Quantum Physics (quant-ph)

Abstract: The quantum network makes use of the quantum states to transmit data, which will revolutionize classical communication and allow for some breakthrough applications. The quantum key distribution (QKD) is one prominent application of quantum networks, and can protect the data transmission through quantum mechanics. In this work, we propose an expandable and cost-effective quantum access network, in which the round-trip structure makes quantum states travel in a circle to carry the information, and the multi-band technique is proposed to support multi-user access. Based on the round-trip multi-band quantum access network, we realize multi-user secure key sharing through the continuous-variable QKD (CV-QKD) protocol. Due to the encoding characteristics of CV-QKD, the quadrature components in different frequency bands can be used to transmit key information for different users. The feasibility of this scheme is confirmed by comprehensive noise analysis, and is verified by a proof-of-principle experiment. The results show that each user can achieve excess noise suppression and 600 bps level secure key generation under 30 km standard fiber transmission. Such networks have the ability of multi-user access theoretically and could be expanded by plugging in simple modules. Therefore, it paves the way for near-term large-scale quantum secure networks.

Abstract: Quantum key distribution (QKD) can theoretically achieve the Holy Grail of cryptography, information-theoretic security against eavesdropping. However, in practice, discrepancies between the mathematical models assumed in security proofs and the actual functioning of the devices used in implementations prevent it from reaching this goal. Since measurement-device-independent QKD guarantees security with arbitrarily flawed receivers, the missing step is securing the sources. So far, all efforts in this regard have come at a price; some proofs are suitable only for particular source imperfections, while others severely compromise the system's performance, i.e., its communication speed and distance. Meanwhile, device-independent QKD is far from being a satisfactory solution, as it is vulnerable to memory attacks, it cannot incorporate information leakage from the user devices in a device-independent manner, and its performance is poor. Here, we solve this crucial problem by presenting a security proof that is robust against all practical source imperfections while maintaining high performance. Moreover, our proof requires minimal state characterization, which facilitates its application to real-life implementations. We anticipate that, thanks to these advantages, it will serve as a basis for the standardization of QKD sources.

Abstract: Quantum information processing and its subfield, quantum image processing, are rapidly growing fields as a result of advancements in the practicality of quantum mechanics. In this paper, we propose a quantum algorithm for processing information, such as one-dimensional time series and two-dimensional images, in the frequency domain. The information of interest is encoded into the magnitude of probability amplitude or the coefficient of each basis state. The oracle for filtering operates based on postselection results, and its explicit circuit design is presented. This oracle is versatile enough to perform all basic filtering, including high pass, low pass, band pass, band stop, and many other processing techniques. Finally, we present two novel schemes for transposing matrices in this paper. They use similar encoding rules but with deliberate choices in terms of selecting basis states. These schemes could potentially be useful for other quantum information processing tasks, such as edge detection. The proposed techniques are implemented on the IBM Qiskit quantum simulator. Some results are compared with traditional information processing results to verify their correctness and are presented in this paper.

Abstract: In this work, we show that a pair of entangled qubits can be used to compute a product privately. More precisely, two participants with a private input from a finite field can perform local operations on a shared, Bell-like quantum state, and when these qubits are later sent to a third participant, the third participant can determine the product of the inputs, but without learning more about the individual inputs. We give a concrete way to realize this product computation for arbitrary finite fields of prime order.

Abstract: We consider the problem of finding the energy minimum of a complex quantum Hamiltonian by employing a non-Markovian bath prepared in a low energy state. The energy minimization problem is thus turned into a thermodynamic cooling protocol in which we repeatedly put the system of interest in contact with a colder auxiliary system. By tuning the internal parameters of the bath, we show that the optimal cooling is obtained in a regime where the bath exhibits a quantum phase transition in the thermodynamic limit. This result highlights the importance of collective effects in thermodynamic devices. We furthermore introduce a two-step protocol that combines the interaction with the bath with a measure of its energy. While this protocol does not destroy coherence in the system of interest, we show that it can further enhance the cooling effect.

Abstract: Trapped-ion has shown great advantages in building quantum computers. While high fidelity entangling-gate has been realized for few ions, how to maintain the high fidelity for large scale trapped-ions still remains an open problem.Here, we present an analysis on arbitrary scale ion chain and focus on motional-related errors, reported as one of the leading error sources in state-of-the-art experiments. We theoretically analyze two-qubit entangling-gate infidelity in a large ion crystal. To verify our result, we develop an efficient numerical simulation algorithm that avoids exponential increases of the Hilbert space dimension. For the motional heating error, We derive a much tighter bound of gate infidelity than previously estimated $O(N\Gamma\tau)$, and we give an intuitive understanding from the trajectories in the phase space of motional modes. Our discoveries may inspire the scheme of pulse design against incoherent errors and shed light on the way toward constructing scalable quantum computers with large ion crystals.

Abstract: A connection is revealed between the superposition principle and locality. A self consistent interpretation of the superposition principle is put forth, from which it is shown that quantum mechanics may be a local statistical theory. Then it is shown how Bell experiments can be satisfactorily explained by assuming local nature for entangled particles, i.e., the violation of Bell inequality cannot distinguish between locality and nonlocality, which is referred to as locality loophole. Moreover, existing experimental results are presented indicating locality in quantum mechanics and new experiments are proposed so that the locality loophole may be closed.

Abstract: In this letter, we consider a simple three-party scenario, with one Dealer (Alice), one Assistant (Bob) and a Reconstructor (Charlie). We find the classical limit of reconstructing the quantum secret under this framework. Interestingly this happens to be 2/3. We formulate the expression for reconstruction fidelity in terms of the Bloch parameters of the shared resource state. It is imperative to say at this point any resource state for which the score is beyond 2/3, gives us a quantum advantage in context of reconstructing the secret. Interestingly, this fidelity not only depends upon the tripartite correlation tensor but also on the existent bipartite channel between the dealer and the reconstructor. This fidelity in a sense is able to measure, the amount of information we are able to reconstruct. As a result of which it also takes into account the contribution of the teleportation capacity of the channel between the dealer and reconstructor in addition to the secret sharing capacity of three qubit resource state. In this work along with example we also discuss the cases when the quantum advantage in the reconstruction fidelity is entirely because of the secret sharing process (where involvement of three party is necessary) or entirely because of the teleportation in subsystem or because of. We are able to show quantum advantages in all possible scenarios with states other than the standard GHZ state. We also obtain necessary conditions based on the correlation tensor between three parties and correlation matrix between the dealer and the reconstructor. In this letter we discover a new kind of interoperability that happens in the process of secret sharing and teleportation for a given tripartite resource. This result benchmarks the identification process of three qubit resources for these tasks while setting up large scale quantum network.

Abstract: Quantum machine learning (QML) has witnessed immense progress recently, with quantum support vector machines (QSVMs) emerging as a promising model. This paper focuses on the two existing QSVM methods: quantum kernel SVM (QK-SVM) and quantum variational SVM (QV-SVM). While both have yielded impressive results, we present a novel approach that synergizes the strengths of QK-SVM and QV-SVM to enhance accuracy. Our proposed model, quantum variational kernel SVM (QVK-SVM), leverages the quantum kernel and quantum variational algorithm. We conducted extensive experiments on the Iris dataset and observed that QVK-SVM outperforms both existing models in terms of accuracy, loss, and confusion matrix indicators. Our results demonstrate that QVK-SVM holds tremendous potential as a reliable and transformative tool for QML applications. Hence, we recommend its adoption in future QML research endeavors.

Abstract: In this paper, an exact solution of the Schr\"odinger equation for the time-dependent potential $U\left( x,t \right)={m\,{{\Omega }^{2}}\left( t \right){{x}^{2}}}/{2}\;$ is constructed, where the frequency $\Omega \left( t \right)$ is a "sufficiently" smooth function of time. For the quantum system under consideration, a set of wave functions and a spectrum of time-dependent energy eigenvalues are obtained. The behavior of the time-dependent potential affects the instability of the quantum system. The dynamics of the system is described by the Hill equation. The time-dependent Wigner function and its generalization to the phase space of higher kinematic values are found in an explicit form. The process of energy "pumping" of a quantum system, which leads to its instability, is considered in detail.

Abstract: The use of multilevel quantum information carriers, also known as qudits, attracts a significant deal of interest as a way for further scalability of quantum computing devices. However, a nontrivial task is to experimentally achieve a gain in the efficiency of realizing quantum algorithms with qudits since higher qudit levels typically have relatively short coherence times compared to qubit states. Here we propose and experimentally demonstrate two approaches for the realization of continuous dynamical decoupling of magnetic-sensitive states with $m_F=\pm1$ for qudits encoded in optical transition of trapped $^{171}$Yb$^{+}$ ions. We achieve improvement in qudit levels coherence time by the order of magnitude (more than 9 ms) without any magnetic shielding, which reveals the potential advantage of the symmetry of the $^{171}$Yb$^{+}$ ion energy structure for counteracting the magnetic field noise. Our results are a step towards the realization of qudit-based algorithms using trapped ions.

Abstract: We exploit complete complementarity relations to characterize quantum correlations encoded in a three-flavor oscillating neutrino system. In particular, we analyze the contributions associated to the twoflavor subsystems, each of which exhibits a particular internal structure. We focus on the behavior of the correlations at large distances, both for an initial electron and muon neutrino state. Our analysis is based on the wave packet approach in which the neutrino is represented by a mixed state: consequently, the bipartite correlations are described by the Quantum Discord.

Abstract: Originated from the work extraction in quantum systems coupled to a heat bath, quantum deficit is a kind of significant quantum correlations like quantum entanglement. It links quantum thermodynamics with quantum information. We analytically calculate the one-way deficit of the generalized $n$-qubit Werner state. We find that the one-way deficit increases as the mixing probability $p$ increases for any $n$. For fixed $p$, we observe that the one-way deficit increases as $n$ increases. For any $n$, the maximum of one-way deficit is attained at $p=1$. Furthermore, for large $n$ ($2^n \rightarrow \infty$), we prove that the curve of one-way deficit versus $p$ approaches to a straight line with slope $1$. We also calculate the Holevo quantity for the generalized $n$-qubit Werner state, and show that it is zero.

Abstract: The quantum simulation of magnetism in trapped-ion systems makes use of the crystal vibrations to mediate pairwise interactions between spins, which are encoded in the internal electronic states of the ions, and measured in experiments that probe the real-time dynamics. These interactions can be accounted for by a long-wavelength relativistic theory, where the phonons are described by a coarse-grained Klein-Gordon field $\phi(x)$ locally coupled to the spins that acts as a carrier, leading to an analogue of pion-mediated Yukawa interactions. In the vicinity of a structural transition of the ion crystal, one must go beyond the Klein-Gordon fields, and include additional $\lambda\phi^4$ terms responsible for phonon-phonon scattering. This leads to quantum effects that can be expressed by Feynman loop integrals that modify the range of the Yukawa-type spin interactions; an effect that could be used to probe the underlying fixed point of this quantum field theory (QFT). Unfortunately, the rigidity of the trapped-ion crystal makes it challenging to observe genuine quantum effects, such as the flow of the critical point with the quartic coupling $\lambda$. We hereby show that thermal effects, which can be controlled by laser cooling, can unveil this flow through the appearance of thermal masses in interacting QFTs. We perform self-consistent calculations that resum certain Feynman diagrams and, additionally, go beyond mean-field theory to predict how measurements on the trapped-ion spin system can probe key properties of the $\lambda\phi^4$ QFT.

Abstract: Quantum sensing with solid-state spins offers the promise of high spatial resolution, bandwidth, and dynamic range at sensitivities comparable to more mature quantum sensing technologies, such as atomic vapor cells and superconducting devices. However, despite comparable theoretical sensitivity limits, the performance of bulk solid-state quantum sensors has so far lagged behind these more mature alternatives. A recent review~\cite{barry2020sensitivity} suggests several paths to improve performance of magnetometers employing nitrogen-vacancy defects in diamond, the most-studied solid-state quantum sensing platform. Implementing several suggested techniques, we demonstrate the most sensitive nitrogen-vacancy-based bulk magnetometer reported to date. Our approach combines tailored diamond growth to achieve low strain and long intrinsic dephasing times, the use of double-quantum Ramsey and Hahn echo magnetometry sequences for broadband and narrowband magnetometry respectively, and P1 driving to further extend dephasing time. Notably, the device does not include a flux concentrator, preserving the fixed response of the NVs to magnetic field. The magnetometer realizes a broadband \textcolor{mhsnew}{near-}DC sensitivity $\sim 460$~fT$\cdot$s$^{1/2}$ and a narrowband AC sensitivity $\sim 210$~fT$\cdot$s$^{1/2}$. We describe the experimental setup in detail and highlight potential paths for future improvement.

Abstract: We study the time dynamics of the twisted quantum states. We find an explicit connection between the well-known stationary Landau state and an evolving twisted state, even if the Hamiltonian accounts for the linear energy dissipation. Utilizing this unitary connection, we analyze nonstationary Landau states and unveil some of their properties. We demonstrate how the proposed transformation enables simple evaluation of the different operator mean values for the evolving twisted state based on the solution to the classical Ermakov equation and matrix elements calculated on the stationary Landau states. We anticipate that suggested formalism may significantly simplify the analysis and become a convenient tool for the further theoretical development of the dissipative evolution of the twisted quantum wave packet.