Quantum Correlations In States Research Articles

Adaptive Phase Estimation with Squeezed Vacuum Approaching the Quantum Limit

Phase estimation plays a central role in communications, sensing, and information processing. Quantum correlated states, such as squeezed states, enable phase estimation beyond the shot-noise limit, and in principle approach the ultimate quantum limit in precision, when paired with optimal quantum measurements. However, physical realizations of optimal quantum measurements for optical phase estimation with quantum-correlated states are still unknown. Here we address this problem by introducing an adaptive Gaussian measurement strategy for optical phase estimation with squeezed vacuum states that, by construction, approaches the quantum limit in precision. This strategy builds from a comprehensive set of locally optimal POVMs through rotations and homodyne measurements and uses the Adaptive Quantum State Estimation framework for optimizing the adaptive measurement process, which, under certain regularity conditions, guarantees asymptotic optimality for this quantum parameter estimation problem. As a result, the adaptive phase estimation strategy based on locally-optimal homodyne measurements achieves the quantum limit within the phase interval of [0,π/2). Furthermore, we generalize this strategy by including heterodyne measurements, enabling phase estimation across the full range of phases from [0,π), where squeezed vacuum allows for unambiguous phase encoding. Remarkably, for this phase interval, which is the maximum range of phases that can be encoded in squeezed vacuum, this estimation strategy maintains an asymptotic quantum-optimal performance, representing a significant advancement in quantum metrology.

Trimer quantum spin liquid in a honeycomb array of Rydberg atoms

Quantum spin liquids are elusive but paradigmatic examples of strongly correlated quantum states that are characterized by long-range quantum entanglement. Recently, signatures of a gapped topological Z2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\mathbb{Z}}}_{2}$$\\end{document} spin liquid have been observed in a system of Rydberg atoms; however, the full capability of these platforms to realize quantum spin liquids extends far beyond this state alone. Here, we propose the realization of a different class of spin liquids in a honeycomb array of Rydberg atoms. Exploring the system’s quantum phase diagram using density-matrix renormalization group and exact diagonalization calculations, we identify several density-wave-ordered phases and a trimer spin liquid ground state with an emergent U(1) × U(1) local symmetry. This liquid state originates from superpositions of classical trimer configurations on the dual triangular lattice in the regime where third-nearest-neighbor atoms lie within the Rydberg blockade radius. Finally, we discuss the conditions to enhance the preparation fidelity of this state by a general Rydberg-blockade-based projection mechanism, test the robustness of the trimer spin liquid phase in a range of realistic parameters, and outline methods for its experimental detection.

Switching global correlations on and off in a many-body quantum state by tuning local entanglement.

A quantum many-body state built on a classical 1D Ising model with locally entangled qubits is considered. This setup can model an infinite player quantum Prisoner's dilemma game with each site representing two entangled players (or qubits). The local entanglement γ between two qubits placed on a site in the 1D Ising model and classical coupling between adjacent sites of the Ising model have an apposite influence on qubits. It points to a counter-intuitive situation wherein local entanglement at a site can exactly cancel global correlations, signaling an artificial quantum many-body state wherein, by locally tuning the entanglement at a particular site, one can transition from a strongly correlated quantum state to an uncorrelated quantum state and then to a correlated classical state. In other words, we can simulate a state similar to a type II superconducting state via local tuning of entanglement in a 1D Ising chain with entangled qubits.

Self-Intercalated 1T-FeSe2 as an Effective Kagome Lattice.

In kagome lattice, with the emergence of Dirac cones and flat band in electronic structure, it provides a versatile ground for exploring intriguing interplay among frustrated geometry, topology and correlation. However, such engaging interest is strongly limited by available kagome materials in nature. Here we report on a synthetic strategy of constructing kagome systems via self-intercalation of Fe atoms into the van der Waals gap of FeSe2 via molecular beam epitaxy. Using low-temperature scanning tunneling microscopy, we unveil a kagome-like morphology upon intercalating a 2 × 2 ordered Fe atoms, resulting in a stoichiometry of Fe5Se8. Both the bias-dependent STM imaging and theoretical modeling calculations suggest that the kagome pattern mainly originates from slight but important reconstruction of topmost Se atoms, incurred by the nonequivalent subsurface Fe sites due to the intercalation. Our study demonstrates an alternative approach of constructing artificial kagome structures, which envisions to be tuned for exploring correlated quantum states.

Machine learning discovery of new phases in programmable quantum simulator snapshots

Machine learning has recently emerged as a promising approach for studying complex phenomena characterized by rich datasets. In particular, data-centric approaches lead to the possibility of automatically discovering structures in experimental datasets that manual inspection may miss. Here, we introduce an interpretable unsupervised-supervised hybrid machine learning approach, the hybrid-correlation convolutional neural network (hybrid-CCNN), and apply it to experimental data generated using a programmable quantum simulator based on Rydberg atom arrays. Specifically, we apply hybrid-CCNN to discover and identify new quantum phases on square lattices with programmable interactions. The initial unsupervised dimensionality reduction and clustering stage first reveals five distinct quantum phase regions. In a second supervised stage, we refine these phase boundaries and seek insights into the phases by training multiple CCNN classifiers. A learned spatial weighting, introduced to the CCNNs in this work, enables discovery of spatial structure at scales beyond the filter size. The characteristic spatial weightings and snippets of correlations specifically recognized in each phase capture quantum fluctuations in the striated phase and identify a previously undetected boundary-ordered phase as well as motifs of more exotic ordered phases. These observations demonstrate that a combination of programmable quantum simulators with machine learning can be used as a powerful tool for detailed exploration of correlated quantum states of matter.

Super-exponential distinguishability of correlated quantum states

In the problem of asymptotic binary i.i.d. state discrimination, the optimal asymptotics of the type I and the type II error probabilities is in general an exponential decrease to zero as a function of the number of samples; the set of achievable exponent pairs is characterized by the quantum Hoeffding bound theorem. A super-exponential decrease for both types of error probabilities is only possible in the trivial case when the two states are orthogonal and hence can be perfectly distinguished using only a single copy of the system. In this paper, we show that a qualitatively different behavior can occur when there is correlation between the samples. Namely, we use gauge-invariant and translation-invariant quasi-free states on the algebra of the canonical anti-commutation relations to exhibit pairs of states on an infinite spin chain with the properties that (a) all finite-size restrictions of the states have invertible density operators and (b) the type I and the type II error probabilities both decrease to zero at least with the speed e^{-nclog n} with some positive constant c, i.e., with a super-exponential speed in the sample size n. Particular examples of such states include the ground states of the XX model corresponding to different transverse magnetic fields. In fact, we prove our result in the setting of binary composite hypothesis testing, and hence, it can be applied to prove super-exponential distinguishability of the hypotheses that the transverse magnetic field is above a certain threshold vs. that it is below a strictly lower value.

Spin liquid state in a rare-earth hyperkagome lattice

Quantum fluctuations enhanced by frustration and subtle interplay between competing degrees of freedom offer an ideal ground to realize novel states with fractional quantum numbers in quantum materials that defy standard theoretical paradigms. Quantum spin liquid (QSL) is a highly entangled state wherein frustration induced strong quantum fluctuations preclude symmetry breaking phase transitions down to zero temperature without any order parameter. Experimental realizations of QSL in quantum materials with spin dimensionality greater than one is very rare. Here, we present our thermodynamic, nuclear magnetic resonance, muon spin relaxation and inelastic neutron scattering studies of a new rare-earth hyperkagome compound Li3Yb3Te2O12 in which Yb3+ ions constitute a three dimensional spin-lattice without any detectable disorder. Our comprehensive experiments evince neither signature of magnetic ordering nor spin freezing down to 38 mK that suggest the realization of dynamic liquid-like ground state in this antiferromagnet. The ground state of this material is interpreted by a low energy Jeff = 1/2 degrees of freedom with short range spin correlations. The present results demonstrate a viable basis to explore spin-orbit driven enigmatic correlated quantum states in a new class of rare-earth based three dimensional frustrated magnets that may open new avenues in theoretical and experimental search for spin liquids.

Atomic-scale thermopower in charge density wave states

The microscopic origins of thermopower have been investigated to design efficient thermoelectric devices, but strongly correlated quantum states such as charge density waves and Mott insulating phase remain to be explored for atomic-scale thermopower engineering. Here, we report on thermopower and phonon puddles in the charge density wave states in 1T-TaS2, probed by scanning thermoelectric microscopy. The Star-of-David clusters of atoms in 1T-TaS2 exhibit counterintuitive variations in thermopower with broken three-fold symmetry at the atomic scale, originating from the localized nature of valence electrons and their interlayer coupling in the Mott insulating charge density waves phase of 1T-TaS2. Additionally, phonon puddles are observed with a spatial range shorter than the conventional mean free path of phonons, revealing the phonon propagation and scattering in the subsurface structures of 1T-TaS2.

Gate-Defined Electron Interferometer in Bilayer Graphene.

We present an electron interferometer defined purely by electrostatic gating in an encapsulated bilayer graphene. This minimizes possible sample degradation introduced by conventional etching methods when preparing quantum devices. The device quality is demonstrated by observing Aharonov-Bohm (AB) oscillations with a period of h/e, h/2e, h/3e, and h/4e, witnessing a coherence length of many microns. The AB oscillations as well as the type of carriers (electrons or holes) are seamlessly tunable with gating. The coherence length longer than the ring perimeter and semiclassical trajectory of the carrier are established from the analysis of the temperature and magnetic field dependence of the oscillations. Our gate-defined ring geometry has the potential to evolve into a platform for exploring correlated quantum states such as superconductivity in interferometers in twisted bilayer graphene.

Multipartite entanglement to boost superadditivity of coherent information in quantum communication lines with polarization-dependent losses

Coherent information quantifies the achievable rate of the reliable quantum information transmission through a communication channel. Use of the correlated quantum states instead of the factorized ones may result in an increase in the coherent information, a phenomenon known as superadditivity. However, even for simple physical models of channels it is rather difficult to detect the superadditivity and find the advantageous multipartite states. Here we consider the case of polarization dependent losses and propose some physically motivated multipartite entangled states which outperform all factorized states in a wide range of the channel parameters. We show that in the asymptotic limit of the infinite number of channel uses the superadditivity phenomenon takes place whenever the channel is neither degradable nor antidegradable. Besides the superadditivity identification, we also provide a method how to modify the proposed states and get a higher quantum communication rate by doubling the number of channel uses. The obtained results give a deeper understanding of coherent information in the multishot scenario and may serve as a benchmark for quantum capacity estimations and future approaches toward an optimal strategy to transfer quantum information.

Exchange coupling-mediated broken symmetries in Ta2NiSe5 revealed from quadrupolar circular photogalvanic effect.

In low–electron density materials, interactions can lead to highly correlated quantum states of matter. Ta2NiSe5, an excitonic insulator (EI) candidate, exists in a novel broken-symmetry phase below 327 K, characterized by robust exchange interaction and electron-lattice coupling. We study this phase of Ta2NiSe5 using the quadrupole circular photogalvanic effect (QCPGE). Light-matter interaction in Ta2NiSe5 mediated by electric quadrupole/magnetic dipole coupling produces helicity-dependent DC response even with centrosymmetry, making it particularly sensitive to certain other broken symmetries. We show that the exchange interaction in Ta2NiSe5 can lead to a triclinic structure with a broken C2 symmetry. Our results provide an incisive probe of the symmetries of the low-temperature phase of Ta2NiSe5 and add new symmetry constraints to the identification of a strongly correlated EI phase. The high sensitivity of QCPGE to subtle symmetry breaking in centrosymmetric systems will enable its use in studying other complex crystalline systems.

Building ground states of the Hubbard model by time-ordered bound-pair injection

According to energy band theory, ground states of a normal conductor and insulator can be obtained by filling electrons individually into energy levels, without any restrictions. It fails when the electron-electron correlation is taken into account. In this work, we investigate the dynamic process of building ground states of a Hubbard model. It is based on time-ordered quantum quenches for unidirectional hopping across a central and an auxiliary Hubbard model. We find that there exists a set of optimal parameters (chemical potentials and pair binding energy) for the auxiliary system, which takes the role of electron-pair reservoir. The exceptional point dynamics in non-Hermitian quantum mechanics allows the perfect transfer of electron pair from the reservoir to the central system, obtaining its ground states at different fillings. The dynamics of time-ordered pair filling not only provides a method for correlated quantum state engineering, but also reveals the feature of the ground state in an alternative way.

A diagrammatic approach to variational quantum ansatz construction

Variational quantum eigensolvers (VQEs) are a promising class of quantum algorithms for preparing approximate ground states in near-term quantum devices. Minimizing the error in such an approximation requires designing ansatzes using physical considerations that target the studied system. One such consideration is size-extensivity, meaning that the ground state quantum correlations are to be compactly represented in the ansatz. On digital quantum computers, however, the size-extensive ansatzes usually require expansion via Trotter-Suzuki methods. These introduce additional costs and errors to the approximation. In this work, we present a diagrammatic scheme for the digital VQE ansatzes, which is size-extensive but does not rely on Trotterization. We start by designing a family of digital ansatzes that explore the entire Hilbert space with the minimum number of free parameters. We then demonstrate how one may compress an arbitrary digital ansatz, by enforcing symmetry constraints of the target system, or by using them as parent ansatzes for a hierarchy of increasingly long but increasingly accurate sub-ansatzes. We apply a perturbative analysis and develop a diagrammatic formalism that ensures the size-extensivity of generated hierarchies. We test our methods on a short spin chain, finding good convergence to the ground state in the paramagnetic and the ferromagnetic phase of the transverse-field Ising model.

Competition of Superconductivity and Charge Density Wave in Selective Oxidized CsV3Sb5 Thin Flakes

The recently discovered layered kagome metals AV$_3$Sb$_5$ (A = K, Rb, and Cs) with vanadium kagome networks provide a novel platform to explore correlated quantum states intertwined with topological band structures. Here we report the prominent effect of hole doping on both superconductivity and charge density wave (CDW) order, achieved by selective oxidation of exfoliated thin flakes. A superconducting dome is revealed as a function of the effective doping content. The superconducting transition temperature ($T_{\mathrm{c}}$) and upper critical field in thin flakes are significantly enhanced compared with the bulk, which is accompanied by the suppression of CDW. Our detailed analyses establish the pivotal role of van Hove singularities (VHSs) in promoting correlated quantum orders in these kagome metals. Our experiment not only demonstrates the intriguing nature of superconducting and CDW orders, but also provides a novel route to tune the carrier concentration through both selective oxidation and electric gating. This establishes AV$_3$Sb$_5$ as a tunable 2D platform for the further exploration of topology and correlation among 3$d$ electrons in kagome lattices.

Few-body nature of Kondo correlated ground states

The quenching of degenerate impurity states in metals generally induces a long-range correlated quantum state known as the Kondo screening cloud. While a macroscopic number of particles clearly take part in forming this extended structure, assessing the number of truly entangled degrees of freedom requires a careful analysis of the relevant many-body wavefunction. For this purpose, we examine the natural single-particle orbitals that are eigenstates of the single-particle density (correlation) matrix for the ground state of two quantum impurity problems: the interacting resonant level model (IRLM) and the single impurity Anderson model (SIAM). As a simple and general probe for few-body versus many-body character we consider the rate of exponential decay of the correlation matrix eigenvalues towards inactive (fully empty or filled) orbitals. We find that this rate remains large in the physically most relevant region of parameter space, implying a few-body character. Genuine many-body correlations emerge only when the Kondo temperature becomes exponentially small, for instance near a quantum critical point. In addition, we demonstrate that a simple numerical diagonalization of the few-body problem restricted to the Fock space of the most correlated orbitals converges exponentially fast with respect to the number of orbitals, to the true ground state of the IRLM. We also show that finite size effects drastically affect the correlation spectrum, shedding light on an apparent paradox arising from previous studies on short chains.

Atomic Line Defects and Topological Superconductivity in Unconventional Superconductors

Topological superconductors (TSCs) are correlated quantum states with simultaneous off-diagonal long-range order and nontrivial topological invariants. They produce gapless or zero energy boundary excitations, including Majorana zero modes and chiral Majorana edge states with topologically protected phase coherence essential for fault-tolerant quantum computing. Candidate TSCs are very rare in nature. Here, we propose a novel route toward emergent quasi-one-dimensional (1D) TSCs in naturally embedded quantum structures such as atomic line defects in unconventional spin-singlet $s$-wave and $d$-wave superconductors. We show that inversion symmetry breaking and charge transfer due to the missing atoms lead to the occupation of incipient impurity bands and mixed parity spin singlet and triplet Cooper pairing of neighboring electrons traversing the line defect. Nontrivial topological invariants arise and occupy a large part of the parameter space, including the time reversal symmetry breaking Zeeman coupling due to applied magnetic field or defect-induced magnetism, creating TSCs in different topological classes with robust Majorana zero modes at both ends of the line defect. Beyond providing a novel mechanism for the recent discovery of zero-energy bound states at both ends of an atomic line defect in monolayer Fe(Te,Se) superconductors, the findings pave the way for new material realizations of the simplest and most robust 1D TSCs using embedded quantum structures in unconventional superconductors with large pairing energy gaps and high transition temperatures.

Quantum correlation swapping between Werner and separable states

In this paper we study quantum correlation (QC) swapping between two different kinds of quantum-correlated states. To be concrete, one is the famous Werner state while another is a kind of separable state. We employ measurement-induced disturbance (Luo 2008 Phys. Rev. A 77 02230) as the quantifier to characterize and quantify QC. We work out QCs in all the concerned states and concretely analyze their behaviors and properties such monotony features and thresholds. We find that a long-distance shared QC can be generated from two short-distance ones via intermediate measure. Moreover, the amount of the long-distance shared QC cannot reach that in the Werner state but that in the separable state.

Dynamical formation of a magnetic polaron in a two-dimensional quantum antiferromagnet

Tremendous recent progress in the quantum simulation of the Hubbard model paves the way to controllably study doped antiferromagnetic Mott insulators. Motivated by these experimental advancements, we numerically study the real-time dynamics of a single hole created in an antiferromagnet on a square lattice, as described by the t–J model. Initially, the hole spreads ballistically with a velocity proportional to the hopping matrix element. At intermediate to long times, the dimensionality as well as the spin background determine the hole dynamics. A hole created in the ground state of a two dimensional (2D) quantum antiferromagnet propagates again ballistically at long times but with a velocity proportional to the spin exchange coupling, showing the formation of a magnetic polaron. We provide an intuitive explanation of this dynamics in terms of a parton construction, which leads to a good quantitative agreement with the numerical tensor network state simulations. In the limit of infinite temperature and no spin exchange couplings, the dynamics can be approximated by a quantum random walk on a Bethe lattice with coordination number . Adding Ising interactions corresponds to an effective disordered potential, which can dramatically slow down the hole propagation, consistent with subdiffusive dynamics. The study of the hole dynamics paves the way for understanding the microscopic constituents of this strongly correlated quantum state.

Probability backflow for correlated quantum states

In its original formulation, quantum backflow (QB) is an interference effect that manifests itself as a negative probability transfer for free-particle states comprised of plane waves with only positive momenta. Quantum reentry (QR) is another interference effect in which a wave packet expanding from a spatial region of its initial confinement partially returns to the region in the absence of any external forces. Here we show that both QB and QR are special cases of a more general classically-forbidden probability flow for quantum states with certain position-momentum correlations. We further demonstrate that it is possible to construct correlated quantum states for which the amount of probability transferred in the "wrong" (classically impossible) direction exceeds the least upper bound on the corresponding probability transfer in the QB and QR problems, known as the Bracken-Melloy constant.

Melting of charge density wave and Mott gap collapse on 1T−TaS2 induced by interfacial water

Microscopically revealing the interactions between interfacial water and the quantum states of matter is an important task from both the materials science and the physics points of view. Here we report a low-temperature scanning tunneling microscopy (STM) and spectroscopy study of water adsorption on the charge density wave compound $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which has a Mott-insulating ground state. Interfacial water forms monolayer islands with $6\ifmmode\times\else\texttimes\fi{}6$ superstructures on the surface of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, and the charge order under water islands can be directly imaged in STM topographies taken with negative bias voltages. Compared with the original $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ charge order in $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, the charge order under water islands becomes significantly disordered and denser. A $\mathsf{V}$-shaped gaplike feature emerges in water-covered $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which may be due to the enhanced dielectric constant of interfacial water, which reduces short-range Coulomb repulsion and induces Mott gap collapse. Our observations open the way to microscopically understanding the interactions between interfacial water and the correlated quantum states of matter.