Multiple Quantum States Research Articles

High-Resolution Spectroscopy of the Intermediate Impurity States near a Quantum Phase Transition.

The intermediate behavior near a quantum phase transition is crucial for understanding the quantum criticality of various competing phases and their separate origins, yet it remains unexplored for the multiple Yu-Shiba-Rusinov (YSR) states. Here, we investigated the detailed spectroscopic change of the exchange-coupling-dependent YSR states near a quantum phase transition. The initially developed one pair of YSR states, induced by the Fe vacancy in monolayer Fe(Te,Se) superconductor, are clearly resolved with high resolution showing an evolution into two pairs of YSR peaks yet with dichotomy in their spectral features as they enter the quantum phase transition region. Spectral-weight analysis suggests that the double YSR pairs occur as a result of field splitting by the magnetic anisotropy. Our findings unveil the intermediate region of a quantum phase transition with a magnetic anisotropy-induced splitting of the YSR resonance, and highlight a prospect for developing functional electronics based on the flexibly controllable multiple quantum states.

Entanglement manipulation through multicore fibres

Multicore fibres are recently gaining considerable attention in the context of quantum communication, where their capability to transmit multiple quantum states along different cores of the same channel makes them a promising candidate for the implementation of scalable quantum networks. Here, we show that multicore fibres can be effectively used not only for the scope of communication but also for the manipulation of entangled states. Exploiting the formalism of completely positive trace-preserving maps, we describe the action of a multicore fibre as a quantum channel and investigate the propagation of a transmitted state under the effect of decoherence and inter-core crosstalk. Then, we propose a novel protocol for the manipulation of the entanglement where, starting from a maximally entangled state of two qudits, we use a multicore fibre to create new families of mixed entangled states. Notably, the presence of crosstalk is fundamental for the generation of such states.

Prior entanglement exponentially improves one-server quantum private information retrieval for quantum messages

Quantum private information retrieval (QPIR) for quantum messages is a quantum communication task, in which a user retrieves one of the multiple quantum states from the server without revealing which state is retrieved. In the one-server setting, we find an exponential gap in the communication complexities between the presence and absence of prior entanglement in this problem with the one-server setting. To achieve this aim, as the first step, we prove that the trivial solution of downloading all messages is optimal under QPIR for quantum messages, which is a similar result to that of classical PIR but different from QPIR for classical messages. As the second step, we propose an efficient one-server one-round QPIR protocol with prior entanglement by constructing a reduction from a QPIR protocol for classical messages to a QPIR protocol for quantum messages in the presence of prior entanglement.

Tuning of charge order by uniaxial stress in a cuprate superconductor

Strongly correlated electron materials are often characterized by competition and interplay of multiple quantum states. For example, in high-temperature cuprate superconductors unconventional superconductivity, spin- and charge-density wave orders coexist. A key question is whether competing states coexist on the atomic scale or if they segregate into distinct regions. Using X-ray diffraction, we investigate the competition between charge order and superconductivity in the archetypal cuprate La2−xBaxCuO4, around x = 1/8-doping, where uniaxial stress restores optimal 3D superconductivity at σ3D ≈ 0.06 GPa. We find that the charge order peaks and the correlation length along the stripe are strongly reduced up to σ3D. Upon the increase of stress beyond this point, no further changes were observed. Simultaneously, the charge order onset temperature only shows a modest decrease. Our findings suggest that optimal 3D superconductivity is not linked to the absence of charge stripes but instead requires their arrangement into smaller regions. Our results provide insight into the length scales over which the interplay between superconductivity and charge order takes place.

Quantum multi-state Swap Test: an algorithm for estimating overlaps of arbitrary number quantum states

Estimating the overlap between two states is an important task with several applications in quantum information. However, the typical swap test circuit can only measure a sole pair of quantum states at a time. In this study, a recursive quantum circuit is designed to measure overlaps of n quantum states |ϕ1〉,|ϕ2〉,…|ϕn〉\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$\\left | {\\phi _{1} } \\right \\rangle ,\\left | {\\phi _{2} } \\right \\rangle ,\\ldots\\left | {\\phi _{n} }\\right \\rangle $\\end{document} concurrently with O(k2k)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$O(k2^{k})$\\end{document} controlled-swap(CSWAP) gates and O(k)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$O(k)$\\end{document} ancillary qubits, where k=⌈logn⌉\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$k=\\left \\lceil {\\log n} \\right \\rceil $\\end{document}. All pairwise overlaps among input quantum states |〈ϕi|ϕj〉|2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$|\\langle \\phi _{i}|\\phi _{j}\\rangle |^{2}$\\end{document} can be obtained in this circuit. Compared with existing scheme for measuring the overlap of multiple quantum states, the circuit provides higher precision and less consumption of ancillary qubits. In addition, some simulation experiments are performed on IBM quantum cloud platform to verify the superiority of this algorithm.

Dynamic and scalable secret sharing schemes based on matrix product compressed states

<sec>Currently, quantum secret sharing (QSS) schemes based on entangled states have not yet fully utilized the potential of the probability amplitude of entangled states. However, the probability amplitude is a key characteristic of quantum information science and possesses significant application prospects in the fields of quantum computing and quantum communication. It is worth noting that entangled states can be effectively represented by matrix product states (MPSs). The representation of entangled states using MPS can precisely reveal the entanglement characteristics closely related to the probability amplitude.</sec><sec>This study first focuses on the representation of the <i>W</i> state by using MPS, an approach that allows us to determine the key conditions for <i>W</i> state to achieve quantum advantage in QSS. Subsequently, this research demonstrates that by representing entangled states with MPS, a <i>W</i> state can be compressed into a single photon state and a simplified matrix form, presenting an innovative technical path.</sec><sec>Moreover, one of the most attractive features of our proposed QSS scheme is its ability to compress multiple different quantum states (represented by photons) into a unified state represented by a single photon. This characteristic endows our scheme with scalability and flexibility, meaning that the group of participants can be easily expanded or reduced according to their specific needs. The addition of new participants is managed by Alice, who is responsible for the distribution of quantum state shares. On the other hand, when a participant leaves the group, their old quantum state share can be simply ignored in the process of recovering the secret's quantum state, thereby simplifying the management process.</sec><sec>Through this strategy, we can not only make effective use of entangled resources but also meet the various requirements of the system, including but not limited to communication security, data transfer rates, and system scalability. This research provides new perspectives and possibilities for the field of quantum information science and may have a significant influence on the development of the field.</sec>

Two (w, ω, n) weighted threshold quantum secret sharing schemes on d-level single quantum systems

Quantum secret sharing occupies an important position in quantum cryptography. In this paper, we propose two weighted threshold quantum secret sharing schemes on d-level single quantum systems that share classical information and quantum states, respectively. The dealer Alice distributes the secret share to each participant using the Chinese Remainder Theorem(CRT) for a polynomial ring, which can realize secret sharing among participants with different weights. In classical information sharing, the dealer can share multiple secrets in one round of the protocol and the participants can verify the correctness of the recovered secrets. In quantum state sharing, the dealer can share multiple single quantum states in one round of protocol. Finally, we demonstrate that our scheme is resistant to common external and internal attacks and compare it with related schemes.

Controlled cyclic remote preparation

Multi-party quantum communication has gradually attracted widespread attention. To realize the perfect transmission of quantum states among multiple participants, a novel multi-party controlled cyclic remote preparation protocol for arbitrary single-qubit states with three senders is proposed. With the permission of one controller, each sender can transmit an arbitrary single-qubit state to its neighbor. In addition, we give a universal protocol for multi-party controlled cyclic remote preparation of arbitrary single-qubit states in the case of multiple senders, which can realize deterministic cyclic preparation of multiple quantum states in one direction. The scheme shows that the communication task can be successfully achieved only if all senders cooperate with the controller, and there is no need for the senders to employ information splitting and additional operations before performing measurements. Finally, we discuss the cyclic remote preparation protocol with three senders under five types of noisy environment, and the closeness between the output state and original state is measured by calculating fidelity.

Emission enhancement of erbium in a reverse nanofocusing waveguide

Since Purcell’s seminal report 75 years ago, electromagnetic resonators have been used to control light-matter interactions to make brighter radiation sources and unleash unprecedented control over quantum states of light and matter. Indeed, optical resonators such as microcavities and plasmonic antennas offer excellent control but only over a limited spectral range. Strategies to mutually tune and match emission and resonator frequency are often required, which is intricate and precludes the possibility of enhancing multiple transitions simultaneously. In this letter, we report a strong radiative emission rate enhancement of Er3+-ions across the telecommunications C-band in a single plasmonic waveguide based on the Purcell effect. Our gap waveguide uses a reverse nanofocusing approach to efficiently enhance, extract and guide emission from the nanoscale to a photonic waveguide while keeping plasmonic losses at a minimum. Remarkably, the large and broadband Purcell enhancement allows us to resolve Stark-split electric dipole transitions, which are typically only observed under cryogenic conditions. Simultaneous radiative emission enhancement of multiple quantum states is of great interest for photonic quantum networks and on-chip data communications.

Organic quantum materials: A review

AbstractInterests in organic quantum materials (OQMs) have been explosively growing in the field of condensed physics of matter due to their rich chemistry and unique quantum properties. They are strongly correlated systems and show novel electromagnetic performance such as high‐temperature superconducting, quantum sensing, spin electronics, quantum dots, topological insulating, quantum Hall effects, spin liquids, qubits, and so forth, which exhibit promising prospects in information communication and thus facilitate the construction of a modern intelligent society. This article reviews recent developments in the research on the electromagnetic characteristics of OQMs. We mainly give an overview on the progress of superconductors and quantum spin liquids based on organic materials and describe their possible mechanisms. Numerous experimental findings exhibit new exciton interactions and provide insights into exotic electronic properties. Finally, their association and strategies for realizing multiple quantum states in one system are discussed.

Multiple Quantum States Induced in 1T‐TaSe2 by Controlling the Stacking Order of Charge Density Waves

AbstractVan der Waals (vdW) materials afford unprecedented opportunities for control of electronic properties by utilizing the stacking degree of freedom. An intriguing frontier, largely unexplored, is the stacking of charge density wave (CDW) phases that is a broken‐symmetry state with periodically modulated charge density and the atomic lattice. Employing density functional theory, it is uncovered that the stacking order can play a significant role in the quantum phase transitions of layered 1T‐TaSe2 with a striking 2D CDW order. By controlling the vertical stacking order of CDWs, bulk 1T‐TaSe2 can host various electronic phases including quasi‐1D and 3D metals and band insulators. Particularly, the ground‐state stacking configuration shows 3D metallicity due to the enhanced intralayer and interlayer electron hopping, and the second lowest energy configuration shows band insulating behavior via interlayer dimerization, implying potential metal‐insulator transition. In ultrathin‐layer 1T‐TaSe2, not only the stacking order but also the thickness dictate the electronic properties. While the monolayer is a Mott insulator, the bilayer (trilayer) is a band insulator (metal). More interestingly, the four‐layer emerges as an insulator or a semimetal dependent on its stacking order. The wide‐tunable electronic properties of 1T‐TaSe2 CDW compound will open a new pathway for designing novel quantum devices.

Topological phase transitions and topological quantum states modulated by the counter-rotating wave terms in a one-dimensional superconducting microwave cavity lattice

We propose a one-dimensional lattice theory scheme based on superconducting microwave cavities, which includes two different types of microwave cavity unit cells. The coupling between unit cells is controlled by flux qubits to simulate and study their topological insulator characteristics. Specifically, a one-dimensional superconducting microwave cavity lattice scheme with a p-wave superconducting pairing term is achieved by mapping the counter-rotating wave terms to the p-wave superconducting pairing term. We found that the p-wave superconducting pairing term can modulate the topological quantum state of the system, allowing for the creation of topological quantum information transmission channels with four edge states. In addition, when the p-wave superconducting pairing term and the nearest-neighbor interaction exist, we find that the energy band undergoes fluctuations, inducing the generation of new energy bands, but the degeneracy of the edge states remains stable, which can achieve multiple topological quantum state transmission paths. However, when its regulatory value exceeds the threshold, the energy gap of the system will close, causing the edge states to annihilate in new energy bands. Furthermore, when considering the existence of defects in the system, we found that when the strength of the defects are small, the edge state produces small fluctuations, but it can be clearly distinguished, indicating its robustness. When the strength of the defect exceeds the threshold, the edge state and energy band cause irregular fluctuations, allowing the edge state to integrate into the energy band. Our research results have important theoretical value and practical significance, and can be applied in quantum optics and quantum information processing in the future.

Topological phase transitions and topological quantum states modulated by the counter-rotating wave terms in a one-dimensional superconducting microwave cavity lattice

We propose a one-dimensional lattice theory scheme based on superconducting microwave cavities, which includes two different types of microwave cavity unit cells. The coupling between unit cells is controlled by flux qubits to simulate and study their topological insulator characteristics. Specifically, a one-dimensional superconducting microwave cavity lattice scheme with a p-wave superconducting pairing term is achieved by mapping the counter-rotating wave terms to the p-wave superconducting pairing term. We found that the p-wave superconducting pairing term can modulate the topological quantum state of the system, allowing for the creation of topological quantum information transmission channels with four edge states. In addition, when the p-wave superconducting pairing term and the nearest-neighbor interaction exist, we find that the energy band undergoes fluctuations, inducing the generation of new energy bands, but the degeneracy of the edge states remains stable, which can achieve multiple topological quantum state transmission paths. However, when its regulatory value exceeds the threshold, the energy gap of the system will close, causing the edge states to annihilate in new energy bands. Furthermore, when considering the existence of defects in the system, we found that when the strength of the defects are small, the edge state produces small fluctuations, but it can be clearly distinguished, indicating its robustness. When the strength of the defect exceeds the threshold, the edge state and energy band cause irregular fluctuations, allowing the edge state to integrate into the energy band. Our research results have important theoretical value and practical significance, and can be applied in quantum optics and quantum information processing in the future.

Simultaneous ground-state cooling of multiple degenerate mechanical modes through the cross-Kerr effect.

Simultaneous ground-state cooling of multiple degenerate mechanical modes is a difficult issue in optomechanical systems, owing to the existence of the dark mode effect. Here we propose a universal and scalable method to break the dark mode effect of two degenerate mechanical modes by introducing cross-Kerr (CK) nonlinearity. At most, four stable steady states can be achieved in our scheme in the presence of the CK effect, unlike the bistable behavior of the standard optomechanical system. Under a constant input laser power, the effective detuning and mechanical resonant frequency can be modulated by the CK nonlinearity, resulting in an optimal CK coupling strength for cooling. Similarly, there will be an optimal input laser power for cooling when the CK coupling strength stays fixed. Our scheme can be extended to break the dark mode effect of multiple degenerate mechanical modes by introducing more than one CK effect. To fulfill the requirement of the simultaneous ground-state cooling of N multiple degenerate mechanical modes, N - 1 CK effects with different strengths are needed. Our proposal provides new, to the best of our knowledge. insights into dark mode control and might pave the way to manipulating multiple quantum states in a macroscopic system.

Quantum state discrimination circuits inspired by Deutschian closed timelike curves

It is known that a party with access to a Deutschian closed timelike curve (DCTC) can perfectly distinguish multiple nonorthogonal quantum states. In this paper we propose a practical method for discriminating multiple nonorthogonal states, by using a previously known quantum circuit designed to simulate DCTCs. This method relies on multiple copies of an input state, multiple iterations of the circuit, and a fixed set of unitary operations. We first characterize the performance of this circuit and study its asymptotic behavior. We also show how it can be equivalently recast as a local adaptive circuit that may be implemented simply in an experiment. Finally, we prove that our state discrimination strategy achieves the multiple Chernoff bound when discriminating an arbitrary set of pure qubit states.

Ground-state cooling of multiple near-degenerate mechanical modes

We propose a general and experimentally feasible approach to realize simultaneous ground-state cooling of arbitrary number of near-degenerate, or even fully degenerate mechanical modes, overcoming the limit imposed by the formation of mechanical dark modes. Multiple optical modes are employed to provide different dissipation channels that prevent complete destructive interference of the cooling pathway, and thus eliminating the dark modes. The cooling rate and limit are explicitly specified, in which the distinguishability of the optical modes to the mechanical modes is found to be critical for an efficient cooling process. In a realistic multi-mode optomechanical system, ground-state cooling of all mechanical modes is demonstrated by sequentially introducing optical drives, proving the feasibility and scalability of the proposed scheme. The work may provide new insights in preparing and manipulating multiple quantum states in macroscopic systems.

Shape-Tuned Multiphoton-Emitting InP Nanotetrapods.

As the properties of a semiconductor material depend on the fate of the excitons, manipulating exciton behavior is the primary objective of nanomaterials. Although nanocrystals exhibit unusual excitonic characteristics owing to strong spatial confinement, studying the interactions between excitons in a single nanoparticle remains challenging due to the rapidly vanishing multiexciton species. Here, a platform for exciton tailoring using a straightforward strategy of shape-tuning of single-crystalline nanocrystals is presented. Spectroscopic and theoretical studies reveal a systematic transition of exciton confinement orientation from 3D to 2D, which is solely tuned by the geometric shape of material. Such a precise shape-effect triggers a multiphoton emission in single nanotetrapods with arms longer than the exciton Bohr radius of material. In consequence, the unique interplay between the multiple quantum states allows a geometric modulation of the quantum-confined Stark effect and nanocrystal memory effect in single nanotetrapods. These results provide a useful metric in designing nanomaterials for future photonic applications.

Reinforcement Learning with Neural Networks for Quantum Multiple Hypothesis Testing

Reinforcement learning with neural networks (RLNN) has recently demonstrated great promise for many problems, including some problems in quantum information theory. In this work, we apply RLNN to quantum hypothesis testing and determine the optimal measurement strategy for distinguishing between multiple quantum states {ρj} while minimizing the error probability. In the case where the candidate states correspond to a quantum system with many qubit subsystems, implementing the optimal measurement on the entire system is experimentally infeasible. We use RLNN to find locally-adaptive measurement strategies that are experimentally feasible, where only one quantum subsystem is measured in each round. We provide numerical results which demonstrate that RLNN successfully finds the optimal local approach, even for candidate states up to 20 subsystems. We additionally demonstrate that the RLNN strategy meets or exceeds the success probability for a modified locally greedy approach in each random trial.While the use of RLNN is highly successful for designing adaptive local measurement strategies, in general a significant gap can exist between the success probability of the optimal locally-adaptive measurement strategy and the optimal collective measurement. We build on previous work to provide a set of necessary and sufficient conditions for collective protocols to strictly outperform locally adaptive protocols. We also provide a new example which, to our knowledge, is the simplest known state set exhibiting a significant gap between local and collective protocols. This result raises interesting new questions about the gap between theoretically optimal measurement strategies and practically implementable measurement strategies.

Improved analytical bounds on delivery times of long-distance entanglement

The ability to distribute high-quality entanglement between remote parties is a necessary primitive for many quantum communication applications. A large range of schemes for realizing the long-distance delivery of remote entanglement has been proposed, both for bipartite and multipartite entanglement. For assessing the viability of these schemes, knowledge of the time at which entanglement is delivered is crucial. Specifically, if the communication task requires multiple remote-entangled quantum states and these states are generated at different times by the scheme, the earlier states will need to wait and thus their quality will decrease while being stored in an (imperfect) memory. For the remote-entanglement delivery schemes which are closest to experimental reach, this time assessment is challenging, as they consist of nondeterministic components such as probabilistic entanglement swaps. For many such protocols even the average time at which entanglement can be distributed is not known exactly, in particular when they consist of feedback loops and forced restarts. In this work, we provide improved analytical bounds on the average and on the quantiles of the completion time of entanglement distribution protocols in the case that all network components have success probabilities lower bounded by a constant. A canonical example of such a protocol is a nested quantum repeater scheme which consists of heralded entanglement generation and entanglement swaps. For this scheme specifically, our results imply that a common approximation to the mean entanglement distribution time, the 3-over-2 formula, is in essence an upper bound to the real time. Our results rely on a novel connection with reliability theory.

Experimental Quantum Principal Component Analysis via Parametrized Quantum Circuits.

Principal component analysis (PCA) is a widely applied but rather time-consuming tool in machine learning techniques. In 2014, Lloyd, Mohseni, and Rebentrost proposed a quantum PCA (qPCA) algorithm [Lloyd, Mohseni, and Rebentrost, Nat. Phys. 10, 631 (2014)NPAHAX1745-247310.1038/nphys3029] that still lacks experimental demonstration due to the experimental challenges in preparing multiple quantum state copies and implementing quantum phase estimations. Here, we propose a new qPCA algorithm using the hybrid classical-quantum control, where parameterized quantum circuits are optimized with simple measurement observables, which significantly reduces the experimental complexity. As one important PCA application, we implement a human face recognition process using the images from the Yale Face Dataset. By training our quantum processor, the eigenface information in the training dataset is encoded into the parameterized quantum circuit, and the quantum processor learns to recognize new face images from the test dataset with high fidelities. Our work paves a new avenue toward the study of qPCA applications in theory and experiment.