Strong Quantum Correlations Research Articles

Generation of phonon quantum states and quantum correlations among single photon emitters in hexagonal boron nitride

Hexagonal boron nitride exhibits two types of defects with great potential for quantum information technologies: single-photon emitters (SPEs) and one-dimensional grain boundaries hosting topologically-protected phonons, termed as topologically-protected phonon lines (TPL). Here, by means of a simple effective model and density functional theory calculations, we show that it is possible to use these phonons for the transmission of information. Particularly, a single SPE can be used to induce single-, two- and qubit-phonon states in the one-dimensional channel, and (ii) two distant SPEs can be coupled by the TPL that acts as a waveguide, thus exhibiting strong quantum correlations. We highlight the possibilities offered by this material-built-in nano-architecture as a phononic device for quantum information technologies.

Quantum correlations in a chain-type quantum network

A quantum network concerns several independent entangled resources and can create strong quantum correlations by performing joint measurements on some observers. In this paper, we discuss an n-partite chain network with each of two neighboring observers sharing an arbitrary Bell state and all intermediate observers performing some positive-operator-valued measurements with parameter λ. The expressions of all post-measurement states between any two observers are obtained, and their quantifications of Bell nonlocality, Einstein–Podolsky–Rosen steering and entanglement with different ranges of λ are respectively detected and analyzed.

Observation of quantum-correlated twin beams in cascaded nonlinear interactions.

We report on the generation of twin beams through a cascaded process of optical parametric oscillation in a doubly resonant second-harmonic generation system. These bright beams exhibit strong quantum correlations, enabling the observation of up to 5 dB of noise reduction in their intensity difference below the standard quantum limit.

Strong quantum correlation in hybrid opto-mechanics

Strong quantum correlation in hybrid opto-mechanics

Multiwavelength Quantum Correlated Photon Pair Generation for Quantum Entanglement Key Distribution

With the rapid advancement of quantum information technology, fully connected multi-user quantum entanglement distribution networks have garnered increasing attention. Among these, multi-wavelength quantum light sources are key devices for establishing connections between multiple users. Despite recent impressive advances, it remains challenging to increase the wavelength number of photon pairs due to limitations in the design and fabrication of nonlinear optical devices. This work explores the potential of silicon nitride (Si<sub>3</sub>N<sub>4</sub>) microring resonators (MRRs) as scalable platforms for multi-wavelength quantum light sources.<br>In this work, we conduct a comprehensive analysis of the key design parameters of the Si<sub>3</sub>N<sub>4</sub> MRRs, including waveguide dimension, resonator dispersion, and coupling condition, to optimize photon-pair generation. Based on these parameters, a Si<sub>3</sub>N<sub>4</sub> MRR with a free spectral range of 20 GHz and an average quality factor of 1.6 million is designed and fabricated. This small free spectral range enables the generation of more channels of correlated photon pairs using the same wavelength resources. The high-quality of the resonator facilitates the high photon pair generation rate, which are critical for quantum entanglement distribution. With a continuous-wave pump laser, correlated photon pairs across a wide spectral range are generated through the spontaneous four-wave mixing (SFWM). The coincidence-to-accidental ratio (CAR) measurements verify the strong quantum correlation between photon pairs, highlighting the reliability of the system for entanglement distribution. Furthermore, the generation and output characteristics of quantum-correlated photon pairs are experimentally investigated with a tunable bandpass filter. The results demonstrate that the successful generation of 71 wavelength-correlated photon pairs within a 25.6 nm spectral range is achieved as shown in the Figure A. Our results pave the way for the development of multi-wavelength quantum light sources with Si<sub>3</sub>N<sub>4</sub> platform, thereby advancing the multi-user quantum networks.

Deterministic manipulation of steering between distant quantum network nodes.

Multipartite Einstein-Podolsky-Rosen (EPR) steering is a key resource in a quantum network. Although EPR steering between spatially separated regions of ultracold atomic systems has been observed, deterministic manipulation of steering between distant quantum network nodes is required for a secure quantum communication network. Here, we propose a feasible scheme to deterministically generate, store, and manipulate one-way EPR steering between distant atomic cells by a cavity-enhanced quantum memory approach. While optical cavities effectively suppress the unavoidable noises in electromagnetically induced transparency, three atomic cells are in a strong Greenberger-Horne-Zeilinger state by faithfully storing three spatially separated entangled optical modes. In this way, the strong quantum correlation of atomic cells guarantees one-to-two node EPR steering is achieved, and can perserve the stored EPR steering in these quantum nodes. Furthermore, the steerability can be actively manipulated by the temperature of the atomic cell. This scheme provides the direct reference for experimental implementation for one-way multipartite steerable states, which enables an asymmetric quantum network protocol.

Fundamental resolution limit of quantum imaging with undetected photons

Quantum imaging with undetected photons relies on the principle of induced coherence without induced emission and uses two sources of photon-pairs with a signal- and an idler photon. Each pair shares strong quantum correlations in both position and momentum, which allows to image an object illuminated with idler photons by just measuring signal photons that never interact with the object. In this work, we theoretically investigate the transverse resolution of this non-local imaging scheme through a general formalism that treats propagating photons beyond the commonly used paraxial approximation. We hereby prove that the resolution of quantum imaging with undetected photons is fundamentally diffraction limited to the longer wavelength of the signal and idler pairs. Moreover, we conclude that this result is also valid for other non-local two-photon imaging schemes.

Nonadiabatic dissociation of molecular Bose-Einstein condensates: Competition between chemical reactions

We provide a framework to solve generic models describing the dissociation of multiple molecular Bose-Einstein condensates in a nonadiabatic regime. The competition between individual chemical reactions can lead to non-trivial dependence on critical components such as path interference and symmetries, thus, affecting the final distribution of atomic population. We find an analytical solution for an illustrative example model involving four atomic modes. When the system parameters satisfy $CPT$ symmetry, where $C$ is charge conjugation, $P$ is parity, and $T$ is time-reversal symmetry, our solution predicts a population imbalance between atomic modes that is exponentially sensitive to system parameters. However, a weakly broken symmetry alters the population in each atomic mode and can reverse the population imbalance. Our solution also demonstrates a strong quantum correlation between atomic modes that leads to the spontaneous production of atoms in a multi-mode squeezed state. Moreover, in our framework, a time-dependent non-Hermitian quantum mechanics naturally manifests which can alternatively be realized experimentally in photonic systems.

Generation of tripartite squeezed state by cascaded four-wave mixing in single hot rubidium atomic system

We report the quantum correlated triple beams via cascaded four-wave mixing (CFWM) amplified in single hot atomic vapor. Experimentally, we show that strong quantum correlation of three light beams, among them any two’s quantum correlation is characterized by the maximum value of intensity-difference squeezing (IDS) about −7.8 ± 0.3 dB. We found there is IDS between two idler beams, because two pairs of Einstein–Podolsky–Rosen injections potentially exist in our system. Besides, CFWM can emit three-mode beams at three different frequencies, in which these beams can be well separated in the spatial domain. Moreover, much difference with other methods, the injecting probe field can manipulate the gain and IDS of output three-mode light beams, which is resulting from competition relationship between cascaded two four-wave mixing processes. More interestingly, Autler–Townes splitting of gain peaks of output signals due to dressing effect of pumping fields, will lead to the evolution of measured two- and three-mode IDS from single-mode to multi-mode at frequency domain. This result will provide a multimode quantum resource which can potentially realize multimode entanglement and quantum networks.

Two-colour high-purity Einstein-Podolsky-Rosen photonic state

We report a high-purity Einstein-Podolsky-Rosen (EPR) state between light modes with the wavelengths separated by more than 200 nm. We demonstrate highly efficient EPR-steering between the modes with the product of conditional variances {{{{{{{{mathcal{E}}}}}}}}}^{2}=0.11pm 0.01ll 1. The modes display − 7.7 ± 0.5 dB of two-mode squeezing and an overall state purity of 0.63 ± 0.16. EPR-steering is observed over five octaves of sideband frequencies from RF down to audio-band. The demonstrated combination of high state purity, strong quantum correlations, and extended frequency range enables new matter-light quantum protocols.

Metastable magnetization plateaus in the S =1 organic spin ladder BIP-TENO induced by a microsecond-pulsed megagauss field

The entire magnetization process in the two-leg organic spin ladder BIP-TENO is studied with an ultrahigh magnetic field of up to 150 T. Nontrivial multiple magnetization plateaus are observed, indicating there exist strong quantum correlations between spins. In addition to the previously reported 1/4 plateau, another plateau appears at 1/3 magnetization only when spins are decoupled to the lattice degree of freedom in the adiabatic limit. Spontaneous symmetry breaking is expected to occur with the spin-lattice decoupling induced by a fast evolution of external magnetic fields in the range of $\ensuremath{\mu}$s. Under the adiabatic condition, five fractional (1/4, 1/3, 1/2, 2/3, and 3/4) magnetization plateaus are likely to appear with an assumption that the magnetization saturates at around 160 T.

Engineering topological states in atom-based semiconductor quantum dots

The realization of controllable fermionic quantum systems via quantum simulation is instrumental for exploring many of the most intriguing effects in condensed-matter physics1–3. Semiconductor quantum dots are particularly promising for quantum simulation as they can be engineered to achieve strong quantum correlations. However, although simulation of the Fermi–Hubbard model4 and Nagaoka ferromagnetism5 have been reported before, the simplest one-dimensional model of strongly correlated topological matter, the many-body Su–Schrieffer–Heeger (SSH) model6–11, has so far remained elusive—mostly owing to the challenge of precisely engineering long-range interactions between electrons to reproduce the chosen Hamiltonian. Here we show that for precision-placed atoms in silicon with strong Coulomb confinement, we can engineer a minimum of six all-epitaxial in-plane gates to tune the energy levels across a linear array of ten quantum dots to realize both the trivial and the topological phases of the many-body SSH model. The strong on-site energies (about 25 millielectronvolts) and the ability to engineer gates with subnanometre precision in a unique staggered design allow us to tune the ratio between intercell and intracell electron transport to observe clear signatures of a topological phase with two conductance peaks at quarter-filling, compared with the ten conductance peaks of the trivial phase. The demonstration of the SSH model in a fermionic system isomorphic to qubits showcases our highly controllable quantum system and its usefulness for future simulations of strongly interacting electrons.

Autoregressive neural-network wavefunctions for ab initio quantum chemistry

In recent years, neural-network quantum states have emerged as powerful tools for the study of quantum many-body systems. Electronic structure calculations are one such canonical many-body problem that have attracted sustained research efforts spanning multiple decades, whilst only recently being attempted with neural-network quantum states. However, the complex non-local interactions and high sample complexity are substantial challenges that call for bespoke solutions. Here, we parameterize the electronic wavefunction with an autoregressive neural network that permits highly efficient and scalable sampling, whilst also embedding physical priors reflecting the structure of molecular systems without sacrificing expressibility. This allows us to perform electronic structure calculations on molecules with up to 30 spin orbitals—at least an order of magnitude more Slater determinants than previous applications of conventional neural-network quantum states—and we find that our ansatz can outperform the de facto gold-standard coupled-cluster methods even in the presence of strong quantum correlations. With a highly expressive neural network for which sampling is no longer a computational bottleneck, we conclude that the barriers to further scaling are not associated with the wavefunction ansatz itself, but rather are inherent to any variational Monte Carlo approach. To perform electronic structure calculations in quantum chemistry systems, methods are needed that are both accurate and scalable as the size of the molecule of interest increases. Barrett and colleagues employ an autoregressive neural-network ansatz that allows them to study larger molecules than previously attempted with neural-network quantum state approaches.

Ultra-strong nonreciprocal amplification with hot atoms

Optical nonreciprocity plays an important role in optical communication and signal processing. Here, we propose and demonstrate a magnetic-free optical nonreciprocal amplification based on a far-detuned four-wave mixing process in hot atoms. The absorption of the probe field can be significantly reduced by the large detuning. In our experiment, with just one pump field, an ultra-strong forward gain of 45 dB is achieved while the backward one keeps a near unit transmission. The experimental realization of ultra-strong nonreciprocal amplification may have applications for the generation of strong nonreciprocal quantum correlations.

The effect of nonequilibrium entropy production on the quantum Fisher information and correlations

In this study, we apply quantum master equations beyond secular approximation and investigate the nonequilibrium thermodynamic cost of enhanced quantum metrology and quantum correlations. We find that the nonequilibrium conditions enhance quantum Fisher information (QFI) and quantum correlations predominantly for weak tunneling scenarios. The enhancement is assisted by a corresponding increase in the thermodynamic cost characterized by the entropy production rate (EPR). For the strong tunneling regimes, the QFI and quantum correlations cannot be unceasingly boosted by higher thermodynamic costs and decay once the system is overburdened with extremely large energy currents. The result indicates that for open systems with weak tunneling rates, thermodynamic cost can be potentially exploited to improve the quantum metrology and quantum correlations.

Dynamics of Quantum Correlation and Entropic Uncertainty in Spin‐12$\frac{1}{2}$ Alternating Transverse Ising Model

AbstractBy implementing the quantum renormalization group method, the effective Hamiltonian of the spin‐ Ising model with regularly altering strengths of both the magnetic fields and the exchange couplings are first derived analytically. Then, tripartite quantum discord (QD) and entropic uncertainty (EU) of the Ising model and their quantum phase transitions are investigated. Moreover, the quantum critical points and correlation length exponents of the model are analyzed, and both the block–block EU and the site–site EU are discussed through numerical calculations. It is found that under the system size increasing, the first derivatives of both tripartite QD and block–block EU show nonanalytic and scaling behaviors near the critical point. Furthermore, the strong quantum correlations among multi‐particles do not necessarily lead to the decrease of EU, as the site–site EU in the vicinity of the critical point increases due to competitions among quantum correlations. The findings may offer new insights into utilizing EU as an observable of the quantum phase transitions.

Excitations of a Bose-Einstein condensate and the quantum geometry of a flat band

The quantum geometry of Bloch states fundamentally affects a wide range of physical phenomena. The quantum Hall effect, for example, is governed by the Chern number, and flat-band superconductivity by the distance between the Bloch states: the quantum metric. While understanding quantum geometry phenomena in the context of fermions is well established, less is known about the role of quantum geometry in bosonic systems where particles can undergo Bose-Einstein condensation (BEC). In conventional single-band or continuum systems, excitations of a weakly interacting BEC are determined by the condensate density and the interparticle interaction energy. In contrast to this, we discover here fundamental connections between the properties of a weakly interacting BEC and the underlying quantum geometry of a multiband lattice system. We show that, in the flat-band limit, the defining physical quantities of BEC, namely, the speed of sound and the quantum depletion, are dictated solely by the quantum geometry. We find that the speed of sound becomes proportional to the quantum metric of the condensed state. Furthermore, the quantum distance between the Bloch functions forces the quantum depletion and the quantum fluctuations of the density-density correlation to obtain finite values for infinitesimally small interactions. This is in striking contrast to dispersive bands where these quantities vanish with the interaction strength. Additionally, we show how in the flat-band limit the supercurrent is carried by the quantum fluctuations and is determined by the Berry connections of the Bloch states. Our results reveal how nontrivial quantum geometry allows reaching strong quantum correlation regime of condensed bosons even with weak interactions. This is highly relevant, for example, for polariton and photon BECs where interparticle interactions are inherently small. Our predictions can be experimentally tested with flat-band lattices already implemented in ultracold gases and various photonic platforms.

Quantum Geometry and Flat Band Bose-Einstein Condensation.

We study the properties of a weakly interacting Bose-Einstein condensate (BEC) in a flat band lattice system by using the multiband Bogoliubov theory and discover fundamental connections to the underlying quantum geometry. In a flat band, the speed of sound and the quantum depletion of the condensate are dictated by the quantum geometry, and a finite quantum distance between the condensed and other states guarantees stability of the BEC. Our results reveal that a suitable quantum geometry allows one to reach the strong quantum correlation regime even with weak interactions.

Theory of exciton thermal radiation in semiconducting single-walled carbon nanotubes.

Spectral control of thermal radiation is an essential strategy for highly efficient and functional utilization of thermal radiation energy. Among the various proposed methods, quantum confinement in low-dimensional materials is promising because of its inherent ability to emit narrowband thermal radiation. Here, we theoretically investigate thermal radiation from one-dimensional (1D) semiconductors characterized by the strong quantum correlation effect due to the Coulomb interaction. We derive a simple and useful formula for the emissivity, which is then used to calculate the thermal radiation spectrum of semiconducting single-walled carbon nanotubes as a representative of 1D semiconductors. The calculations show that the exciton state, which is an electron-hole pair mutually bound by the Coulomb interaction, causes enhancement of the radiation spectrum peak and significant narrowing of its linewidth in the near-infrared wavelength range. The theory developed here will be a firm foundation for exciton thermal radiation in 1D semiconductors, which is expected to lead to new energy harvesting technologies.

Asymptotic Entanglement Sudden Death in Two Atoms with Dipole-Dipole and Ising Interactions Coupled to a Radiation Field at Non-Zero Detuning.

We investigate the time evolution and asymptotic behavior of a system of two two-level atoms (qubits) interacting off-resonance with a single mode radiation field. The two atoms are coupled to each other through dipole–dipole as well as Ising interactions. An exact analytic solution for the system dynamics that spans the entire phase space is provided. We focus on initial states that cause the system to evolve to entanglement sudden death (ESD) between the two atoms. We find that combining the Ising and dipole–dipole interactions is very powerful in controlling the entanglement dynamics and ESD compared with either one of them separately. Their effects on eliminating ESD may add up constructively or destructively depending on the type of Ising interaction (Ferromagnetic or anti-Ferromagnetic), the detuning parameter value, and the initial state of the system. The asymptotic behavior of the ESD is found to depend substantially on the initial state of the system, where ESD can be entirely eliminated by tuning the system parameters except in the case of an initial correlated Bell state. Interestingly, the entanglement, atomic population and quantum correlation between the two atoms and the field synchronize and reach asymptotically quasi-steady dynamic states. Each one of them ends up as a continuous irregular oscillation, where the collapse periods vanish, with a limited amplitude and an approximately constant mean value that depend on the initial state and the system parameters choice. This indicates an asymptotic continuous exchange of energy (and strong quantum correlation) between the atoms and the field takes place, accompanied by diminished ESD for these chosen setups of the system. This system can be realized in spin states of quantum dots or Rydberg atoms in optical cavities, and superconducting or hybrid qubits in linear resonators.