Kwek: Our experiment uses Gaussian boson sampling, a technique that showcases quantum advantage, with quantum computers claiming to surpass supercomputers significantly. This method parallels the transition probability of molecules between vibronic states and demonstrates that the Franck-Condon Factor, which measures the transition probability, and the concept of "permanence" within matrices are mathematically related. Permanence is the idea that algebraic operations should behave consistently within every number system.

The complexity of boson sampling is computationally challenging, and we use this technique in our experiment to calculate the probabilities of vibronic transitions in molecules.

LFW: Any interesting design work involved?

Zhang: Our design is simple yet elegant. To slow down light, we attached devices called resonators to the edge of the photonic topological insulator to generate the required flat bands -- which slows down the chiral edge states within the photonic topological insulator.

Kwek: To increase the sensitivity of detection, we made use of "squeezed" coherent light with low quantum noise. Light in a coherent state is equivalent to a displaced vacuum state. In the original scheme, researchers use light that is displaced and then squeezed to remove noise. But we found that the analysis worked with light that is first squeezed then displaced.

LFW: Main thing you'd like people to know about your work? Key benefits?

Zhang: The key benefit of our research is achieving broadband slow light without backscattering. Slow light is typically constrained by narrow bandwidth, and the slower the light the more it suffers from backscattering. Our demonstration proves it's possible to enable slow, unidirectional light propagation across a much broader bandwidth than previously possible.

Kwek: The main message is the possibility of harnessing quantum theory as a new technique for chemistry and possibly molecular biology. The technique can be extended to many more chemistry problems amenable to graph theoretic analysis -- the analysis of interconnected networks to solve complex problems. The key benefit will be the insights gained in chemistry and molecular biology through our investigations.

LFW: Most surprising/coolest aspects of your work?

Zhang: The coolest aspect of our work is that we addressed a long-standing challenge in slow-light engineering using an exceptionally simple design. The "a-ha" moment came with the realization: "A-ha, it's just this simple!"

Kwek: We also show in the paper that we can probe the molecular spectrum with and without excitations. We simulated the vibronic spectra of formic acid (CHO) for four vibronic modes and thymine (CHNO) for seven modes using a 16-mode integrated photonic chip. We simulated spectra for naphthalene (CH), phenanthrene (CH), and benzene (CH) for more diverse transitions. We hope such techniques will provide insights into bigger and more complicated molecules.

LFW: Any big challenges to overcome?

Zhang: There are several challenges ahead. Since the photonic topological insulator requires a magnetic response, scaling the approach to optical frequencies for other applications is difficult. And extending the working bandwidth of the system is another area that demands further investigation.

Kwek: Making the chip larger, which is necessary to simulate bigger molecules, involves more heating to adjust the phase shifters and it becomes more difficult manually. It's really an engineering problem, but I think it should be possible.

LFW: What's next?

Zhang: Our findings have the potential to be applied to quantum computers operating at microwave frequencies. The next step will be integrating these designs with qubits -- basic units of information used to encode data in quantum computing -- for quantum information processing.

Kwek: We're exploring more applications of the chip to solve chemistry, molecular biology, pharmaceutical, and logistics problems.