We construct optimal measurements, achieving the ultimate precision predicted by quantum theory, for the simultaneous estimation of centroid, separation, and relative intensities of two incoherent point sources using a linear optical system. We discuss the physical feasibility of the scheme, which could pave the way for future practical implementations of quantum-inspired imaging.

Understanding out-of-equilibrium quantum dynamics is a critical outstanding problem, with key questions regarding characterizing adiabaticity for applications in quantum technologies. We show how the metric-space approach to quantum mechanics naturally characterizes regimes of quantum dynamics and provides an appealingly visual tool for assessing their degree of adiabaticity. Further, the dynamic trajectories of quantum systems in metric space suggest a lack of ergodicity, thus providing a better understanding of the fundamental one-to-one mapping between densities and wave functions.

Quantum cellular automata are important tools in understanding quantum dynamics, thanks to their simple and effective list of rules. Here we consider a class of noisy, one-dimensional quantum cellular automata that allow one to shift from unitary dynamics to completely positive maps, investigating the evolution of coherence as well as the notion of reversibility in such a setting. To this aim, we associate an approximate reverse automaton to each noisy automaton, and assess its effect, and we define an irreversibility time based on the distance from the maximally mixed state, which is shown to be the only attractor of the automaton map in the presence of dephasing. Our analysis illustrates the interplay between unitary and noisy dynamics.

We provide an analysis on the critical property of quantum coherence in XY spin systems based on the quantum renormalization group theory. We find that the quantum coherence obeys a conservation relation, i.e., the quantum coherence of a three-block state that is the sum of its reduced state coherence. The quantum coherence of the whole block state and its reduced state obey similar scaling properties with identical quantum coherence exponent 0.999. This means that the quantum critical behavior of a system may gain from its reduced state. With the unitary operator approaches, we evaluated the evolutionary characteristics of the large-scale system and illustrate its behavior with different initial states. The result shows that quantum coherence periodically fluctuates over time. The period decreases under the system size increasing.

We present a technique for the synthesis of ultrashort laser pulses with approximately one cycle (FWHM) of temporal duration. These pulses are characterized by a certain degree of chirp. We show that these pulses produce both an enhancement of the high-order harmonic generation (HHG) cutoff and a noticeable increase of the yield, when interacting with an atomic system. Additionally, the asymmetric nature of the driven pulses plays an important role in the efficiency and cutoff extension of the high-order harmonics generated. Starting from the HHG spectra, we demonstrate it is possible to retrieve isolated attosecond pulses by spectral filtering. The analysis and interpretation of the different characteristics present in the HHG driven by this kind of pulse was carried out invoking classic arguments. Furthermore, a more complete description and validation of the HHG properties is performed by a quantum analysis, based on the integration of the time-dependent Schrödinger equation in full dimensionality.

We theoretically investigate the high-precision estimation of the coupling rate, which can reflect an actual observable, in a hybrid optomechanical system (OMS). In our scheme, apart from an optical cavity, the mechanical vibrator is also coupled to a two-level system (TLS) via a direct dispersive coupling. As for the initial state, we take into account coherent states for both optical and mechanical modes. After evolving a period of the vibrator, the final state then carries the information about the estimated coupling rate. We find that through measuring the internal state of the TLS, the precision can achieve a Heisenberg limit that indicates remarkable improvement of estimation in quantum metrology. This result is obviously shown in a simple analytical expression under the approximate condition, and further analysis demonstrates that the Heisenberg limit can be attained indeed. Moreover, we also study the dependence of the precision on the estimated parameter to reveal the appropriate measurement ranges.

We demonstrate 8.5 dB thermal squeezing of a membrane oscillator using the dynamical backaction effect and electrostatic feedback in an optomechanical membrane-in-the-middle setup. We show that strong squeezing can be obtained even in the far-detuning regime of a sideband-resolved system. By using the dielectrophoretic force of a metallic needle kept in close proximity to the membrane, we implement the one-quadrature active feedback scheme to prevent the divergence of the amplified quadrature and surpass the 3 dB limit of mechanical squeezing. We also discuss different regions of the sideband spectrum where strong squeezing can be obtained. Although the demonstration here is classical, this technique is equally applicable to prepare the mechanical oscillator in a quantum squeezed state.

We formulate a multiple scattering theory of light in media spatially disordered along two directions and homogeneous along the third one without making any paraxial approximation on the wave equation and fully treating the vector character of light. With this formalism, we calculate the distribution of transverse momenta of a beam as it evolves along the optical axis and unveil a phenomenon not captured by the paraxial equation: a crossover from a scalar to a vector regime, visible in the coherent backscattering peak as polarization gets randomized.

In this paper, we investigate the impact of the electromagnetic scattering caused by other objects in nonfree space on the time-averaged force exerted upon a Rayleigh particle, which is conventionally referred to as the backaction effect. We show that backaction modifies the gradient force, radiation pressure, and spin curl force exerted upon Rayleigh particles, and gives rise to an additional force term which stems from the gradient of the backaction field in nonfree space. As a numerical example, we look into the trapping of a dielectric nanoparticle at the center of curvature of a spherical mirror, and study how it is affected by the backaction effect. We show that backaction can enhance the force exerted upon the particle, reshape the trapping potential, and shift the equilibrium position of the particle.

High-precision radio-frequency, microwave, and infrared spectroscopic measurements of the antihydrogen molecular ion ${\overline{\text{H}}}_2^{-}$ ($\overline{p}\overline{p}{e}^{+}$) compared with its normal matter counterpart provide direct tests of the $CPT$ theorem. The sensitivity to a difference between the positron-antiproton and electron-proton mass ratios and to a difference between the positron-antiproton and electron-proton hyperfine interactions can exceed that obtained by comparing antihydrogen with hydrogen by several orders of magnitude. Practical schemes are outlined for measurements on a single ${\overline{\text{H}}}_2^{-}$ ion in a cryogenic Penning trap that use nondestructive state identification by measuring the cyclotron frequency and bound-positron spin-flip frequency and also for creating an ${\overline{\text{H}}}_2^{-}$ ion and initializing its quantum state.

Adding excitations on a coherent state provides an effective way to observe the nonclassical properties of radiation fields. Here, we describe and analyze how to apply this concept to the motional state of a mechanical oscillator and present a full scheme to prepare non-Gaussian phonon-added coherent states of the mechanical motion in cavity optomechanics. We first generate a mechanical coherent state using electromagnetically induced transparency. We then add a single phonon onto the coherent state via optomechanical parametric down-conversion combined with single-photon detection. We validate this single-phonon-added coherent state by using a red-detuned beam and reading out the state of the optical output field. This approach allows us to verify nonclassical properties of the phonon state, such as sub-Poissonian character and quadrature squeezing. We further show that our scheme can be directly implemented using existing devices, and is generic in nature and hence applicable to a variety of systems in opto- and electromechanics.

Quantum illumination is a technique for detecting the presence of a target in a noisy environment by means of a quantum probe. We prove that the two-mode squeezed vacuum state is the optimal probe for quantum illumination in the scenario of asymmetric discrimination, where the goal is to minimize the decay rate of the probability of a false positive with a given probability of a false negative. Quantum illumination with two-mode squeezed vacuum states offers a 6 dB advantage in the error probability exponent compared to illumination with coherent states. Whether more advanced quantum illumination strategies may offer further improvements had been a longstanding open question. Our fundamental result proves that nothing can be gained by considering more exotic quantum states, such as, e.g., multimode entangled states. Our proof is based on a fundamental entropic inequality for the noisy quantum Gaussian attenuators. We also prove that without access to a quantum memory, the optimal probes for quantum illumination are the coherent states.

The generation of steady quantumness in the presence of an environment is of utmost importance if we are to build practical quantum devices. We propose a scheme of generating steady coherence and magic in a qubit system attached to a heat bath at a certain temperature through its interaction with another qubit system attached to a spin bath. Coherence generation in the reduced qubit is always possible in this model. The steady coherence in the reduced qubit attached to the heat bath may be used to enhance the subsequent transient performance of a quantum absorption refrigerator. For the case of generation of magic, which is the quantum resource responsible for implementation of gates which are not simulatable via stabilizer computation, we show that there exists a critical temperature of the heat bath beyond which it is not possible to create magic in the reduced qubit attached to the heat bath. Below the critical temperature, the strength of interaction between the qubits must lie within a certain region for the creation of magic. We further note that by increasing the strength of coupling of the second qubit to the spin bath, typified by the reset probability, keeping the coupling strength of the first qubit to the heat bath, it is possible to increase the critical temperature of the heat bath for the creation of magic.

Dynamically correcting for unwanted interactions between a quantum system and its environment is vital to achieving the high-fidelity quantum control necessary for a broad range of quantum information technologies. In recent work, we uncovered the complete solution space of all possible driving fields that suppress transverse quasistatic noise errors while performing single-qubit phase gates. This solution space lives within a simple geometrical framework that makes it possible to obtain globally optimal pulses subject to a set of experimental constraints by solving certain geometrical optimization problems. In this work, we solve such a geometrical optimization problem to find the fastest possible pulses that implement single-qubit phase gates while canceling transverse quasistatic noise to second order. Because the time-optimized pulses are not smooth, we provide a method based on our geometrical approach to obtain experimentally feasible smooth pulses that approximate the time-optimal ones with minimal loss in gate speed. We show that in the presence of realistic constraints on pulse rise times, our smooth pulses significantly outperform sequences based on ideal pulse shapes, highlighting the benefits of building experimental waveform constraints directly into dynamically corrected gate designs.

Toric codes and color codes are two important classes of topological codes. Kubica et al. [A. Kubica et al., New J. Phys. 17, 083026 (2015)] showed that any $D$-dimensional color code can be mapped to a finite number of toric codes in $D$ dimensions. We propose an alternate map of three-dimensional (3D) color codes to 3D toric codes with a view to decoding 3D color codes. Our approach builds on Delfosse's result [N. Delfosse, Phys. Rev. A 89, 012317 (2014)] for 2D color codes and exploits the topological properties of these codes. Our result reduces the decoding of 3D color codes to that of 3D toric codes. Bit-flip errors are decoded by projecting on one set of 3D toric codes, while phase-flip errors are decoded by projecting onto another set of 3D toric codes. We use these projections to study the performance of a class of 3D color codes called stacked codes.

The mass spectrum of cationic boron clusters, ${{\mathrm{B}}_N}^{+}$ ($N=5-20$), after photoexcitation demonstrates that radiative cooling is an important, though often neglected, process in determining the relative stability of small and isolated particles. The observed intensities in mass spectra suggest that ${{\mathrm{B}}_5}^{+},{{\mathrm{B}}_{11}}^{+},{{\mathrm{B}}_{13}}^{+}$, and ${{\mathrm{B}}_{15}}^{+}$ are particularly stable clusters, consistent with density-functional theory calculations. Quantitative agreement, however, is only obtained if radiative cooling is included in the analysis. All clusters are found to radiate on microsecond timescales, suggesting recurrent fluorescence as the dominant photon emission process.

We present a combined theoretical and experimental study of primary and postcollision mechanisms involved when colliding low-energy multiply charged ions with van der Waals dimers. The collision dynamics is investigated using a classical calculation based on the Coulombic Over-the-Barrier Model adapted to rare gas dimer targets. Despite its simplicity, the model predictions are found in very good agreement with experimental results obtained using COLd Target Recoil Ion Momentum Spectroscopy, both for the relative yields of the different relaxation processes and for the associated transverse momentum exchange distributions between the projectile and the target. This agreement shows to what extent van der Waals dimers can be assimilated to independent atoms.