publication at AIP Advances

We describe the design of a cryogenic rotation stage (CRS) for use with the cryogenic half-wave plate (CHWP) polarization modulator on the POLARBEAR-2b and POLARBEAR-2c (PB2b/c) cosmic microwave background (CMB) experiments, the second and third installments of the Simons Array. Rapid modulation of the CMB polarization signal using a CHWP suppresses 1/f contamination due to atmospheric turbulence and allows a single polarimeter to measure both polarization states, mitigating systematic effects that arise when differencing orthogonal detectors. To modulate the full detector array while avoiding excess photon loading due to thermal emission, the CHWP must have a clear-aperture diameter of > 450 mm and be cooled to < 100 K. We have designed a 454-mm-clear-aperture, < 65 K CRS using a superconducting magnetic bearing driven by a synchronous magnetic motor. We present the specifications for the CRS, its interfacing to the PB2b/c receiver cryostat, its performance in a stand-alone test, and plans for future work., PREPRINT VERSION. Accepted to the Journal of Low Temperature Physics, Article JLTP-D-17-00265R1 See DOI link, supplied by Springer Publishing, for the fully-reviewed published article and citation information