Quantum Information Processing Technologies Research Articles

Challenges for exploiting nanomagnet properties on surfaces.

Molecular complexes with single-molecule magnet (SMM) or qubit properties, commonly called molecular nanomagnets, are great candidates for information storage or quantum information processing technologies. However, the implementation of molecular nanomagnets in devices for the above-mentioned applications requires controlled surface deposition and addressing the nanomagnets' properties on the surface. This Perspectives paper gives a brief overview of molecular properties on a surface relevant for magnetic molecules and how they are affected when the molecules interact with a surface; then, we focus on systems of increasing complexity, where the relevant SMMs and qubit properties have been observed for the molecules deposited on surfaces; finally, future perspectives, including possible ways of overcoming the problems encountered so far are discussed.

Quantum estimation of tripartite coupling in spin-magnon-mechanical hybrid systems

Tripartite interactions play a fundamental role in the quantum information processing and quantum technology. However, it is generally difficult to realize strong tripartite coupling. We investigate the estimation of a tripartite coupling strength in a hybrid setup composed of a single nitrogen-vacancy (NV) center and a micromagnet. A time-independent parametric drive can be utilized to increase the estimation precision of the tripartite coupling strength. By calculating the quantum Fisher information (QFI), we can obtain the optimal estimation precision by measuring the eigenstate of the tripartite system. At the critical position, the QFI is divergent due to that the preparation time of the eigenstate is divergent. When the system is subjected to a dissipation, the QFI near the critical point of the driven-dissipation phase transition is analytically obtained. The direct intensity measurement is the optimal measurement near the dissipation phase transition point. In addition, we quantify the robustness of an imperfect measurement operator by the measurement noise susceptibility based on the error propagation formula. We find that the direct intensity measurement is robust enough against small measurement disturbance from a coherent drive. But it can be disturbed by the nonlinear anti-harmonic measurement noise, especially near the critical point.

Laser-Induced Ultrafast Spin Injection in All-Semiconductor Magnetic CrI3/WSe2 Heterobilayer.

Spin injection stands out as a crucial method employed for initializing, manipulating, and measuring the spin states of electrons, which are fundamental to the creation of qubits in quantum computing. However, ensuring efficient spin injection while maintaining compatibility with standard semiconductor processing techniques is a significant challenge. Herein, we demonstrate the capability of inducing an ultrafast spin injection into a WSe2 layer from a magnetic CrI3 layer on a femtosecond time scale, achieved through real-time time-dependent density functional theory calculations upon a laser pulse. Following the peak of the magnetic moment in the CrI3 sublayer, the magnetic moment of the WSe2 layer reaches a maximum of 0.89 μB (per unit cell containing 4 WSe2 and 1 CrI3 units). During the spin dynamics, spin-polarized excited electrons transfer from the WSe2 layer to the CrI3 layer via type-II band alignment. The large spin splitting in conduction bands and the difference in the number of spin-polarized local unoccupied states available in the CrI3 layer lead to a net spin in the WSe2 layer. Furthermore, we confirmed that the number of available states, the spin-flip process, and the laser pulse parameters play important roles during the spin injection process. This work highlights the dynamic and rapid nature of spin manipulation in layered all-semiconductor systems, offering significant implications for the development and enhancement of quantum information processing technologies.

Quantum Information Processing Technology Based on Quantum Optics

We outline ongoing endeavors in the development of quantum information processing technology utilizing quantum optics. We highlight the distinctive attributes of quantum optical platforms and explore two distinct approaches: discrete variable and continuous variable quantum optics, for the realization of quantum information processing. In addition, we showcase recent achievements in the implementation of quantum simulators, aiming to address practical challenges using today’s available technologies.

Nonequilibrium dynamics of the Jaynes-Cummings dimer.

We investigate the nonequilibrium dynamics of a Josephson-coupled Jaynes-Cummings dimer in the presence of Kerr nonlinearity, which can be realized in the cavity and circuit quantum electrodynamics systems. The semiclassical dynamics is analyzed systematically to chart out a variety of photonic Josephson oscillations and their regime of stability. Different types of transitions between the dynamical states lead to the self-trapping phenomenon, which results in photon population imbalance between the two cavities. We also study the dynamics quantum mechanically to identify characteristic features of different steady states and to explore fascinating quantum effects, such as spin dephasing, phase fluctuation, and revival phenomena of the photon field, as well as the entanglement of spin qubits. For a particular "self-trapped" state, the mutual information between the atomic qubits exhibits a direct correlation with the photon population imbalance, which is promising for generating photon mediated entanglement between two non interacting qubits in a controlled manner. Under a sudden quench from stable to unstable regime, the photon distribution exhibits phase space mixing with a rapid loss of coherence, resembling a thermal state. Finally, we discuss the relevance of the new results in experiments, which can have applications in quantum information processing and quantum technologies.

General quantum correlation from nonreal values of Kirkwood–Dirac quasiprobability over orthonormal product bases

We propose a characterization and a quantification of the general quantum correlation which is exhibited even by a separable (unentangled) mixed bipartite state in terms of the nonclassical values of the associated Kirkwood–Dirac (KD) quasiprobability. Such a general quantum correlation, wherein entanglement is a subset, is not only intriguing from a fundamental point of view, but it has also been recognized as a resource in a variety of schemes of quantum information processing and quantum technology. Given a bipartite state, we construct a quantity based on the imaginary part the associated KD quasiprobability defined over a pair of orthonormal product bases and an optimization procedure over all pairs of such bases. We show that it satisfies certain requirements expected for a quantifier of general quantum correlations. It gives a lower bound to the total sum of the quantum standard deviation of all the elements of the product (local) basis, minimized over all such bases. It suggests an interpretation as the minimum genuine quantum share of uncertainty in all local von-Neumann projective measurements. Moreover, it is a faithful witness for entanglement and measurement-induced nonlocality of pure bipartite states. We then discuss a variational scheme for its estimation, and based on this, we offer information theoretical meanings of the general quantum correlation. Our results suggest a deep connection between the nonclassical concept of general quantum correlation and the nonclassical values of the KD quasiprobability and the associated strange weak values.

Decentralized quantum anonymous veto voting scheme based on measurement-device-independence

To remove any trusted third party from the voting scheme, a new primitive quantum secure multiparty exclusive OR protocol based on Measurement-Device-Independence (MDI) is presented by utilizing entanglement swapping of Bell states. Based on this primitive protocol of quantum secure multiparty protocol, a novel quantum anonymous veto voting scheme is constructed, which can meet complete security attributes. Especially, this scheme does not need any third party to count the votes, while all voters can tally the voting results by themselves. At the same time, blockchain is introduced for supervision, so that the voters can verify the validity of the voting content under extreme circumstances. The voting scheme takes Bell states as quantum resources and only performs Bell states identification based on the principle of MDI, so it is feasible to implement this scheme with the present quantum information processing technologies. Finally, the proposed voting scheme is simulated in IBM Qiskit, and the simulation results show that this scheme is correct and feasible.

Material-Inherent Noise Sources in Quantum Information Architecture.

NISQ is a representative keyword at present as an acronym for "noisy intermediate-scale quantum", which identifies the current era of quantum information processing (QIP) technologies. QIP science and technologies aim to accomplish unprecedented performance in computation, communications, simulations, and sensing by exploiting the infinite capacity of parallelism, coherence, and entanglement as governing quantum mechanical principles. For the last several decades, quantum computing has reached to the technology readiness level 5, where components are integrated to build mid-sized commercial products. While this is a celebrated and triumphant achievement, we are still a great distance away from quantum-superior, fault-tolerant architecture. To reach this goal, we need to harness technologies that recognize undesirable factors to lower fidelity and induce errors from various sources of noise with controllable correction capabilities. This review surveys noisy processes arising from materials upon which several quantum architectures have been constructed, and it summarizes leading research activities in searching for origins of noise and noise reduction methods to build advanced, large-scale quantum technologies in the near future.

Microwave-to-optical transduction with erbium ions coupled to planar photonic and superconducting resonators

Optical quantum networks can connect distant quantum processors to enable secure quantum communication and distributed quantum computing. Superconducting qubits are a leading technology for quantum information processing but cannot couple to long-distance optical networks without an efficient, coherent, and low noise interface between microwave and optical photons. Here, we demonstrate a microwave-to-optical transducer using an ensemble of erbium ions that is simultaneously coupled to a superconducting microwave resonator and a nanophotonic optical resonator. The coherent atomic transitions of the ions mediate the frequency conversion from microwave photons to optical photons and using photon counting we observed device conversion efficiency approaching 10−7. With pulsed operation at a low duty cycle, the device maintained a spin temperature below 100 mK and microwave resonator heating of less than 0.15 quanta.

Theoretical analysis and experimental preparation of nanodiamond yttrium vacancy color center

This study conducts a theoretical analysis on the stable structure, charge transfer, energy level transition, and electronic structure of a diamond yttrium vacancy color center. The color center in a diamond is very important as a single photon source material for quantum information processing technology and applications, such as temperature sensors, solid-state quantum systems, photonics, lighting, imaging technology, and biomedicine. Nanodiamond yttrium vacancy color centers are prepared using the solid substitution method with physical vapor deposition and microwave plasma chemical vapor deposition equipment. The density functional theory (DFT) is used to analyze the stability, electronic structure, and co-doping of N, Si, and O on the related defect structure of a rare Earth element yttrium-doped diamond using first-principle calculations, demonstrating consistency with the experimental results. The research shows that the fluorescence wavelength of an yttrium vacancy color center measured using the solid substitution method is 701.2 nm, which is located in the red region. The system is most stable when a Y atom replaces a carbon atom and there are two vacancies around it. The zero-phonon line of the diamond YV2 defect is predicted to be 708.855 nm. Additionally, the influence doping is analyzed with N, Si, and O atoms. An yttrium-doped diamond emits light in the visible light region, which provides a theoretical basis for the application of diamond yttrium vacancy color centers.

Substrate engineering of plasmonic nanocavity antenna modes.

Plasmonic nanocavities have emerged as a promising platform for next-generation spectroscopy, sensing and photonic quantum information processing technologies, benefiting from a unique confluence of nanoscale compactness and integrability, ultrafast functionality and room-temperature viability. Harnessing their unprecedented optical field confinement and enhancement properties for such diverse application domains, however, demands continued innovation in cavity design and robust strategies for engineering their plasmonic mode characteristics, with the aim of optimizing spatial and spectral matching conditions for strong light-matter interaction involving embedded quantum emitters. Adopting the canonical gold bowtie nanoantenna, we show that the complex refractive index, n + ik, of the substrate material provides additional design flexibility in tailoring the properties of plasmonic nanocavity modes, including their resonance wavelengths, hotspot locations, intracavity field polarization and radiative decay rates. In particular, we predict that highly refractive (n ≥ 4) or highly absorptive (k ≥ 4) substrates provide two complementary approaches to engineering nanocavity modes that are especially desirable for coupling two-dimensional quantum materials, featuring namely an elevated hotspot with a dominantly in-plane polarized near-field, as well as a strongly radiative character. Our study elucidates the benefits and intricacies of a largely unexplored facet of nanocavity mode manipulation, beyond the widely practiced synthetic control over the cavity topology or physical dimensions, and paves the way for plasmonic cavity quantum electrodynamics with two-dimensional excitonic matter.

Phonon Engineering of Micro‐ and Nanophononic Crystals and Acoustic Metamaterials: A Review

Phononic crystals (PnCs) and acoustic metamaterials (AMMs) are artificially architected materials endowed with the capabilities of wave manipulation. This review specifically covers the theoretical fundamentals and recent development of PnCs and AMMs at the micro‐ and nanoscales due to their potential significance in modern electronic applications. Similarities and differences in phonon transport at these two scales are discussed. Research gaps between the macro‐ and small‐scale PnC and AMM development are identified from a theoretical point of view. A future outlook is also presented to propose possible applications of micro‐ and nano‐PnCs and AMMs, including advancing quantum information processing technologies, such as quantum computing.

Simple, Reliable, and Noise-Resilient Continuous-Variable Quantum State Tomography with Convex Optimization

Precise reconstruction of unknown quantum states from measurement data, a process commonly called quantum state tomography, is a crucial component in the development of quantum information processing technologies. Many different tomography methods have been proposed over the years. Maximum likelihood estimation is a prominent example, being the most popular method for a long period of time. Recently, more advanced neural network methods have started to emerge. Here, we go back to basics and present a method for continuous variable state reconstruction that is both conceptually and practically simple, based on convex optimization. Convex optimization has been used for process tomography and qubit state tomography, but seems to have been overlooked for continuous variable quantum state tomography. We demonstrate high-fidelity reconstruction of an underlying state from data corrupted by thermal noise and imperfect detection, for both homodyne and heterodyne measurements. A major advantage over other methods is that convex optimization algorithms are guaranteed to converge to the optimal solution.

Operational interpretation of bipartite quantum correlation

Operational interpretations of quantum resource measures lie at the heart of quantum information processing technologies. In this paper, we show that the bipartite quantum correlation in terms of Wigner-Yanase skew information, is the minimal failure probability of a particular state ensemble discrimination task via the associated least square measurement. We then derive a general relation between quantum coherence with respect to quantum operation and discriminating probability specific to least square measurement. Furthermore, we provide a wave-particle duality relation between bipartite quantum correlation and which-path information.

Identifying optimal cycles in quantum thermal machines with reinforcement-learning

The optimal control of open quantum systems is a challenging task but has a key role in improving existing quantum information processing technologies. We introduce a general framework based on reinforcement learning to discover optimal thermodynamic cycles that maximize the power of out-of-equilibrium quantum heat engines and refrigerators. We apply our method, based on the soft actor-critic algorithm, to three systems: a benchmark two-level system heat engine, where we find the optimal known cycle; an experimentally realistic refrigerator based on a superconducting qubit that generates coherence, where we find a non-intuitive control sequence that outperforms previous cycles proposed in literature; a heat engine based on a quantum harmonic oscillator, where we find a cycle with an elaborate structure that outperforms the optimized Otto cycle. We then evaluate the corresponding efficiency at maximum power.

FPGA Accelerated Post-Quantum Cryptography

Recent advancement in quantum information processing technology has led to the emergence of advanced cryptography in the post-quantum era. Next generation cryptographic techniques aim to be mathematically resistant against any known attacks related to quantum computing, and can be easily implemented on traditional hardware platforms. The National Institutes of Standards and Technology (NIST) has entered the fourth-round standardization process of post-quantum cryptography (PQC). Software implementations of PQC candidates have been widely investigated. Interests in domain-specific hardware acceleration of PQC algorithms have risen, in particular using field-programmable gate arrays (FPGAs). While conventional general-purpose hardware platforms have been used for PQC implementations, modern FPGAs promise software-hardware co-optimisation, deep pipeline parallelism and trivial support for custom-precision arithmetic. Therefore, the time is ripe for reviewing recent FPGA-based PQC implementations. This article first surveys state-of-the-art advances in PQC implementations on FPGAs, including fast arithmetic, algorithm-hardware codesign approaches and open-source PQC hardware projects, then gives a brief review of recent attacks on PQC algorithms and their hardware implementations. Finally, we summarise the challenges for hardware implementations along with potential research directions in this promising field.

A Flexible Design Platform for Si/SiGe Exchange-Only Qubits with Low Disorder.

Spin-based silicon quantum dots are an attractive qubit technology for quantum information processing with respect to coherence time, control, and engineering. Here we present an exchange-only Si qubit device platform that combines the throughput of CMOS-like wafer processing with the versatility of direct-write lithography. The technology, which we coin "SLEDGE", features dot-shaped gates that are patterned simultaneously on one topographical plane and subsequently connected by vias to interconnect metal lines. The process design enables nontrivial layouts as well as flexibility in gate dimensions, material selection, and additional device features such as for rf qubit control. We show that the SLEDGE process has reduced electrostatic disorder with respect to traditional overlapping gate devices with lift-off metallization, and we present spin coherent exchange oscillations and single qubit blind randomized benchmarking data.

On-Demand Storage and Retrieval of Microwave Photons Using a Superconducting Multiresonator Quantum Memory.

We realize on-demand storage and retrieval of weak coherent microwave photon pulses at the single-photon level. A superconducting multiresonator system which is composed of a set of frequency-tunable coplanar waveguide resonators is implemented as the quantum memory. By dynamically tuning the resonant frequencies of the resonators, we achieve tunable memory bandwidth from 10 to 55MHz, with well preserved phase coherence. We further demonstrate on-demand storage and retrieval of a time-bin flying qubit. This result opens up a prospect to integrate our chip-based quantum memory with the state-of-the-art superconducting quantum circuit technology for quantum information processing.

Multi‐hop non‐destructive qudit teleportation via non‐maximally entangled GHZ channels

A scheme for the multihop non-destructive teleportation of an arbitrary single-qudit state based on various non-maximally entangled Greenberger–Horne–Zeilinger channels is proposed here. The parallel entanglement swap is used to form a direct d-dimensional ( d ≥ 2 ) entangled channel shared between the sender and the receiver. The authors’ propose a novel idea that the third-party Charlie with high quantum information processing technology executes the channel modulation, which reduces the technical requirements of the sender and the receiver. In addition, if the teleportation fails, the unknown state will be restored by the sender. This work provides a feasible method for teleportation and ensures the integrity of the unknown quantum information.

Fault-tolerant Technology for Quantum Information Processing and Its Implementation Methods

Fault-tolerant Technology for Quantum Information Processing and Its Implementation Methods