Multilevel quantum systems lose coherence due to quantum jumps. One example is the spontaneous decay between the internal levels of an atom. In this work, we propose and implement a way to simulate experimentally a three-level system under a quantum jump using a three-mode photonic system. We simulated three different dynamics of spontaneous decay in a three-level atomic system: cascade decay, $\mathrm{\Lambda }$ decay, and $V$ decay. With an attenuated light coherent source at photon level, we have prepared photons in a three-path superposition state, which was encoded in parallel Gaussian modes. By programming a spatial light modulator (SLM) for producing periodical phase modulation, we were able to implement the corresponding dynamical maps for quantum jumps in terms of the Kraus operator decomposition. Our results have verified experimentally the variation of the population levels and the decoherence effects caused by quantum jumps in the three decay configurations of the three-level system.

It is challenging to observe the superbunching effect with true chaotic light. Here we propose and demonstrate a method to achieve the superbunching effect of $g^{\left(2\right)}\left(0\right)=2.42\pm 0.02$ with broadband stationary chaotic light based on a cascaded Michelson interferometer (CMI), exceeding the theoretical upper limit of 2 for the two-photon bunching effect of chaotic light. The superbunching correlation peak is measured with an ultrafast two-photon absorption detector whose full width at half-maximum reaches about 95 fs. Two-photon superbunching theory in a CMI is developed to interpret the effect and is in agreement with experimental results. The theory also predicts that the degree of second-order coherence can be much greater than 2 if chaotic light propagates $N$ times in a CMI. Finally, a type of weak signal detection setup that employs broadband chaotic light circulating in a CMI is proposed. Theoretically, it can increase the detection sensitivity of weak signals 79 times after the chaotic light circulates 100 times in the CMI.

The process of emission of electromagnetic radiation does not occur instantaneously, but is “formed” over a finite time known as the radiation formation time. In the ultrarelativistic regime, the corresponding (longitudinal) formation length is given by the formation time times the speed of light and controls several features of radiation. Here, we elucidate the importance of the transverse formation length (TFL) by investigating nonlinear Compton scattering by an electron initially counterpropagating with respect to a flying focus laser beam. The TFL is related to the transverse size of the radiation formation “volume” and, unlike the longitudinal formation length, has a quantum origin. Since the TFL is typically of the order of the Compton wavelength, where any laser field can be assumed to be approximately uniform, related quantum interference effects have been ignored. However, we show analytically that if the focus in a flying focus beam with $n_L\gg 1$ cycles moves at the speed of light and backwards with respect to the beam propagation direction, the effects of the TFL undergo a large enhancement proportional to $n_L$ and may substantially alter the differential emission probability for feasible flying focus pulses.

The explicit expression for the inclusive probability to record a photon created in transition radiation from a one-Dirac-particle wave packet traversing an ideally conducting plate is derived in the leading order of perturbation theory. The anomalous magnetic moment of the Dirac particle is taken into account. It is shown that the quantum corrections to transition radiation from an electrically charged particle give rise to production of photons with polarization vector orthogonal to the reaction plane ($E$ plane). These corrections result from both the quantum recoil and the finite size of a wave packet. As for the transition radiation produced by a neutron falling normally onto the conducting plate, the probability to detect a photon with polarization vector lying in the reaction plane does not depend on the observation angle and the energy of the incident particle. The peculiarities of transition radiation stemming from different shapes of the particle wave packet are investigated. In particular, the transition radiation produced by the wave packet of one twisted Dirac particle is described. A comparison with the classical approach to transition radiation is given and the quantum formula for the inclusive probability to detect a photon radiated by the $N$-particle wave packet is derived.

Free-space channels provide the possibility of establishing continuous-variable quantum key distribution in global communication networks. However, the fluctuating nature of transmissivity in these channels introduces an extra noise which reduces the achievable secret key rate. We consider two classical postprocessing strategies, postselection of high-transmissivity data and data clusterization, to reduce the fluctuation-induced noise of the channel. We undertake the investigation of such strategies utilizing a composable security proof in a realistic finite-size regime against both collective and individual attacks. We also present an efficient parameter estimation approach to estimate the effective Gaussian parameters over the postselected data or the clustered data. Although the composable finite-size effects become more significant with the postselection and clusterization both reducing the size of the data, our results show that these strategies are still able to enhance the finite-size key rate against both individual and collective attacks with a remarkable improvement against collective attacks, even moving the protocol from an insecure regime to a secure regime under certain conditions.

Optical homodyne detection has been widely used in continuous-variable (CV) quantum information processing for measuring field quadrature. In this paper we explore the possibility of operating a conjugate homodyne detection system in “photon counting” mode to implement discrete-variable (DV) quantum key distribution (QKD). A conjugate homodyne detection system, which consists of a beam splitter followed by two optical homodyne detectors, can simultaneously measure a pair of conjugate quadratures $X$ and $P$ of the incoming quantum state. In classical electrodynamics, $X^2+P^2$ is proportional to the energy (the photon number) of the input light. In quantum optics, $X$ and $P$ do not commute and thus the above photon-number measurement is intrinsically noisy. This implies that a blind application of standard security proofs of QKD could result in pessimistic performance. We overcome this obstacle by taking advantage of two special features of the proposed detection scheme. First, the fundamental detection noise associated with vacuum fluctuations cannot be manipulated by an external adversary. Second, the ability to reconstruct the photon number distribution at the receiver's end can place additional constraints on possible attacks from the adversary. As an example, we study the security of the BB84 QKD using conjugate homodyne detection and evaluate its performance through numerical simulations. This study may open the door to a family of QKD protocols, complementary to the well-established DV-QKD based on single-photon detection and CV-QKD based on coherent detection.

Due to their long coherence times, nuclear spins have gained considerable attention as physical qubits. Two-qubit gates between nuclear spins of distinct resonance frequencies can be mediated by electron spins, usually employing a sequence of electron-nuclear gates. Here we present a different approach inspired by, but not limited to, NV centers in diamond and discuss possible applications. To this end we generalize external electron spin control sequences for nuclear spin initialization and hyperpolarization to achieve the simultaneous control of distinct nuclear spins via an electron spin. This approach results in efficient entangling gates that, compared to standard techniques, reduce the gate time by more than 50% when the gate time is limited by off-resonant coupling to other spins and by up to 22% when the gate time is limited by small electron-nuclear coupling.

Strong molecule-light interaction enables the control of molecular structures and dynamical processes. A model with a strong laser drive is proposed to greatly enhance the intermolecular distance of resonant energy transfer, where the molecules are strongly driven by an optical cavity. The optimal Rabi frequency and quantum yield of energy transfer are observed, resulting from the trade-off between dipole-dipole interaction and molecule-cavity coupling. When the strong drive at a certain Rabi frequency is applied, a larger spatial range of effective energy transfer and a slower decay rate with the distance compared to the Förster mechanism of resonant energy transfer are observed in our model. Our work sheds light on the spectroscopic study of cooperative energy transfer in molecular polaritons.

Quantum state smoothing is a technique to construct an estimate of the quantum state at a particular time, conditioned on a measurement record from both before and after that time. The technique assumes that an observer, Alice, monitors part of the environment of a quantum system and that the remaining part of the environment, unobserved by Alice, is measured by a secondary observer, Bob, who may have a choice in how he monitors it. The effect of Bob's measurement choice on the effectiveness of Alice's smoothing has been studied in a number of recent papers. Here we expand upon the Letter which introduced linear Gaussian quantum (LGQ) state smoothing [Phys. Rev. Lett. 122, 190402 (2019)]. In the current paper we provide a more detailed derivation of the LGQ smoothing equations and address an open question about Bob's optimal measurement strategy. Specifically, we develop a simple hypothesis that allows one to approximate the optimal measurement choice for Bob given Alice's measurement choice. By “optimal choice” we mean the choice for Bob that will maximize the purity improvement of Alice's smoothed state compared to her filtered state (an estimated state based only on Alice's past measurement record). The hypothesis, that Bob should choose his measurement so that he observes the back-action on the system from Alice's measurement, seems contrary to one's intuition about quantum state smoothing. Nevertheless, we show that it works even beyond a linear Gaussian setting.

There is a controversy of how to interpret interactions of electrons with a large spatial coherence with light and matter. When such an electron emits a photon, it can do so either as if its charge were confined to a point within a coherence length, the region where a square modulus of a wave function ${\left|\psi \right|}^2$ is localized, or as a continuous cloud of space charge spread over it. This problem was addressed in a recent study [R. Remez et al., Phys. Rev. Lett. 123, 060401 (2019)] where a conclusion was drawn in favor of the first (point) interpretation. Here we argue that there is an alternative explanation for the measurements reported in that paper, which relies on a purely classical concept of a so-called prewave zone and does not allow one to refute the second interpretation. We propose an experiment of Smith-Purcell radiation from a nonrelativistic vortex electron carrying orbital angular momentum, which can unambiguously lead to the opposite conclusion. Beyond the paraxial approximation, the vortex packet has a nonpoint electric quadrupole moment, which grows as the packet spreads and results in a nonlinear $L^3$ growth of the radiation intensity with the length $L$ of the grating when $L$ is much larger than the packet's Rayleigh length. Such a nonlinear effect has never been observed for single electrons and, if detected, it would be a hallmark of the nonpoint nature of charge in a wave packet. Thus, two views on ${\left|\psi \right|}^2$ are complementary to each other and an electron radiates either as a point charge or as a continuous charge flow depending on the experimental conditions and on its quantum state. Our conclusions hold for a large class of non-Gaussian packets and emission processes for which the radiation formation length can exceed the Rayleigh length, such as Cherenkov radiation, transition radiation, diffraction radiation, and so forth.

The entanglement structure between different frequency components within broadband quantum light pulses, forged at entanglement creation, represents a promising route to the practical delivery of many multipartite quantum information applications. However, the scalability of such applications is largely limited by the entanglement decoherence caused by photon loss. One promising method to combat such losses is noiseless linear amplification. However, while there have been various procedures that implement noiseless linear amplification on single-mode states, no realization has thus far been proposed for noiseless linear amplification on quantum states carrying a multimode structure. In this work we close this gap, proposing a noiseless linear amplifier (NLA) with photon catalysis (PC), namely, the PC-NLA. Constructing a multimode version of an existing NLA that uses quantum scissors (QS), the QS-NLA, we then show how the PC-NLA is compatible with the QS-NLA, even though the former uses half the physical resources of the latter. We then apply our multimode NLA frameworks to the problem of continuous-variable (CV) entanglement distillation, determining how the multimode structure of the entanglement impacts the performance of the NLAs. Different from single-mode NLA analyses, we find that a multimode NLA is only effective as a CV entanglement distillation strategy when the channel loss is beyond some threshold—a threshold largely dependent on the multimode structure. The results provided here will be valuable for real-world implementations of multipartite quantum information applications that utilize complex entanglement structure within broadband light pulses.

Arrays of neutral-atom qubits in optical tweezers are a promising platform for quantum computation. Despite experimental progress, a major roadblock for realizing neutral-atom quantum computation is the qubit initialization. Here we propose that supersymmetry, a theoretical framework developed in particle physics, can be used for ultrahigh-fidelity initialization of neutral-atom qubits. We show that a single atom can be deterministically prepared in the vibrational ground state of an optical tweezer by adiabatically extracting all excited atoms to a supersymmetric auxiliary tweezer. The scheme works for both bosonic and fermionic atom qubits trapped in realistic Gaussian optical tweezers and may pave the way for realizing large-scale quantum computation, simulation, and information processing with neutral atoms.

We calculate interaction constants for the contributions from $\mathcal{P},\mathcal{T}$-odd scalar-pseudoscalar and tensor-pseudotensor operators to the electric dipole moment of ${}^{129}\mathrm{Xe}$ using relativistic many-body theory including the effects of dynamical electron correlations. These interaction constants are necessary ingredients to relating the corresponding measurements to fundamental parameters in models of physics beyond the standard model. We obtain ${\alpha }_{C_S}=\left(0.71\pm 0.18\right)\left({10}^{-23}e\text{cm}\right)$ and ${\alpha }_{C_T}=\left(0.520\pm 0.049\right)\left({10}^{-20}{〈\mathrm{\Sigma }〉}_{\text{Xe}}e\text{cm}\right)$, respectively. We apply our results to test a phenomenological relation between the two quantities, commonly used in the literature, and discuss their present and future phenomenological impact.

Interatomic Coulombic electron capture (ICEC) is an environment-assisted process in which a free electron can efficiently attach to a quantum system by transferring the excess energy of the electron capture to a neighbor ionizing it. Using the ab initio $R$-matrix method, we show that Fano profiles, resulting from interferences between the ICEC final states and resonant states, appear in the ICEC cross sections even at extremely large system-neighbor separations. We identify several types of resonant states depending on their decay pathways which may involve long-range electron and energy transfer. ICEC is a fundamental process and the interferences lead to substantial enhancement or decrease of the cross sections. The present investigation is therefore of general relevance in many contexts wherever electron capture in an environment takes place.

We check the ab initio $GW$ approximation and Bethe-Salpeter equation (BSE) many-body methodology against the exact solution benchmark of the hydrogen molecule ${\mathrm{H}}_2$ ground state and excitation spectrum, and in comparison with the configuration interaction (CI) and time-dependent Hartree-Fock methods. The comparison is made on all the states we could unambiguously identify from the excitonic wave functions' symmetry. At the equilibrium distance $R=1.4a_0$, the $GW+\mathrm{BSE}$ energy levels are in good agreement with the exact results, with an accuracy of $\sim 0.1\text{–}0.2\mathrm{eV}$. $GW+\mathrm{BSE}$ potential-energy curves are also in good agreement with the CI and the exact result up to $2.3a_0$. The solution no longer exists beyond $3.0a_0$ for triplets ($4.3a_0$ for singlets) due to instability of the ground state. We tried to improve the $GW$ reference ground state by a renormalized random-phase approximation (r-RPA), but this did not solve the problem.

We experimentally investigated the delayed fragmentation processes of ethylene and allene dications induced by 300-eV electrons with the cold target recoil ion momentum spectroscopy in an observation window up to ∼2 μs. For each molecule, several distinct pathways involving prompt or delayed fragmentations were observed and relative branching ratios among them were analyzed. According to the time-of-flight coincidence measurements of corresponding recoil ionic fragments, the survival times and thus lifetimes from nanoseconds to microseconds of different delayed-dissociative precursor dications were retrieved. Besides the two-body channels, i.e., ${\mathrm{H}}^{+}+{\mathrm{C}}_2{{\mathrm{H}}_3}^{+}$ and ${\mathrm{H}}^{+}+{\mathrm{C}}_3{{\mathrm{H}}_3}^{+}$, the delayed deprotonation was also observed in many-body channels, i.e., $\mathrm{H}+{\mathrm{H}}^{+}+{\mathrm{C}}_n{{\mathrm{H}}_2}^{+}$ ($n=2$, 3) and $2\mathrm{H}/{\mathrm{H}}_2+{\mathrm{H}}^{+}+{\mathrm{C}}_3{\mathrm{H}}^{+}$. The lifetimes of the metastable intermediate diacations ${\mathrm{C}}_n{{\mathrm{H}}_3}^{2+}$ and ${\mathrm{C}}_3{{\mathrm{H}}_2}^{2+}$ were estimated.

We present accurate values of the atomic properties that are sensitive to an external electric field in an experiment, such as static electric dipole and quadrupole polarizabilities, hyperpolarizabilities, and quadrupole moments of the low-lying $5s^2{}^{1}S_0$, $5s5p{}^{3}P_{0,1,2}$, and $5s5p{}^{1}P_1$ states of the Cd atom. The four-component relativistic coupled-cluster theory is employed to determine the atomic wave functions and energies. The finite-field approach is adopted to estimate the above quantities from the evaluated energies. The roles of electron correlation effects in the calculations are demonstrated by giving results that are obtained with different approximations. Precise knowledge of the above spectroscopic properties is useful for estimating typical systematics of the optical clocks built using the Cd atoms.

We use path integrals to calculate perturbative corrections to the correlation function of a particle under the action of nonlinear optical tweezers, both in the overdamped and underdamped regimes. In both cases, it is found that to leading order, nonlinearities manifest as shifts in the characteristic frequency of the system. The results are compared to numerical simulations. The present calculations enable a direct experimental method to access the nonlinear optical trap parameters by analyzing position data, similarly to standard harmonic tweezers.

The interference between Compton scattering and nonlinear Compton scattering from a two-color field in the x-ray regime is theoretically examined for bound electrons. The underlying phase shifts are analyzed using a perturbative approach in the incoming classical field. The perturbative approach is bench marked with a nonperturbative approach in the classical field. The interference for different combinations of linear polarization of the two fields is examined when the Compton and the nonlinear Compton scattered waves have the same wave vector and polarization. Only two cases exhibit interference. When there is interference, the calculations reveal an intrinsic phase difference between the Compton scattered wave function and the nonlinear Compton scattered wave function of either 0 or $\pi$ depending on the scattering angle.