Yin Cai

Contrôle cohérent quantique avec un peigne de fréquence: Les états quantiques multimodes sont au coeur des protocoles detraitement quantique de l’information et de métrologie quantique. Àpartir d’un peigne de fréquence optique injectant un oscillateurparamétrique optique pompé en mode synchrone (SPOPO) nousavons généré des états multimodes en temps/fréquence. Unsimulateur quantique est alors mis en place à partir de ce SPOPO et demise en forme d’impulsion, et permet de mettre en évidence de étatsclusters pouvant compter jusque 12 noeuds et un protocole departage de secret quantique à six partenaires. De plus, une détectionmultipixel résolue en fréquence est développée et utilisée pourréaliser un état cluster linéaire à 8 noeuds. Nous avons égalementutilisé cette source pour développer un spectromètre ayant unesensibilité allant au delà de celle imposée par les fluctuations du videquantique.

Einstein-Podolsky-Rosen (EPR) steering, a distinct class of quantum correlation intermediate between entanglement and Bell nonlocality, exhibits unique asymmetry and directionality. EPR steering swapping, analogous to entanglement swapping, enables two initially independent particles to establish EPR steering correlations. Recent advances in all-optical entanglement and EPR steering swapping have overcome the limitation of mutual electro-optical conversions. However, for quantum information systems required in real-world applications, extending these capabilities to multimode regimes remains largely underexplored. Here, we propose an m -to- n multimode all-optical EPR steering swapping architecture based on energy-level-cascaded four-wave mixing processes. By harnessing fundamental light-atom interactions, we demonstrate scalable multimode EPR steering swapping and realize non-Hermitian control over dressing-induced coherent channels. Crucially, our multimode approach demonstrates distinctive, mode-dependent features of non-monotonicity and asymmetry, which are unattainable in conventional two-mode schemes. The flexible controllability afforded by such multimode quantum correlations also facilitates interfaces between non-Hermitian light-matter systems, paving the way for practical implementations of EPR steering and all-optical quantum networks.

Quantum metrology harnesses nonclassical resources including squeezed states and quantum entanglement to overcome fundamental limitations imposed by the shot-noise limit (SNL). While conventional optical interferometers for phase detection typically operate in the MHz-frequency regime due to technical noise constraints at lower frequencies, numerous practical applications demand operation in the kHz regime or below. Here, we present a truncated SU(1,1) interferometric configuration that achieves enhanced phase sensitivity in the low-frequency (kHz) domain by exploiting quantum entanglement generated through two-mode squeezed states. Our approach incorporates quantum sideband control to achieve quantum correlations across multiple low-frequency bands while simultaneously suppressing technical noise. Through precise locking of optimal phase, we demonstrate a significant improvement in signal-to-noise ratio within the kHz range, with phase sensitivity surpassing the SNL by 2.0 ± 0.2 dB. These experimental results provide crucial validation for extending entanglement-enhanced optical interferometry into low-frequency metrological applications and establish novel methodologies for optical modulation and sideband-based quantum information processing and quantum sensing protocols.

While biphoton generation holds significant importance in quantum entanglement and quantum technology, its dynamics involving non‐Hermitian characteristics have experimentally been untouched so far. Nevertheless, it is found that the non‐Hermitian features induced by the atomic coherence can be well employed for the direct control of biphoton generation and entanglement. By harnessing such natural non‐Hermitian interaction, a method is devised to expand a non‐Hermitian Hao biphoton generation processes. It is experimentally demonstrated such biphoton correlation can be adjusted within an unprecedentedly wide range, ranging from Rabi oscillation to antibunching‐exponential‐decay. Typically, the exceptional point at the transition boundary between the two regimes is distinctly observed.

The concept of non‐Hermitian systems, also known as exceptional points (EPs), has recently emerged as a novel approach for designing the response of systems that exchange energy with the external environment. The system behaves as a non‐Hermitian Hamiltonian (NH Hamiltonian), the real part of which is related to the internal dynamics of the system, and the imaginary part reflects the energy exchange between the system and the outside environment. By utilizing double EIT windows and adjusting the intensity of the dressing field, the number and positions of eigenvalues can be modulated to construct quantum entanglement sources for non‐Hermitian systems, thereby laying the foundation for quantum communication and quantum computing. To prepare photon entanglement sources, it is necessary to study the correlation of high‐dimensional quantized energy levels under PT symmetry and symmetry breaking induced by the dressing field; Hamiltonian correlation matrix of the non‐Hermitian system characterizes the relationship between gain and loss, detuning and dephasing rate in the reaction energy‐level system, and the coupling relationship of the dressing field. The energy‐level system itself is non‐Hermitian, and the study of symmetry breaking is conducted by controlling EPs during the level control process through the dressing field, thus eliminating the need for introducing artificial photonic structures. The third‐order non‐Hermitian characteristics of the PT‐NH system reflect the positional relationship between the cascade field and the nested field. When the Rabi frequency (dephase rate) of the dressing field is the dominant factor, the real (imaginary) part of the eigenvalues is split, resulting in in‐phase (out‐of‐phase) constructive (destructive) transformations of the dressing order.

Einstein–Podolsky–Rosen (EPR) steering is a type of directional quantum correlation that holds immense significance and broad applications in quantum information processing. While EPR steering has been achieved across various physical systems, research into its implementation in non-Hermitian systems remains in its early stages. In this study, we delve into the realm of non-Hermitian control of EPR steering by leveraging atomic coherence-controlled energy-level cascaded four-wave mixing (ELC-FWM) processes. We derive analytical expressions for the generation of EPR steering within such non-Hermitian nonlinear systems, demonstrating that exceptional points (EPs) and multimode EPR steering can be realized through introducing dressing-control fields. Furthermore, we illustrate that nonlinear coherent channels and the associated steerability distribution of the output modes can be tailored during the EPR steering generation process, which is directly linked to the eigenvalues of non-Hermitian processes. Additionally, we analyze the impact of loss effects on generated multimode EPR steering. Our findings suggest that non-Hermitian control offers a promising all-optical approach for constructing practical quantum networks.

We report super broad non-Hermitian line shape from out-of-phase and in-phase photon-phonon dressing (quantization) in Eu³⁺: NaYF₄ and Eu³⁺: BiPO₄ nanocrystals. The line shape is controlled by changing time gate position, time gate width, power, temperature, sample, photomultiplier tubes, and laser. We observed that the fluorescence (FL) line-shape contrasts are 69.23% for Eu³⁺: BiPO₄ and 43.75% for Eu³⁺: NaYF₄, owing to the stronger out-of-phase photon-phonon dressing (destructive quantization). Moreover, we observed that the spontaneous four-wave mixing (SFWM) line shape was approximately three times wider at 300 K than at 77 K for the [(12:1)-phase] Eu³⁺: NaYF₄ due to more high-frequency in-phase phonon dressing (strong constructive quantization). Furthermore, we showed that the noise line-shape width remains unchanged for Eu³⁺: BiPO₄ (16 nm) and Eu³⁺: NaYF₄ (12 nm) due to out-of-phase and in-phase photon-phonon dressing balance. Such results have potential applications in multi-channel band stop filter.

Multimode quantum light fields significantly enhance quantum state manipulation and communication capabilities by expanding the dimensionality of the Hilbert space. In this work, we elaborately design a non-horizontal cavity suitable for a Gouy phase-compensated synchronously pumped optical parametric oscillator. By injecting an optical frequency comb in HG₁₀ mode into the cavity as the seed, we demonstrate the simultaneous generation of a bright squeezed optical frequency comb in HG₁₀ mode and a vacuum squeezed optical frequency comb in HG₀₁ mode. The coexistence of amplitude quadrature squeezed fields in these two orthogonal first-order Hermite-Gaussian transverse modes also suggest the presence of quadrature entanglement between the two first-order Laguerre-Gaussian modes, LG₀ ^{+ 1} and LG₀ ^{- 1}. This research underscores the capability of one-step generation of multi-transverse-mode squeezed optical frequency combs using a single cavity, marking a significant stride in the realm of quantum optics.

Non-Hermitian degeneracy, also known as exceptional point, has been recently seen as a new way to engineer the response of open physical systems. Based on natural non-Hermitian atomic coherence, we investigate spontaneous six-wave mixing (SSWM) and eight-wave mixing (SEWM) processes under both parity–time (PT) and anti-parity–time (anti-PT) symmetry, and we obtain high-dimensional coherent channels under third-order energy-level splitting. Finally, we reveal that the third-order non-Hermitian interaction between two dressing fields of the nested scheme is the strongest, and the parallel scheme is the weakest, with the cascade scheme considered intermediate between them. It can also be used to develop quantum memory devices with enhanced sensitivity in the atom-like system.