Levitated dipolar scatterers exhibit exceptional performance as optomechanical systems for observing quantum mechanics at the mesoscopic and atomic scales. These systems scatter photons in almost every direction, and, with generally limited light collection efficiency, only partial information about the state of the system can be extracted. In this article, we present a setup to enhance information extraction of optomechanical measurements by constraining light scattering to a specific spatial direction. The setup consists of a hollow hemispherical mirror that controls the light scattered by a dipolar scatterer at high scattering angles outside the hole's region. Control of light scattering at high angles can be used to suppress the corresponding optomechanical backaction, which results in focusing of the measurement information at low angles. This mirror is compatible with existing setups commonly employed in levitated optomechanics, including confocal lenses and optical resonators.

Space-time wave packets (STWPs) have received significant attention since they can propagate in free space at arbitrary group velocity without dispersion and diffraction. However, at present, the generation of STWPs has been limited to the paraxial regime. Here we show that conventional optical elements can be used to extend STWPs beyond the paraxial regime. A dispersion magnifier, consisting of two lenses and a beam expander, applies spatiotemporal shaping to paraxial STWPs to create nonparaxial STWPs. The control of the magnification ratio results in versatile engineering capabilities on group velocity, beam diameter, and propagation distance. As an example, we numerically demonstrate long-distance propagation or slow group velocity of the output wave packet with subwavelength cross sections.

Cylindrical vector (CV) beams have garnered widespread research interest owing to their polarization symmetry and unique properties. The advent of high-intensity ultrafast CV laser sources is poised to advance ultrafast science and attosecond physics. The type-II phase-matched, wavelength-degenerate optical parametric amplification (OPA) configuration can realize polarization-independent parametric amplification for the signal. When combined with chirped-pulse amplification (CPA) technology, this method shows promise for producing ultrafast vector beams with customizable polarization profiles at ultrahigh intensities. However, in addition to the linear spectral phase retardance introduced by the birefringent OPA crystal, known as the wave-plate effect, the optical parametric phase (OPP) inherently induced in optical parametric CPA (OPCPA) may cause another type of spectral phase retardation between the ordinary and extraordinary polarized components of the signal. This nonlinear spectral phase retardance depends significantly on the parametric process. This paper presents the results of a comprehensive theoretical investigation of nonlinear spectral phase retardance and correlates it with its linear counterpart. The dynamic behavior of nonlinear spectral phase retardance and its impact on the temporal compensation of compressed signal pulses are comprehensively discussed. These findings are expected to guide the design of CV-polarized OPCPA systems and their future applications.

Ground-state cooling of mechanical resonators is a prerequisite for the observation of various quantum effects in optomechanical systems and thus has always been a crucial task in quantum optomechanics. In this paper, we study how to realize ground-state cooling of the mechanical mode in a Fano-mirror optomechanical setup, which allows for enhanced effective optomechanical interaction but typically works in the (deeply) unresolved-sideband regime. We reveal that for such a two-sided cavity geometry with very different decay rates at the two cavity mirrors, it is possible to cool the mechanical mode down to its ground state within a broad range of parameters by using an appropriate single-sided coherent feedback. This is possible even if the total optical loss is more than seven orders of magnitude larger than the mechanical frequency and the feedback efficiency is relatively low. Importantly, we show that a more standard double-sided feedback scheme is not appropriate to cooperate with a Fano-mirror system.

Quantum correlations in four-wave-mixing from ensembles of cold two-level atoms may prevail without filtering over background light with well-known classical interpretations, such as Rayleigh scattering, as recently experimentally demonstrated in Phys. Rev. Lett. 128, 083601 (2022). Here we provide an extended investigation of this effect, in which we detail the experimental procedure and the variation of the quantum correlation with various parameters of the system. Particularly, we show that the decay rate of the quantum correlations changes with the number of atoms in the sample, providing another indication of its superradiance-like nature. The nonclassical aspects of the signal occur for short timescales, but the long timescales carry as well a lot of information on the classical correlations of the system. This slow classical regime presents also two clearly distinct timescales, which we explain by two different pathways for the creation of biphotons. From the global analysis of the data in all its timescales, we are able to derive an empirical expression to fit the data, resulting in information on, among other parameters, the sample's temperature and superradiant-like acceleration. In general, the reported quantum correlations present a dependence on critical parameters of the system, such as optical depth and excitation power, that is quite different from other systems used for biphoton generation, and are more robust to changes in these parameters. This opens the possibility of exploring this process for efficient generation of narrow-band biphotons or of other quantum-correlated photonic states of higher order.

Optical-microwave quantum correlations are pivotal for hybrid quantum network construction. Here, we propose a scheme to achieve nonreciprocal optical-microwave steering via Bogoliubov cooling in a cavity optomagnonic system, where optical and microwave modes couple to magnons in a YIG sphere through optomagnonic and magnetic dipole interactions, respectively. By selectively applying bias magnetic fields to the YIG sphere in different directions, Kerr nonlinearity induces nonreciprocal Bogoliubov cooling effects, thereby engendering highly nonreciprocal steering. Our study presents a novel avenue for achieving nonreciprocal steering between optical and microwave fields, with potential applications in chiral quantum networking.

Active atomic clocks are predicted to provide far better short-term stability and robustness against thermal fluctuations than typical feedback-based optical atomic clocks. However, continuous laser operation using an ensemble of clock atoms still remains an experimentally challenging task. We study spatial self-organization in a transversely driven ensemble of clock atoms inside an optical resonator and coherent light emission from the cavity. We focus on the spectral properties of the emitted light in the narrow atomic linewidth regime, where the phase coherence providing frequency stability is stored in the atomic dipoles rather than the cavity field. The atoms are off-resonantly driven by a standing-wave coherent laser transversely to the cavity axis, allowing for atomic motion along the cavity axis as well as along the pump. In order to treat larger atom numbers we employ a second-order cumulant expansion which allows us to calculate the spectrum of the cavity light field. We identify the self-organization threshold where the atoms align themselves in a checkerboard pattern, thus maximizing light scattering into the cavity, which simultaneously induces cooling. For a larger driving intensity, more atoms are transferred to the excited state, reducing cooling but increasing light emission from the excited atoms. This can be enhanced via a second cavity mode at the atomic frequency spatially shifted by a quarter wavelength. For large enough atom numbers we observe laser-like emission close to the bare atomic transition frequency.

An analytical study of a feedback-driven quantum engine based on a two-level system is presented. This engine is similar to a quantum Otto engine but substitutes the quantum isochoric process with a feedback-driven operation based on a measured observable $A\left(\theta \right)$, whose eigenstates are coherent superpositions of energy eigenstates. Projective measurements control the feedback operations, while work extraction or heat absorption occurs during driven operations. We derive the general time-dependent efficiency, power, and power fluctuations. Quantum coherence is examined through three mechanisms: (i) coherence associated with $\theta \ne 0$, (ii) residual coherence after incomplete thermalization, and (iii) coherence generated by the interaction between the system and a coherent laser pulse on resonance. We investigate how the parameter $\theta$, quantum coherence, and timing of unitary and feedback-driven strokes influence engine performance. We demonstrate that quantum coherence can significantly enhance power output and efficiency while reducing fluctuations through optimal cycle parameter adjustments.

The optimal measurement that discriminates nonorthogonal quantum states with fixed rates of inconclusive outcomes (FRIO) can be decomposed into an assisted separation of the inputs, yielding conclusive and inconclusive outputs, followed by a minimum-error (ME) measurement for the conclusive ones (standard FRIO) or both ones (concatenated FRIO). The implementation of these measurements is underpinned by quantum resources, and here we investigate coherence based on positive operator-valued measures (POVMs) as a resource for both strategies in discriminating equally probable symmetric states of arbitrary dimension. First, we show that the POVM coherence in the assisted separation stage decomposes into the coherence of the ancillary state and the quantum discord between the system and the ancilla, evidencing coherence as a more elementary resource than quantum correlations. Next, it is demonstrated that the POVM coherence for standard and concatenated FRIO decomposes into the POVM coherence measures for state separation and ME measurement, weighted by the probabilities of occurrence of each event. Due to the ME discrimination of inconclusive states, the coherence required for the concatenated scheme is shown to be greater than that of the standard one. We discuss other general aspects of our results by characterizing the POVM coherence in the discrimination of qutrit states, with respect to the distinguishability of the inputs and the inconclusive rate. Finally, by exploiting POVM-based coherence as a quantifier of cryptographic randomness gain, we discuss the standard and concatenated FRIO strategies from the perspective of generating random bits that are secret to an eavesdropper.

We show that the teleportation protocol can be efficiently used to detect quantum critical points using finite-temperature data even if all resources needed for its implementation lie within the system under investigation. Contrary to a previous proposal, there is no need to use an external qubit as the input state to be teleported to one of the qubits within the system. Here we use a pair of nearest-neighbor spins from an infinite spin-$\frac{1}{2}$ chain in equilibrium with a heat bath as the entangled resource of the quantum-teleportation protocol and a third adjacent qubit within the chain itself as the input state to be teleported. For several spin-chain models subjected to an external magnetic field, we show that the efficiency of the teleportation protocol is severely affected as we cross the quantum critical points associated with those spin chains. This abrupt change in efficiency gives us a clear indication of a quantum phase transition.

Nonstabilizerness (“magic”) is an essential part of quantum computation, so the quantitative study of nonstabilizerness has become a very important issue. Since researchers have defined the Pauli spectrum of quantum states, they can use it to calculate other nonstabilizerness measures such as stabilizer Rényi entropy and stabilizer nullity. At the same time, the study of the magic-channel resource theory has become a main direction in recent years. In this article we extend the definition of the Pauli spectrum to quantum channels and define the definitions of stabilizer Rényi entropy and stabilizer nullity for quantum channels based on the Pauli spectrum. Meanwhile, we study the properties of stabilizer Rényi entropy and stabilizer nullity of quantum channels and give the corresponding proofs. Additionally, this article gives the necessary and sufficient condition that the depolarizing channel is a completely stabilizer-preserving operation (CSPO), and we can calculate other channels. Furthermore, we verify that stabilizer Rényi entropy of quantum channels satisfies the amortization inequality so that the maximum value of the distillation bounds of the magic channels provided by the stabilizer Rényi entropy can be established. Therefore, we can give an alternative method to calculate the magic-channel distillation bounds. Moreover, we compare the channel distillation rate provided by stabilizer Rényi entropy with that provided by thauma and find that the channel distillation rate proposed in this article has higher distillation efficiency. Finally, the nonstabilizerness cost of the gate synthesis is given.

Recent studies have shown that quantum information may be effectively transmitted by a finite collection of completely depolarizing channels in a coherent superposition of different orders, via an operation known as the quantum $\mathtt{SWITCH}$. Such results are quite remarkable, as completely depolarizing channels taken in isolation and in a definite order can only output white noise. For general channels, however, little is known about the potential communication enhancement provided by the quantum $\mathtt{SWITCH}$. In this work, we define an easily computable quantity ${\mathcal{P}}_n$ associated with the quantum $\mathtt{SWITCH}$ of $n$ copies of a fixed channel, and we conjecture that ${\mathcal{P}}_n>0$ is both a necessary and sufficient condition for communication enhancement via the quantum $\mathtt{SWITCH}$. In support of our conjecture, we derive a simple analytic expression for the classical capacity of the quantum $\mathtt{SWITCH}$ of $n$ copies of an arbitrary Pauli channel in terms of the quantity ${\mathcal{P}}_n$, which we then use to show that our conjecture indeed holds in the space of all Pauli channels. Utilizing such results, we then formulate a communication protocol involving the quantum $\mathtt{SWITCH}$ which enhances the private capacity of the BB84 channel.

Quantum coherence of collective states plays a pivotal role in understanding many-body cooperative behaviors. Collective coherences between the ground state and excited states in potassium atoms were previously revealed; however, the coherence between collectively excited states has hitherto been overlooked, and its observation has remained challenging. Here we present a joint experimental and theoretical investigation of coherence effects in K atomic vapor using two-dimensional coherent spectroscopy. By collecting zero-quantum, double-quantum, and double-quantum–zero-quantum two-dimensional spectra, distinct coherence signatures are spectroscopically accessible. The experimental spectra are well reproduced by numerically solving the time dependence of the Lindblad equation, and further analyses unveil nonradiative coherence as the origin of the observations. Our work provides comprehensive insight into the collective coherence effect in multiatom systems.

Solid's harmonic spectra can be controlled and optimized by the driving laser pulse; especially the laser's chirp apparently affects the harmonic waveform. In this work, we investigate the harmonic spectrum structure and frequency modulation of ZnO crystal under the influence of the laser pulses with a small chirp. It is found that by fine-tuning the laser chirp, not only the monochromaticity of the harmonic spectrum can be improved, eliminating the subpeak structures in the ZnO harmonic spectrum, but also the double-peak or triple-peak structure of the harmonics can be generated in the ZnO crystals with the electronic population almost unchanged. Through the analysis of the harmonic intensity and phase contributed by the rising and falling parts of the laser pulse, we elucidate the reasons behind the harmonic modulation of the ZnO crystal under the laser pulses with a small chirp. Leveraging the chirped pulse-induced frequency shifts in harmonic emission from the rising and falling parts of the laser, we further manipulate the harmonic spectrum structure of the ZnO crystal. This study provides insights into solid harmonic regulation, facilitating the optimization of solid spectral structures.

We theoretically demonstrate the existence of subcycle solitons in a medium with several optical resonances, with the soliton duration smaller than the periods of all resonant transitions in the medium. We show that the evolution of the electric field in this case is governed by the generalized sine-Gordon equation. We find the solitary-wave solutions of this equation both in the case of a conservative and dissipative optical medium. It is shown that stable subcycle solitons in general can only exist in an initially preexcited medium with nonresonant losses. In this case, half-cycle dissipative solitons can be formed and steadily propagate in the resonant medium. In contrast, the stable conservative solitary waves are obtained to only exist at integer ratios of the transition dipole moments of all resonances that are involved.

Scalable quantum technologies may be applied in prospective architectures employing traditional information processing elements, such as transistors, rectifiers, or switches modulated by low-power inputs. In this respect, recently developed quantum processors based, e.g., on superconducting circuits may alternatively be employed as the basic platform for ultra-low-power consumption classical processors, in addition to obvious applications in quantum information processing and quantum computing. Here we propose a single-photon microwave switch based on a circuit quantum electrodynamics setup, in which a single control photon in a transmission line is able to switch on or off the propagation of another single photon in a separate line. The performances of this single-photon switch are quantified in terms of the photon flux through the output channel, providing a direct comparison of our results with available data. Furthermore, we show how the design of this microwave switch enables the recovery of the single control photon after the switching process. This proposal may be readily realized in state-of-art superconducting circuit technology.

We show nonreciprocal light propagation for single-photon inputs due to quantum noise in coupled optical systems with gain and loss. We consider two parity-time $\left(\mathcal{PT}\right)$-symmetric linear optical systems consisting of either two directly coupled resonators or two finite-length waveguides evanescently coupled in parallel. One resonator or waveguide is filled with an active gain medium and the other with a passive loss medium. The light propagation is reciprocal in such $\mathcal{PT}$-symmetric linear systems without quantum noise. We show here that light transmission becomes nonreciprocal when we include quantum noises in our modeling, which is essential for a proper physical description. The quantum nonreciprocity is especially pronounced in the $\mathcal{PT}$ broken phase. Transmitted light intensity in the waveguide of incidence is asymmetric for two waveguides, even without noise. Quantum noise significantly enhances such asymmetry in the broken phase.