How to probe the collective excitations and control the properties of quantum materials via non-equilibrium processes is one of the central open questions in quantum science. In this two-part talk, I will attempt to shed some light on this question by considering two systems of interest. I will first show how in the spin liquid phase of toric code ladders dynamical correlators can be used to detect the presence of semionic excitations. Then, in the second portion of my talk, I will demonstrate how charge and spin currents can be induced and controlled via nonreciprocal dissipative processes in two-dimensional spinful fermionic systems with broken inversion and time-reversal symmetries. With these two examples, I hope to provide some insights into the extremely rich physics of correlated systems away from equilibrium.

This presentation focuses on two primary objectives that examine distinct aspects of nonequilibrium behavior: identifying characteristic dissipative timescales in Markovian systems and evaluating the applicability of equilibrium thermodynamic potentials in active matter systems. Through these projects, I demonstrate how coarse-graining irreversible dynamics and studying quasistatic work can generate physical insights into complex nonequilibrium systems.

Supermassive black holes are ubiquitous in massive galaxies, and are observed at the earliest epochs of the universe.  The process of forming them remains a completely open question, and likely relies on the existence of creating an intermediate mass black hole (IMBH) as an intermediary step.  Discovering and characterizing IMBHs is therefore a vital step in understanding overall black hole formation.   I will discuss results from several high-resolution cosmological zoom-in simulations focusing on the formation and evolution of IMBHs.  Local dwarf galaxies are often hosts of IMBHs, though detecting radiation from these black holes as they accrete is challenging.  In addition, about 50% of the IMBHs in dwarfs are not centrally located, but rather are wandering within a few kpc of the galaxy center.   The reason for off-center locations is mainly due to galaxy-galaxy mergers, and the black holes remain off-center due to very long dynamical friction timescales.  IMBHs may also wander through massive galaxy halos after their hosts are disrupted, occasionally inspiraling and merging with a central supermassive black hole.  IMBH mergers, either with each other or with supermassive black holes, are optimal for gravitational wave detection by the LISA mission, which will launch in the mid-2030s and revolutionize black hole astronomy.

Photon counting opens a door into a new world. Quantum effects emerge. Beyond fundamental interest, those effects lead to unprecedented accuracy in measuring light, often surpassing the capabilities of classical sensors. Quantum-enabled photonic techniques can enhance nearly every traditional application in optics: from astronomy to biology and from communications to imaging. I will talk about our recent experiments with faint light that enable practical quantum advantage by demonstrating below-the-shot-noise sensitivity and super-resolution. Let me show you a new world that connects fundamental laws of nature with everyday optical technologies.

Bio: Dr. Sergey V. Polyakov is the Chief of the Quantum Measurement Division, Physical Measurement Laboratory at NIST. He oversees the broad spectrum of research projects, from fundamental and applied quantum information science to the identification and dissemination of fundamental physical constants and units that form the international measurement system (also known as Système international d'unités, or SI). His own research aims to develop quantum methods for characterizing faint light. Sergey contributed to early research efforts in quantum repeaters. He developed innovative methods of single-photon source characterization that led to in situ, non-invasive measurement of the underlying physics of single-photon emitters. He holds the world record in the verification of the accuracy of single-photon detector calibration. Recently, he invented and developed a new class of optical receivers for classical communications that use quantum measurement. Sergey is a Fellow of the Optical Society of America and has served as a General Chair of CLEO (2021), and Nonlinear Photonics topical meeting (2022).

In recent years, research at the interface of material science, chemistry, and quantum optics has surged and now offers new possibilities to study light-matter interactions. The combination of theoretical concepts from these fields presents an opportunity to create a predictive theoretical and computational approach from first principles that describes the correlated dynamics of electrons, nuclei, and the electromagnetic field on the same quantized footing.
In this talk, we discuss how density-functional theory can be generalized to quantum-electrodynamical density-functional theory (QEDFT) and introduce electron-photon exchange-correlation functionals. The presented approach, photon MBD (pMBD), is inspired by the many-body dispersion (MBD) method for weak intermolecular interactions, which is generalized to include both electronic and photonic (electromagnetic) degrees of freedom on the same footing. We demonstrate that pMBD accurately captures effects that arise in the context of strong light-matter interactions, such as anisotropic electron-photon interactions, beyond single-photon effects, and cavity modulated van der Waals interactions. Moreover, we show that pMBD is computationally efficient and allows simulations of large complex systems coupled to optical cavities.

Melting and Re-Freezing in Markovian and non-Markovian Open Quantum Time Crystals

Time crystals - many-body systems whose observables oscillate persistently, spontaneously breaking time-translation symmetry - have moved from curiosity to laboratory reality, but almost exclusively on atom-cavity and optical-lattice platforms, and almost exclusively under the Born–Markov assumption of a memoryless environment. This talk challenges both restrictions.

I begin with detection. Discrete time-crystalline (DTC) order is conventionally read out optically, through spin-dependent fluorescence or magnetization. We showed [1] that a periodically driven spinless Hubbard ladder, emulable in a gate-defined quantum-dot array, hosts a dissipative DTC visible directly in the charge current, as a subharmonic peak in its Fourier transform, and that the system can be tuned reversibly out of and back into the time-crystal phase. Extending this to spinful transport [2], we used dynamical symmetries to identify analytically when DTC and discrete time-quasi-crystal order survive coupling to electrodes — a general and invasive environment providing single-particle gain and loss — and showed that the spin-polarized current is itself a direct measure of time-crystallinity. Turning to molecular platforms [3], we predicted DTC behavior in arrays of exchange-coupled single-molecule magnets, where the response frequency is set by the anisotropy-defined level structure and is essentially independent of the exchange coupling, while the many-body character surfaces as a pulse-like magnetization envelope.

The second half concerns bath memory. Real reservoirs have a finite correlation time. We find [4] that non-Markovianity re-freezes a dissipative time crystal that Markovian damping would melt: information backflow opens a spectral bottleneck that suppresses decay channels while leaving oscillation frequencies nearly fixed, with the damping rate vanishing as a power law in the memory time. Finally [5], we prove that the Floquet spectrum of a post-Markovian DTC splits exactly into the ordinary Markovian spectrum plus a replica of the purely unitary spectrum, uniformly contracted by exp(−T/τ_m). Once the memory time exceeds the Markovian lifetime, this replica sets the subharmonic decay, which then becomes independent of drive amplitude, interaction strength, dissipation rate and system size — a rigid, calibration-free read-out of the reservoir correlation time. The time crystal thereby witnesses the non-Markovianity of its own environment.

References

[1] S. Sarkar and Y. Dubi, Nano Lett. 22, 4445 (2022)
[2] S. Sarkar and Y. Dubi, Commun. Phys. 5, 155 (2022)
[3] S. Sarkar and Y. Dubi, ACS Nano 18, 27988 (2024)
[4] R. Ghosh, S. Sarkar, and Y. Dubi, Freezing a molten dissipative time crystal with non-Markovianity (2026, under review)
[5] R. Ghosh, V. Mukherjee, and Y. Dubi, Experimentally relevant witness for non-Markovianity in discrete time crystals (2026, in preperation)

Christiaan Huygens made foundational contributions to both wave optics and classical mechanics—two fields that appear remarkably different. In this talk, I will present our recent discovery of an unexpected quantitative connection between them, linking optical polarization, coherence, and entanglement to mechanical concepts such as the center of mass and moment of inertia. Interestingly, distributions of classical point masses can provide a mechanical representation of entanglement in classically nonseparable optical fields. I will further show how this correspondence extends to quantum two-qubit states, establishing a conceptual bridge from classical mechanics to classical optical and quantum entanglement. These results reveal a surprising unity among mechanics, optics, and quantum physics, and offer an intuitive perspective on the nature of entanglement. Our original optics–mechanics connection was recognized by Physics World as one of its ten most popular physics stories of 2023.

The intermediate band solar cell (IBSC) concept has been proposed to overcome the efficiency limits of single-junction devices. In IBSCs, mid-gap states in the host material create an additional intermediate absorption pathway for below-bandgap photons, increasing the short-circuit current without compromising the open-circuit voltage. We have shown that submonolayer CdTe QDs grown by MBE-MEE and embedded in ZnCdSe, latticed matched to InP, can be employed in IBSCs without the formation of a detrimental wetting layer be fabricated with near-optimal IBSC parameters. Hundreds of QD-containing layers can be grown, which is particularly important for addressing the inherently low absorption in IBSCs, and especially those using type-II QDs.  We present results from recently grown CdTe/ZnCdSe type-II QDs that exhibit deeper PL emission, indicating the formation of larger QDs required for an optimized IB energy position.

In addition, we present theoretical considerations for increasing light absorption in these solar cells. Distributed Bragg reflectors (DBRs) are an attractive choice to recycle photons and enhance absorption. We demonstrate optimization techniques for the inverse design of broadband (1 to 1.8 µm) and wide-angle (0 to 70°) DBRs operating in this spectral range, specifically modeling a DBR using ZnCdSe/ZnCdMgSe lattice-matched to InP. The total photon absorption was chosen as the figure of merit for the optimized structures. We show that the proposed design offers excellent broadband reflectivity with strong robustness against fabrication errors.