Here we provide a theoretical framework to analyze discrete time-crystalline (DTC) phases in the dissipative dipolar systems subjected to a two-pulse excitation scheme. As a particular realization, we choose a quantum many-body system that exhibits prethermalization due to the presence of the quasiconserved quantities. The analysis uses a fluctuation-regulated quantum master equation, which captures the dissipative effects of the drive and dipolar coupling on the dynamics regularized by the thermal fluctuations. We find that the effects of such dissipation lend stability to the dynamics and are directly responsible for the robustness. Specifically, we find that longer fluctuation correlation time enhances the stability of DTC. We also obtain the lifetime of such a robust period-doubling response, a salient feature of the DTC phase, by varying several system parameters. Our results are in good agreement with the recent experimental findings in dipolar systems using nuclear magnetic resonance spectroscopy. Finally, we provide an estimate how the DTC performance degrades with temperature.

We explore a degenerate four-wave mixing process induced by transversely structured light beams in a rubidium vapor cell. In particular, we consider the nonlinear interaction driven by optical modes contained in the orbital angular momentum Poincaré sphere, which can be parametrized in terms of a polar and an azimuthal angle. In this context we investigate the transfer of spatial structure to two distinct four-wave mixing (FWM) signals, possessing different propagation directions in space. We show that under usual assumptions, the output fields can also be described by modes belonging to Poincaré spheres, and that the angles describing the input and output modes are related according to well-defined rules. Our experimental results show good agreement with the calculations, which predict intricate field structures and a transition of the FWM transverse profile between the near- and far-field regions.

An ensemble of rubidium atoms can be excited with lasers such that it evolves into an entangled state with just one collective excitation within the Rydberg-blockade radius. The decay of this state leads to the emission of a single antibunched photon. For a hot vapor of rubidium atoms in a microcell, we numerically study the feasibility of such a single-photon source under different experimental conditions like the atomic density distribution and the choice of electronic states addressed by the lasers. For the excitation process with three rectangular lasers pulses, we simulate the coherent dynamics of the system in a truncated Hilbert space. We investigate the radiative behavior of the moving rubidium atoms and optimize the laser pulse sequence accordingly. We find that the collective decay of the single excitation leads to a fast and directed photon emission and further that a pulse sequence similar to a spin echo increases the directionality of the photon. Finally, we analyze the residual double excitations and find that they do not exhibit these collective decay properties and play only a minor deleterious role.

The single-atom version of the quantum heat engine based on electromagnetically induced transparency is theoretically investigated in this paper. In contact with a hot and a cold bath, the atom trapped in an optical cavity is coherently coupled by a coherent field, thus generating photons as output through a process that mimics a heat engine. Using semiclassical theory, we obtain the conditions for the classical gain parameter being positive. Then by taking a full quantum approach, we discuss the statistical property of the generated photons depicted by the photon number distribution and Wigner function, and show that it can be effectively tuned by the coupling field. Investigations on the energy and entropy balances of the heat engine process lead to an interesting result that the power of the engine is simply the energy per time provided by the hot reservoir. The coupling field plays a critical role in supporting the heat-engine process. However, the energy that it provides is all dumped into the cold reservoir. The total energy exchange conforms to the first law of thermodynamics. The efficiency of the engine depends on the temperatures of the two reservoirs and the strength of the coupling field. The entropy change of the heat-engine process suggests that when working far above the threshold of lasing, the efficiency approaches that of its atomic-ensemble-based partner.

We introduce and analyze the entanglement properties of randomized hypergraph (RH) states, as an extended notion of the randomization procedure in the quantum logic gates for the usual graph states, recently proposed in the literature. The probabilities of applying imperfect generalized controlled-$Z$ gates simulate the noisy operations over the qubits. We obtain entanglement measures as negativity, concurrence, and genuine multiparticle negativity, and show that entanglement exhibits a nonmonotonic behavior in terms of the randomness parameters, which is a consequence of the nonuniformity of the associated hypergraphs, reinforcing the claim that the entanglement of randomized graph states is monotonic since they are related to 2-uniform hypergraphs. Moreover, we observed the phenomena of entanglement sudden death and entanglement sudden birth in RH states. This work reveals a connection between the nonuniformity of hypergraphs and loss of entanglement.

Coherent control errors, for which ideal Hamiltonians are perturbed by unknown multiplicative noise terms, are a major obstacle for reliable quantum computing. In this paper we present a framework for analyzing the robustness of quantum algorithms against coherent control errors using Lipschitz bounds. We derive worst-case fidelity bounds which show that the resilience against coherent control errors is mainly influenced by the norms of the Hamiltonians generating the individual gates. These bounds are explicitly computable even for large circuits and they can be used to guarantee fault tolerance via threshold theorems. Moreover, we apply our theoretical framework to derive a guideline for robust quantum algorithm design and transpilation, which amounts to reducing the norms of the Hamiltonians. Using the three-qubit quantum Fourier transform as an example application, we demonstrate that this guideline targets robustness more effectively than existing ones based on circuit depth or gate count. Furthermore, we apply our framework to study the effect of parameter regularization in variational quantum algorithms. The practicality of the theoretical results is demonstrated via implementations in simulation and on a quantum computer.

Spectrotemporal processing is essential in reaching ultimate per-photon information capacity in optical communication and metrology. In contrast to the spatial domain, multimode processing in the time-frequency domain is, however, challenging. Here, we propose a protocol for spectrum-to-position conversion using the spatial spin wave modulation technique in gradient echo quantum memory. This way we link the two domains and allow the processing to be performed purely on the spatial modes using conventional optics. We present the characterization of our interface as well as the frequency estimation uncertainty discussion, including comparison with the Cramér-Rao bound. The experimental results are backed up by numerical simulations. The measurements were performed on a single-photon level demonstrating low added noise and proving applicability in a photon-starved regime. Our results hold prospects for ultraprecise spectroscopy and present an opportunity to enhance many protocols in quantum and classical communication, sensing, and computing.

We extend our previous calculations of the vibrational excitation and the dissociative electron attachment in the low-energy electron collisions with the HNCO molecule to include both stretching and bending modes in the motion of the hydrogen atom. These calculations confirm the assumption, made in our previous work, that the bending motion does not significantly alter the cross sections for the excitation of hydrogen stretching and dissociation. On the other hand, the extension of the model gives additional insight into the vibrational dynamics of the intermediate collision complex. We calculate the cross sections for the excitation of the hydrogen bending motion and we compare them with the recent experiments. The excitation curves for higher bending overtones show signs of interaction with the stretching motion. We also calculate the two-dimensional electron energy-loss spectrum within the model.

We report an experimental study of the highly excited $3{}^1{\mathrm{\Delta }}_g$ and $4{}^1{\mathrm{\Delta }}_g$ electronic states of the ${\mathrm{Rb}}_2$ molecule. Rovibrational levels of the two electronic states were probed using the high-resolution optical-optical double resonance technique by exciting ${}^{85}{\mathrm{Rb}}_2$ molecules from thermally populated levels of the $X{}^1{\mathrm{\Sigma }}_g^{+}$ ground state through intermediate levels of the $B{}^1{\mathrm{\Pi }}_u$ electronic state. The ${}^1\mathrm{\Delta }_g$ resonances induced by the probe laser were observed by detecting laser induced fluorescence from collisionally populated triplet states lying near the upper ${}^1\mathrm{\Delta }_g$ states to the $a^3{\mathrm{\Sigma }}_u^{+}$ triplet ground state. The ${}^1\mathrm{\Delta }_g$ character of the two electronic states was confirmed by showing that the probe transitions to these states abide by ${}^1\mathrm{\Delta }\leftarrow {}^1\mathrm{\Pi }$ dipole selection rules and by observing that their lowest rotational level is $J=2$. A set of molecular constants and a Rydberg-Klein-Rees potential-energy curve were calculated from the observed term values for each electronic state and compared with ab initio predictions.

We report absolute frequency, isotope shift, radiative lifetime, and hyperfine structure measurements of the $\left(4s^2\right){}^{1}S_0\to \left(4s4p\right){}^{1}P_1$ (213.8 nm) transition in Zn I using a cryogenic buffer gas beam. Laser-induced fluorescence is collected with two orthogonally oriented detectors to take advantage of differences in the emission pattern of the isotopes. This enables a clear distinction between isotopes whose resonances are otherwise unresolved, and a measurement of the ${}^{67}\mathrm{Zn}$ hyperfine structure parameters, $A\left({}^{67}\mathrm{Zn}\right)=20\left(2\right)\mathrm{MHz}$ and $B\left({}^{67}\mathrm{Zn}\right)=10\left(5\right)\mathrm{MHz}$. We reference our frequency measurements to an ultralow expansion cavity and achieve an uncertainty at the level of 1 MHz, about 1 percent of the natural linewidth of the transition.

We introduce an approach to investigate the spatial distribution of positrons generated from the Dirac vacuum state through a localized supercritical electric field. The well-known degeneracy of energy states in the upper and lower continua of a supercritical Dirac Hamiltonian has posed a significant challenge in distinguishing between the appropriate subspaces of electronic and positronic states within the interaction zone. Consequently, accurately partitioning the fully coupled quantum field operator into its electronic and positronic contributions has been widely viewed as problematic. However, this partitioning would be beneficial for determining positronic densities, which would provide valuable insights into the particles’ birth positions and production rate within the interaction zone. Naturally, the computed quasidensities obtained by projecting onto force-free (or partially dressed) energy eigenstates exhibit spatially dependent mathematical corrections, resulting in deviations from the true physical positronic density. These corrections are not attributable to real particles during the interaction. Our work focuses on developing a quantitative understanding of these corrections, allowing us to effectively subtract them out and gain insights into the genuine dynamics within the pair-creation interaction zone.

We show how to achieve strong squeezing of a microwave output field by reservoir engineering a cavity magnomechanical system, consisting of a microwave cavity, a magnon mode, and a mechanical vibration mode. The magnon mode is simultaneously driven by two microwave fields at the blue and red sidebands associated with the vibration mode. The two-tone drive induces a squeezed magnonic reservoir for the intracavity field, leading to a squeezed cavity mode due to the cavity-magnon state swapping, which further yields a squeezed cavity output field. The squeezing of the output field is stationary and substantial by using the currently available parameters in cavity magnomechanics. The work indicates the potential of the cavity magnomechanical system in preparing squeezed microwave fields, and may find promising applications in quantum information science and quantum metrology.

We investigate the phenomenon of disorder-free localization in quantum systems with global permutation symmetry. We use permutation group theory to systematically construct permutation-symmetric many-fermion Hamiltonians and interpret them as generators of continuous-time quantum walks. When the number of fermions is very large we find that all the canonical basis states localize at all times, without the introduction of any disorder coefficients. This time-independent localization is not the result of any emergent disorder, distinguishing it from existing mechanisms for disorder-free localization. Next we establish the conditions under which the localization is preserved. We find that interactions that preserve and break the global permutation symmetry sustain localization. Furthermore, the basis states of systems with reduced permutation symmetry localize even for a small number of fermions when the symmetry-reducing parameters are tuned accordingly. We show that similar localization also occurs for a permutation-symmetric Heisenberg spin chain and permutation-symmetric bosonic systems, implying that the localization is independent of the superselected symmetry. Finally, we make connections of the Hamiltonians studied here to the adjacency matrices of graphs and use this to propose a prescription for disorder-free localization in continuous-time quantum walk systems. Many of the models proposed here feature all-to-all connectivity and can be potentially realized on superconducting quantum circuits, trapped ion systems, and ultracold atoms.

Superoscillatory, band-limited functions oscillate faster than their fastest Fourier component. Superoscillations have been intensively explored recently as they give rise to many out-of-the-spectrum phenomena entailing both fundamental and applied significance. We experimentally demonstrate a form of superoscillations which is manifested by light apparently coming from a source located far away from the actual one. These superoscillations are sensed through sharp transverse shifts in the local wave vector at the minima of a pinhole diffraction pattern. We call this phenomenon “optical ventriloquism.”

The quantum approximate optimization algorithm (QAOA) is promising for achieving quantum computational advantage with near-term quantum devices. It was numerically shown that the QAOA cost functions exhibit a phenomenon called reachability deficit (RD), where the success probability cannot reach unity until the circuit depth exceeds a certain critical value. However, an in-depth theoretical understanding of RD remains lacking. Here we focus on a variational variant of Grover search on multiple marked solutions as a prototype for analyzing the RD problem, where we further relax the criterion of reachability by tolerating a certain probability of failure. Specifically, we obtain a general analytical expression relating the critical depth of the quantum circuit to the solution density. In the dilute limit, the critical depth is consistent with the Grover bound, exhibiting a robust quadratic scaling that is insensitive to the failure probability. Moreover, we also find that the projective mixing Hamiltonian performs significantly better than the traditional mixing Hamiltonian in the QAOA, although it is less favorable in terms of physical implementation. However, by taking into account two-body interactions in the mixing Hamiltonian, the performance becomes on par with the projective mixing Hamiltonian at the cost of $O\left(n^2\right)$ additional terms. These results represent a simplified but insightful model of the QAOA, fully addressing the dependence of required circuit depth over the ground-state degeneracy.

The uncertainty principle may be considered as giving rise to the notion of incompatibility of observables, a property that has been carefully analyzed in the literature for single systems. A pack of quantum measurements that cannot be measured simultaneously is said to form a set of incompatible measurements. Every set of incompatible measurements has an advantage over the compatible ones in a quantum state discrimination task where one prepares a state from an ensemble and sends it to another party, and the latter tries to detect the state using available measurements. Comparison between global and local quantum state discriminations is known to lead to a phenomenon of “nonlocality.” In this work, we seal a connection between the domains of local quantum state discrimination and incompatible quantum measurements. We consider the local quantum state discrimination task where a sender prepares a bipartite state and sends the subsystems to two receivers. The receivers try to detect the sent state using locally incompatible measurements. We analyze the ratio of the probability of successfully guessing the state using incompatible measurements and the maximum probability of successfully guessing the state using compatible measurements. We find that this ratio is upper bounded by a simple function of robustnesses of incompatibilities of the local measurements. Interestingly, corresponding to every pair of sets of incompatible measurements, there exists at least one local state discrimination task where this bound can be achieved. We argue that the optimal local quantum state discrimination task does not present any “nonlocality,” where the term is used in the sense of a difference between the ratios, of probabilities of successful detection via incompatible and compatible measurements, in global and local state discriminations. The results can be generalized to the regime of multipartite local quantum state distinguishing tasks.

We revisit the derivation of multipole contributions to the atom-wall interaction previously presented in Łach et al. [G. Łach, M. DeKieviet, and U. D. Jentschura, Phys. Rev. A 81, 052507 (2010)]. A careful reconsideration of the angular momentum decomposition of the second-, third-, and fourth-rank tensors composed of the derivatives of the electric-field modes leads to a modification for the results for the quadrupole, octupole, and hexadecupole contributions to the atom-wall interaction. Asymptotic results are given for the asymptotic long-range forms of the multipole terms, in both the short-range and long-range limits. Calculations are carried out for hydrogen and positronium in contact with $\alpha$-quartz; a reanalysis of analytic models of the dielectric function of $\alpha$-quartz is performed. Analytic results are provided for the multipole polarizabilities of hydrogen and positronium. The quadrupole correction is shown to be numerically significant for atom-surface interactions. The expansion into multipoles is shown to constitute a divergent, asymptotic series. Connections to van der Waals corrected density-functional theory and applications to physisorption are described.

We demonstrate that genuine multipartite entangled states can be generated using frequency bin encoding in integrated photonic platforms. We introduce a source of four-photon GHZ states and a source of three-photon W states. We predict generation rates on the order of ${10}^4$ Hz for a silicon microring source with milliwatt pump powers. These results, along with the versatility and scalability of integrated structures, identify this as a promising approach for the generation of higher-dimensional and larger entangled states.