We consider the problem of engineering the two-excitation Dicke state $|D_2^3\rangle$ in a three-qubit system with all-to-all Ising-type qubit-qubit interaction, which is also subject to global transverse (Zeeman-type) control fields. The theoretical underpinning for our envisioned state-preparation scheme, in which $|000\rangle$ is adopted as the initial state of the system, is provided by a Lie-algebraic result that guarantees state-to-state controllability of this system for an arbitrary choice of initial and final states that are invariant with respect to permutations of qubits. This scheme is envisaged in the form of a pulse sequence that involves three instantaneous control pulses, which are equivalent to global qubit rotations, and two Ising-interaction pulses of finite durations between consecutive control pulses. The design of this pulse sequence (whose total duration is $T\approx 0.95\hslash /J$, where $J$ is the Ising-coupling strength) leans heavily on the concept of the symmetric sector, a four-dimensional, permutationally invariant subspace of the three-qubit Hilbert space. We demonstrate the feasibility of the proposed state-preparation scheme by carrying out a detailed numerical analysis of its robustness to systematic errors, i.e., deviations from the optimal values of the eight parameters that characterize the underlying pulse sequence. Finally, we discuss how our proposed scheme can be generalized for engineering Dicke states in systems with $N\ge 4$ qubits. For the sake of illustration, we describe the preparation of the two-excitation Dicke state $|D_2^4\rangle$ in a four-qubit system.

In this paper we present a genetic-algorithm-based search technique for linear optics schemes, performing two-qubit quantum gates. We successfully apply this technique to find heralded two-qubit gates and obtain schemes with performance parameters equal to the best currently known. Simple metrics are introduced which enable a comparison of schemes with different heralding mechanisms. The scheme performance degradation is discussed for the cases when detectors in the heralding part of the scheme are not photon-number resolving. We propose a procedure for overcoming this drawback which allows us to restore the reliable heralding signal even with non-photon-number-resolving detectors.

The relativistic configuration interaction plus many-body perturbation (RCI $+$ MBPT) method is utilized to calculate the wave functions, energy levels, and reduced $E1$ transition matrix elements of Ca atoms in detail. The static and dynamic polarizabilities of the $4s^2{}^{1}S_0, 4s4p{}^{1}P_1$, and $4s4p{}^{3}P_{0,1,2}$ states are determined by the sum-over-states method. A number of magic-zero wavelengths for the ground and $4s4p{}^{3}P_0$ states, as well as magic wavelengths for the $4s^2{}^{1}S_0\to 4s4p{}^{3}P_0$ and $4s^2{}^{1}S_0\to 4s4p{}^{3}P_1$ transitions, are obtained. We suggest that the measurements of the magic-zero wavelengths of the $4s4p{}^{3}P_0$ states can be used to determine oscillator strengths with high precision. In addition, the angle-dependent magic wavelengths of each magnetic sublevel transition of the $4s^2{}^{1}S_0\to 4s4p{}^{3}P_1$ transition are provided, which are of particular interest for optical frequency measurements, Bose-Einstein condensate generation, and superradiant laser technology.

We propose a protocol to retrieve the photoionization time delay that employs a pair of oppositely circularly polarized attosecond extreme ultraviolet pulses with frequencies $\omega$ and $2\omega$. By numerically solving the three-dimensional time-dependent Schrödinger equation, we observe an oscillation in the energy-resolved photoionization time delay encoded in the relative optical phase between $\omega$ and $2\omega$ pulses when the emission asymmetry of the photoelectron is maximal. Our results show that this oscillation arises from the interference between two one-photon ionization pathways from the ground state due to the broad bandwidth effect of an ultrashort pulse: the absorption of one left-handed circularly polarized photon and the emission of one right-handed circularly polarized photon. The latter is regarded as a consequence of the breakdown of the rotating-wave approximation (RWA). We compare the perturbation theory with and without RWA and demonstrate that photon emission plays a crucial role in retrieving photoionization time delay. Our study enriches the underlying physics of attosecond spectroscopy and provides a framework for accurately measuring photoionization time delays in atoms and molecules.

In recent years, moiré lattice has become a hot topic and has inspired the research upsurge of moiré lattice. In this work, we propose a method of constructing a multicomposite moiré lattice, which is composed of more than three periodic component structures. Moreover, we propose the moiré lattice–metasurface structure, which can realize the multiwavelength interface states between these kinds of moiré lattices and metasurfaces. The wavelength, polarization, and number of moiré interface states can be manipulated flexibly, with anisotropic metasurfaces. These multiwavelength interface states are employed to enhance quantum emission (QE) and over 20 times QE efficiency can be obtained.

We analyze the dynamics of one and two two-level atoms interacting with the electromagnetic field in the vicinity of an optical nanofiber without making either the Born or the Markov approximations. We model the dielectric response of the nanofiber with a constant dielectric function and the Drude-Lorentz model, observing deviations from the standard super- and subradiant decays. We discuss the validity of approximating the speed of atom-atom communication to the group velocity of the guided field in the presence of nontrivial environment correlations. Our work presents a deeper understanding of the validity of commonly used approximations in recent waveguide QED platforms.

We construct and study the simplest universal dissipative Lindblad master equation for many-body systems with the purpose of a new dissipative extension of existing nonrelativistic theories of fundamental spontaneous decoherence and spontaneous wave function collapse in nature. It is universal as it is written in terms of second-quantized mass density $\widehat{\varrho }$ and current $\widehat{\mathbf{J}}$, thus making it independent of the material structure and its parameters. Assuming linear friction in the current, we find that the dissipative structure is strictly constrained. Following the general structure of our dissipative Lindblad equation, we derive and analyze the dissipative extensions of the two most known spontaneous wave function collapse models, the Diósi-Penrose and the continuous spontaneous localization models.

We propose a quantum algorithm to investigate finite-temperature properties of quantum many-body systems by implementing the microcanonical thermal pure quantum (TPQ) states [S. Sugiura and A. Shimizu, Phys. Rev. Lett. 108, 240401 (2012)] on a quantum computer. In the formalism, the TPQ state is constructed by applying the power of a rescaled Hamiltonian to a random state. We clarify that, using techniques based on quantum signal processing, the power operation can be efficiently realized when the ground-state energy of a quantum system of interest is given with some precision. Our algorithm has an advantage in calculating thermodynamic quantities at low temperatures.

Locally indistinguishable states are useful to distribute information among spatially separated parties such that the information is locked. This implies that the parties are not able to extract the information completely via local operations and classical communication (LOCC), while it might be possible via LOCC when the parties share entanglement. In this work, we consider an information distribution protocol using orthogonal states for $m\ge 3$ spatially separated parties such that even if any $k\le (m-1)$ parties collaborate still the information cannot be revealed completely. Such a protocol is useful to understand up to what extent the encoded information remains locked. However, if required, the parties can share entanglement and extract the information completely by LOCC. To make the process resource efficient, it should consume less number of entangled states. We show that though the set of states, which are locally indistinguishable across every bipartition, are sufficient for the above protocol, they may consume a higher number of entangled states when aiming for complete information extraction. We establish this by constructing a class of locally indistinguishable sets of orthogonal states which can be employed to accomplish the above protocol and these sets consume less number of entangled states, compared with the former sets, for complete information extraction. In fact, this difference in the number of required entangled states for complete information extraction grows linearly with the number of parties. This study sheds light on suitable use of local indistinguishability property of quantum states as a resource, and thus we demonstrate an efficient way of information distribution.

This work examines secret key rates of key distribution based on quantum repeaters in a broad parameter space of the communication distance and coherence time of the quantum memories. As the first step in this task, a Markov decision process modeling the distribution of entangled quantum states via quantum repeaters is developed. Based on this model, a simulation is implemented, which is employed to determine secret key rates under naively controlled, limited memory storage times for a wide range of parameters. The complexity of the quantum state evolution in a multiple-segment quantum repeater chain motivates the use of deep reinforcement learning to search for optimal solutions for the memory storage time limits, typically referred to as memory cutoffs. The main contribution in this work is to explore very general cutoff strategies which dynamically adapt to the state of the quantum repeater. An implementation of this approach is presented, with our focus on four-segment quantum repeaters, achieving a proof of concept of its validity by finding exemplary solutions that outperform the naive strategies.

We analyze two ways to obtain distinguishability measures between quantum maps by employing the square root of the quantum Jensen-Shannon divergence, which forms a true distance in the space of density operators. The arising measures are the transmission distance between quantum channels and the entropic channel divergence. We investigate their mathematical properties and discuss their physical meaning. Additionally, we establish a chain rule for the entropic channel divergence, which implies the amortization collapse, a relevant result with potential applications in the field of discrimination of quantum channels and converse bounds. Finally, we analyze the distinguishability between two given Pauli channels and study exemplary Hamiltonian dynamics under decoherence.

The rotational dynamics of the asymmetric-top ${\mathrm{H}}_2\mathrm{O}$ molecule induced by an intense nonresonant laser pulse is investigated by a time-resolved pump-probe technique. The periodic rotational revival structure is not identified differently from the rotational dynamics of linear, symmetric, and near-symmetric top molecules. The rotational excitation processes are revealed based on the Fourier-transform spectrum of the ${\mathrm{H}}_2{\mathrm{O}}^{+}$ yield observed as a function of the delay between the pump and probe pulses. In addition, by comparing with calculations, it is discussed whether one of two possibilities of the polarizability of ${\mathrm{H}}_2\mathrm{O}$ reported previously is plausible.

Hong-Ou-Mandel (HOM) interference is a compelling quantum phenomenon that demonstrates the nonclassical nature of single photons. In this study, we investigate an electromagnetically induced transparency-based double-$\mathrm{\Lambda }$ four-wave mixing system from the perspective of quantized light fields. The system can be used to realize efficient HOM interference in the frequency domain. By using the reduced density operator theory, we demonstrate that although the double-$\mathrm{\Lambda }$ medium does not exhibit phase-dependent properties for the closed-loop case of two incident single photons, frequency-domain HOM two-photon interference occurs. For experimentally achievable optical depth conditions, our theory indicates that this double-$\mathrm{\Lambda }$ scheme can perform high-fidelity Hadamard gate operations on frequency-encoded single-photon qubits, and thereby generate HOM two-photon NOON states with a fidelity greater than 0.99. Furthermore, we demonstrate that this scheme can be used to realize arbitrary single-qubit gates and two-qubit SWAP gates by simply controlling the laser detuning and phase, exhibiting its multifunctional properties and providing a different route to scalable optical quantum computing.

The quantum switch implements indefinite causal order via a coherent control of the orderings of multiple quantum operations, leading to various advantages and applications, one of which being in work extraction where enhancements to daemonic ergotropy is possible. Motivated by recent developments in the connections between non-Markovianity and the quantum switch, we construct a non-Markovian process that reduces to the two-party quantum switch in the fully non-Markovian limit. By controlling the amount of non-Markovianity in the process, we identified two operational regimes with differing behaviors. One has its daemonic ergotropy dependent on the presence and amount of non-Markovianity, achieving the maximum in the quantum switch case of full non-Markovianity. The other regime, however, has no advantages from non-Markovianity. We compare this non-Markovian process with the case of a superposition of independent channels, where two channels are placed in a coherent superposition without indefinite causal order, uncovering the advantages of non-Markovianity. Finally, the conditions required for the production of positive daemonic ergotropy are also derived for the case of fully non-Markovian and fully Markovian limits, where we compare against the conditions required for the superposition of independent channels.

Quantum hypothesis testing (QHT) provides an effective method to discriminate between two quantum states using a two-outcome positive operator-valued measure (POVM). Two types of decision errors in a QHT can occur. In this paper we focus on the asymmetric setting of QHT, where the two types of decision errors are treated unequally, considering the operational limitations arising from the lack of a reference frame for chirality. This reference frame is associated with the group ${\mathbb{Z}}_2$ consisting of the identity transformation and the parity transformation. Thus, we have to discriminate between two qubit states by performing the ${\mathbb{Z}}_2$-invariant POVMs only. We start from the discrimination between two pure states. By solving the specific optimization problem we completely characterize the asymptotic behavior of the minimal probability of type-II error which occurs when the null hypothesis is accepted when it is false. Our results reveal that the minimal probability reduces to zero in a finite number of copies, if the ${\mathbb{Z}}_2$-twirlings of such two pure states are different. We further derive the critical number of copies such that the minimal probability reduces to zero. Finally, we replace one of the two pure states with a maximally mixed state, and similarly characterize the asymptotic behavior of the minimal probability of type-II error.

Standard Bell inequalities hold when distant parties are not allowed to communicate. Barrett et al. [Phys. Rev. A 75, 012103 (2007)] found correlations from Pauli measurements on certain network graphs to refute a local-hidden-variable (LHV) description, even allowing some communication along the graph. This has recently found applications in proving separation between classical and quantum computing, in terms of shallow circuits, and distributed computing. The correlations presented by Barrett et al. can be understood as coming from an extension of three party Greenberger-Horne-Zeilinger state correlations which can be embedded on a graph state. In this paper, we propose systematic extensions of any graph state, which we dub inflated graph states, such that they exhibit correlations which refute any communication-assisted LHV model. We further show the smallest possible such example, with a seven-qubit linear graph state, as well as specially crafted smaller examples with five and four qubits. The latter is the smallest possible violation using binary inputs and outputs.

We consider a quantum computation that only extracts one bit of information per $N$-qubit quantum state preparation. This is relevant for error mitigation schemes where the remainder of the system is measured to detect errors. We optimize the estimation of the expectation value of an operator by its linear decomposition into bitwise-measurable terms. We prove that optimal decompositions must be in terms of reflections with eigenvalues $\pm 1$. We find the optimal reflection decomposition of a fast-forwardable operator, and show a numerical improvement over a simple Pauli decomposition by a factor $N^{0.7}$.

With rapid progress and significant successes in quantum hacking attacks, machine learning is being applied in continuous-variable quantum key distribution (CVQKD) systems. However, most machine-learning networks are shown to be vulnerable to adversarial examples. Thus, we propose a defense scheme with artificial key fingerprints for CVQKD systems. We first embed artificial key fingerprints into quadrature values, then exploit changes of fingerprints in the communication process to probe whether the samples have been manipulated. The paper verifies the effectiveness of our proposal based on four groups of samples that consist of normal measurement values and abnormal measurement values under four attacks. Simulation results show that the model can monitor sample changes in real time with high precision, and our scheme stays robust against adversarial examples attack. This proposal provides a different idea to address the potential vulnerability between machine-learning models used in CVQKD systems and their possible attacked training and test data by adversarial examples.

Here we relate two methods of subdiffraction imaging: bandwidth extrapolation for spatially bounded sources and quantum-metrology-inspired methods under assumptions about source structure. We present a quantum estimation theoretical approach in which the source is modeled in terms of unknown parameters corresponding to the Fourier components, whose impact on resolution is very intuitive. Using this method, we find that imaging spatially bounded sources faces an unavoidable fundamental resolution limit, but that in the small-source limit, certain measurement approaches can significantly improve the sensitivity over conventional methods.

In the field of fault-tolerant quantum computing, continuous-variable systems can be utilized to protect quantum information from noise through the use of bosonic codes. These codes map qubit-type quantum information onto the larger bosonic Hilbert space, and can be divided into two main categories: translational-symmetric codes, such as Gottesman-Kitaev-Preskill (GKP) codes, and rotational-symmetric codes, including cat and binomial codes. The relationship between these families of codes has not yet been fully understood. We present an iterative protocol for converting between two instances of these codes—generalized GKP (so-called qunaught) states and fourfold-symmetric binomial states corresponding to a zero-logical encoded qubit—using only Gaussian operations. This conversion demonstrates the potential for universality of binomial states for all-Gaussian quantum computation and provides a method for the heralded preparation of GKP states. Through numerical simulation, we obtain GKP qunaught states with a fidelity of over $98\%$ and a probability of approximately $3.14\%$, after only two steps of our iterative protocol, though higher fidelities can be achieved with additional iterations at the cost of lower success probabilities.