We investigate the quantum sensing performance of a dephasing qubit as a probe in Ohmic-family environments, characterized by the coupling strength $\eta$, the Ohmicity parameter $s$, and the cutoff frequency ${\omega }_c$ to be estimated. The performance is quantified by the dimensionless quantum signal-to-noise ratio $\mathcal{Q}$. We show that the evolution of $\mathcal{Q}$ with the scaled time ${\omega }_{c}t$ is independent of ${\omega }_c$, and peaks at an optimal time $t_{\text{opt}}$, yielding optimal sensitivity ${\mathcal{Q}}_{\text{opt}}$. We analyze how ${\mathcal{Q}}_{\text{opt}}$ depends on $\eta , s$, and the temperature $T$. Our results demonstrate that, for any Ohmic-family environment, provided that ${\omega }_c{t}_{\text{opt}}\ll 1, {\mathcal{Q}}_{\text{opt}}$ always reaches the upper bound, ${\mathcal{Q}}_{\text{max}}=0.648$ at zero temperature, and consistently attains ${\mathcal{Q}}_{\text{max}}/4$ at high temperatures. Remarkably, we find that increasing the scaled temperature $T/{\omega }_c$ can enhance ${\mathcal{Q}}_{\text{opt}}$ by nearly two orders of magnitude compared to its zero-temperature counterpart for certain Ohmic-family environments. Our work reveals that temperature can serve as a resource to enhance sensing precision, as it accelerates the encoding of the cutoff frequency information into the probe state, thereby enabling optimal measurement within a short time window.

Bose-Einstein condensation (BEC) in the grand-canonical ensemble admits a formulation in terms of a phase-density decomposition of the condensate mode operator ${\widehat{\psi }}_{\mathbf{0}}$. In the presence of macroscopic condensate number fluctuations this representation presents nontrivial implications. In particular, we show that, for the ideal gas, under the assumption of a well-defined phase and a fluctuating condensate density, the square modulus of the anomalous average $\langle {\widehat{\psi }}_{\mathbf{0}}\rangle$ can provide only a fraction of the whole condensate density ${\rho }_{\mathbf{0}}$, and for the grand-canonical statistics of the ideal Bose gas one obtains the value $|\langle {\widehat{\psi }}_{\mathbf{0}}\rangle {|}^2=(\pi /4){\rho }_{\mathbf{0}}$. The remaining part is supplemented by the (macroscopic) fluctuations of ${\widehat{\psi }}_{\mathbf{0}}$, which become a distinctive feature of the BEC in this setting. This provides a transparent physical picture of a condensate of photons with a well-defined phase but large number fluctuations, as observed in dye-filled-microcavity photon experiments. We also propose a way to access the square modulus of the anomalous average to test theoretical predictions.

Light-matter interactions in resonant media often occur on femtosecond timescales and are typically characterized using ultrafast laser systems. We employ continuous-wave (cw) Hong-Ou-Mandel (HOM) quantum interferometry using indistinguishable photon pairs generated via spontaneous parametric down-conversion to measure molecular dephasing times of the solvatochromic polymethine dye IR-797 across different solvents. Here variations in the surrounding solvent modify the electronic transition energy and linewidth of the dye, and these environment-induced changes directly influence the molecular dephasing time, which governs electronic coherence on femtosecond timescales. By inserting the dye solution into one arm of the interferometer, we observe solvent-induced modifications of the HOM interference dip arising from the coherent linear response of the medium. Fitting the coincidence profiles using a susceptibility-based Lorentzian model allows extraction of the dephasing time. At a concentration of $20\mu \mathrm{g}\text{/}\mathrm{ml}$, the retrieved coherence times range from 25 to 60 fs depending on the solvent. For comparison, classical absorption spectroscopy yields effective coherence times of 20–30 fs. These results establish cw-pumped HOM interferometry as a quantum-light approach to ultrafast spectroscopy at ultralow photon flux, enabling femtosecond-scale measurements without femtosecond laser sources. In addition, the technique inherently cancels even-order dispersion and offers promising applications in materials science, chemistry, and biological systems.

Essential-state models are central for quantum control and technology in broad regimes of light-matter interaction. The canonical effective Hamiltonian is obtained equivalently from adiabatic elimination, the Markov approximation, and the pole approximation. These approximations are known to break down at high intensities, significantly limiting their applicability to moderate light-matter interaction. We show how this limitation can be addressed by applying quasidegenerate Rayleigh-Schrödinger perturbation theory (QDRSPT). We reconcile QDRSPT with adiabatic elimination and propose a quasidegenerate extension of adiabatic elimination that is robust when the detuning of the essential states is non-negligible. The accuracy of QDRSPT is demonstrated in both the low- and high-frequency regimes, showing excellent agreement with Floquet calculations at high intensities. The crucial corrections to adiabatic elimination make the eigenvectors of the effective Hamiltonian nonorthogonal. Physically, this allows us to account for the asymmetric strength with which different essential states couple to the nonessential states. We expect that our systematic approach to effective Hamiltonians from QDRSPT will constitute a new state of the art in intense light-matter interaction and quantum optics with novel forms of strong coupling and quantum control phenomena being conceivable.

Wigner's thought experiment illustrates quantum theory's measurement problem by considering an observer who measures a quantum system inside a sealed laboratory, modeled unitarily by an outsider. Recent extensions of this thought experiment, referred to as extended Wigner's friend arguments, question how different observers can reason consistently about each other in quantum setups, and challenge the absoluteness of the outcome value obtained by the friend under a notion of locality. In this work, we present an argument against the absoluteness of free choices under the same notion of locality, using an extended Wigner's friend scenario based on the Pusey–Barrett–Rudolph theorem. Similar arguments based on other contextuality or nonlocality models are possible.

Determining the minimum number of measurement bases required for the complete characterization of unknown quantum states is a central problem in quantum state tomography, especially in high-dimensional systems. For a $d$-dimensional quantum state, parameter counting implies that at least $d+1$ projective measurement bases are required for full reconstruction. However, explicit constructions achieving this information-theoretic minimum are not generally available beyond special cases based on mutually unbiased bases (MUBs). Here, we present a quantum state tomography scheme that reconstructs an arbitrary $d$-dimensional quantum state using exactly $d+1$ projective measurement bases. Our construction is explicit, applies to all finite dimensions, and does not rely on the existence of a complete set of MUBs. We further demonstrate the scheme on a silicon photonic chip by reconstructing six-dimensional quantum states, obtaining fidelities above 0.96 in a dimension where a complete set of MUBs is not known to exist. These results establish an explicit minimal-basis construction for high-dimensional quantum state tomography and provide a different perspective on the design of informationally complete quantum measurements at the information-theoretic limit.

Double microwave shielding employs ${\sigma }^{+}$- and $\pi$-polarized microwave fields, tuned close to the lowest rotational transition, to engineer a long-range repulsive barrier between polar molecules. By preventing molecules from reaching the short range, this technique suppresses detrimental two-body losses and recently enabled the realization of molecular Bose-Einstein condensates and self-bound droplets. Yet, the optimal operating regimes of the shielding mechanism remain largely unexplored. Here, by leveraging the underlying universality of the scattering problem, we systematically map the four-dimensional microwave parameter space—spanned by the detunings and intensities of the two fields—to identify configurations that maximize both shielding efficiency and interaction tunability. We define optimal operating regimes as configurations that are strictly free of field-linked bound states while sufficiently suppressing two-body losses to exceed typical lifetimes of ultracold samples. In these regimes, we evaluate the elastic-to-inelastic collision ratios required for efficient evaporative cooling and explore the accessible tuning range of the effective dipolar interactions. Finally, to identify the best platforms for future quantum simulation experiments, we conduct a global survey of candidate molecular species under realistic field constraints. We identify heavy, strongly dipolar molecules as the most promising candidates, demonstrating that they can achieve extreme loss suppression—reaching two-body loss rates as low as ${10}^{-18}{\mathrm{cm}}^3/\mathrm{s}$—alongside robust interaction tunability using only moderate field strengths.

The defining feature of ideal Gottesman-Kitaev-Preskill (GKP) states is that they are unchanged by stabilizers, which allow them to detect and correct for common errors without destroying the quantum information encoded in the states. Given this property, can one use the amount to which a state is unchanged by the stabilizers as a proxy for the quality of a GKP state? This is shown to hold in the opposite manner to which it is routinely assumed because, in fact, the fidelity a state has to an ideal GKP state is only upper bounded by the stabilizer expectation values. This means that, for qubits encoded in harmonic oscillators via the GKP code, a good stabilizer expectation value does not guarantee proximity to an ideal GKP state in terms of any distance based on fidelity.

We develop a unified instantaneous-mode description for lasers with dispersive cavities, exploiting the separation of timescales between fast cavity fields and slow carrier dynamics. The resulting reduced rate equations retain the essential effects of frequency-dependent mirrors through a dynamic modal gain and an effective confinement factor determined directly by the mirror reflectivity. Applied to a Fano laser, the reduced description captures the essential dynamics, closely reproduces the full-model behavior, and clarifies the physical origin of dispersive instabilities. More generally, the approach provides a transparent framework for deriving reduced models and simplifying the stability analysis of dispersive laser cavities.

We introduce the concept of optical coherence squeezing in double-slit interference. We construct Hermitian operators that characterize the coherence at the slits, leading to coherence uncertainty relations and a corresponding squeezing condition. We also analyze states exhibiting such squeezing and show its manifestations in the uncertainty of the magnitudes and positions of the intensity fringes. Our work suggests that coherence may be regarded as a degree of freedom for squeezing, complementing phase, amplitude, and polarization, which could benefit quantum-enhanced interferometry.

We report on the realization of a quantum-degenerate Bose-Bose mixture of ${}^{162}\mathrm{Dy}$ and ${}^{164}\mathrm{Dy}$. Owing to the near-identical mass and polarizability of the two isotopes, the mixture thermalizes efficiently, with evaporation trajectories closely following those of the single-isotope case. Using a broad interspecies Feshbach resonance, we explore a miscible-immiscible transition between the two Bose-Einstein condensates. The tunability of the interspecies interaction, combined with the large magnetic dipole moment of Dy, makes this platform well suited for exploring dipolar effects in ultracold mixtures, including multicomponent supersolidity.

We examine the conservation law of coherence which, for partially coherent scalar wave fields, connects a coherence density to a coherence flux. A relation is established between this law and the law of energy conservation. In addition, the physical meaning of the coherence density and its associated flux is clarified. Our findings are illustrated by analyzing partially coherent fields in the focal region of a converging lens.

Expectation values of measurement operators, interpreted as measurement probabilities, arise frequently throughout quantum information sciences. When quantum states are randomly distributed, their expectation values are also randomly distributed. In this work, with the goal of understanding nonunitary dynamics, we generalize previous derivations for distributions of expectation values [Campos et al., Phys. Lett. A 377, 1854 (2013)] to the case of sets of measurement operators and random mixed quantum states within variable sized environments. Using combinatorics approaches, we derive expressions for their moments. We proceed to construct empirical distributions of simulated Haar random brickwork quantum circuits with local depolarizing noise, and compare their form to a proposed effective global-depolarizing-like model with variable effective noise scales and environment dimensions. The fitted effective distributions reproduce peak behavior across circuit depths, noise scales, and system sizes, while deviations in the distribution tails arise from local noise effects. The fit effective model parameters are also shown to vary smoothly and consistently with circuit depth and noise scale. Finally, sets of nonsymmetric measurement operators are shown to exhibit distinct multimodal distributions relative to unimodal distributions for symmetric measurement operators, opening up questions about their simulability.

Quantum memories integrated in a modular quantum processing architecture can rationalize the resources required for quantum computation. This work focuses on spin-based quantum memories, where itinerant electromagnetic fields are stored in large ensembles of effective two-level systems, such as atomic or solid-state spin ensembles, embedded in a cavity. Using a mean-field framework, we model the ensemble as an effective spin communication channel and describe both absorption and emission processes using a cascaded quantum model. We derive optimal time-dependent modulations of the cavity linewidth that maximize storage and retrieval efficiency for fast incoming pulses. Our analysis yields an upper bound on efficiency, which can be met in the narrow bandwidth regime. It also shows the existence of a critical bandwidth above which the efficiency severely decreases. Numerical simulations are presented in the context of microwave-frequency quantum memories interfaced with superconducting quantum processors, highlighting the protocol's relevance for modular quantum architectures.

We present an analytical theory for the most subradiant modes in a finite one-dimensional emitter array coupled to either an ideal or a nonideal waveguide. Using an effective non-Hermitian Hamiltonian together with a Bragg-edge open-boundary ansatz, we derive compact expressions for the full complex collective eigenvalues, including both the linewidths and the collective energy shifts. The linewidths of the most subradiant states exhibit the characteristic $N^{-3}$ scaling in both cases, while in the deep-subwavelength regime they display even-odd oscillations due to boundary interference. In contrast, the collective energy shift approaches a separation-dependent asymptotic value with a leading finite-size correction scaling as $N^{-2}$. These results highlight the distinct physical origins of the imaginary and real parts of the subradiant eigenvalue: Bragg-edge destructive interference controls the linewidth, whereas near-field dipole-dipole interactions dominate the collective shift. Our theory provides a transparent framework beyond the ideal-waveguide limit and opens potential applications in subradiant spectroscopy and waveguide-QED-based sensing.

We study the out-of-equilibrium dynamics of a weakly interacting one-dimensional Bose gas in a box trap, subjected to a drive realized by a periodically oscillating linear potential. After a transient regime, the gas reaches a quasisteady state, characterized by the presence of several solitons. At weak driving amplitude, the solitons are only weakly perturbed by one another, while at strong driving amplitude a regime analogous to turbulence is reached, where the solitons are strongly intertwined with each other. We show that a hallmark of both regimes can be found in the momentum distribution, which displays a power-law decay $n\left(k\right)\sim k^{-2}$ at weak driving amplitude and $n\left(k\right)\sim k^{-\alpha }$ with a power-law exponent $\alpha \in [7,9]$ at large amplitude. We further characterize each of the two regimes by following the space-time maps and characterizing the solitons using the inverse scattering transform. The protocol analyzed in this study is amenable to experimental realization in current experimental setups.

Unsharp measurements are key resources for tasks that balance information gain and disturbance, but certifying them without device assumptions remains a challenge. We propose a fully device-independent protocol for characterizing unsharp instruments, based on an entanglement-assisted sequential quantum random access code, where the first decoder is allowed to communicate her measurement setting to the second. This communication-enhanced scheme creates a decoding regime in which both decoders surpass classical bounds, enabling tight quantification of sharpness and direct quantification of measurement incompatibility beyond noncommunicating protocols. Experimentally, we implement tunable unsharp measurements using a Mach-Zehnder interferometer, observing the predicted sequential enhancement in decoding probability. Additionally, we achieve significantly narrower sharpness intervals and incompatibility quantification across multiple target sharpness values. Our results show that communication is a powerful operational resource for certifying precisely unsharp instruments and advancing device-independent quantum information protocols.

Bosonic quantum error-correcting codes encode logical information in a harmonic oscillator, with the Gottesman-Kitaev-Preskill (GKP) and number-phase (NP) codes representing two fundamentally different encoding paradigms. Although both have been extensively studied, it remains unclear under what physical noise conditions (including photon loss and dephasing) one encoding intrinsically outperforms the other. Here we estimate a quantitative performance boundary between GKP and NP codes under general photon loss-dephasing noise. By optimizing code parameters within each encoding family, we identify the noise regimes in which each code exhibits a fundamental advantage. In particular, we find that the crossover occurs when the dephasing strength is approximately two orders of magnitude smaller than the loss strength, revealing a sharp separation between operational regimes. Beyond this specific comparison, our work establishes a practical and extensible methodology for benchmarking bosonic codes and optimizing their parameters, providing concrete guidance for the experimental selection and deployment of bosonic encodings in realistic noise environments.

We demonstrate efficient sub-Doppler laser cooling of the three eigenmodes of a ${}^{40}\mathrm{Ca}^{+}$ ion confined in a compact Penning trap operating with a magnetic field of 0.91 T. Using the same set of laser beams as required for the initial Doppler laser cooling operation, we detune the laser frequencies to produce a narrow two-photon dark resonance. The process achieves a $1/e$ cooling time constant of $108\left(8\right)\mu \text{s}$, ultimately reducing the mean thermal axial mode occupation from 72(23) to 1.5(3) in $800\mu \mathrm{s}$ as measured by resonantly probing an electric quadrupole transition near 729 nm. A parametric drive is applied to the trap electrodes, which coherently exchanges the axial mode occupation with that of each radial mode, allowing for three-dimensional sub-Doppler cooling using only the axially propagating laser beams. This sub-Doppler cooling is achieved for an axial oscillation frequency of ${\omega }_z=2\pi \times 221\mathrm{kHz}$, which places the motion well outside the Lamb-Dicke confinement regime at the Doppler laser cooling limit. Our measured cooling rate and final mode occupation are in good agreement with a semiclassical model which combines a Lindblad master equation solution for ion-photon interactions with classical harmonic-oscillator motion of the trapped ion.