Quantum illumination uses quantum correlations to improve the detection of objects in the presence of background noise. A simple and common practical approach to quantum illumination is to use threshold detectors to measure the idler mode. We investigate the optimization of these direct measurement protocols. Surprisingly, we find that there can be an advantage to having a signal detector whose quantum efficiency is significantly less than perfect, one that does not vanish for low object reflectivities or signal strengths. We also show that decreasing the separation between pulses, while keeping the rate of photons transmitted per second fixed, can improve the performance, under appropriate conditions. We further show that postselecting on the idler detector firing is less efficient at object detection than not postselecting.

We have developed a method for which a variety of reactive scattering processes involving the ${\mathrm{H}}_2$ reaction complex can be studied using the same set of potential curves and couplings. The method is based on a close-coupling approach in a strict diabatic representation. By rigorously incorporating nonadiabatic couplings among bound states, we enable the computation of final-state distributions. Loss into the ionization continuum is accounted for with a nonlocal complex potential matrix. The method has successfully been applied in studies of ${\mathrm{H}}^{+}+{\mathrm{H}}^{-}$ mutual neutralization and $\mathrm{H}\left(1s\right)+\mathrm{H}\left(ns\right)$ associative ionization. In this paper, we investigate the applicability of this method to dissociative recombination and resonant ion-pair formation in electron collisions with ${\mathrm{HD}}^{+}$. The importance of a nonlocal description of autoionization is demonstrated. Calculated cross sections and final-state distributions are compared with results from experiments and previous theoretical studies.

We analyze the radiative forces between two dielectric nanospheres mediated via the quantum and thermal fluctuations of the electromagnetic field in the presence of an external drive. We generalize the scattering theory description of fluctuation forces to include external quantum fields, allowing them to be in an arbitrary quantum state. The known trapping and optical binding potentials are recovered for an external coherent state. We demonstrate that an external squeezed vacuum state creates similar potentials to a laser, despite its zero average intensity. Moreover, Schrödinger cat states of the field can enhance or suppress the optical potential depending on whether they are odd or even. Considering the nanospheres trapped by optical tweezers, we examine the total interparticle potential as a function of various experimentally relevant parameters, such as the field intensity, polarization, and phase of the trapping lasers. We demonstrate that an appropriate set of parameters could produce mutual bound states of the two nanospheres with potential depth as large as $\sim 200\mathrm{K}$. Our results are pertinent to ongoing experiments with trapped nanospheres in the macroscopic quantum regime, paving the way for engineering interactions among macroscopic quantum systems.

We consider ionization of the hydrogen atom by fast electron impact in the presence of a laser field with circular or linear polarization, in the first-order Born approximation in the scattering potential. We use a semiperturbative approach in which the interaction of the laser field with the incident and outgoing electrons is treated nonperturbatively by Gordon-Volkov wave functions, while the laser-atom interaction is treated in first-order perturbation theory. Analytical expressions, in a closed form, for the direct and exchange atomic transition amplitudes are employed. A simplified formula of the total laser-assisted triple-differential cross section (TDCS) is obtained at low values of the photon energy and small momentum of the residual ion. We study the role of laser field polarization and provide a comparative analysis of TDCSs for circularly and linearly polarized laser fields as a function of polar angles of the outgoing electrons at different photon energies. An asymmetric noncoplanar scattering geometry is examined, in which the polar and azimuthal angles of the scattered electron are fixed, while the ejected electron is detected at different angles. The numerical results for TDCSs by a circularly polarized laser field are compared to those derived by a linearly polarized laser field and notable differences are found in both magnitude and angular distributions of the TDCSs.

The study of chiral molecules using Compton scattering is an area that holds promise for further exploration. The current paper presents a preliminary and relatively simple discussion on the possibility of applying Compton ionization to enantiomeric recognition. By focusing on two different types of chiral hydrogen, we find that the momentum spectrum of Compton electrons can encode chiral features, whereas the light-scattering spectra may not necessarily do so. In particular, the momentum spectrum can also exhibit chirality. These results could provide valuable insights and recommendations for the investigation of actual chiral molecules.

We have observed the decoherence of a lithium atomic wave during its propagation in the presence of the radiation emitted by tungsten-halogen lamps, i.e., decoherence induced by blackbody radiation. We used our atom interferometer to detect this decoherence by measuring the atom fringe-visibility loss. The absorption of a photon excites the atom, which spontaneously emits a fluorescence photon. The momenta of these two photons have random directions, and this random character is the main source of decoherence. All previous similar experiments used small-bandwidth coherent excitation by a laser, whereas incoherent radiation involves several technical and conceptual differences. Our approach is interesting as blackbody radiation is omnipresent and decoherence should be considered if particles resonant to electromagnetic fields are used.

Accurate temperature measurement is critical in many scientific and engineering fields, so that researchers continuously strive to improve the accuracy, sensitivity, and robustness of the current measurement methods. In this paper, we propose a theoretical approach for temperature measurement using an optomechanical system in which the position of a mechanical oscillator is coupled to the cavity field. Our approach enables precise control and manipulation of both, resulting in highly accurate temperature measurements. We evaluate the accuracy of temperature estimation by using classical and quantum Fisher information, considering both open and closed systems, and investigate entanglement effects of the primary field mode. Our findings indicate that increasing entanglement at the input made reduces measurement time and increases sensitivity in estimating the temperature. However, we observe that quantum coherence is destroyed by decoherence, leading to reduced performance of quantum systems. Furthermore, we show that the Fisher information of the system is robust against mechanical decoherence, but significantly damped due to optical decoherence. We discuss the limitations and challenges of our method and suggest possible applications and future directions for our research. Finally, we determine the accuracy of temperature estimation for a typical optomechanical system based on phase values measured in the closed system. Our results demonstrate the potential of optomechanical systems for highly accurate temperature measurement and their robustness against decoherence. This study can provide insights into the field of temperature measurement, offering a theoretical approach that can be applied in many scientific and engineering applications.

The aim of the DeLLight (deflection of light by light) experiment is to observe optical nonlinearity in vacuum, as predicted by quantum electrodynamics, by measuring the refraction of a low-intensity focused laser pulse (probe) when crossing the effective vacuum index gradient induced by a high-intensity focused laser pulse (pump). The deflection signal is amplified by using a Sagnac interferometer. Here, we report a measurement performed with the DeLLight pilot interferometer, of the deflection of light by light in air, with a low-intensity pump. We show that the deflection signal measured by the interferometer is amplified, and is in agreement with the expected signal induced by the optical Kerr effect in air. Moreover, we verify that the signal varies as expected as a function of the pump intensity, the temporal delay between the pump and the probe, and their relative polarization. These results represent a proof of concept of the DeLLight experimental method based on interferometric amplification.

The initial states of a quantum system can significantly influence its future dynamics, especially in non-Markovian quantum processes due to the environmental memory effects. Based on a previous work of ours, we propose a method to quantify the memory effects of a non-Markovian quantum process conditioned on a particular system initial state. With this method, we analytically study the early-time memory effects of a superradiance model consisting of $N$ atoms (the system) interacting with a single-mode vacuum cavity (the environment) with two types of initial states: the Dicke states and the factorized identical states. We find that the radiation intensity are closely related to the memory effects, and correspondingly, the enhancement of memory effects (from independent radiation to collective radiation) is important for the degree of superradiance, especially for the Dicke states. Furthermore, numerical results in other regimes show that the characteristics of memory effects and superradiance in longer-time dynamics can be reflected through its early-time dynamics.

A quantum system which can store energy, and from which one can extract useful work, is known as a quantum battery. Such a device raises interesting issues surrounding how quantum physics can provide certain advantages in the charging, energy storage, or discharging of the quantum battery as compared to their classical equivalents. However, the pernicious effect of dissipation degrades the performance of any realistic battery. Here, we show how one can circumvent this problem of energy loss by proposing a quantum battery model which benefits from quantum squeezing. Namely, charging the battery quadratically with a short temporal pulse induces a hyperbolic enhancement in the stored energy, such that the dissipation present becomes essentially negligible in comparison. Furthermore, we show that when the driving is strong enough, the useful work which can be extracted from the quantum battery, that is the ergotropy, is exactly equal to the stored energy. These impressive properties imply a highly efficient quantum energetic device with abundant amounts of ergotropy. Our theoretical results suggest a possible route to realizing high-performance quantum batteries, which could be realized in a variety of platforms exploiting quantum continuous variables.

We show via explicit construction that all the extreme rays of both the three-party quantum entropy cone and the four-party stabilizer entropy cone can be obtained from subsystem coarse grainings of specific higher-party quantum states, namely, extreme states characterized by saturating a (nontrivial) maximal set of instances of subadditivity. This suggests that the study of the “subadditivity cone,” and the set of its extreme rays realizable in quantum mechanics, provides a powerful tool for deriving inner bounds for the quantum and stabilizer entropy cones, as well as constraints on new inequalities for the von Neumann entropy.

A protocol for regulating the distribution of quantum information between multiple parties is put forward. To prohibit the unrestricted distribution of quantum-resource states in a public quantum network, agents can apply a resource-destroying map to each sender's channel. Since resource-destroying maps only exist for affine quantum resource theories, censorship of a nonaffine resource theory is established on an operationally motivated subspace of free states. This is achieved by using what we name a resource-censoring map. The protocol is applied to censoring coherence, reference frames, and entanglement. Because of the local nature of the censorship protocol, it is, in principle, possible for collaborating parties to bypass censorship. Thus, we additionally derive necessary and sufficient conditions under which the censorship protocol is unbreakable.

We study the squeezing dynamics in a Kerr-nonlinear oscillator, and quantify the metrological usefulness of the resulting states. Even if the nonlinearity limits the attainable squeezing by making the evolution non-Gaussian, the states obtained still have a high quantum Fisher information for sensing displacements. However, contrary to the Gaussian case, the amplitude of the displacement cannot be estimated by simple quadrature measurements. Therefore, we propose the use of a measurement-after-interaction protocol where a linear quadrature measurement is preceded by an additional nonlinear evolution, and show the significant sensitivity enhancement that can be obtained. Our results are robust when considering realistic imperfections such as energy relaxation, and can be implemented in state-of-the-art experimental setups.

We use a random search technique to find quantum gate sequences that implement perfect quantum state preparation or unitary operator synthesis with arbitrary targets. This approach is based on the recent discovery that there is a large multiplicity of quantum circuits that achieve unit fidelity in performing a given target operation, even at the minimum number of single-qubit and two-qubit gates needed to achieve unit fidelity. We show that the fraction of perfect-fidelity quantum circuits increases rapidly as soon as the circuit size exceeds the minimum circuit size required for achieving unit fidelity. This result implies that near-optimal quantum circuits for a variety of quantum information processing tasks can be identified relatively easily by trying only a few randomly chosen quantum circuits and optimizing their parameters. In addition to analyzing the case where the cnot gate is the elementary two-qubit gate, we consider the possibility of using alternative two-qubit gates. In particular, we analyze the case where the two-qubit gate is the $B$ gate, which is known to reduce the minimum quantum circuit size for two-qubit operations. We apply the random search method to the problem of decomposing the four-qubit Toffoli gate and find a 15-cnot-gate decomposition.

Quantum teleportation in the presence of noisy channels acting on a bipartite resource state is considered. We consider a family of generalized noisy channels that continuously connect the amplitude damping and the dephasing channels, encompassing a wide family of in-between scenarios, to delve into the relation between the teleportation success and the amount of three- and four-partite entanglement (distributed among the qubits of the resource state and those representing local environments) generated during the evolution. Our analysis reveals that for a fixed entanglement of the resource state, the channels that better protect the teleportation fidelity against the detrimental effects of noise are those that generate higher amounts of (Greenberger-Horne-Zeilinger–type) multipartite entanglement. This suggests that the dynamically induced multipartite correlations may serve as an additional resource for teleportation and provides insight into the characterization of processes and of the type of induced entanglement according to their ability to assist the protocol.

Efficient and reliable verification of quantum states is central to quantum information processing applications. If using well-characterized measurement devices, effective methods have been developed for quantum state verification (QSV). In reality, however, measurement devices are generally imperfect or untrusted, which limits significantly the application of standard QSV protocols. Here, we propose the measurement-device-independent QSV (MDI-QSV) scheme for practice. With the help of trusted quantum inputs, we have developed a systematical approach to design MDI-QSV strategies for an arbitrary pure target state. We find that the number of required measurements has an optimal scaling with required accuracy and confidence level, similar to the standard QSV where trusted measurement devices are available. Our results offer a sample-efficient and realistic method for quantum state verification with virtues of a measurement-device-independent manner, and are within reach of current technology.

Quantum resources may provide an advantage over their classical counterparts. Theoretically, in certain tasks, this advantage can be very high. In this work, we construct such a task based on a game, mediated by the Referee and played between Alice and Bob. The Referee sends Alice a value of a random variable. At the same time, the Referee also sends Bob some partial information regarding that value. Here partial information can be defined in the following way. Bob gets the information of a random set that must contain the value of the variable, which is sent to Alice by the Referee, along with other value(s). Alice is not allowed to know what information is sent to Bob by the Referee. Again, Bob does not know which value of the random variable is sent to Alice. Now, the game can be won if and only if Bob can unambiguously identify the value of the variable that is sent to Alice, with some nonzero probability, no matter what information Bob receives or which value is sent to Alice. However, to help Bob, Alice sends some limited amount of information to him, based on any strategy that is fixed by Alice and Bob before the game begins. We show that if Alice sends a limited amount of classical information, then the game cannot be won, while the quantum analog of the “limited amount of classical information” is sufficient for winning the game. Thus, it establishes a quantum advantage. We further analyze several variants of the game and provide certain bounds on the success probabilities. Moreover, we establish connections between the trine ensemble, mutually unbiased bases, and the encoding-decoding strategies of those variants. We also discuss the role of quantum coherence in the present context.

Single-photon interference is the essential key for the recently well-known twin-field quantum key distribution (TF-QKD) to break the linear rate-distance limit and requires beams with identical polarizations. Inspired by this fact and aiming to improve the secret key rate, we propose a hybrid high-dimensional QKD named the interfering-or-not-interfering QKD (INI-QKD), in which both the polarization and phase degrees of freedom (DOFs) are adopted as information carriers. The protocol's security proof is analyzed based on entanglement distillation and three defined effective events, $X_1$, $X_2$, and $X_3$. The simulation shows that the $X_1$ event, from which only the phase information is extracted, exceeds TF-QKD's variants, while the $X_2$ and $X_3$ events, in which both types of information are decoded, can achieve twice the secret key rate as measurement-device-independent QKD (MDI-QKD). It is also proven that by adding the polarization DOF, INI-QKD obtains more resistance to phase mismatch than TF-QKD. Remarkably, these can all be achieved by simply altering the TF-QKD's measurement setup to that of MDI-QKD.