We propose a theoretical scheme to achieve a nonreciprocal universal photon blockade (PB) in a chiral coupled-cavity system. This approach utilizes the synergistic effect of chiral asymmetry and two-photon quantum interference: By embedding a two-level atom described by the two-photon Jaynes-Cummings (JC) model within one of the optical cavities, specific excitation pathways can be induced by tuning the phase and intensity of the external driving field. We demonstrate that under forward driving, the external driving field and the internal two-photon JC model coupling produce destructive quantum interference, thereby effectively suppressing two-photon excitation and achieving a high-quality single-photon PB. However, under backward driving, due to the chiral asymmetry of the system, the phase-matching condition for destructive interference is completely broken, and the PB phenomenon subsequently disappears. Our findings provide a promising approach for designing high-performance nonreciprocal single-photon sources and quantum isolation devices.

We discuss the experimental realization of a rotating radio-frequency (rrf) trap, proposed by Hasegawa and Bollinger [Phys. Rev. A 72, 043403 (2005)]. Compared to a traditional linear rf (lrf) Paul trap, the rrf trap is a closer analogy to the popular mechanical lecture demonstration for a Paul trap. In an ion trap with realistic, nonideal electrode geometry, the rrf trap averages over angular variations in the effective potential. This averaging provides more uniform confinement and reduces ion loss compared with the lrf trap at equal trap strength near the center. This feature makes the rrf trap configuration advantageous for precision metrology applications, such as electron electric dipole moment (eEDM) measurements.

We present a scheme for semidevice-independent quantum randomness certification from an untrusted measurement device and a trusted source and demonstrate it experimentally. No assumptions about noise or imperfections in the measurement are required, and the scheme is simple to implement with existing technology. The measurement device is probed with a few trusted states, and the output entropy can be lower bounded conditioned on the observed outcome distribution. The protocol can be applied to measurements with any finite number of outcomes and, in particular, can be realized by homodyne measurements of the vacuum using a detector probed by coherent states, as we experimentally demonstrate by intensity modulation of a telecom-wavelength pilot laser followed by homodyne detection and discretization by analog-to-digital conversion. We show that randomness can be certified in the presence of both Gaussian additive noise and non-Gaussian imperfections.

We demonstrate that the 337-nm emission from neutral nitrogen molecules, pumped via incoherent electron-impact excitation in circularly polarized femtosecond filaments, is oscillatory superfluorescence (SF) rather than amplified spontaneous emission. Time-resolved measurements reveal that both the pulse duration and the emission delay scale as $P^{-1/2}$ with nitrogen pressure, providing a definitive signature of gain-swept SF. These dynamics are quantitatively reproduced by bidirectional Maxwell-Bloch simulations that incorporate impulsive electron-impact excitation and fine-structure quantum beats. Our results demonstrate that macroscopic quantum coherence emerges following ultrafast electron-impact pumping and persists in atmospheric-pressure plasmas despite rapid collisional dephasing. These results establish femtosecond filaments as a robust platform for studying collective molecular phenomena and provide a framework for generating remote, coherent UV sources in the atmosphere.

We experimentally demonstrate atom interferometry using the transverse phase profile of an optical mode. As proof of principle, we use the helical phase windings of hypergeometric-Gaussian beams for Ramsey interferometry with ensembles of ballistically expanding cold ${}^{87}\mathrm{Rb}\mathrm{atoms}$, and we show that the interferometer can measure rotations induced by a motor with a sensitivity that scales linearly with orbital angular momentum and interferometer time. We characterize the thermal decoherence of the interferometer, deriving and experimentally confirming a closed-form expression for the spatially varying interferometer visibility arising near the singularity of the helical phase winding, motivating the use of condensed atoms in ring-shaped traps.

Ghost imaging was first demonstrated with entangled photon pairs and is well known for its peculiar properties. The signal beam that illuminates the object possesses no spatial resolution, whereas the reference beam, which never interacts with the object, is spatially resolved. Either beam alone cannot reconstruct the image, which can only be obtained when the signal and reference beams are correlated. Here, we report a thermal light ghost imaging experiment with even more peculiar properties, in which the image is reconstructed via the correlation of time bins containing zero photons, a process referred to as “ghost imaging with zero photons” hereafter. All the photons that have interacted with the object are discarded. The reason why the ghost image can be reconstructed with zero photons is attributed to the photon-number projection measurement and the photon statistics of thermal light. The information about the object is contained not only in the detected photons, but also in the zero photons. The results are helpful in resolving the debate on the physics of ghost imaging and understanding the relation between quantum and classical correlations.

The emergence of intrinsic probability is one of the central problems in quantum mechanics, and investigating the origin of the Born rule provides the most direct approach to this problem. Many derivations of the Born rule have been proposed based on different postulates and frameworks. Identifying which postulates are genuinely necessary is therefore essential for understanding the origin of quantum probability. Here we prove that two postulates commonly employed in derivations of the Born rule, additivity and noncontextuality, are not equivalent and thus cannot be unified into a single postulate. We further show that the additivity postulate is indispensable in five major derivations of the Born rule: Gleason's theorem, Busch's extension of Gleason's theorem, the Deutsch-Wallace theorem, Zurek's envariance derivation, and the Finkelstein-Hartle theorem. We demonstrate that these derivations rely either on explicit additivity assumptions or on implicit forms of additivity embedded in the original arguments and that removing or weakening additivity gives rise to significant problems in the derivations. Our results support the view that additional postulates with intrinsic probabilistic properties should be introduced to derive the Born rule.

Entangling the motion of massive mechanical objects is a central challenge in macroscopic quantum physics. In this work, we propose a cavity-free scheme for generating entanglement between the center-of-mass motions of two levitated yttrium iron garnet microspheres. Each mechanical mode is coupled to a magnon mode through a spatially inhomogeneous magnetic drive, and the two magnon modes are coupled via the magnetic dipolar interaction. By engineering asymmetric magnomechanical interactions, with a squeezing-type coupling in one microsphere and a beam-splitter-type coupling in the other, entanglement between the two mechanical modes is generated through mediation by the magnon subsystem. We further show that the motional entanglement remains long-lived after the drives are switched off, whereas magnon-related entanglement rapidly decays. This work provides an alternative approach to generating motional entanglement between massive objects.

Integrated frequency-comb sources are a key enabling technology for frequency-metrology applications. Their on-chip integration promises to bring metrology capacity outside of the laboratory, particularly since they can operate at low continuous-wave pump laser power in the dissipative Kerr soliton (DKS) regime. Yet such a small footprint and low power come at a cost: higher noise and overall lower comb power. In particular, this translates to highly challenging detection and locking of the carrier-envelope offset, which is necessary for complete stabilization of the comb. Recently, Kerr-induced synchronization (KIS) of a DKS to a reference laser was demonstrated as a tool for passive all-optical stabilization of DKS microcombs, with fundamental modification to the DKS and microcomb properties. Here, we demonstrate that the combination of additional power from the reference laser (now part of the DKS) and the KIS phase locking that pins the repetition rate together fundamentally alters the DKS, forcing an energy redistribution to maintain its center of mass. We demonstrate this self-balancing effect theoretically, which in a pure quadratic dispersion resonator leads to reference-dependent recoil. With higher-order dispersion through which the DKS generates phase-matched dispersive waves (DWs), we show that self-balancing results in the enhancement of the high-frequency DW power, offering an all-optical pathway for efficient carrier-envelope-offset detection in octave-spanning microcombs.

Quantum error-correcting codes are at the heart of fault-tolerant quantum computing. As the size of quantum platforms is expected to grow, one of the open questions is to design new optimal codes of ever-increasing size. A related challenge is to “certify” the quality of a given code by evaluating its minimum distance, a quantity characterizing the code's capacity to preserve quantum information. This problem is known to be NP-hard. Here we propose to harness the power of contemporary quantum platforms to address this question and, in this way, help design quantum platforms of the future. Namely, we introduce an approach to compute the minimum distance of quantum stabilizer codes by reformulating the problem as a quadratic unconstrained binary optimization (QUBO) problem and leveraging established QUBO algorithms and heuristics as well as quantum annealing (QA) to address the latter. The reformulation as a QUBO introduces only a logarithmic multiplicative overhead in the required number of variables. We demonstrate the formulation using the D-Wave Advantage 4.1 quantum annealer and the hybrid quantum-classical algorithm Qbsolv, and benchmark the resulting QUBO-based implementations against specialized classical distance-finding methods. The QA-assisted and simulated-annealing-assisted implementations show broadly comparable performance, while the classical comparison clarifies the relative strengths of heuristic optimization and exact distance certification. Our results establish a solver-independent QUBO formulation with only a logarithmic multiplicative overhead in the number of variables and characterize connectivity and minor embedding as central limitations of its present implementation on quantum annealing hardware.

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.