High-Efficiency, Ultra-Broadband, and Low-Noise Quantum Memory in Atomic Barium Vapor

Introduction.-Optical quantum memory describes the process of on-demand storage and retrieval of single-photon-level quantum states, and is a critical enabling technology for many quantum applications. Memory bandwidth plays an important role in these applications, as it determines the pulse durations compatible with the memory and places an upper bound on the clock rate and processing speed of a quantum device. Here we present experimental results of an atomic barium quantum memory that enables storage and retrieval of ultra-broadband (>800 GHz) signal photons with high storage efficiency [95.6(3)%] and low noise [3.8(6) × 10⁻⁵ noise photons]. Experimental Results.-The quantum memory operation is based on the ground (6s^{2 1}S₀), excited (6s6p ¹P₁), and metastable (6s5d ¹D₂) orbital states of atomic barium in a Λ-type configuration. The barium vapor is created in an 800-900 ^◦C barium heat pipe oven with 0-1000 torr tunable argon buffer gas pressure. The ground-excited transition at 553.5 nm features large and tunable homogeneous collisional broadening due to the argon buffer gas and a peak optical depth of d = 50. The memory operates in the so-called absorb-then-transfer (ATT) regime, in which the signal field is linearly absorbed along the ground-excited transition, whose collisionally broadened lineshape enables efficient absorption of ultra-broadband photons; the resulting atomic polarization is transferred into a so-called spin wave by application of a strong [O(10 uJ), 100 fs] control pulse along the excited-metastable transition at 1500 nm. The storage state has a O(0.1) second coherence lifetime in the bare atom [1], but this is reduced to the O(ns) level due to motional dephasing [0.49(1) ns measured memory lifetime]. The memory experiment is repeated at a repetition rate of 1 kHz. The total end-to-end efficiency of the memory at 900 ^◦C is 31(1)%, which is limited by available control field power.