Collaborative Research: Photonic Chip-Scale Time Crystals

普通的晶体，如盐粒和雪花，或构成现代电子产品支柱的硅晶体，都是由大量的原子或分子组成的。就像雪花中的水分子一样有序对称，六边形晶体代表的不是对称，而是它突破了真空的完美对称，变成了分子阵列的离散对称。虽然少数原子可以通过化学键形成具有新的物理和化学性质的分子，但只有真正大量的（~10^23，数十万亿亿）这种相互作用的构建块聚集在一起才能形成晶体。合成的晶体再次具有独特的性质，这使它们具有实用价值。大约十年前，科学家们开始理论化时间晶体——由大量相互作用的构件组成的系统，它们打破了空间上的对称，而是时间维度上的对称。研究表明，与固态空间晶体相比，时间晶体提供了理想的特性（例如，时间稳稳性），以及全新的物理效应（例如，当加热预期会破坏晶体时，避免晶体秩序的丧失）。这些固有的时间晶体特性对于未来的应用至关重要，比如量子计算，在量子计算中，量子比特的数据有望随着时间的推移和多次读取操作后保持其信息。到目前为止，时间晶体的实验证明仍然很少，特别是局限于不利于实际应用的孤立系统。它们也很大程度上局限于“小”晶体，通常只有2个时间基本细胞。提出的研究旨在通过在小型化非线性光学器件中使用光子在非孤立系统中实现时间晶体来克服这些限制。这个平台授权调查未探索的方面的时间晶体，并展示其在精密计时的应用。此外，它可以容纳“大”时间晶体，因此提供了实现凝聚态物理效应的时间类似物的可能性，并使用成熟的光子技术解决悬而未决的问题。为了进一步发挥作用，我们提议的计划包括科学专业教育推广和研讨会组成部分，这些组成部分将为有抱负的高中生和工程师开发和提供课程。该团队将在耗散克尔非线性腔中展示离散时间晶体。利用集成光子结构的纳米制造所提供的灵活性，时间晶体将在多色激光泵浦的氮化硅微环谐振腔中实现和研究。色散工程、合理设计和抽运谐振器是实现大时间晶体的保证。所创造的时间晶体的状态将被控制，不同相位之间的过渡将通过频率调制和扫描来实现。时间晶体的稳定将通过自注入实现，将两个激光器锁定在微环谐振器的两个非相邻的同族光学模式上，并通过监测泵浦激光器之间的拍音与产生的次谐波的相位噪声来跟踪。在该平台上实现离散时间晶体所固有的分频导致相位噪声的降低。该系统可以通过将泵浦激光器锁定到外部频率参考来进行频率参考传输。该平台的成功演示结合了光子学和凝聚态物理的概念，极大地加速了对时间晶体作为一种新物质相的研究，并揭示了它们的一些实际应用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Garden variety crystals like salt grains and snowflakes, or silicon crystals forming the backbone of modern-day electronics all consist of large numbers of atoms or molecules. As orderly and symmetric as water molecules sit in a snowflake, the hexagonal crystal represents not symmetry but its breaking through the reduction of the perfect symmetry of empty space into the discrete symmetry of an array of molecules. While a small number of atoms can form molecules with new physical and chemical properties by chemical bonding, only a truly large number (~10^23, hundreds of thousands of billions of billions) of such interacting building blocks coming together can form crystals. The resultant crystals possess yet again unique properties, giving rise to their utility. About a decade ago, scientists started theorizing time crystals – systems consisting of a large number of interacting building blocks which break symmetry, not in space but in the time dimension. It was shown that, paralleling solid-state spatial crystals, time crystals offer desirable characteristics (e.g., temporal robustness), as well as totally new physical effects (e.g., avoiding loss of crystalline order when typically heating is expected to destroy a crystal). These inherent time crystal properties are crucial for future applications such as quantum computation where quantum bits of data are expected to preserve their information over time and after several reading operations. Experimental demonstrations of time crystals have thus far remained scarce and particularly limited to isolated systems which are not conducive to real-world applications. They have also been largely confined to “small” crystals, typically with only 2 temporal elementary cells. The proposed research aims to surmount these limitations by realizing time crystals in non-isolated systems using photons in miniaturized nonlinear optical devices. This platform empowers investigation of unexplored aspects of time crystals and demonstrating their application in precision timekeeping. Additionally, it accommodates “big” time crystals, hence offering the possibility of realizing temporal analogues of condensed matter physical effects and addressing open questions using the mature photonic technology. For further impact, our proposed program includes scientific professional education outreach and workshop components which will develop and deliver a curriculum to aspiring high school students and engineers.The team will demonstrate discrete time crystals in dissipative Kerr nonlinear cavities. Leveraging the flexibilities afforded by nanofabrication of integrated photonic structures, time crystals will be realized and investigated in silicon nitride microring resonators pumped by polychromatic lasers. Dispersion engineering and judicious design and pumping of the resonator will ensure realizing big time crystals. The state of the created time crystals will be controlled and transition between different phases will be achieved by means of frequency modulation and sweep. Stabilization of time crystals will be achieved by self-injection locking two lasers to two non-adjacent same-family optical modes of the microring resonator and tracked through monitoring the phase noise of the beatnote between the pump lasers versus that of the generated subharmonics. The frequency division inherent to the realization of discrete time crystals in this platform results in the reduction of the phase noise. The system can be used for frequency reference transfer by locking the pump lasers to external frequency references. Successful demonstration of the proposed platform combining concepts from photonics and condensed matter physics significantly accelerates the investigation of time crystals as a new phase of matter and reveals some of their practical applications.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.