Ultra-Broadband, Low-Noise Quantum Memory in Atomic Barium Vapor with 95% Storage Efficiency

Quantum memory bandwidth plays an important role in many quantum 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 per pulse]. Our memory is unique in its high efficiency and simultaneously broad bandwidth, and its noise performance is comparable to state-of-the-art noise measurements in ladder-type atomic systems.

Our quantum memory 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 heat pipe oven with 0-1000 torr argon buffer gas. The ground-excited transition at 553.5 nm features large and tunable homogeneous collisional broadening, controlled via the buffer gas pressure, 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 [O(1 photon), 500 fs] is linearly absorbed along the ground-excited transition, then the resulting atomic polarization is transferred to a so-called spin wave by application of a strong [O(10 uJ), 100 fs] π-pulse control field along the excited-metastable transition at 1500 nm. Our memory lifetimes are limited 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, and the total end-to-end efficiency of the memory at 900 °C is 31(1)%, limited by available control field power. We observe the largest total memory efficiency at small, non-zero two-photon detuning. We call this effect near-off-resonant memory (or NORM) operation, which we believe balances resonant reabsorption loss and finite available control field power.