近日，美国马里兰大学的Vladimir E. Manucharyan&Roman Kuzmin及其研究团队取得一项新进展。经过不懈努力，他们成功观察到施密德-布尔加达耶夫耗散量子相变。相关研究成果已于2024年12月2日在国际知名学术期刊《自然—物理学》上发表。
该研究团队通过将欧姆环境实现为多模腔，并探测结对腔中驻波模式谱的影响，来观察这一相变。研究人员发现，根据腔的特征阻抗，足够弱的结会以电感或电容的形式散射腔光子。这两种情况分别对应于超导相和绝缘相，且临界阻抗与预期值相符。在相边界处，量子涨落增强了结的非线性，使得结表现为电阻。这种损耗机制调和了超导相和绝缘相，并可能为量子临界动力学提供了一个有用的指示。
据悉，在周期性势场中运动的经典粒子可以局限于单个势阱最小值内，而量子粒子则通过隧穿到相邻的最小值形成扩展态。这两种极限之间由黏性摩擦力驱动的量子施密德-布尔加达耶夫（Schmid-Bulgadaev）相变所分隔。这一物理现象对约瑟夫森结器件具有重要影响，因为约瑟夫森结器件的超导相位动力学可以用周期性势场中的虚拟粒子来模拟。因此，人们预期当连接两个超导体的电阻值超过某一阈值时，任何这样的结都可能经历施密德-布尔加达耶夫相变。
附：英文原文
Title: Observation of the Schmid–Bulgadaev dissipative quantum phase transition
Author: Kuzmin, Roman, Mehta, Nitish, Grabon, Nicholas, Mencia, Raymond A., Burshtein, Amir, Goldstein, Moshe, Manucharyan, Vladimir E.
Issue&Volume: 2024-12-02
Abstract: A classical particle moving in a periodic potential can localize inside a single potential minimum, but a quantum particle forms extended states by tunnelling to neighbouring minima. These two limits are separated by a quantum Schmid–Bulgadaev phase transition driven by a viscous friction force. This physics has implications for Josephson junction devices, which feature superconducting phase dynamics that can be modelled by a fictitious particle in a periodic potential. As a result, it has been anticipated that any junction of two superconductors connected to a resistor can undergo a Schmid–Bulgadaev transition when the value of the resistor exceeds a threshold. Here we observe this transition by implementing the ohmic environment as a massively multimode cavity and probing the effect of the junction on the standing-wave mode spectrum of the cavity. We find that, depending on the characteristic impedance of the cavity, sufficiently weak junctions scatter cavity photons as either inductors or capacitors. These regimes correspond to the superconducting and insulating phases, respectively, and the critical impedance matches the expected value. At the phase boundary, quantum fluctuations boost the junction nonlinearity so that the junction behaves as a resistor. This loss mechanism reconciles the superconducting and insulating phases and provides a possibly useful indication of quantum-critical dynamics.
DOI: 10.1038/s41567-024-02695-7
Source: https://www.nature.com/articles/s41567-024-02695-7