We experimentally observed a spectral broadening of the third harmonic in the long-wavelength range accompanied by intensity enhancement as the laser repetition rates increased. A correction for the simulation of the third harmonic at different repetition rates is proposed. Simulation and analysis confirm that the pulse accumulation effect changes the spatiotemporal evolution of the electric field within the filament, thus enhancing the cross-phase modulation and self-phase modulation of the third harmonic, leading to the broadening of the third harmonic, with a quantitative analysis establishing the cross-phase modulation as the dominant mechanism. These findings provide a comprehensive physical framework for understanding third harmonic generation in a high-repetition-rate filament. The demonstrated repetition-rate-dependent mechanism presents new opportunities for advancing intense UV pulse generation techniques, with promising applications in precision laser machining and remote UV detection technology.

Extended Wigner's friend no-go theorems provide a modern lens for investigating the measurement problem, by making precise the challenges that arise when one attempts to model agents as dynamical quantum systems. Most such no-go theorems studied to date, such as the Frauchiger-Renner argument and the Local Friendliness argument, are explicitly constructed using quantum correlations that violate Bell inequalities. In this work, we show that such correlations are not necessary for having extended Wigner's friend paradoxes, by constructing a no-go theorem utilizing a proof of the failure of noncontextuality. The argument hinges on an alternative metaphysical assumption (which we term commutation irrelevance) that is a natural extension of a key assumption going into the Frauchiger and Renner's no-go theorem.

Recent results on constant overhead LDPC code-based fault-tolerance against i.i.d. errors naturally lead to the question of fault-tolerance against errors with long-range correlations. Ideally, any correlation can be captured by a joint (system and bath) Hamiltonian. However, an arbitrary joint Hamiltonian is often intractable, and hence, the joint Hamiltonian model with pairwise terms was introduced and developed in a seminal series of papers by Terhal and Burkard [Phys. Rev. A 71, 012336 (2005)], Aliferis, Gottesman, and Preskill [Quantum Inf. Comput. 6, 97 (2005)], and Aharonov, Kitaev, and Preskill [Phys. Rev. Lett. 96, 050504 (2006)]. However, the analysis of the new constant overhead codes in that error model appears to be quite challenging. In this paper, to model correlated errors in quantum memory, we introduce a correlation model which is a generalization of the well-known hidden random fields. This proposed model, which includes stationary and ergodic (and possibly non-Markov) error distributions, is shown to capture correlations not captured by the joint Hamiltonian model with pairwise terms. On the other hand, building on non-i.i.d. measure concentration, we show that for a broad class of (possibly) non-Markov and nonstationary error distributions, quantum Tanner codes ensure an exponential retention time (in the number of physical qubits), when the error rate is below a threshold. An implication of this work is that the rate of decay of the correlation with distance does not necessarily differentiate between good and bad correlations.

We report the image current detection of electrons in a room-temperature Paul trap at microwave frequencies. By selectively leveraging distinct cavity modes for trapping and detection, our approach efficiently extracts electron signals otherwise buried in the microwave drive used for pseudopotential formation. When the trapped electrons resonate with the cavity mode, we observe a mode excitation and its exponential decay attributed to resistive cooling. Detuning electrons from the cavity resonance halts this decay, and sweeping electrons' secular frequency reveals their oscillatory spectrum. Implementing this experiment at cryogenic temperatures could enable the image current detection and ground-state cooling of a single electron in Paul traps.

Topological quantum computing holding global anti-interference ability is realized by braiding some anyons, such as well-known Fibonacci anyons. Here, based on $\text{SO}{\left(3\right)}_2$ theory we obtain a total of six anyon models utilizing $F$ matrices, $R$ symbols, and fusion rules of metaplectic anyon. We obtain the elementary braiding matrices (EBMs) by means of unconventional encoding. After braiding $X$ and $X^{\prime }$, we insert a pair of $Z$ anyons into them to ensure that the initial order of anyons remains unchanged. In this process only fusion is required, and measurement is not necessary. Three of them $\{V_3^{113},V_3^{131},V_1^{133}\}$ are studied in detail. We study systematically the compilation of these three models through EBMs obtained analytically. For the one-qubit case, the classical $H$ and $T$ gate can be well constructed using the Genetic Algorithm-enhanced Solovay-Kitaev Algorithm (GA-enhanced SKA) by $\{V_3^{113},V_3^{131},V_1^{133}\}$. The obtained accuracy of the $H$ and $T$ gates by $\{V_3^{113},V_1^{133}\}$ is slightly inferior to the corresponding gates of the Fibonacci anyon model, but it also can meet the requirements of fault-tolerant quantum computing,with $V_3^{131}$ giving the best performance of these four models. For the two-qubit case, we use the exhaustive method for short lengths and the GA for long lengths to obtain a braidword for $\{V_3^{113},V_3^{131},V_1^{133}\}$ models. The resulting matrices can well approximate the local equivalence class of the controlled-not (cnot) gate, while demonstrating a much smaller error than the Fibonacci model, especially for the $V_3^{113}$. The braiding processes of conventional encoding (using identical anyons) and unconventional encoding (using distinct anyons) are compared. Finally, we attempt to generalize the model to the $N$-qubit case.

The Petz recovery map provides a near-optimal reversal of quantum noise, yet proposals for its implementation are only recent. We propose a physical realization of the exact state-specific Petz map in an ion trap for qubit decoherence channels. Our circuit constructions require at most $1\left(2\right)$ ancilla qubits and $3\left(20\right)$ cnot gates for channels with Kraus rank $2(>2)$. We analyze typical ion trap errors and construct corresponding Petz maps, simulating their performance under realistic noise modeled by residual spin-motion coupling. Quantum circuits are provided for depolarizing, dephasing, and amplitude damping channels. Focusing on single-shot recovery, suited for present-day devices, we also quantify the precision of prior knowledge required to achieve a recovery error below 0.01 across varying decoherence levels and state purities.

The finite operators are derived for the nonrecoil [leading order in the $(m/M)$ expansion] relativistic corrections in hydrogenlike atoms and ions at order $m{\alpha }^6$ in the two- and three-body formalisms beyond the adiabatic approximation. Singular operators that appear in the derivation are analyzed, and various regularization methods are discussed.

We present a novel spectroscopic technique, two-color two-photon spin-noise spectroscopy, and demonstrate its application to ${}^{133}\mathrm{Cs}$ atoms. This approach encompasses both the pump-probe and nonlinear two-photon schemes, as well as an interference noise arising due to Doppler shift. In the pump-probe scheme, the pump laser populates the excited states, while the probe laser monitors spin fluctuations, enabling the study of density fluctuations in these states. The pure two-photon scheme can be used to study a spin noise in the ground state, where fluctuations in the probe beam polarization are governed by the third-order susceptibility ${\chi }^{\left(3\right)}$, offering additional advantages. We also theoretically analyze the structure of the susceptibility tensor to qualitatively interpret observed effects, such as a dependence on the pump-laser polarization plane and appearance of high harmonics of the Larmor frequency in the spin noise.

The Fermi polaron problem, which describes a mobile impurity that interacts with a spin-polarized Fermi sea, is a paradigmatic system in quantum many-body physics and has been challenging to address quantitatively in its strong-coupling regime. We present controlled thermodynamic calculations for the Fermi polaron at strong coupling using finite-temperature auxiliary-field quantum Monte Carlo methods in the framework of the canonical ensemble. Modeled as a spin-imbalanced system, the Fermi polaron has a Monte Carlo sign problem, but we show that it is moderate over a wide range of temperatures and coupling strengths beyond the unitary limit of the BCS-BEC crossover. We calculate the contact, a quantity which measures the strength of the short-range correlations, as a function of temperature at unitarity and as a function of the coupling strength at fixed temperature and find good agreement with a variational approach based on a single particle-hole excitation of the Fermi sea. We compare our results for the contact with recent experiments and find good agreement at unitarity (within error bars) but we observe discrepancies away from unitarity on the BEC side of the crossover. We also calculate the impurity thermal energy at unitarity as a function of temperature.

We investigate the performance of the quantum natural gradient (QNG) optimizer in the presence of noise. Specifically, we evaluate the efficacy of QNG within the quantum approximate optimization algorithm for finding the ground state of the transverse field Ising model. Its performance is benchmarked against the vanilla gradient descent optimizer across two prominent quantum computing platforms: Rydberg atoms and superconducting circuits. Our analysis includes simulations under both idealized noise-free conditions and realistic noisy environments based on calibration data from actual devices. Results demonstrate that QNG consistently outperforms vanilla gradient descent, exhibiting faster convergence on average and greater robustness against random initializations of parameters. This robustness is attributed to the distance regularization in parameter space inherent to QNG. Additionally, QNG achieves a higher convergence rate to the solution, effectively avoiding certain local minima. These findings highlight QNG as a promising tool for optimizing variational quantum algorithms in noisy intermediate-scale quantum devices.

In this work, we develop a system plus environment approach for interacting chiral molecules under a quantum-classical description of the spin-spin model using a Caldeira-Leggett-like coupling. After presenting the interacting model, we show that a long-ranged parity-nonconserving interaction, encoded within a nonlinear Schrödinger equation, produces an energy difference between the two enantiomers of the central chiral molecule when it interacts with a chiral environment. Three examples of such interactions are considered, with particular focus on $Z^0$-photon vacuum polarization. Finally, we reveal a chirality transmission effect phenomenon, where the time-averaged population difference of the central molecule is amplified when the chiral asymmetry of the environment is considered.

The search for variations of the fine-structure constant $\alpha$ by using atomic clocks has driven the development of novel atomic clock technologies. Among these, the ${}^{229}\mathrm{Th}$ nuclear clock based on nuclear transition stands out due to its high sensitivity. Previous studies on the $\alpha$ variation in atomic clocks have typically focused purely either on a nuclear or electronic transition but not on the hyperfine electronic bridge (HEB) transitions, which involve simultaneous changes in both nuclear and electronic structures. In this work, we propose to search for the $\alpha$ variation by measuring the 3.14-eV HEB transition between the hyperfine levels $[I_e,\left(4f^{12}\right)J_e=4,F_e]$ and $[I_g,\left(4f^{12}\right)J_e=2,F_g]$ of ${}^{229}\mathrm{Th}^{32+}$ ions, and the 2.39-eV HEB transition between $[I_e,\left(4f^4\right)J_g=4,F_e]$ and $[I_g,\left(4f^4\right)J_e=2,F_g]$ of ${}^{229}\mathrm{Th}^{40+}$ ions. These two HEB transitions exhibit remarkably large sensitivity factors $K_{\alpha }$ of about $-2.2\times {10}^4$ and $-2.9\times {10}^4$, respectively. Compared to the nuclear-clock transition $(I_e \mathrm{to} I_g)$ in bare ions, the sensitivity factors are significantly enhanced by factors of 2.7 and 3.5. Moreover, both the 3.14- and 2.39-eV HEB transitions are well supported by current high-precision laser technology, making our approach promising for improving the precision of $\alpha$ variation detection.

Electron-ion coincidence method, in particular cold-target recoil-ion momentum spectroscopy (COLTRIMS), is currently the most advanced tool for detailed study of the processes of Compton ionization of atoms. We have performed a theoretical analysis of the capabilities of this method as applied to Compton ionization of an atom by a twisted x-ray photon. Our analysis uses the nonstationary formalism of quantum scattering theory, which, unlike the generally accepted stationary formalism, allows one to take into account the specifics of real initial states (wave packets) of a photon and an atom. We have shown that the energy and angular distribution of ionized electrons measured in coincidence with recoil ions does not depend on the orbital angular momentum of the twisted photon, but depends on its cone angle (opening angle), which determines the characteristic angular shift in the distribution.

Control of decoherence in open quantum systems has become a topic of great interest due to the emergence of quantum technologies that depend on quantum coherence effects. In this work, we investigate the decoherence dynamics of systems coupled to multiple baths through noncommuting systems' operators and beyond the weak system-bath coupling limit. By building on cooperative effects between baths, we propose a strategy to mitigate rapid decoherence. Concretely, we study the dynamics of a qubit coupled to multiple environments with arbitrary interaction strengths and along different coordinates. Based on insights gained on the decoherence dynamics from the analytical effective Hamiltonian method, we carry out numerical simulations using the reaction coordinate quantum master equation method. We show that when the system strongly interacts with a decohering bath, increasing its coupling to a second, dissipative bath can slow down the decoherence dynamics in a certain parameter regime. Our work offers insights into the preservation of quantum coherence in open quantum systems based on frustration effects, by utilizing cooperative effects between different heat baths.

We study the nonequilibrium thermodynamics of pure decoherence processes in open quantum systems coupled to a thermal reservoir. We review various definitions of central quantities, such as internal energy, work, heat, and entropy production, developed within local and global approaches to quantum thermodynamics. Within local approaches, thermodynamic quantities only refer to the open system's degrees of freedom, while in the global approaches, certain quantities are defined by referring explicitly to the reservoir degrees of freedom. Employing a microscopic, analytically solvable model, we perform a comparison of these two perspectives, revealing substantial differences in the thermodynamic quantities and in the formulations of the first and second laws. The main reason for these discrepancies is the fact that the global approaches involve the system-reservoir interaction which exchanges a large amount of energy with the environment, while the average open system energy is constant in time because the dynamics represents pure decoherence and does not affect the open system populations.

The composition of individual quantum system can mathematically be defined in various ways. However, the no-signaling principle alongside local tomography criteria delineates two possible extreme compositions, commonly referred to as the minimal and maximal compositions. The former (latter) allows for the effects (states) that are beyond-quantum composition. A recent work [Phys. Rev. Lett. 130, 110202 (2023)] demonstrates that the information causality (IC) principle rules out extreme compositions. In this work, we introduce a communication game to show that there are states and effects within both extreme compositions for which the success probability of the game surpasses the maximum achievable quantum bound. However, the IC principle remains satisfied, meaning that it cannot rule out all beyond-quantum correlations resulting from the extreme compositions. This again opens up the question of the effectiveness of the IC principle in ruling out beyond-quantum correlation.

The implementation of practical error correction protocols is essential for deployment of quantum information technologies. Ways of exploiting high-spin nuclei, which have multilevel quantum resources, have attracted interest in this context because they offer additional Hilbert space dimensions in a spatially compact and theoretically efficient structure. We present a quantitative analysis of the performance of a spin-qudit-based error-correctable quantum memory, with reference to the actual Hamiltonians of several potential candidate systems. First, the ideal code-word implemented on a spin-7/2 nucleus, which provides first-order Pauli-$X, Y$ and $Z$ error correction, has intrinsic infidelity due to mixed eigenstates under realistic conditions. We confirm that expansion to a spin-9/2 system with tailored code words can compensate for this infidelity. Second, we claim that electric field fluctuations, which are inevitable in real systems, should also be considered as a noise source, and we illustrate an encoding and decoding scheme for a multi-spin-qudit-based error correction code that can simultaneously compensate for both electric- and magnetic-field perturbations. Such strategies are important as we move beyond the current noisy-intermediate quantum era, and fidelities above two or three nines become crucial for the implementation of quantum technologies.

Improving the performance of quantum algorithms is a fundamental task to achieve quantum advantage. In many cases, extracting information from quantum systems poses an important challenge for practical implementations of real-world quantum computers, given the high resource cost of performing state tomography. In this scenario, randomized measurements have emerged as a promising tool. In particular, the classical shadow protocol allows one to retrieve expected values of low-weight Pauli observables by performing only local measurements. In this paper, we present an implementation of the recently introduced feedback-based algorithm for quantum optimization using the Ket quantum programming platform to solve the MaxCut optimization problem. We employ classical shadows for the feedback routine of parameter estimation and compare this approach with the direct estimation of observables. Our results show that depending on the graph geometry of the MaxCut problem, the number of measurements required to estimate expected values of observables with classical shadows can be up to 16 times lower than with direct-observable estimation. Furthermore, by analyzing complete graphs, we numerically confirm logarithmic growth in the required number of measurements relative to the number of observables, reinforcing that classical shadows can be a useful tool for estimating low-locality Pauli observables in quantum algorithms.

Multidimensional rotation serves as a powerful tool for enhancing information reconciliation and extending the transmission distance in continuous-variable quantum key distribution (CV-QKD). However, the lack of closed-form orthogonal transformations for high-dimensional rotations has limited the maximum reconciliation efficiency to channels with eight dimensions over the past decade. This paper presents a cross-rotation scheme to overcome this limitation and enable reconciliation in arbitrarily high dimensions, constrained to even multiples of eight. The key treatment involves reshaping the string vector into matrix form and applying orthogonal transformations to its columns and rows in a cross manner, thereby increasing the reconciliation dimension by one order per cross rotation while significantly reducing the communication overhead over the classical channel. A rigorous performance analysis is also presented from the perspective of achievable sum-rate. Simulation results demonstrate that 64-dimensional cross rotation nearly approaches the upper bound, making it a recommended choice for practical implementations.