A. P. Lund, T. C. Ralph
We discuss techniques for producing, manipulating and measureing qubits encoded optically as vacuum and single photon states. We show that a universal set of non-deterministic gates can be constructed using linear optics and photon counting. We investigate the efficacy of a test gate given realistic detector efficiencies.
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Entangling Quantum Memories via Heralded Photonic Bell Measurement
本文比较单轨与双轨光子编码在量子存储器纠缠交换中的性能，分析非理想因素对可蒸馏纠缠和保真度的影响，并研究纠缠蒸馏方案。
We propose a quantum noise eater for a single qubit and experimentally verify its performance for recovery of a superposition carried by a dual-rail photonic qubit. We consider a case when only one of the rails (e.g., one of interferometric arms) is vulnerable to noise. A coherent but randomly arriving photon penetrating into this single rail causes a change of its state, which results in an error in a subsequent quantum information processing. We theoretically prove and experimentally demonstrate a conditional full recovery of the superposition by this quantum noise eater.
Generation of frequency-bin-encoded dual-rail cluster states via time-frequency multiplexing of microwave photonic qubits
本文提出通过微波光子量子比特的时间-频率复用生成频率桶编码的双轨簇状态，实现了高保真度的多量子比特纠缠态生成。
Photon-Number Conserved Universal Quantum Logic Employing Continuous-Time Quantum Walk on Dual-Rail Qubit Arrays
利用双轨量子比特编码与连续时间量子行走的协同作用，在超导电路中实现通用量子逻辑，通过可调耦合强度实现单、双、三量子比特操作，抑制泄漏并提升保真度。
Photonic interface between subcarrier wave and dual rail encodings
本文提出并演示了一种基于线性光学电路的副载波与双轨编码光子接口，保真度超过92%，为长距离异构量子网络中结合量子密钥分发与量子中继器提供了可行方案。
Logical multi-qubit entanglement with dual-rail superconducting qubits
本研究通过双轨超导量子比特实现了逻辑多量子比特纠缠，展示了高保真度的纠缠态和通用门集，为量子纠错提供了新方案。
Deterministic sources of high-fidelity entangled qubit pairs encoded in the dual-rail photonic basis, i.e., presence of a single photon in one of two orthogonal modes, are a key enabling technology of many applications of quantum information processing, including high-rate high-fidelity quantum communications over long distances. The most popular and mature sources of such photonic entanglement, e.g., those that leverage spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (sFWM), generate an entangled (so-called, continuous-variable) quantum state that contains contri…
Demonstrating a superconducting dual-rail cavity qubit with erasure-detected logical measurements
本文实现了超导双轨腔量子比特，通过集成擦除检测的投影逻辑测量，以0.01%级精度制备和测量逻辑态，检测超99%腔衰减事件，并区分误差类型，首次确认了级联双轨擦除量子比特所需的误差层级。
Entanglement is the fuel of advanced quantum technology. It is for instance consumed in measurement-based quantum computing and allows loss-tolerant encoding of quantum information. In photonics, entanglement has traditionally been generated probabilistically, requiring massive multiplexing for scaling up to many photons. An alternative approach utilizes quantum emitters in nanophotonic devices for deterministic generation of single photons, which an be extended to two- and multi-photon generation on demand. The proposed polarization-entanglement sources are, however, incompatible with spatial…
Doubling Qubits in a Trapped-Ion System via Vibrational Dual-Rail Encoding
本文提出了一种在囚离子系统中通过振动双轨编码实现量子比特倍增的方法，结合内部量子比特和双轨量子比特，提升逻辑量子比特数量并保持全连接性。
Using only linear optical elements, the creation of dual-rail photonic entangled states is inherently probabilistic. Known entanglement generation schemes have low success probabilities, requiring large-scale multiplexing to achieve near-deterministic operation of quantum information processing protocols. In this paper, we introduce multiple techniques and methods to generate photonic entangled states with high probability, which have the potential to reduce the footprint of Linear Optical Quantum Computing (LOQC) architectures drastically. Most notably, we are showing how to improve Bell stat…
We analyze the problem of increasing the efficiency of single-photon sources or single-rail photonic qubits via linear optical processing and destructive conditional measurements. In contrast to previous work we allow for the use of coherent states and do not limit to photon-counting measurements. We conjecture that it is not possible to increase the efficiency, prove this conjecture for several important special cases, and provide extensive numerical results for the general case.
The potential for artificial intelligence (AI) to take over the work of physicists should be treated with glee. Here I evaluate one of the scientific discoveries in quantum photonics made by a leading AI in the field, in order to try and gain insight into when I will be allowed to go spend my days sipping mezcal margaritas on a warm beach. My analysis leads me to the distressing conclusion that it may, in fact, be quite a while yet.
Bias-Preserving Gates and Quantum Error Correction With Dual-Rail Cat Codes
提出双轨猫码（DRCC），结合内层猫码与外层双轨结构，实现偏压保持逻辑门和确定性单光子损失纠错，为硬件高效、抗擦除的容错量子计算提供新方案。
We solve the Jaynes-Cummings Hamiltonian with time-dependent coupling parameters under dipole and rotating-wave approximation for a three-dimensional (3D) photonic crystal (PC) single mode cavity with a sufficiently high quality (Q) factor. We then exploit the results to show how to create a maximally entangled state of two atoms, and how to implement several quantum logic gates: a dual-rail Hadamard gate, a dual-rail NOT gate, and a SWAP gate. The atoms in all of these operations are syncronized, which is not the case in previous studies [1,2] in PCs. Our method has the potential for extensio…
We investigate the performance of a Kennedy receiver, which is known as a beneficial tool in optical coherent communications, to the quantum state discrimination of the two superpositions of vacuum and single photon states corresponding to the $\hatσ_x$ eigenstates in the single-rail encoding of photonic qubits. We experimentally characterize the Kennedy receiver in vacuum-single photon two-dimensional space using quantum detector tomography and evaluate the achievable discrimination error probability from the reconstructed measurement operators. We furthermore derive the minimum error rate ob…
Teleportation plays an important role in the communication of quantum information between the nodes of a quantum network and is viewed as an essential ingredient for long-distance Quantum Cryptography. We describe a method to teleport the quantum information carried by a photon in a superposition of a number $d$ of light modes (a "qudit") by the help of $d$ additional photons based on transcription. A qudit encoded into a single excitation of $d$ light modes (in our case Laguerre-Gauss modes which carry orbital angular momentum) is transcribed to $d$ single-rail photonic qubits, which are spat…
Photonic quantum systems are among the most promising architectures for quantum computers. It is well known that for dual-rail photons effective non-linearities and near-deterministic non-trivial two-qubit gates can be achieved via the measurement process and by introducing ancillary photons. While in principle this opens a legitimate path to scalable linear optical quantum computing, the technical requirements are still very challenging and thus other optical encodings are being actively investigated. One of the alternatives is to use single-rail encoded photons, where entangled states can be…
Photon counting induces an effective nonlinear optical phase shift on certain states derived by linear optics from single photons. Although this no nlinearity is nondeterministic, it is sufficient in principle to allow scalable linear optics quantum computation (LOQC). The most obvious way to encode a qubit optically is as a superposition of the vacuum and a single photon in one mode -- so-called "single-rail" logic. Until now this approach was thought to be prohibitively expensive (in resources) compared to "dual-rail" logic where a qubit is stored by a photon across two modes. Here we attack…
Teleportation of a genuine single-rail vacuum-one-photon qubit generated via a quantum dot source
本文利用微腔中共振激发半导体量子点的相干控制，实现了单一空间模式内真正单轨真空-单光子量子比特的隐形传态与纠缠交换。
All-photonic quantum key distribution beyond the single-repeater bound
本文提出一种全光量子密钥分发协议，在不进行错误校正的情况下超越单中继限制，通过调整量子信号速度实现更高的密钥率。
Dynamical error reshaping for dual-rail erasure qubits
本文提出通过抑制色散量子比特引入的噪声来提升双轨擦除量子比特的性能，显著降低擦除检查误差和逻辑两量子比特门保真度。
Erasure detection of a dual-rail qubit encoded in a double-post superconducting cavity
本文在双柱超导腔中实现双轨量子比特，利用辅助transmon进行擦除检测，并通过改进的联合Wigner层析成像表征码空间，测得擦除率远高于残余退相干率，展示了该架构在量子纠错中的潜力。
Error-detected coherence metrology of a dual-rail encoded fixed-frequency multimode superconducting qubit
本研究在单个固定频率超导多模transmon量子比特中实现双轨编码，将振幅阻尼误差转换为可检测泄漏态，使逻辑误差率比物理误差率低一个数量级以上，并验证了架构的稳定性与可重复性。
Dual-rail encoding with superconducting cavities
本文提出电路QED双轨量子比特，利用超导腔单光子子空间编码，将光子损失转为易纠正的擦除错误，实现通用门操作，错误率层级有利，有望低于QEC阈值。
Logic-qubit entanglement is a promising resource in quantum information processing, especially in future large-scale quantum networks. In the paper, we put forward an efficient entanglement purification protocol (EPP) for nonlocal mixed logic entangled states with the bit-flip error in the logic qubits of the logic Bell state, resorting to the photon-atom interaction in low-quality (Q) cavity and atomic state measurement. Different from existing EPPs, this protocol can also purify the logic phase-flip error, and the bit-flip error and the phase-flip error in physic qubit. During the protocol, …
Fault-tolerant quantum computing requires gates which function correctly despite the presence of errors, and are scalable if the error probability-per-gate is below a threshold value. To date, no method has been described for calculating this probability from measurements on a gate. Here we introduce a technique enabling quantitative benchmarking of quantum-logic gates against fault-tolerance thresholds for any architecture. We demonstrate our technique experimentally using a photonic entangling-gate. The relationship between experimental errors and their quantum logic effect is non-trivial: r…
Several emerging quantum technologies, including quantum networks, modular and fusion-based quantum computing, rely crucially on the ability to perform photonic Bell state measurements. Therefore, photon losses and the 50\% success probablity upper bound of Bell state measurements pose a critical limitation to photonic quantum technologies. Here, we develop protocols that overcome these two key challenges through logical encoding of photonic qubits. Our approach uses a tree graph state logical encoding, which can be produced deterministically with a few quantum emitters, and achieves near-dete…
Recent progress in nonlinear optical materials and microresonators has brought quantum computing with bulk optical nonlinearities into the realm of possibility. This platform is of great interest, not only because photonics is an obvious choice for quantum networks, but also because it may be the only feasible route to quantum information processing at room temperature. We introduce a paradigm for room-temperature photonic quantum logic that significantly simplifies the realization of various quantum circuits, and in particular, of error correction. It uses only the strongest available bulk no…
Increasing control of single photons enables new applications of photonic quantum-enhanced technology and further experimental exploration of fundamental quantum phenomena. Here, we demonstrate quantum logic using narrow linewidth photons that are produced under nearly perfect quantum control from a single ^87Rb atom strongly coupled to a high-finesse cavity. We use a controlled- NOT gate integrated into a photonic chip to entangle these photons, and we observe non-classical correlations between events separated by periods exceeding the travel time across the chip by three orders of magnitude.…
Deterministic entangling gates with nonlinear quantum photonic interferometers
针对线性光子量子计算的概率性局限，本文提出利用弱非线性干涉元件串联构造通用单、双量子比特门，理论上可实现保真度接近100%的确定性纠缠门。
Boosted linear-optical measurements on single-rail qubits with unentangled ancillas
利用8端口干涉仪和光子检测，实现单轨量子比特在XY布洛赫平面的测量，成功概率达147/256，超越传统1/2限制。
This note reviews the model of computation generated by photonic circuits, comprising edges that are traversed by photons in a single time-bin and vertices given by idealised lossless beam splitters and phase shifters. The circuit model is abstracted as a representation of the symmetric monoidal category of unitary matrices on the bosonic Fock space of multimode photons. A diagrammatic language, designed to aid with the understanding and development of photonic algorithms, is presented that encapsulates the category properties of this representation. As demonstrations of the formalism, the bos…
To realize fault-tolerant quantum computing, it is necessary to store quantum information in logical qubits with error correction functions, realized by distributing a logical state among multiple physical qubits or by encoding it in the Hilbert space of a high-dimensional system. Quantum gate operations between these error-correctable logical qubits, which are essential for implementation of any practical quantum computational task, have not been experimentally demonstrated yet. Here we demonstrate a geometric method for realizing controlled-phase gates between two logical qubits encoded in p…
Squeezing-enhanced Pairwise Fusion of Photonic Qudits
通过在线性光学融合门中引入单模压缩器，回收部分失败测量结果，提高了对d-轨单光子量子比特的贝尔投影成功概率，并推导了精确的POVM表达式。
Surface acoustic waves (SAWs) strongly modulate the shallow electric potential in piezoelectric materials. In semiconductor heterostructures such as GaAs/AlGaAs, SAWs can thus be employed to transfer individual electrons between distant quantum dots. This transfer mechanism makes SAW technologies a promising candidate to convey quantum information through a circuit of quantum logic gates. Here we present two essential building blocks of such a SAW-driven quantum circuit. First, we implement a directional coupler allowing to partition a flying electron arbitrarily into two paths of transportati…
As primitives for entanglement generation, controlled phase gates take a central role in quantum computing. Especially in ideas realizing instances of quantum computation in linear optical gate arrays a closer look can be rewarding. In such architectures, all effective non-linearities are induced by measurements: Hence the probability of success is a crucial parameter of such quantum gates. In this note, we discuss this question for controlled phase gates that implement an arbitrary phase with one and two control qubits. Within the class of post-selected gates in dual-rail encoding with vacuum…
Fast, High-Fidelity Erasure Detection of Dual-Rail Qubits with Symmetrically Coupled Readout
本文提出一种快速、高保真且可扩展的双轨量子比特擦除检测方法，通过单个读出谐振器对称耦合双轨量子比特，实现384ns内单次擦除检测，显著降低误差率。
We develop a theory for the interaction of multi-level atoms with multi-mode cavities yielding cavity-enhanced multi-photon resonances. The locations of the resonances are predicted from the use of effective two- and three-level Hamiltonians. As an application we show that quantum gates can be realised when photonic qubits are encoded on the cavity modes in arrangements where ancilla atoms transit the cavity. The fidelity of operations is increased by conditional measurements on the atom and by the use of a selected, dual-rail, Hilbert space. A universal set of gates is proposed, including the…
Lithium niobate photonic circuits have the salutary property of permitting the generation, transmission, and processing of photons to be accommodated on a single chip. Compact photonic circuits such as these, with multiple components integrated on a single chip, are crucial for efficiently implementing quantum information processing schemes. We present a set of basic transformations that are useful for manipulating modal qubits in Ti:LiNbO$_3$ photonic quantum circuits. These include the mode analyzer, a device that separates the even and odd components of a state into two separate spatial pat…
Integrated photonic platform with high-speed entanglement generation and witnessing
本研究在硅光子芯片上实现基于诱骗态技术的多轨单光子纠缠产生与验证，采用12.5 GHz带宽片上平衡零拍探测器室温运行，经损耗补偿后保真度达92%、CHSH违反下界2.59，为集成室温量子光子系统提供可行方案。
Robust photonic quantum gates with large number of waveguide segments
本研究通过数值模拟分析多段波导几何设计对光子量子门保真度与功率损耗的影响，发现增加段数可降低误差并呈现渐近标度，在硅和铌酸锂平台上验证了其鲁棒性与效率提升。
Motivated by rapidly advancing capabilities for extensive nanoscale patterning of optical materials, I propose an approach to implementing photonic sequential logic that exploits circuit-scale phase coherence for efficient realizations of fundamental components such as a NAND-gate-with-fanout and a bistable latch. Kerr-nonlinear optical resonators are utilized in combination with interference effects to drive the binary logic. Quantum-optical input-output models are characterized numerically using design parameters that yield attojoule-scale energy separation between the latch states.
Variational quantum cloning machine on an integrated photonic interferometer
本研究在6模可编程集成光子器件上实验实现双轨编码光子量子比特的1→2变分量子克隆机，结合经典反馈达到近最优克隆性能，验证了集成光子平台用于量子算法变分自学习的潜力。
Finite-energy GKP-QPC architectures for photonic quantum memories and repeaters
该研究提出有限能量GKP-QPC级联架构，经实验验证其可降低量子存储器的压缩阈值、提升平均保真度比，还能大幅提高量子中继器的密钥率并扩展通信距离，为相关器件提供设计准则。
LiDMaS: Architecture-Level Modeling of Fault-Tolerant Magic-State Injection in GKP Photonic Qubits
LiDMaS通过架构级建模研究GKP编码光子量子比特中逻辑T门魔态的容错注入，结合RUS协议和外表面码保护，提供可扩展光子容错架构的设计指导。
An Improved Design for All-Photonic Quantum Repeaters
本研究针对全光量子中继器的RGS资源开销问题，提出采用树编码链路量子比特的新型RGS设计与自适应逻辑BSM方案，以更少量子比特实现更高纠缠速率，降低光学模式需求。
Heisenberg spin chains can act as quantum wires transferring quantum states either perfectly or with high fidelity. Gaussian packets of excitations passing through dual rails can encode the two states of a logical qubit, depending on which rail is empty and which rail is carrying the packet. With extra interactions in one or between different chains, one can introduce interaction zones in arrays of such chains, where specific one or two qubit gates act on any qubit which passes through these interaction zones. Therefore universal quantum computation is made possible in a static way where no ex…
Entanglement is a fundamental property of quantum mechanics, and is a primary resource in quantum information systems. Its manipulation remains a central challenge in the development of quantum technology. In this work, we demonstrate a device which can generate, manipulate, and analyse two-qubit entangled states, using miniature and mass-manufacturable silicon photonics. By combining four photon-pair sources with a reconfigurable six-mode interferometer, embedding a switchable entangling gate, we generate two-qubit entangled states, manipulate their entanglement, and analyse them, all in the …
We present the dipole induced transparency (DIT) of a diamond nitrogen-vacancy center embedded in a photonic crystal cavity coupled to two waveguides, and it is obvious with the robust and flexible reflectance and transmittance difference of circularly polarized lights between the uncoupled and the coupled cavities even in the bad cavity regime (the Purcell regime). With this DIT, we propose two universal hyperparallel hybrid photonic quantum logic gates, including a hybrid hyper-controlled-not gate and a hybrid hyper-Toffoli gate, on photon systems in both the polarization and the spatial-mod…
Erasure Minesweeper: exploring hybrid-erasure surface code architectures for efficient quantum error correction
本研究提出混合擦除表面码架构，将部分量子比特设为双轨擦除量子比特，在固定transmon预算下提升表面码纠错性能，平衡资源成本与逻辑指标，适配近期量子硬件条件。
Photonic Qubit Gates via 1D Scattering from an Array of Two-Level Emitters
本文提出利用1D波导和二维发射体阵列实现双轨数编码光子量子位的确定性相门，并探讨双量子位操作。
Programmable nonlinear function synthesis on a photonic processor with quantum signal processing
该研究将量子信号处理映射到可编程光子集成电路，通过相位控制合成非线性函数，实验验证了深度达L=11的QSP序列，实现算法到硬件的非线性函数生成。
Heralded three-photon entanglement from a single-photon source on a photonic chip
本文首次在可编程光子芯片上实验实现了可预报的双轨编码三光子GHZ态，保真度达0.573±0.024，为容错光子量子计算提供了关键资源态。
A hardware-native time-frequency GKP logical qubit toward fault-tolerant photonic operation
该研究提出了一种基于时间-频率GKP逻辑量子比特的硬件原生实现，通过频率梳参考和干涉测量实现容错光子操作。
We outline a proposal for a method of preparing an encoded two-state system (logical qubit) that is immune to collective noise acting on the Hilbert space of the states supporting it. The logical qubit is comprised of three photonic three-state systems (qutrits) and is generated by the process of spontaneous parametric down conversion. The states are constructed using linear optical elements along with three down-conversion sources, and are deemed successful by the simultaneous detection of six events. We also show how to select a maximally entangled state of two qutrits by similar methods. Fo…
Simulating photonic devices with noisy optical elements
本文应用noisy gates方法直接模拟dual rail框架下的含噪声光学电路，无需映射到密度矩阵，并在基于门和测量的量子计算模型上验证其通用性，同时设计了抗光子损失的光子变分量子算法求解MAX-2-CUT问题。
Experimental demonstration of entanglement pumping with bosonic logical qubits
本研究通过玻色子量子纠错码实现纠缠泵浦，利用长相干时间存储量子比特，通过反复生成纠缠并注入可检测逻辑量子比特，有效延长纠缠存在时间，同时提出用于双轨码的擦除检测协议。
Quantum memory for flying optical qubits is a key enabler for a wide range of applications in quantum information science and technology. A critical figure of merit is the overall storage-and-retrieval efficiency. So far, despite the recent achievements of efficient memories for light pulses, the storage of qubits has suffered from limited efficiency. Here we report on a quantum memory for polarization qubits that combines an average conditional fidelity above 99% and an efficiency equal to (68$\pm$ 2)%, thereby demonstrating a reversible qubit mapping where more information is retrieved than …
N-qubit universal quantum logic with a photonic qudit and O(N) linear optics elements
本研究提出用单个时间箱qudit和O(N)个线性光学元件实现N量子比特通用逻辑，元件规模远低于传统方案，可构建低成本小规模量子处理器，并通过腔QED系统连接物质量子比特实现可扩展与量子互联功能。
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