Spontaneous parametric down-conversion (SPDC) has been a key enabling technology in exploring quantum phenomena and their applications for decades. For instance, traditional SPDC, which splits a high-energy pump photon into two lower-energy photons, is a common way to produce entangled photon pairs. Since the early realizations of SPDC, researchers have thought to generalize it to higher order, e.g., to produce entangled photon triplets. However, directly generating photon triplets through a single SPDC process has remained elusive. Here, using a flux-pumped superconducting parametric cavity, we demonstrate direct three-photon SPDC, with photon triplets generated in a single cavity mode or split between multiple modes. With strong pumping, the states can be quite bright, with flux densities exceeding 60 photons per second per hertz. The observed states are strongly non-Gaussian, which has important implications for potential applications. In the single-mode case, we observe a triangular star-shaped distribution of quadrature voltages, indicative of the long-predicted “star state.” The observed state shows strong third-order correlations, as expected for a state generated by a cubic Hamiltonian. By pumping at the sum frequency of multiple modes, we observe strong three-body correlations between multiple modes, strikingly, in the absence of second-order correlations. We further analyze the third-order correlations under mode transformations by the symplectic symmetry group, showing that the observed transformation properties serve to “fingerprint” the specific cubic Hamiltonian that generates them. The observed non-Gaussian, third-order correlations represent an important step forward in quantum optics and may have a strong impact on quantum communication with microwave fields as well as continuous-variable quantum computation.

中文翻译：

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超导参量腔中三光子自发参量下转换的观察

自发的参数下转换（SPDC）成为数十年来探索量子现象及其应用的关键技术。例如，将高能泵浦光子分成两个低能光子的传统SPDC是产生纠缠光子对的常见方法。自从SPDC的早期实现以来，研究人员就一直考虑将其推广到更高的阶数，例如产生纠缠的光子三重态。但是，通过单个SPDC过程直接生成光子三重态仍然难以捉摸。在这里，我们使用通量泵浦的超导参量腔，演示了直接三光子SPDC，其中光子三元组在单个腔模式下生成，或在多个模式间分裂。通过强泵浦，状态可以非常明亮，通量密度超过每秒60个光子/赫兹。观察到的状态是强烈的非高斯状态，这对潜在的应用具有重要意义。在单模情况下，我们观察到正交电压呈三角形星形分布，这表明人们早已预测到“星形状态”。观察到的状态显示出很强的三阶相关性，这是立方哈密顿量生成的状态所期望的。通过以多种模式的总频率泵浦，我们可以显着地观察到多种模式之间的强三体相关性，而没有二阶相关性。我们通过辛对称组进一步分析了模式转换下的三阶相关性，表明所观察到的转换特性有助于“指纹”生成它们的特定三次哈密顿量。观察到的非高斯