<p>Phase-change materials (PCMs)-based integrated photonic memory offers a viable pathway for the development of neuromorphic computing chips. The sizable optical contrast in the telecom band between amorphous and crystalline phases of PCM, in particular, Ge₂Sb₂Te₅ (GST), is used for multilevel programming. However, the high extinction coefficient <i>k</i> of crystalline GST leads to high optical loss, posing a serious challenge for scaling up the device array for practical use. In this work, we focus on the atomic understanding and application of the so-called low-loss PCM, Sb₂Se₃, through multiscale simulations. First, we elucidate the bonding origin of the wavelength-dependent optical properties of amorphous and crystalline Sb₂Se₃ via ab initio calculations. Given the suppressed <i>k</i> in the telecom band, we design a programmable mode converter (PMC) waveguide device that utilizes only the contrast in refractive index <i>n</i> between amorphous and crystalline Sb₂Se₃ to encode multiple optical levels per waveguide device. The finite-difference time-domain simulations show that a single PMC device can achieve 5-bit programming precision (32 levels) via direct laser writing, and the photonic tensor core formed by the PMC array could possibly be scaled to 128 × 128. Finally, a thorough comparison between low-loss PCM and conventional PCM is provided.</p>