Session T34: Quantum Photonics and Nonlinear Optics II

Charge state dynamics of ensemble silicon vacancies in bulk diamond

The manipulation of the charge states of solid-state defects is of importance in applications to quantum technology and precision sensing. Relevant examples include the silicon vacancy (SiV) and nitrogen vacancy (NV) centers in diamond, both of which display non-trivial, partly-understood charge state interconversion pathways. Here we investigate the charge state dynamics of ensemble SiVs in a bulk type 1b diamond using multicolor confocal microscopy. We create photoionized carriers via optical illumination, and track their diffusion by monitoring the charge state of the surrounding defects, changing upon carrier capture. Leveraging electric fields to bias carrier diffusion¹, we show that the preferential dark state of silicon vacancies is SiV^2-, which can convert to a bright SiV^- state upon capturing a free hole. These results are relevant to ongoing efforts aimed at exploiting the properties of SiV centers in diamond as spin qubits or nanoscale probes.

1. Lozovoi, A., Jayakumar, H., Daw, D., Lakra, A. & Meriles, C. A. Probing metastable space-charge potentials in a wide bandgap semiconductor. Phys. Rev. Lett. 125, 256602 (2020).   

Quantum Description of Cavity Raman Elucidates "Anomalous" anti-Stokes SERS

Gold nanospheres, (~90 nm in diameter), plasmonically confine and enhance incident radiation sufficient to approach the limit of single-molecule Raman detection.  In this regime, we record the power-dependent anti-Stokes-to-Stokes  (AS-to-S) ratio of a BPE molecule and find that there is no satisfactory map to the canonical quantum model with vibrational pumping + laser-induced heating. Non-thermal anti-Stokes exceed three orders of magnitude enhancement over conservative estimates of the pumping effect. With impetus from this anomalous AS behavior, we revisit optomechanical models of Raman both with explicit numerical simulation as well as a perturbation algorithm to generate analytical expressions for higher-order scattering terms. These tools let us characterize novel pathways that harness the heterodyning effect of the cavity luminescence spectrum. We show that these channels may far exceed the feeble pumping component, thus elucidating the anomaly.  Notably, the novel pathways are second order in the pump but only first order in the Raman cross-section, a unique attribute that is reflected in the data.   

Towards a quantum memory based on an excited-state absorption in Tm:YAP

Quantum memories compatible with telecommunication-wavelength photons are a key element towards building a large-scale quantum network, since they allow remote quantum information exchange through existing low-loss fibre infrastructure. Rare-earth ion (REI) doped crystals exhibit broad inhomogenous lines (∼GHz), allowing for large spectral multiplexing, while preserving very good coherence properties such as long coherence times (∼ms-s) leading to long storage times. Given the appeal of a quantum memory at telecom wavelength, and the difficulty in reaching high efficiencies with erbium-doped crystals, we performed spectroscopic investigations of  ³F₄↔³H₄ excited-state transition at around 1450 nm wavelength in various Tm-doped crystals. In this talk we will present recent results for Thulium-doped yttrium aluminium perovskite (Tm:YAP). We will discuss an optical hole burning scheme that allows us to assess the coherence of this transition and show that an atomic frequency comb (AFC) based quantum memory can be achieved in this platform.   

The Effect of Emitters on Quantum State Transfer in Coupled Cavity Arrays

I will report an investigation into perfect quantum state transfer (QST) across coupled-cavity emitter arrays to provide a comprehensive understanding of the effects of system geometry and physical attributes on quantum state transfer as a function of time. By performing simulations with disordered emitter geometries and couplings to the cavity array, our group was able to determine the ways in which QST is degraded. This is unique as much of the existing literature on perfect QST and the effects of disorder focuses on cavity-only systems.  We will show fidelity plots which illustrate the probability of transit from one side of the lattice to another for specific geometries, and also 'phase diagrams' which give the optimal QST fidelity as a function of the placement of a single emitter and its coupling to the cavity.   

Polariton creation in photonic quantum simulators

A scalable, tunable quantum simulator based on coupled cavity arrays in the strong cavity QED regime could provide a sandbox for modelling complex, strongly correlated materials. Of the many proposed platforms for such a quantum simulator, we focus on solid state emitters in nanophotonic cavities for the scalability of these systems. However, while these systems can be easily scaled, the effects of the emitters’ spectral inhomogeneous broadening on the light-matter hybridization have not yet been fully explored, nor have the effects of multi-emitter-cavity interaction which are of current interest for achieving strong cavity QED coupling in color center systems. In order to understand these effects we first exactly solve small open quantum systems based on the Tavis-Cummings-Hubbard model using a quantum master equation. Then we develop an effective Hamiltonian method to simulate large open quantum systems which includes two new metrics, polaritonic and nodal participation ratios. These findings can be used for photonic engineering of versatile quantum simulators.   

Engineering symmetry-selective couplings of a superconducting artificial molecule to microwave waveguides

Tailoring the decay rate of structured quantum emitters into their environment opens possibilities for quantum-state stabilization, amplification, and nonlinear quantum optics. Here we demonstrate a novel coupling scheme between an artificial molecule comprising two identical, strongly coupled transmon qubits, and two microwave waveguides. In our scheme, the coupling is engineered so that transitions between states of the same (opposite) parity are predominantly coupled to one (the other) waveguide. In a first-generation device, we achieve a selectivity of 2 orders of magnitude for the coupling rates. In addition, we implement a two-photon Raman process activated by simultaneously driving both waveguides and show that it can be used to coherently couple states of different parity in the single-excitation manifold of the molecule. Using that process, we implement frequency conversion across the waveguides, mediated by the molecule, with efficiency in excess of 90%. Finally, we show that this coupling arrangement makes it possible to straightforwardly generate spatially-separated Bell states propagating across the waveguides. We envisage further applications to quantum thermodynamics, microwave photodetection, and photon-photon gates.   

Tuning coupled cavity arrays Hamiltonians with Monte-Carlo methods

We used Monte-Carlo techniques to tune coupled cavity array hopping parameters for perfect quantum state transfer (QST). For coupled cavity arrays with single excitations, we were able to find multiple cavity-emitter topologies which exhibited perfect QST. This is unique, as most existing work is cavity only. We developed two methods to tune the array hopping parameters. One set the Hamiltonian eigenvalues to a list of known eigenvalues, while the second set the time evolution operator to a predetermined matrix. The second method is of considerable interest, as it is easily generalized to multiple excitations. We will also show that using these methods, it is possible to "program" a coupled cavity array for a quite general time dependent behavior.   

Poynting vector singularities in scattering from cylinders with curl-free gauge fields

We study Mie scattering from a cylinder comprised of engineered refractive index mimicking a free vortex leading to a magneto-electric coupling for stationary material. We find Poynting vector singularities in the near field distribution and predict an additional phase change in the far-field for magneto-electric-cylinder compared to conventional Mie scattering. We show that the magnitude of the artificial curl-free gauge fields can be used to control the angular position of the phase change in the far-field and the position of the Poynting vector singularities. Furthermore, we investigate the space-time reversal symmetry of material containing artificial curl-free gauge fields and dispersion relation for such spatially dispersive medium. The high k-vector region around the singularity can be exploited for near-field superresolution imaging and the far-field phase for scattered field holography.   

Exciton-polaron-polariton condensation

Exciton-polaritons created in microcavity semiconductors are highly tunable quantum states that, thanks to their hybrid character, allow the transfer of features between light and matter. Polariton interactions make it possible to create quantum fluids, exhibiting macroscopic quantum states like condensation and superfluidity. Because of this, they constitute a fruitful field to exchange ideas with atomic physics and to unveil novel no-linear optical effects. Recent experiments have demonstrated that, by doping the semiconductor with itinerant electrons, the exciton-polaritons get dressed in electronic excitations to create polarons, opening a new venue to explore Bose-Fermi mixtures. Here, we describe the condensation of exciton-polaritons in the presence of a two-dimensional electron gas by employing a non-perturbative many-body theory to treat exciton-electron correlations combined with a non-equilibrium theory for the condensate.   

Multi-axis Coherent Control of the Tin Vacancy Center Spin in Diamond

Group-IV color centers in diamond are a promising building block candidate for future quantum networks. The negatively charged tin-vacancy center (SnV) offers the benefits of strong protection against phonon dephasing and robust cyclicity of its optical transitions arising from its strong spin-orbit interaction. In the work covered in this talk, we deploy an all-optical stimulated Raman drive between the ground and excited states to realize multi-axis coherent control of the SnV spin qubit. We utilize coherent population trapping and optically driven electronic spin resonance to confirm coherent access to the qubit at 1.7 K, and obtain spin Rabi oscillations at a rate of Omega/2pi = 19.0(1) MHz. All-optical Ramsey interferometry reveals a spin dephasing time of T2* = 1.3(3) us and a CPMG-4 dynamical decoupling extends the spin coherence time to T_2 = 0.30(8) ms. These results, paired with previously reported transform-limited photons and integration into photonic nanostructures, establish the SnV as a competitive candidate for quantum network applications.     

Frequency multiplexed entangled photon pairs and detectors for quantum repeaters

Large-scale entanglement-based quantum networks rely on efficient quantum repeaters that can distribute entanglement in a multi-mode and heralded fashion over multiple network nodes. Next to optical quantum memories, most quantum repeater architectures also require suitably multiplexed sources of entangled photon pairs as well as detectors that can distinguish between the different modes—including temporal, spatial and spectral modes— used to encode quantum information.

Here we demonstrate the generation and characterization of frequency multiplexed photon pairs at 795 nm (signal) and 1532 nm (idler) wavelength by employing cavity-enhanced spontaneous parametric down-conversion in a nonlinear crystal and a virtually-imaged phase array (VIPA) that allows mapping of spectral modes onto distinct spatial modes. Coincidence measurements reveal non-classical correlation between spectrally correlated signal and idler photons, with strongly reduced correlations between photons belonging to non-matched spectral channels. This demonstrates that both our photon-pair source as well as the novel detection setup are suitable for creating a frequency multiplexed quantum repeater   

Unconventional saturation effects at intermediate drive in a lossy cavity coupled to few emitters

The loss rates in highly dissipative cavity-emitter systems render the realization of many desirable nonlinear effects, such as saturation and photon blockade, problematic.

Here we present another effect occurring within the Tavis-Cummings model: a nonlinear response of the cavity for resonant external driving of intermediate strength, which makes use of large cavity dissipation rates. In this regime, (N+1)-photon processes dominate when the cavity couples to N emitters. We provide a picture of how the effect occurs due to destructive interference between the emitter ensemble and the external drive. We find that a central condition for the observed effect is large cooperativity, i.e., the product of the cavity and emitter decay rates is much smaller than the collective cavity-emitter interaction strength squared. Importantly, this condition does not require strong coupling. We also find an analytical expression for the critical drive strength at which the effect appears. Our results have potential for quantum state engineering and could be used for the characterization of cavity-emitter systems with unknown emitter number. In particular, our results open the way for investigations of unique quantum-optics applications in a variety of platforms that neither require high-quality cavities nor strong coupling.   

Strong reverse saturable absorption and negative nonlinear refractive index in NiFe2O4 nanoparticles.

Transition metal oxides (AB₂O₄) are an important class of materials due to their wide potential applications, such as energy storage, sensors, high-density magnetic recordings. We study the third-order nonlinear optical response of NiFe₂O₄ (NFO) by using the open and close aperture Z-scan technique to get a comprehensive study of nonlinear absorption and refraction. NFO has an indirect bandgap around 1.48eV and two direct bandgaps around 2.49eV and 3.12eV respectively. We excite the sample with a nanosecond laser at a wavelength of 532nm, and a pulse width of 7ns. We observe reverse saturable absorption as a nonlinear optical response of NFO. From the study of nanosecond excitation in NFO, we get a strong nonlinear optical response, a nonlinear absorption coefficient of around 1.3×10² cm GW^-1, and a nonlinear refractive index of 3.36 × 10^-4cm² GW^-1 at intensity 0.31GWcm^-2The observation of the negative nonlinear refractive index reveals the occurrence of self -defocusing effect. From the strong nonlinear optical response, it is clear that NiFe₂O₄ can be used for optical limiting applications.   

Strong squeezing and bunching effect by a dipolariton cavity interacting with nonclassical light

Dipolaritons are quasiparticles created in microcavities containing two coupled quantum wells driven by a laser. In this work, we investigate the photon correlations and noise properties of the emergent light from a dipolariton system interacting with squeezed light produced by a nonlinear crystal. The present analysis shows that the considered scheme generates a strong and resistant squeezing outside the cavity in the presence of direct and indirect excitonic baths with non-vanishing temperatures. The weak coupling regime is favorable to a perfect squeezing and, in addition, exhibits a linear variation of the nonclassical effect as a function of the squeezed pumping amplitude. Furthermore, the study of the second-order correlation predicts a bunched transmitted light from the dipolariton cavity. For an off-resonant interaction, the autocorrelation function exhibits higher oscillation amplitudes than the resonant case. The second-order correlations analysis also predicts a bunched transmitted light from the dipolariton cavity. The autocorrelation function for an off-resonant interaction has larger oscillation amplitudes than the resonant case.   

Superradiance in dynamically modulated Tavis-Cumming model with spectral disorder

Superradiance is a cooperative phenomenon in quantum optical systems wherein the emission of photons from a quantum emitter is enhanced due to other quantum emitters coupling to the same electromagnetic mode. However, disorder in the resonant frequencies of the quantum emitters can perturb this effect. In this work, we study the interplay between superradiance, spectral disorder and dynamical modulation in a Tavis-Cumming model wherein all the emitters couple to a single resonant electromagnetic mode. As a consequence of the all-to-all coupling between the emitters, the presence of spectral disorder never prohibits the formation of a superradiant state over an extensive number of emitters. Furthermore, in such disordered systems, we show that a quantum control protocol modulating the resonant frequency of the optical mode leads to a multiplicative enhancement in the superradiance even in the limit of large number of emitters. Our results are relevant to experimental demonstration and application of superradiant effects in solid-state quantum optical systems such as color centers or rare-earth ions.