Fluctuation theorems are one of the pillars of nonequilibrium thermodynamics. Broadly speaking, they concern the statistical distribution of quantities such as heat, work, or entropy production. Quantum experiments, however, usually can only assess these distributions indirectly, or reconstruct them a posteriori. In this Letter we report an experiment where the distribution of entropy production is obtained directly from the outcomes (clicks) of an optical experiment simulating the interaction between a two-level system and a thermal reservoir. The setup consists of entangled photon pairs, one of which is sent to an interferometer implementing a finite-temperature amplitude damping channel, and is designed so as to allow full access to the two-point measurement statistics of both system and reservoir. First, by measuring the entangled pair, we directly implement the two-point measurement scheme in the system, avoiding the destructive nature of photodetection. Second, each optical path of the interferometer is associated with a specific transition of the reservoir. Thus, by blocking all but one of the paths, we can measure the conditional entropy production, given a specific reservoir trajectory.

Many complex systems can spontaneously oscillate under nonperiodic forcing. Such self-oscillators are commonplace in biological and technological assemblies where temporal periodicity is needed, such as the beating of a human heart or the vibration of a cello string. While self-oscillation is well understood in classical nonlinear systems and their quantized counterparts, the spontaneous emergence of periodicity in quantum systems is more elusive. Here, we show that this behavior can emerge within the repeated-interaction description of open quantum systems. Specifically, we consider a many-body quantum system that undergoes dissipation due to sequential coupling with auxiliary systems at random times. We develop dynamical symmetry conditions that guarantee an oscillatory long-time state in this setting. Our rigorous results are illustrated with specific spin models, which could be implemented in trapped-ion quantum simulators.

Inspired by the recent work [Nature (London) 521, 336 (2015)], we propose a scheme to suppress the backaction-induced dephasing of the spin state in a spin-oscillator system by squeezing the quantum fluctuation of the two-mode oscillator. We show specifically that squeezing either on a single mode individually or on both modes simultaneously could be used to effectively suppress spin dephasing. Therefore, with a spin-oscillator interferometer input by the spin-dependent squeezing cat state, the sensitivity of the rotational parameter estimation can be significantly enhanced. Given that various squeezings on the two-mode vibrations of the oscillator have been experimentally demonstrated, the proposal could provide a possibility to implement the sensitive parameter estimation, particularly in the ion-trap system.

Realizing nonunitary transformations on unitary-gate-based quantum devices is critically important for simulating a variety of physical problems, including open quantum systems and subnormalized quantum states. We present a dilation-based algorithm to simulate nonunitary operations using probabilistic quantum computing with only one ancilla qubit. We utilize the singular-value decomposition (SVD) to decompose any general quantum operator into a product of two unitary operators and a diagonal nonunitary operator, which we show can be implemented by a diagonal unitary operator in a one-qubit dilated space. While dilation techniques increase the number of qubits in the calculation, and thus the gate complexity, our algorithm limits the operations required in the dilated space to a diagonal unitary operator, which has known circuit decompositions. We use this algorithm to prepare random subnormalized two-level states on a quantum device with high fidelity. Furthermore, we present the accurate nonunitary dynamics of two-level open quantum systems in a dephasing channel and an amplitude-damping channel computed on a quantum device. The algorithm presented will be most useful for implementing general nonunitary operations when the SVD can be readily computed, which is the case for most operators in the noisy intermediate-scale quantum computing era.

We report experimental observations of the rotational structure in photoassociative spectroscopy of ultralong-range Rydberg molecules (ULRRMs) in an ultracold gas of ${}^{86}\mathrm{Sr}$. ULRRM spectroscopy probes scattering wave functions at much larger internuclear separations than photoassociative spectroscopy of low-lying electronic states. At such separations photon recoil momentum can lead to the transfer of significant angular momentum to the Rydberg molecule, evidence of which is provided through the distribution of excited rotational states. The visibility of the rotational structure is enhanced because, for collisions between ground-state ${}^{86}\mathrm{Sr}$ atoms, the large, near-resonant s-wave scattering length approaches the size of the ULRRM. Similar enhancement is not seen for ${}^{84}\mathrm{Sr}$, which has a much smaller scattering length. Results are interpreted with the aid of a theory that accounts for the recoil momentum associated with photoexcitation and the large $s$-wave scattering length in the entrance channel. While the observed rotational splittings and the qualitative trends in the relative intensities of the spectral features are well described by theory, the effects of recoil momentum are more prominent in the measured data than in the theory.

We solved analytically the three-body mass-imbalanced problem embedded in $D$ dimensions for zero-range resonantly interacting particles. We derived the negative energy eigenstates of the three-body Schrödinger equation by imposing the Bethe-Peierls boundary conditions in $D$ dimensions for zero-energy two-body bound states. The solution retrieves the Efimov-like discrete scaling factor dependence with dimension. The analytical form of the mass-imbalanced three-body bound-state wave function can be used to probe the effective dimension of asymmetric cold atomic traps for Feshbach resonances tuned close to the Efimov limit.

We consider the non-Hermitian, parity-time- [$\left(\mathcal{PT}\right)$-] symmetric extensions of the one-dimensional Su-Schrieffer-Heeger model in the topological nontrivial configuration. We study the properties of the topologically protected edge states and develop an effective two-state analytical description of the system that accurately predicts the $\mathcal{PT}$-symmetry-breaking point for the edge states. We verify our analytical results by exact numerical calculations.

Advancing table-top attosecond sources in brightness and pulse duration is of immense interest and importance for an expanding sphere of applications. Recent theoretical studies [New J. Phys. 22, 093030 (2020)] found that high-order frequency mixing (HFM) in a two-color laser field can be much more efficient than high-order harmonic generation (HHG). Here we study the attosecond properties of the coherent extreme ultraviolet (XUV) generated via HFM analytically and numerically, focusing on the practically important case when one of the fields has much lower frequency and much lower intensity than the other one. We derive simple analytical equations describing intensities and phase locking of the HFM spectral components. We show that the duration of attosecond pulses generated via HFM, while being very similar to that obtained via HHG in the plateau, is shortened for the cutoff region. Moreover, our study demonstrates that the carrier-envelope phase of the attopulses produced via HFM, in contrast to HHG, can be easily controlled by the phases of the generating fields.

The notions of cross-spectral purity and spectral invariance of light impose specific structures for the field coherence. Such concepts were originally introduced for stationary light and recently extended to nonstationary fields. In this work, we establish general conditions for transforming scalar, pulsed, isodiffracting light beams produced, for instance, in usual spherical-mirror laser resonators, to nonstationary secondary sources that exhibit cross-spectral purity or spectral invariance. Further, we introduce hybrid refractive-diffractive imaging systems which perform the desired transformation accurately over a wide spectral range irrespective of the spatial coherence of the incident isodiffracting beam.

Multipartite nonlocality and Bell-type inequalities are used to characterize topological quantum phase transitions (QPTs) in a spinless fermion quantum wire, where both uniform potentials and incommensurate potentials are considered. First, the nonlocality measures show clear signals at the critical points in both the uniform model and the incommensurate model. It indicates that these QPTs are accompanied by dramatic changes of multipartite quantum correlations in the ground states. Second, finite-size scaling analysis is carried out. In particular, in the incommensurate model where translation invariance is broken, with some rescaling techniques, we successfully establish the scaling formula in the large-$L$ limit. Finally, the full phase diagram of the model with mixed potentials is figured out. We find a region which is featured with strong randomness in the large-$L$ limit. The structure of this region is revealed by analyzing the energy spectrum, and an efficient approach to characterize this region is proposed.

For a quantum system undergoing non-Markovian open quantum dynamics, we demonstrate a tomography algorithm based on multitime measurements of the system, which reconstructs a minimal environment coupled to the system, such that the system plus environment undergoes unitary evolution and that the reduced dynamics of the system is identical to the observed dynamics of it. The reconstructed open quantum evolution model can be used to predict any future dynamics of the system when it is further assumed to be time-independent. We define the memory size and memory complexity for the non-Markovian open quantum dynamics which characterize the complexity of the reconstruction algorithm.

Quantum information processing with multilevel systems (qudits) provides additional features and applications than the two-level systems. However, qudits are more prone to dephasing and dynamical decoupling for qudits has never been experimentally demonstrated. Here, as a proof-of-principle demonstration, we experimentally apply dynamical decoupling to protect superpositions with three levels of a trapped ${}^9\mathrm{Be}^{+}$ ion from ambient noisy magnetic field, prolonging coherence by up to approximately an order of magnitude. Our demonstration, straightforwardly scalable to more levels, may open up a path toward long coherence quantum memory, metrology, and information processing with qudits.

We study photon-mediated interactions between molecular spin qudits in the dispersive regime of operation. We derive from a microscopic model the effective interaction between molecular spins, including their crystal field anisotropy (i.e., the presence of nonlinear spin terms) and their multilevel structure. Finally, we calculate the long-time dynamics for a pair of interacting molecular spins using the method of multiple-scale analysis. This allows us to find a set of two-qudit gates that can be realized for a specific choice of molecular spins and to determine the time required for their implementation. Our results are relevant for the implementation of logical gates in general systems of qudits with unequally spaced levels or to determine an adequate computational subspace to encode and process the information.

Quantum interfaces (QIs) that generate entanglement between a multimode atomic memory and a photon form a multiplexed repeater node and could greatly improve quantum repeater rates. Recently, a temporal multimode spin-wave memory entangled with a photon was demonstrated with cold atoms [Wen et al., Phys. Rev. A 100, 012342 (2019)]. However, due to the additional noise generated in a multimode operation, the fidelity of the spin-wave–photon entanglement significantly decreases with the mode number. So far, improvements in the temporal-multimode entanglement fidelity associated with suppressing the additional noise have not been explored. In this work, we propose and experimentally demonstrate a scheme that can suppress the additional noise of a temporally multiplexed QI. The scheme uses an asymmetric channel to retrieve the photons coming from the temporally multiplexed QI. For comparison purposes, we also set up a QI that uses a symmetric channel for photon collection. When the QIs store 14 modes, the measured Bell parameters $S$ for the QIs using the asymmetric and the symmetric photon-collection channels are 2.36 ± 0.03 and 2.24 ± 0.04, respectively, which means that a QI using an asymmetric channel provides a 3% increase in the entanglement fidelity, i.e., a 1.7-fold decrease in the additional noise, compared with a QI using a symmetric channel. In addition, the 14-mode entanglement QIs that use the asymmetric and symmetric collections preserve the violation of a Bell inequality for storage times of up to ∼25 and ∼20 μs, respectively, showing that the asymmetric QI has better entanglement storage performance.

The best limit on the electron electric dipole moment (eEDM) comes from the ACME II experiment [Nature (London) 562, 355 (2018)] which probes physics beyond the standard model at energy scales well above 1 TeV. ACME II measured the eEDM by monitoring electron spin precession in a cold beam of thorium monoxide (ThO) molecules in the metastable $H{}^3{\mathrm{\Delta }}_1$ state, with an observation time $\tau \approx 1$ ms for each molecule. We report here a new measurement of the lifetime of the ThO ($H{}^3{\mathrm{\Delta }}_1$) state, ${\tau }_H=4.2\pm 0.5$ ms. Using an apparatus within which $\tau \approx {\tau }_H$ will enable a substantial reduction in uncertainty of an eEDM measurement.

Quantum systems with multiharmonic dressing have been extensively investigated and exhibited various novel phenomena and technologies by virtue of their robust electromagnetic response. Here we mainly investigate the self-generated longitudinal Floquet modulation phenomenon of an alignment-based magnetic-resonance system with pump-probe structure dressed by two different transverse radio-frequency (rf) fields in a warm atomic cesium ensemble, both theoretically and experimentally. The self-generated Floquet modulation includes first and second harmonics, which are contributed to by linear and nonlinear processes from the weak-dressed field, respectively. The self-generated Floquet modulation dresses the alignment absorption spectra into two continua. The asymmetry of each dressed continuum is due to the generalized parity symmetry of the system being broken by the second harmonic modulation. Moreover, the nonlinear modification effect of the weak rf field on the probe verifies the existence of the four-wave mixing process in the rf band. The analytical expression, establishment process, and asymmetry analysis of the continua are given concisely. Our results can be applied to the fields of precision magnetic-field measurement, quantum control, quantum simulation, and so on. In addition, because the physical mechanism is universal, it can be applied not only to an atomic ensemble, but also to solid-state devices.

Molecular dimers have attracted much attention in the study of molecular structure and dynamics due to their complex interactions involving both weak van der Waals and strong covalent bonds. We investigate the dissociative ionization of carbon dioxide dimers exposed to intense femtosecond laser fields. The angular distributions of ionic fragments of the breakup channels ${\left({\mathrm{CO}}_2\right)}_2{}^{2+}\to {\mathrm{CO}}_2^{+}+{\mathrm{CO}}_2^{+}$ and ${\left({\mathrm{CO}}_2\right)}_2{}^{3+}\to {\mathrm{CO}}_2{}^{2+}+{\mathrm{CO}}_2^{+}$ strongly depend on the laser intensity. Simulations based on time-dependent density-functional theory reproduce the experimental observations qualitatively and show that the angular distribution of fragments is determined by the angle-dependent orbital ionization probability and the relative contributions of different orbitals, both of which are intensity sensitive. By comparing the ionization of the dimer with ${\mathrm{CO}}_2$ monomer, we find that the weak van der Waals bond and molecular geometry in the dimer play considerable roles. This work extends significantly earlier studies of simple linear covalent bond molecules.

We report on an experimental and theoretical study of postcollision interaction (PCI) effects on $L_2-M_{4,5}{N}_{2,3}$ Auger electrons measured above the Kr $K$-edge in which $L_2$ vacancies are primarily generated by $K{L}_2$ x-ray emission. Such cascade processes, in which a deep inner-shell vacancy decays first by x-ray emission followed by Auger electron emission, is a strong decay mode in heavy atoms. The $L_2-M_{4,5}{N}_{2,3}$ Auger electron peak is observed to become increasingly asymmetric with a shifting peak maximum as the absorbed x-ray energy approaches the $K$-shell ionization threshold. This is attributed to PCI energy exchanges of the Auger electron with the $1s$ photoelectron. To model the PCI effects, we have applied a semiclassical approach modified to account for the combined lifetimes of the $K$ and $L_2$ hole states. In addition, our analysis treats several closely spaced Auger transitions with final ionic states having different terms and angular momenta.

A prerequisite for laser cooling a molecular anion, which has not been achieved so far, is the precise knowledge of the relevant transition frequencies in the cooling scheme. To determine these frequencies we present a versatile method that uses one pump and one photodetachment light beam. We apply this approach to ${\mathrm{C}}_2{}^{-}$ and study the laser-cooling transitions between the electronic ground state and the second electronic excited state in their respective vibrational ground levels, $B{}^2{\mathrm{\Sigma }}_u^{+}(v=0)\leftarrow X{}^2{\mathrm{\Sigma }}_g^{+}(v=0)$. Measurements of the $R\left(0\right), R\left(2\right)$, and $P\left(2\right)$ transitions are presented, which determine the transition frequencies with a wave-meter-based accuracy of $0.7\times {10}^{-3}{\mathrm{cm}}^{-1}$ or 20 MHz. The spin-rotation splitting is resolved, which allows for a more precise determination of the splitting constants to ${\gamma }^{\prime }=7.15\left(19\right)\times {10}^{-3} {\mathrm{cm}}^{-1}$ and ${\gamma }^{\prime \prime }=4.10\left(27\right)\times {10}^{-3} {\mathrm{cm}}^{-1}$. These results are used to characterize the ions in the cryogenic 16-pole wire trap employed in this experiment. The translational and rotational temperature of the ions cooled by helium buffer gas are derived from the Doppler widths and the amplitude ratios of the measured transitions. The results support the common observation that the translational temperature is higher than the buffer gas temperature due to collisional heating under micromotion, in particular, at low temperatures. Additionally, a rotational temperature significantly lower than the translational temperature is measured, which agrees with the notion that the mass-weighted collision temperature of the ${\mathrm{C}}_2{}^{-}$-He system defines the internal rotational state population.