Hyperbolic metamaterials (HMM) are artificially engineered materials that exhibit hyperbolic dispersion of light propagating through them. These have been extensively studied for tailoring light propagation. Most studies use an effective medium approach that is extremely useful, though it misses out on properties that can arise from the microscopic details of the HMM. In particular, the HMM can have cavity-like modes, and it is important to understand such modes and their relevance in light propagation and coupling of HMM to quantum emitters. In this work, we bring out the cavity-like modes of the silver nanowire-alumina two-dimensional HMM, which remain on top of the broad response of the HMM. These modes define the characteristic reflection spectra. The observed resonances and their widths are in good agreement with our simulations. These well-defined modes occur even though the metallic part of the HMM has Ohmic losses. Then, we present experimental results on the coupling of quantum emitters to the cavity-like modes of the HMM. We present results for both steady-state and time-resolved photoluminescence. Using these, we extract the corresponding Purcell factors for radiative rate enhancement. Theoretical analyses of the experimental data allow the determination of the cavity coupling parameters and mode volumes. These experimental results are confirmed by the FDTD calculations for the HMM mode volume. This work elucidates the pathway to precise engineering for future applications of HMM modes in strong light-matter interactions.

Breaking the standard quantum limit in the sensing of parameters at different spatial locations, such as in a quantum network, is of great importance. Using the framework of quantum Fisher information, many strategies based on squeezed quantum probes and multipath multiphoton or multiqubit entangled states have been considered. In this context there is always the question of what is the simplest measurement that would saturate quantum Cramer-Rao bound (QCRB). The simplest quantity to measure would be characteristics of photon flux or population distribution in case of qubits. Previous studies have shown that the error sensitivity in SU(1,1) interferometry, also known by several other names as nonlinear interferometry, time reversed measurements; does saturate QCRB for single parameters like phase, displacement, loss. In this work we bring out great utility of generalized SU(1,1) interferometry in distributed sensing. The generalized SU(1,1) interferometry is a combination of SU(m) and SU(1,1) elements, where m is the number of nodes in the network. The SU(m) element is used to produce distributed entanglement starting from a squeezed photonic or matter probe. We demonstrate how error sensitivity measurement at just one output port can saturate or nearly saturate QCRB and thus results in Heisenberg sensitivity of network sensing.

Analyzing the kinetics of biological processes plays a significant role in understanding fundamental cellular functions. Many physics-based technologies used to study such processes are limited by the shot noise inherent to the coherent states of light. These technologies can greatly benefit by leveraging quantum probes to improve the sensitivity of measurements in cellular biology. Surface Plasmon Resonance (SPR) technique has been used effectively to achieve label-free, real-time measurements of protein binding kinetics, which constitutes an important biological phenomenon occurring near the cell membrane. Here, we demonstrate the integration of this technique with the two-mode bright squeezed state having fewer fluctuations as compared to the coherent state to improve the sensitivity of measurement in studying a protein-gold adsorption process. We show 4dB of squeezing as we record the signal-to-noise ratio as the function of time and it is maintained throughout the kinetic process. The quantum advantage as shown in terms of squeezing is achieved despite the total absorption of 74 detection after the sensor. Overall, we provide the most practical setup for improving the sensitivity of the time-dependent measurements involved in various biological processes at the molecular level.

Precise estimation of the atomic resonance frequency is fundamental for the characterization and control of quantum systems. The resonance experiment is a standard method for this measurement, wherein the drive field frequency is swept to invert the system population. We analyze the classical and quantum Fisher information for the resonance experiment driven by hyperbolic-secant-shaped n pulses, setting a fundamental limit on the precision obtainable using the resonance method. We show that measurements using sequences of pulses with alternating phases nearly saturate the quantum Cram & eacute;r-Rao bound, improving the precision of atomic resonance frequency measurements.

Stimulated Raman scattering (SRS) microscopy using picosecond near-IR pulses has provided a great penetration depth with reduced fluorescence interference when imaging biological samples for bioenergy applications. These tools have provided insight into 1) tracking the degradation of chemical composites in biomass feedstocks to investigate the recalcitrant factors during the deconstruction processes, 2) monitoring the production of chemicals in photosynthetic plants and wood-digesting microorganisms, and 3) probing plant-bacteria interactions. However, the above processes are usually slow and require continuous imaging for an extended period. This is challenging for classic SRS because the laser power needed to achieve enough sensitivity causes photodamage in the samples during such long experiments. Quantum-squeezed light with reduced noise in the intensity quadrature can improve the sensitivity of classic SRS microscopy beyond the shot noise limit. The successful squeezing of one of the picosecond pulses in the above SRS will improve sensitivity and reduce photodamage, greatly expanding the range of studies available to SRS microscopy.

Cavity-electromechanical systems are extensively used for sensing and controlling the vibrations of mechanical resonators down to their quantum limit. The nonlinear radiation-pressure interaction in these systems could result in an unstable response of the mechanical resonator showing features such as frequency-combs, period-doubling bifurcations and chaos. However, due to weak light-matter interaction, typically these effects appear at very high driving strengths. By using polariton modes formed by a strongly coupled flux-tunable transmon and a microwave cavity, here we demonstrate an electromechanical device and achieve a single-photon coupling rate g 0 / 2 π of 160 kHz, which is nearly 4% of the mechanical frequency ωm. Due to large g0/ωm ratio, the device shows an unstable mechanical response resulting in frequency combs in sub-single photon limit. We systematically investigate the boundary of the unstable response and identify two important regimes governed by the optomechanical backaction and the nonlinearity of the electromagnetic mode. Such an improvement in the single-photon coupling rate and the observations of microwave frequency combs at single-photon levels may have applications in the quantum control of the motional states and critical parametric sensing. Our experiments strongly suggest the requirement of newer approaches to understand instabilities.

We study the heat transfer between N coupled quantum resonators with applied synthetic electric and magnetic fields realized by changing the resonators parameters by external drivings. To this end we develop two general methods, based on the quantum optical master equation and on the Langevin equation for $N$ coupled oscillators where all quantum oscillators can have their own heat baths. The synthetic electric and magnetic fields are generated by a dynamical modulation of the oscillator resonance with a given phase. Using Floquet theory we solve the dynamical equations with both methods which allow us to determine the heat flux spectra and the transferred power. With apply these methods to study the specific case of a linear tight-binding chain of four quantum coupled resonators. We find that in that case, in addition to a non-reciprocal heat flux spectrum already predicted in previous investigations, the synthetic fields induce here non-reciprocity in the total heat flux hence realizing a net heat flux rectification.

It is now well appreciated that quantum physics can be used to build better sensors. Such sensors can be based on unitary systems [1,2] like interferometers or open systems based on scattering and lossy transmission channels [3-5]. The framework of the quantum Fisher information enables one to obtain best estimates of the parameters and then one can design possible experiments that can reach Cramer- Rao bounds. I would bring out not only the importance of the quantum states used as probes, but also the importance of the ‘quantum’ measurement schemes especially the ones that depend on time reversed arrangements. I would illustrate the great usefulness of squeezed states of matter and light for metrology. [1] S. C. Burd et al., Quantum amplification of mechanical oscillator motion, Science 364, 1163 (2019). [2] G. S. Agarwal, and L. Davidovich, Quantifying quantum-amplified metrology via Fisher information, Phys. Rev. Res. 4, L 012014 (2022). [3] J. Wang, L. Davidovich, and G. S. Agarwal, Quantum sensing of open systems: Estimation of damping constants and temperature, Phys. Rev. Res. 2, 033389 (2020). [4] F. Li, T. Li, M. O. Scully, and G. S. Agarwal, Quantum advantage with seeded squeezed light for absorption measurement, Phys. Rev. Applied 15, 044030 (2021). [5] T.Li, F. Li, X. Liu, V. Yakovlev and G. S. Agarwal, Quantum-enhanced stimulated Brillouin scattering spectroscopy and imaging, OPTICA 9, 959 (2022).

We discuss the possibility of converting a simple pole in the radiative decay of a state into a pole of higher order by using resonant electromagnetic fields. This process of creation of higher order pole is controllable by the intensity of the laser field. We use density matrix and Liouville space and present the modification of the Lorentzian line shapes (Breit-Wigner formula) for example to ones involving square of Lorentzian and derivatives of Lorentzians.

Brillouin microscopy is an emerging label-free imaging technique used to assess local viscoelastic properties. Quantum-enhanced stimulated Brillouin scattering is demonstrated using low power continuous-wave lasers at 795 nm. A signal-to-noise ratio enhancement of 3.4 dB is reported by using two-mode intensity-difference squeezed light generated with the four-wave mixing process in atomic rubidium vapor. The low optical power and the excitation wavelengths in the water transparency window have the potential to provide a powerful bio-imaging technique for probing mechanical properties of biological samples prone to phototoxicity and thermal effects. The performance enhancement affordable through the use of quantum light may pave the way for significantly improved sensitivity that cannot be achieved classically. The proposed method for utilizing squeezed light for enhanced stimulated Brillouin scattering can be easily adapted for both spectroscopic and imaging applications in biology.

Cavity-mediated coupling can lead to remote exchange of energy between two spatially separated bosonic modes. Here, we demonstrate how the application of a two-photon parametric drive to the cavity can bring about a prodigious enhancement in this transfer efficiency.

We present a brief overview of the transport of quantum light across a one-dimensional waveguide which is integrated with a periodic string of quantum-scale dipoles. We demonstrate a scheme to implement transparency by suitably tuning the atomic frequencies without applying a coupling field and bring out the pronounced non-reciprocity of this optical device. The fiber-mediated interaction between integrated dipoles allows one to achieve both dispersive and dissipative couplings, level repulsion and attraction, and enhanced sensing capabilities. All these ideas can be translated to a wide variety of experimental setups of topical interest such as resonators on a transmission line, cold atoms near a fiber and quantum dots coupled to plasmonic excitations in a nanowire or photonic crystal waveguides.

Single photon sources (SPS), especially those based on solid state quantum emitters, are key elements in future quantum technologies. What is required is the development of broadband, high quantum efficiency, room temperature SPS which can also be tunably coupled to optical cavities which could lead to development of all-optical quantum communication platforms. In this regard deterministic coupling of SPS to plasmonic nanocavity arrays has great advantage due to long propagation length and delocalized nature of surface lattice resonances (SLRs). Guided by these considerations, we report experiments on the room temperature tunable coupling of single photon emitting colloidal quantum dots (CQDs) to localised and delocalised modes in plasmonic nanocavity arrays. Using time-resolved photo-luminescence measurement on isolated CQD, we report significant advantage of SLRs in realizing much higher Purcell effect, despite large dephasing of CQDs, with values of ~22 and ~ 6 for coupling to the lattice and localised modes, respectively. We present measurements on the antibunching of CQDs coupled to these modes with g(2)(0) values in quantum domain providing evidence for an effective cooperative behavior. We present a density matrix treatment of the coupling of CQDs to plasmonic and lattice modes enabling us to model the experimental results on Purcell factors as well as on the antibunching. We also provide experimental evidence of indirect excitation of remote CQDs mediated by the lattice modes and propose a model to explain these observations. Our study demonstrates the possibility of developing nanophotonic platforms for single photon operations and communications with broadband quantum emitters and plasmonic nanocavity arrays since these arrays can generate entanglement between to spatially separated quantum emitters.

We develop an ultrafast frequency-resolved Raman spectroscopy with entangled photons for polyatomic molecules in condensed phases, to probe the electronic and vibrational coherences. Using quantum correlation between the photons, the signal shows the capability of both temporal and spectral resolutions that are not accessible by either classical pulses or the fields without entanglement. We develop a microscopic theory for this Raman spectroscopy, revealing the electronic coherence dynamics which often shows a rapid decay within $\sim$50fs. The heterodyne-detected Raman signal is further developed to capture the phases of electronic coherence and emission in real-time domain.

Tian Li, 2, ∗ Fu Li, 3 Xinghua Liu, 3 Vladislav V. Yakovlev, 4, 5 and Girish S. Agarwal 2, 3 Institute for Quantum Science and Engineering, Texas A&M University, College Station, TX 77843, USA Department of Biological and Agricultural Engineering, Texas A&M University, College Station, TX 77843, USA Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA Department of Biomedical Engineering, Texas A&M University, College Station, TX 77843, USA Department of Electrical and Computer Engineering, Texas A&M University, College Station, TX 77843, USA

Chiral interfaces provide a new platform to execute quantum control of light-matter interactions. One phenomenon which has emerged from engineering such nanophotonic interfaces is spin-momentum locking akin to similar reports in electronic topological materials and phases. While there are reports of spin-momentum locking with combination of chiral emitters and/or chiral metamaterials with directional far field excitation it is not readily observable with both achiral emitters and metamaterials. Here, we report the observation of photonic spin-momentum locking in the form of directional and chiral emission from achiral quantum dots (QDs) evanescently coupled to achiral hyperbolic metamaterials (HMM). Efficient coupling between QDs and the metamaterial leads to emergence of these photonic topological modes which can be detected in the far field. We provide theoretical explanation for the emergence of spin-momentum locking through rigorous modeling based on photon Green's function where pseudo spin of light arises from coupling of QDs to evanescent modes of HMM.

In the 1960s, computer engineers had to address the tyranny of numbers problem in which improvements in computing and its applications required integrating an increasing number of electronic components. From the first computers powered by vacuum tubes to the billions of transistors fabricated on a single microprocessor chip today, transformational advances in integration have led to remarkable processing performance and new unforeseen applications in computing. Today, quantum scientists and engineers are facing similar integration challenges. Research labs packed with benchtop components, such as tunable lasers, tables filled with optics, and racks of control hardware, are needed to prepare, manipulate, and read out quantum states from a modest number of qubits. Analogous to electronic circuit design and fabrication nearly five decades ago, scaling quantum systems (i.e. to thousands or millions of components and quantum elements) with the required functionality, high performance, and stability will only be realized through novel design architectures and fabrication techniques that enable the chip-scale integration of electronic and quantum photonic integrated circuits (QPIC). In the next decade, with sustained research, development, and investment in the quantum photonic ecosystem (i.e. PIC-based platforms, devices and circuits, fabrication and integration processes, packaging, and testing and benchmarking), we will witness the transition from single- and few-function prototypes to the large-scale integration of multi-functional and reconfigurable QPICs that will define how information is processed, stored, transmitted, and utilized for quantum computing, communications, metrology, and sensing. This roadmap highlights the current progress in the field of integrated quantum photonics, future challenges, and advances in science and technology needed to meet these challenges.

The robust spin and momentum valley locking of electrons in two-dimensional semiconductors make the valley degree of freedom of great utility for functional optoelectronic devices. Owing to the difference in optical selection rules for the different valleys, these valley electrons can be addressed optically. The electrons and excitons in these materials exhibit valley Hall effect, where the carriers from specific valleys are directed to different directions under electrical or thermal bias. Here we report the optical valley Hall effect where the light emission from the valley polarized excitons in monolayer WS2 propagates in different directions owing to the preferential coupling of excitonic emission to the high momentum states of the hyperbolic metamaterial. The experimentally observed effects are corroborated with theoretical modeling of excitonic emission in the near field of hyperbolic media. The demonstration of the optical valley Hall effect using a bulk artificial photonic media without the need for nanostructuring opens the possibility of realizing valley-based excitonic circuits operating at room temperature.

C. J. Zhu, 2, 3 W. Li, Y. P. Yang, ∗ and G. S. Agarwal † School of Physical Science and Technology, Soochow University, Suzhou 215006, China MOE Key Laboratory of Advanced Micro-Structured Materials, School of Physics Science and Engineering, Tongji University, Shanghai, China 200092 Collaborative Innovation Center of Light Manipulations and Applications, Shandong Normal University, Jinan 250358, China Institute for Quantum Science and Engineering, Department of Biological and Agricultural Engineering, Department of Physics and Astronomy, Texas A&M University, College Station, Texas 77843, USA (Dated: June 22, 2021)