Collaborative Research: Photonic Chip-Scale Time Crystals

普通晶体，如盐粒和雪花，或构成现代电子产品支柱的硅晶体，都由大量的原子或分子组成。就像雪花中的水分子一样有序和对称，六角形晶体代表的不是对称性，而是它突破了空白空间的完美对称性的约简，变成了分子阵列的离散对称性。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(约10^23，数万亿亿)的这种相互作用的构件聚集在一起才能形成晶体。合成的晶体又一次具有独特的性质，从而产生了它们的用途。大约十年前，科学家开始建立时间晶体的理论--由大量相互作用的积木组成的系统，这些积木打破了对称性，不是在空间上，而是在时间维度上。结果表明，与固态空间晶体平行，时间晶体提供了理想的特性(例如，时间稳健性)，以及全新的物理效应(例如，避免在通常预期加热会破坏晶体的情况下失去晶体有序)。这些固有的时间晶体性质对未来的应用至关重要，例如量子计算，在量子计算中，数据的量子比特有望在一段时间内和几次读取操作后保存它们的信息。到目前为止，时间晶体的实验演示仍然很少，特别是限于孤立的系统，这不利于现实世界的应用。它们在很大程度上也被限制在“小”晶体中，通常只有两个临时基本细胞。这项研究旨在通过在小型化的非线性光学器件中使用光子在非隔离系统中实现时间晶体来克服这些限制。该平台支持对时间晶体尚未探索的方面的研究，并展示了它们在精确计时中的应用。此外，它还容纳了“大”的时间晶体，因此有可能实现凝聚态物理效应的时间模拟，并使用成熟的光子技术解决开放的问题。为了产生进一步的影响，我们提议的计划包括科学专业教育推广和研讨会部分，这些部分将为有抱负的高中生和工程师开发和提供课程。该团队将演示耗散克尔非线性腔中的离散时间晶体。利用纳米制造集成光子结构所提供的灵活性，时间晶体将在多色激光泵浦的氮化硅微环谐振器中实现和研究。色散工程和合理的谐振腔设计和泵浦是实现大时间晶体的保证。产生的时间晶体的状态将被控制，并将通过调频和扫频实现不同相位之间的转换。时间晶体的稳定将通过将两个激光器自注入锁定到微环谐振腔的两个不相邻的同族光学模式来实现，并通过监测泵浦激光器之间的脉动噪声与产生的亚谐波的相位噪声来跟踪时间晶体的稳定。在该平台中实现离散时间晶体所固有的分频导致了相位噪声的降低。通过将泵浦激光器锁定到外部频率基准，该系统可用于频率基准传输。结合光子学和凝聚态物理概念的拟议平台的成功演示显著加快了将时间晶体作为一种新的物质阶段的研究，并揭示了它们的一些实际应用。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.