Quantum Interconnects Research Articles

Hybrid Approach to Mitigate Errors in Linear Photonic Bell-State Measurement for Quantum Interconnects

Optical quantum information processing relies critically on Bell-state measurement, a ubiquitous operation for quantum communication and computing. Its practical realization involves the interference of optical modes and the detection of a single photon in an indistinguishable manner. Yet, in the absence of efficient photon-number-resolution capabilities, errors arise from multiphoton components, decreasing the overall process fidelity. Here, we introduce a hybrid detection scheme for Bell-state measurement, leveraging both on-off single-photon detection and quadrature conditioning via homodyne detection. We derive explicit fidelities for quantum teleportation and entanglement-swapping processes employing this strategy, demonstrating its efficacy. We also compare with photon-number-resolving detectors and find a strong advantage of the hybrid scheme in a wide range of parameters. This work provides a new tool for linear-optics schemes, with applications to quantum state engineering and quantum interconnects. Published by the American Physical Society 2024

Important Elements of Spin-Exciton and Magnon-Exciton Coupling.

The recent discovery of spin-exciton and magnon-exciton coupling in a layered antiferromagnetic semiconductor, CrSBr, is both fundamentally intriguing and technologically significant. This discovery unveils a unique capability to optically access and manipulate spin information using excitons, opening doors to applications in quantum interconnects, quantum photonics, and opto-spintronics. Despite their remarkable potential, materials exhibiting spin-exciton and magnon-exciton coupling remain limited. To broaden the library of such materials, we explore key parameters for achieving and tuning spin-exciton and magnon-exciton couplings. We begin by examining the mechanisms of couplings in CrSBr and drawing comparisons with other recently identified two-dimensional magnetic semiconductors. Furthermore, we propose various promising scenarios for spin-exciton coupling, laying the groundwork for future research endeavors.

Remote cross-resonance gate between superconducting fixed-frequency qubits

High-fidelity quantum state transfer and remote entanglement between superconducting fixed-frequency qubits have not yet been realized. In this study, we propose an alternative remote cross-resonance gate. Considering multiple modes of a superconducting coaxial cable connecting qubits, we must find conditions under which the cross-resonance gate operates with a certain accuracy even in the presence of qubit frequency shifts due to manufacturing errors. For 0.25- and 0.5 m cables, remote cross-resonance gates with a concurrence of >99.9% in entanglement generation are obtained even with ±10 MHz frequency shifts. For a 1 m cable with a narrow mode spacing, a concurrence of 99.5% is achieved by reducing the coupling between the qubits and cable. The optimized echoed raised-cosine pulse duration is 150–400 ns, which is similar to the operation time of cross-resonance gates between neighboring qubits on a chip. The dissipation through the cable modes does not considerably affect the obtained results. Such high-precision quantum interconnects pave the way not only for scaling up quantum computer systems but also for nonlocal connections on a chip.

Non-Markovian quantum interconnect formed by a surface plasmon polariton waveguide

Non-Markovian quantum interconnect formed by a surface plasmon polariton waveguide

Design of Nanoscale Quantum Interconnects Aided by Conditional Generative Adversarial Networks

Design of Nanoscale Quantum Interconnects Aided by Conditional Generative Adversarial Networks

Quantum frequency conversion using 4-port fiber-pigtailed PPLN module.

Quantum frequency conversion (QFC), which involves the exchange of frequency modes of photons, is a prerequisite for quantum interconnects among various quantum systems, primarily those based on telecom photonic network infrastructures. Compact and fiber-closed QFC modules are in high demand for such applications. In this paper, we report such a QFC module based on a fiber-coupled 4-port frequency converter with a periodically poled lithium niobate (PPLN) waveguide. The demonstrated QFC shifted the wavelength of a single photon from 780 to 1541 nm. The single photon was prepared via spontaneous parametric down-conversion (SPDC) with heralding photon detection, for which the cross-correlation function was 40.45 ± 0.09. The observed cross-correlation function of the photon pairs had a nonclassical value of 13.7 ± 0.4 after QFC at the maximum device efficiency of 0.73, which preserved the quantum statistical property. Such an efficient QFC module is useful for interfacing atomic systems and fiber-optic communication.

A chip-scale polarization-spatial-momentum quantum SWAP gate in silicon nanophotonics

Recent progress in quantum computing and networking has enabled high-performance, large-scale quantum processors by connecting different quantum modules. Optical quantum systems show advantages in both computing and communications, and integrated quantum photonics further increases the level of scaling and complexity. Here we demonstrate an efficient SWAP gate that deterministically swaps a photon’s polarization qubit with its spatial-momentum qubit on a nanofabricated two-level silicon photonics chip containing three cascaded gates. The on-chip SWAP gate is comprehensively characterized by tomographic measurements with high fidelity for both single-qubit and two-qubit operation. The coherence preservation of the SWAP gate process is verified by single-photon and two-photon quantum interference. The coherent reversible conversion of our SWAP gate facilitates examinations of a quantum interconnect between two chip-scale photonic subsystems with different degrees of freedom, now demonstrated by distributing four Bell states between the two chips. We also elucidate the source of decoherence in the SWAP operation in pursuit of near-unity fidelity. Our deterministic SWAP gate in the silicon platform provides a pathway towards integrated quantum information processing for interconnected modular systems. A deterministic single-photon two-qubit SWAP gate between polarization and spatial-momentum is demonstrated on a silicon chip. A two-qubit swapping process fidelity of 94.9% is obtained. The coherence preservation of the SWAP gate process is verified by two-photon interference.

Jaynes-Cummings interaction between low-energy free electrons and cavity photons.

The Jaynes-Cummings Hamiltonian is at the core of cavity quantum electrodynamics; however, it relies on bound-electron emitters fundamentally limited by the binding Coulomb potential. In this work, we propose theoretically a new approach to realizing the Jaynes-Cummings model using low-energy free electrons coupled to dielectric microcavities and exemplify several quantum technologies made possible by this approach. Using quantum recoil, a large detuning inhibits the emission of multiple consecutive photons, effectively transforming the free electron into a few-level system coupled to the cavity mode. We show that this approach can be used for generation of single photons, photon pairs, and even a quantum SWAP gate between a photon and a free electron, with unity efficiency and high fidelity. Tunable by their kinetic energy, quantum free electrons are inherently versatile emitters with an engineerable emission wavelength. Hence, they pave the way toward new possibilities for quantum interconnects between photonic platforms at disparate spectral regimes.

Disordered topological graphs enhancing nonlinear phenomena.

Complex networks play a fundamental role in understanding phenomena from the collective behavior of spins, neural networks, and power grids to the spread of diseases. Topological phenomena in such networks have recently been exploited to preserve the response of systems in the presence of disorder. We propose and demonstrate topological structurally disordered systems with a modal structure that enhances nonlinear phenomena in the topological channels by inhibiting the ultrafast leakage of energy from edge modes to bulk modes. We present the construction of the graph and show that its dynamics enhances the topologically protected photon pair generation rate by an order of magnitude. Disordered nonlinear topological graphs will enable advanced quantum interconnects, efficient nonlinear sources, and light-based information processing for artificial intelligence.

Simulating time evolution on distributed quantum computers

We study a variation of the Trotter-Suzuki decomposition, in which a Hamiltonian exponential is approximated by an ordered product of two-qubit operator exponentials such that the Trotter step size is enhanced for a small number of terms. Such decomposition directly reflects hardware constraints of distributed quantum computers, where operations on monolithic quantum devices are fast compared to entanglement distribution across separate nodes using interconnects. We simulate non-equilibrium dynamics of transverse-field Ising and XY spin chain models and investigate the impact of locally increased Trotter step sizes that are associated with an increasingly sparse use of the quantum interconnect. We find that the overall quality of the approximation depends smoothly on the local sparsity and that the proliferation of local errors is slow. As a consequence, we show that fast local operations on monolithic devices can be leveraged to obtain an overall improved result fidelity even on distributed quantum computers where the use of interconnects is costly.

A quantum-bit encoding converter

From telecommunications to computing architectures, the realm of classical information hinges on converter technology to enable the exchange of data between digital and analogue formats, a process now routinely performed across a variety of electronic devices. A similar exigency also exists in quantum information technology, where different frameworks are being developed for quantum computing, communication and sensing. Thus, efficient quantum interconnects are a major need to bring these parallel approaches together and scale up quantum information systems. So far, however, the conversion between different optical quantum-bit encodings has remained challenging due to the difficulty of preserving fragile quantum superpositions and the demanding requirements for postselection-free implementations. Here we demonstrate such a conversion of quantum information between the two main paradigms, namely discrete- and continuous-variable qubits. We certify the protocol on a complete set of single-photon qubits, successfully converting them to cat-state qubits with fidelities exceeding the classical limit. Our result demonstrates an essential tool for enabling interconnected quantum devices and architectures with enhanced versatility and scalability. A conversion of quantum information between single-photon and cat-state qubits is demonstrated by teleportation using optical hybrid entanglement. The classical limit of conversion is exceeded over the full Bloch sphere, with an average fidelity above 79%.

Strong coupling of quantum emitters and the exciton polariton in MoS2 nanodisks

As a quasiparticle formed by light and excitons in semiconductors, the exciton polariton (EP) as a quantum bus is promising for the development of quantum interconnect devices at room temperature. However, the significant damping of EPs in the material generally causes a loss of quantum information. We propose a mechanism to overcome the destructive effect of a damping EP on its mediated correlation dynamics of quantum emitters (QEs). Via an investigation of the near-field coupling between two QEs and the EP in a monolayer ${\mathrm{MoS}}_{2}$ nanodisk, we find that, with the complete dissipation of the QEs efficiently avoided, a persistent quantum correlation between the QEs can be generated and stabilized even to their steady state. This is due to the fact that, upon decreasing the QE-${\mathrm{MoS}}_{2}$ distance, the QEs become so hybridized with the EP that one or two bound states are formed between them. Our result supplies a useful way to avoid the destructive impact of EP damping, and it refreshes our understanding of the light-matter interaction in an absorbing medium.

Exciton-coupled coherent magnons in a 2D semiconductor.

The recent discoveries of two-dimensional (2D) magnets1-6 and their stacking into van der Waals structures7-11 have expanded the horizon of 2D phenomena. One exciting application is to exploit coherent magnons12 as energy-efficient information carriers in spintronics and magnonics13,14 or as interconnects in hybrid quantum systems15-17. A particular opportunity arises when a 2D magnet is also a semiconductor, as reported recently for CrSBr (refs. 18-20) and NiPS3 (refs. 21-23) that feature both tightly bound excitons with a large oscillator strength and potentially long-lived coherent magnons owing to the bandgap and spatial confinement. Although magnons and excitons are energetically mismatched by orders of magnitude, their coupling can lead to efficient optical access to spin information. Here we report strong magnon-exciton coupling in the 2D A-type antiferromagnetic semiconductor CrSBr. Coherent magnons launched by above-gap excitation modulate the exciton energies. Time-resolved exciton sensing reveals magnons that can coherently travel beyond seven micrometres, with a coherence time of above five nanoseconds. We observe these exciton-coupled coherent magnons in both even and odd numbers of layers, with and without compensated magnetization, down to the bilayer limit. Given the versatility of van der Waals heterostructures, these coherent 2D magnons may be a basis for optically accessible spintronics, magnonics and quantum interconnects.

Er-doped anatase TiO2 thin films on LaAlO3 (001) for quantum interconnects (QuICs)

Rare-earth ions (REIs) doped into solid-state crystal hosts offer an attractive platform for realizing quantum interconnects that can function as quantum memories and quantum repeaters. The 4f valence electrons of REIs are shielded by 5s and 5p electrons and undergo highly coherent transitions even when embedded in host crystals. In particular, Er3+ has an optical transition in the telecom band that is suitable for low-loss communication. Recently, REIs in thin film systems have gained interest due to potential advantages in providing a flexible host crystal environment, enabling scalable on-chip integration with other quantum devices. Here, we investigate the structural and optical properties of Er-doped anatase TiO2 thin films on LaAlO3 (001) substrates. By choosing a system with minimal lattice mismatch and adjusting Er-dopant concentration, we achieve optical inhomogeneous linewidths of 5 GHz at 4.5 K. We show that 9 nm-thick buffer and capping layers can reduce the linewidth by more than 40%, suggesting a pathway to further narrowing linewidths in this system. We also identify that Er3+ ions mainly incorporate into substitutional Ti4+ sites with non-polar D2d symmetry, which makes Er dopants insensitive to the first order to local electric fields from impurities and is desirable for coherence properties of Er3+ spins.

Quantum-enabled operation of a microwave-optical interface

Solid-state microwave systems offer strong interactions for fast quantum logic and sensing but photons at telecom wavelength are the ideal choice for high-density low-loss quantum interconnects. A general-purpose interface that can make use of single photon effects requires < 1 input noise quanta, which has remained elusive due to either low efficiency or pump induced heating. Here we demonstrate coherent electro-optic modulation on nanosecond-timescales with only 0.1{6}_{-0.01}^{+0.02} microwave input noise photons with a total bidirectional transduction efficiency of 8.7% (or up to 15% with 0.4{1}_{-0.02}^{+0.02}), as required for near-term heralded quantum network protocols. The use of short and high-power optical pump pulses also enables near-unity cooperativity of the electro-optic interaction leading to an internal pure conversion efficiency of up to 99.5%. Together with the low mode occupancy this provides evidence for electro-optic laser cooling and vacuum amplification as predicted a decade ago.

Quantum coherent microwave-optical transduction using high-overtone bulk acoustic resonances

A device capable of converting single quanta of the microwave field to the optical domain is an outstanding endeavour in the context of quantum interconnects between distant superconducting qubits, but likewise can have applications in other fields, such as radio astronomy or, in the classical realm, microwave photonics. A variety of transduction approaches, based on optomechanical or electro-optical interactions, have been proposed and realized, yet the required vanishing added noises and an efficiency approaching unity, have not yet been attained. Here we present a new transduction scheme that could in theory satisfy the requirements for quantum coherent bidirectional transduction. Our scheme relies on an intermediary mechanical mode, a high overtone bulk acoustic resonance (HBAR), to coherently couple microwave and optical photons through the piezoelectric and strain-optical effects. Its efficiency results from the combination of integrated Si3N4 photonic circuits with ultra low loss sustaining high intracavity photon numbers with the highly efficient microwave to mechanical transduction offered by piezoelectrically coupled HBAR. We develop a quantum theory for this multipartite system by first introducing a quantization method for the piezoelectric interaction between the microwave mode and the mechanical mode from first principles (which to our knowledge has not been presented in this form), and link the latter to the conventional Butterworth-Van Dyke model. The HBAR is subsequently coupled to a pair of hybridized optical modes from coupled optical ring cavities via the strain-optical effect. We analyze the conversion capabilities of the proposed device using signal flow graphs, and demonstrate that near quantum coherent transduction is possible, with realistic experimental parameters.

Entanglement across separate silicon dies in a modular superconducting qubit device

Assembling future large-scale quantum computers out of smaller, specialized modules promises to simplify a number of formidable science and engineering challenges. One of the primary challenges in developing a modular architecture is in engineering high fidelity, low-latency quantum interconnects between modules. Here we demonstrate a modular solid state architecture with deterministic inter-module coupling between four physically separate, interchangeable superconducting qubit integrated circuits, achieving two-qubit gate fidelities as high as 99.1 ± 0.5% and 98.3 ± 0.3% for iSWAP and CZ entangling gates, respectively. The quality of the inter-module entanglement is further confirmed by a demonstration of Bell-inequality violation for disjoint pairs of entangled qubits across the four separate silicon dies. Having proven out the fundamental building blocks, this work provides the technological foundations for a modular quantum processor: technology which will accelerate near-term experimental efforts and open up new paths to the fault-tolerant era for solid state qubit architectures.

Development of Quantum Interconnects (QuICs) for Next-Generation Information Technologies

Just as classical information technology rests on a foundation built of interconnected information-processing systems, quantum information technology (QIT) must do the same. A critical component of such systems is the interconnect, a device or process that allows transfer of information between disparate physical media, for example, semiconductor electronics, individual atoms, light pulses in optical fiber, or microwave fields. While interconnects have been well engineered for decades in the realm of classical information technology, quantum interconnects (QuICs) present special challenges, as they must allow the transfer of fragile quantum states between different physical parts or degrees of freedom of the system. The diversity of QIT platforms (superconducting, atomic, solid-state color center, optical, etc.) that will form a quantum internet poses additional challenges. As quantum systems scale to larger size, the quantum interconnect bottleneck is imminent, and is emerging as a grand challenge for QIT. For these reasons, it is the position of the community represented by participants of the NSF workshop on Quantum Interconnects that accelerating QuIC research is crucial for sustained development of a national quantum science and technology program. Given the diversity of QIT platforms, materials used, applications, and infrastructure required, a convergent research program including partnership between academia, industry and national laboratories is required. This document is a summary from a U.S. National Science Foundation supported workshop held on 31 October - 1 November 2019 in Alexandria, VA. Attendees were charged to identify the scientific and community needs, opportunities, and significant challenges for quantum interconnects over the next 2-5 years.

Quantum Engineering With Hybrid Magnonic Systems and Materials (Invited Paper)

Quantum technology has made tremendous strides over the past two decades with remarkable advances in materials engineering, circuit design, and dynamic operation. In particular, the integration of different quantum modules has benefited from hybrid quantum systems, which provide an important pathway for harnessing different natural advantages of complementary quantum systems and for engineering new functionalities. This review article focuses on the current frontiers with respect to utilizing magnons for novel quantum functionalities. Magnons are the fundamental excitations of magnetically ordered solid-state materials and provide great tunability and flexibility for interacting with various quantum modules for integration in diverse quantum systems. The concomitant-rich variety of physics and material selection enable exploration of novel quantum phenomena in materials science and engineering. In addition, the ease of generating strong coupling with other excitations makes hybrid magnonics a unique platform for quantum engineering. We start our discussion with circuit-based hybrid magnonic systems, which are coupled with microwave photons and acoustic phonons. Subsequently, we focus on the recent progress of magnon–magnon coupling within confined magnetic systems. Next, we highlight new opportunities for understanding the interactions between magnons and nitrogen-vacancy centers for quantum sensing and implementing quantum interconnects. Lastly, we focus on the spin excitations and magnon spectra of novel quantum materials investigated with advanced optical characterization.

Flying couplers above spinning resonators generate irreversible refraction.

Creating optical components that allow light to propagate in only one direction-that is, that allow non-reciprocal propagation or 'isolation' of light-is important for a range of applications. Non-reciprocal propagation of sound can be achieved simply by using mechanical components that spin1,2. Spinning also affects de Broglie waves 3 , so a similar idea could be applied in optics. However, the extreme rotation rates that would be required, owing to light travelling much faster than sound, lead to unwanted wobbling. This wobbling makes it difficult to maintain the separation between the spinning devices and the couplers to within tolerance ranges of several nanometres, which is essential for critical coupling4,5. Consequently, previous applications of optical6-17 and optomechanical10,17-20 isolation have used alternative methods. In hard-drive technology, the magnetic read heads of a hard-disk drive fly aerodynamically above the rapidly rotating disk with nanometre precision, separated by a thin film of air with near-zerodrag that acts as a lubrication layer 21 . Inspired by this, here we report the fabrication of photonic couplers (tapered fibresthat couple light into the resonators) that similarly fly above spherical resonators with a separation of only a few nanometres. The resonators spin fast enough to split their counter-circulating optical modes, making the fibre coupler transparent from one side while simultaneously opaque from the other-that is, generating irreversible transmission. Our setup provides 99.6 per cent isolation of light in standard telecommunication fibres, of the type used for fibre-based quantum interconnects 22 . Unlike flat geometries, such as between a magnetic head and spinning disk, the saddle-like, convex geometry of the fibre and sphere in our setup makes it relatively easy to bring the two closer together, which could enable surface-science studies at nanometre-scale separations.