Publications

Quantum causality is an emerging field of study that has the potential to greatly advance our understanding of quantum systems. In this paper, we put forth a theoretical framework for merging quantum information science and causal inference by exploiting entropic principles. For this purpose, we leverage the tradeoff between the entropy of hidden cause and the conditional mutual information of observed variables to develop a scalable algorithmic approach for inferring causality in the presence of latent confounders (common causes) in quantum systems. As an application, we consider a system of three entangled qubits and transmit the second and third qubits over separate noisy quantum channels. In this model, we validate that the first qubit is a latent confounder and the common cause of the second and third qubits. In contrast, when two entangled qubits are prepared and one of them is sent over a noisy channel, there is no common confounder. We also demonstrate that the proposed approach outperforms the results of classical causal inference for the Tubingen database when the variables are classical by exploiting quantum dependence between variables through density matrices rather than joint probability distributions. Thus, the proposed approach unifies classical and quantum causal inference in a principled way.

Engineering symmetries in nanostructures and metasurfaces provides a new paradigm to control incoherent heat radiation for applications in energy conversion, thermal sources, infrared imaging, and radiative cooling.

The concept of photonic frequency-momentum (ω-q) dispersion has been extensively studied in artificial dielectric structures such as photonic crystals and metamaterials. However, the ω-q dispersion of electrodynamic waves hosted in natural materials at the atomistic level is far less explored. Here, we develop a Maxwell Hamiltonian theory of matter combined with the quantum theory of atomistic polarization to obtain the electrodynamic dispersion of natural materials interacting with the photon field. We apply this theory to silicon and discover the existence of anomalous atomistic waves. These waves occur in the spectral region where propagating waves are conventionally forbidden in a macroscopic theory. Our findings demonstrate that natural media can host a variety of yet to be discovered waves with subnanometer effective wavelengths in the picophotonics regime.

We discover the quantum analog of the well-known classical maximum power transfer theorem. Our theoretical framework considers the continuous steady-state problem of coherent energy transfer through an N-node bosonic network coupled to an external dissipative load. We present an exact solution for optimal power transfer in the form of the maximum power transfer theorem known in the design of electrical circuits. Furthermore, we introduce the concept of quantum impedance matching with Thevenin equivalent networks, which are shown to be exact analogs to their classical counterparts. Our results are applicable to both ordered and disordered quantum networks with graph-like structures ranging from nearest-neighbor to all-to-all connectivities. This work points towards universal design principles adapting ideas from the classical regime to the quantum domain for various quantum optical applications in energy harvesting, wireless power transfer, and energy transduction.

We derive a unified quantum theory of coherent and incoherent energy transfer between two atoms (donor and acceptor) valid in arbitrary Markovian nanophotonic environments. Our theory predicts a fundamental bound ????=????+??ηmax=γaγd+γa for energy transfer efficiency arising from the spontaneous emission rates γ_d and γ_a of the donor and acceptor. We propose the control of the acceptor spontaneous emission rate as a new design principle for enhancing energy transfer efficiency. We predict an experiment using mirrors to enhance the efficiency bound by exploiting the dipole orientations of the donor and acceptor. Of fundamental interest, we show that while quantum coherence implies the ultimate efficiency bound has been reached, reaching the ultimate efficiency does not require quantum coherence. Our work paves the way towards nanophotonic analogues of efficiency-enhancing environments known in quantum biological systems.

The fluctuational electrodynamic investigation of thermal radiation from nonequilibrium or nonisothermal bodies remains largely unexplored because it necessarily requires volume integration over the fluctuating currents inside the emitter, which quickly becomes computationally intractable. Here, we put forth a formalism combining fast calculations based on modal expansion and fluctuational electrodynamics to accelerate research at this frontier. We employ our formalism on a sample problem: a long silica wire held under temperature gradient within its cross section. We discover that the far-field thermal emission carries a nonzero spin, which is constant in direction and sign, and interestingly, is transverse to the direction of the power flow. We clearly establish the origin of this transverse spin as arising from the nonequilibrium intermixing of the cylindrical modes of the wire, and not from any previously studied or intuitively expected origins such as chiral or nonisotropic materials and geometries, magnetic materials or fields, and mechanical rotations. This finding of nonequilibrium spin texture of emitted heat radiation can prove useful for advancing the noninvasive thermal metrology or infrared-imaging techniques.

Generation of a local magnetic field at the nanoscale is desirable for many applications such as spin-qubit-based quantum memories. However, this is a challenge due to the slow decay of static magnetic fields. Here, we demonstrate a photonic spin density (PSD)-induced effective static magnetic field for an ensemble of nitrogen-vacancy (NV) centers in bulk diamond. This locally induced magnetic field is a result of coherent interaction between the optical excitation and the NV centers. We demonstrate an optically induced spin rotation on the Bloch sphere exceeding 10 degrees which has potential applications in all-optical coherent control of spin qubits.

All elementary particles in nature can be classified as fermions with half-integer spin and bosons with integer
spin. Within quantum electrodynamics (QED), even though the spin of the Dirac particle is well defined, there
exist open questions on the quantized description of the spin of the gauge field particle—the photon. Using quantum field theory, we discover the quantum operators for the spin angular momentum (SAM) SM = (1/c) d3xπ × A and orbital angular momentum (OAM) LM = −(1/c) d3xπμx × ∇Aμ of the photon, where πμ is the conjugate canonical momentum of the gauge field Aμ. We also reveal a perfect symmetry between the angular momentum commutation relations for Dirac fields and Maxwell fields. We derive the well-known OAM and SAM of classical electromagnetic fields from the above-defined quantum operators. Our work shows that the spin and OAM operators commute, which is important for simultaneously observing and separating the SAM and OAM. The correct commutation relations of orbital and spin angular momentum of the photon have applications in quantum optics, topological photonics as well as nanophotonics and can be extended in the future for the spin structure of nucleons.

Thermal radiation is traditionally an incoherent radiative signal, where the radiated heat is highly unpolarized, spectrally broad, and omnidirectional. The recent extensive interests in thermal photonics focus on tailoring the temporal coherence (spectrum) and spatial coherence (directivity) of thermal radiation. Here, we investigate the photon spin characteristics of the radiation excited by thermal fluctuations using a symmetrybroken metasurface. Utilizing spin-polarized angle-resolved thermal emission spectroscopy (SPARTES), we explicitly show when both mirror- and inversion- symmetries are broken, the summation of spin-angular momentum projected on wavevectors, namely the optical helicity, can be non-vanishing even without applying a magnetic field. We find the photon spin and the energy-momentum dispersion of thermal radiation can be effectively tailored through symmetry engineering. Our results firmly suggest the symmetry-based strategy provides a general pathway for comprehensively controlling the temporal, spatial, and especially spin coherence of thermal radiation.

Generation of local magnetic field at the nanoscale is desired for many applications such as spinqubit-based quantum memories. However, this is a challenge due to the slow decay of static magnetic fields. Here, we demonstrate photonic spin density (PSD) induced effective static magnetic field for an ensemble of nitrogen-vacancy (NV) centers in bulk diamond. This locally induced magnetic field is a result of coherent interaction between the optical excitation and the NV centers. We demonstrate an optically induced spin rotation on the Bloch sphere exceeding 10 degrees which has potential applications in all optical coherent control of spin qubits.