Reconfiguration of Quantum Photonic Integrated Circuits Using Auxiliary Fields

Optical quantum computing (QC) increasingly uses integrated optics based experiments which permit circuit compactness and phase stability. However, despite the rapid adaptations of integrated waveguide devices for quantum photonics, initial gate demonstrations operate in post-selection, thus not allowing scaling of a quantum circuit beyond the depth of a single gate. Recently, a number of quantum circuits have been demonstrated using the femtosecond laser direct write (FLDW) technique. This technique induces refractive index change in glass substrates which can form three-dimensional waveguide devices. Here we demonstrate a potentially scalable waveguide gate for QC, a controlled-phase gate or Knill gate, produced using the FLDW technique. This gate produces a phase shift on a target qubit conditional on the state of a control qubit, as shown in Fig. 1(a). It requires four photons for operation, two of which act as the target and control path-encoded qubits and two ancillas which herald the successful operation of the circuit.

A time-bin qubit is a promising candidate qubit for advanced quantum information processing systems operating over optical fibers or integrated quantum photonic circuits. However, the single- and two-qubit operations of time-bin qubits have been largely unexplored compared with those of polarization qubits. Here, I present a simple scheme for implementing a basic two-qubit operation, namely an ``entangling'' operation of two independent time-bin qubits, based on Hong-Ou-Mandel quantum interference in a high-speed $2\ifmmode\times\else\texttimes\fi{}2$ optical switch. With the proposed scheme, the formation of a time-bin entanglement was confirmed experimentally.

Integrated photonic quantum chip provides a promising platform to perform quantum computation, quantum simulation, quantum metrology and quantum communication. Femtosecond laser direct writing (FLDW) is a potential technique to fabricate various integrated photonic quantum chips in glass. Several quantum logic gates fabricated by FLDW have been reported, such as polarization and path encoded quantum controlled-NOT (CNOT) gates. By combining several single qubit gates and two qubit gates, the quantum circuit can realize different functions, such as generating quantum entangled states and performing quantum computation algorithms. Here we demonstrate the FLDW of integrated photonic quantum chips composed of one Hadamard gate and one CNOT gate for generating all four path-encoded Bell states. The experimental results show that the average fidelity of the reconstructed truth table reaches as high as 98.8 0.3%. Our work is of great importance to be widely applied in many quantum circuits, therefore this technique would offer great potential to fabricate more complex circuits to realize more advanced functions.

III-nitride quantum dots (QD) based single photon emitters are promising for a broad range of quantum photonic applications including secure quantum communication, quantum metrology and quantum computing. III-nitride QDs exhibit wide and tunable energy bandgap, therefore suitable for high temperature operations over a wide spectral range for on demand single photon and entangled photon pairs. The III-nitride nanophotonic waveguides are also preferred due to the extremely low optical losses. An efficient coupling of the quantum dot emissions to the photonic waveguides will play a crucial role towards realizing all nitride quantum photonic integrated circuits (QPIC) allowing the integration of quantum emitters in conventional nanophotonic devices.

Recent advancements in photonic-electronic integration push towards denser multichannel fiber to silicon photonic chip coupling solutions. However, current packaging schemes based on suitably polished fiber arrays do not provide sufficient scalability. Alternatively, lithographically-patterned fused silica glass interposers have been proposed, allowing for the integration of fanout waveguides between a dense array of on-chip silicon waveguides and a cleaved fiber ribbon. In this paper, we propose the use of femtosecond laser inscription for the fabrication of the fused silica glass interposer, allowing for a monolithic integration of waveguides and V-grooves for fiber alignment. The waveguides obtained by Femtosecond Laser Direct Writing (FLDW) have a propagation loss of 0.88 dB/cm at 1550 nm. The mode-field diameter is 12.8 ± 0.4 μm, allowing for a coupling loss of 1.24 ± 0.32 dB when coupling to a standard single mode optical fiber, passively aligned to the fused silica waveguide by insertion in a V-groove created by Femtosecond Laser Irradiation followed by Chemical Etching (FLICE). The average surface roughness of the etched waveguide facet is 160 ± 5 nm. Scattering loss when coupling to fiber is reduced by use of an index-matching adhesive for fiber fixation. A polished out-of-plane coupling mirror at an angle of 41.5° injects the light into standard grating couplers, providing a quasi-planar fiber-to-chip package. The excess loss of the proposed solution is limited to 2 dB per interface, including mirror, waveguide and fiber coupling losses.

Error correction in existing point-to-point communication networks is done on a link-by-link basis, which is referred to in this paper as classical error correction. Inspired by network coding, we introduce in this two-part paper a new paradigm called network error correction. The theory thus developed subsumes classical algebraic coding theory as a special case. In Part I, we discuss the basic concepts and prove the network generalizations of the Hamming bound and the Singleton bound in classical algebraic coding theory. By studying a few elementary examples, the relation between network error correction and classical error correction is investigated.

A new differential interference contrast microscopy (DICM), which uses Savart prisms as a shearing plate and a phase-shifting device, is proposed. The system consists of a phase-shifting module (PSM) and a DICM module (DICMM). The PSM has two Savart prisms: the first prism separates the incident beam into two parallel beams, and the second prism recombines these two beams. The optical path difference (OPD) of the two beams, which is represented by a biased OPD, can be adjusted by rotating the angle of the normal surface of the second prism. In the DICMM, the other Savart prism is used to replace the Nomarski prism (NP) in conventional DICM. It combines with an afocal microscopic system (AMS) to produce a Savart-DICM system, which is able to perform a phase-shifting technique by changing the biased OPD to produce a phase shift of π/2 for each step. This paper describes the configuration and measurement theory of the microscope. The experimental results confirm the validity and capability of the proposed microscope.

The main objective of this paper is to provide a thorough analysis of currently used modulation control schemes for single-phase bidirectional dual active bridge DC/DC converters. In this article, it will be shown that single phase shift, extended phase shift and dual phase shift modulation schemes are special cases of the triple phase shift (TPS) modulation scheme. The article aims to highlight six TPS switching modes and their complements with operational constraints. Unlike previous studies that regarded TPS as a complex scheme, this paper simplifies the analysis of each mode and aims to standardize the understanding of TPS modulation for dual active bridge (DAB) converters. Power equations, range of power transferred and zero-voltage switching (ZVS) are derived for all the modes under TPS without assuming fundamental component analysis. Additionally, a generic optimization algorithm is developed to show the advantages of TPS modulation, and thus, the analysis in this paper offers a valuable insight for single-phase DAB converter designers in identifying a wide range of optimization algorithms to achieve higher efficiency under TPS modulation. This analysis contributes to the advancement of bidirectional DAB converter technology and facilitates its application in various power electronic systems.

<p indent="0mm">Recently, the achievement, entitled “lifetime enhancement for a logic qubit by bosonic encoding error correction”, has been selected as one of “The Ten Major Advances in Chinese Science 2023”. Here, we will introduce briefly the importance of this work. It is known that quantum states are fragile due to decoherence induced by environment. Error will happen on a qubit, resulting in continuously changes of the parameters for this qubit, such as amplitude and phase parameters. The aim of quantum error correction is to amend those errors by using various quantum error correction codes, in which a logic qubit can be represented by several physical qubits. It should be noted that the quantum error correction can be realized by correcting a discrete set of errors, such as bit-flip error, phase-flip error or both of them. On the other hand, types of errors occurred depend on specific platforms of quantum computation. For example, photon loss is more inclined to occur in photonic systems. In this case, the approach of bosonic encoding of quantum error correction is more efficient. The work “lifetime enhancement for a logic qubit by bosonic encoding error correction” is for such a scenario. The challenge of quantum error correction is that operation itself is not perfect which may incur new errors. So it is actually difficult to demonstrate that we can benefit from quantum error correction. The achievement of lifetime enhancement by bosonic encoding error correction succeeds in beating this break-even point. The experiments are performed by circuit quantum electrodynamics on a superconducting processor with a qubit and a coupled resonate cavity. The logic qubit is realized by the bosonic mode of the cavity, while the superconducting qubit plays the role of the ancillary qubit in error correction. The initial states are prepared by the superconducting qubit, and are transferred to the logic qubit in the cavity. Then the error correction can be performed based on detection of photon loss by the superconducting qubit. The results show that the lifetime of the logic qubit increases from <sc>694 μs</sc> to <sc>805 μs</sc> with 16% enhancement. Besides high quality devices, the success of this experiment depends on the feedback control to detect and correct the error of photon loss in a short time. The result of beating the break-even point of bosonic encoding error correction is for one logic qubit. In the near future, it is hopeful to implement logic gates, such as single-qubit rotation gate and two-qubit gate, on a couple of logic qubits by bosonic encoding. The importance of the results will depend on whether higher fidelity can be achieved compared with those of gates with physical qubits. In this way, the advantage of the logic qubits for quantum computation is presented. It will be significant that the approach of bosonic encoding with a superconducting processor is scalable to hundreds of logic qubits, which may take a few years to realize. Our aim is to realize a fault-tolerant universal quantum computer based on a large quantity of high-precision logic qubits. The work “lifetime enhancement for a logic qubit by bosonic encoding error correction” is an important step toward this aim.

Integrated photonic circuits with many input and output modes are essential in applications ranging from conventional optical telecommunication networks, to the elaboration of photonic qubits in the integrated quantum information framework. In particular, the latter field has been object in the recent years of an increasing interest: the compactness and phase stability of integrated waveguide circuits are enabling experiments unconceivable with bulk-optics set-ups. Linear photonic devices for quantum information are based on quantum and classical interference effects: the desired circuit operation can be achieved only with tight fabrication control on both power repartition in splitting elements and phase retardance in the various paths. Here we report on a novel three-dimensional circuit architecture, made possible by the unique capabilities of femtosecond laser waveguide writing, which enables us to realize integrated multimode devices implementing arbitrary linear transformations. Networks of cascaded directional couplers can be built with independent control on the splitting ratios and the phase shifts in each branch. In detail, we show an arbitrarily designed 5&times;5 integrated interferometer: characterization with one- and two-photon experiments confirms the accuracy of our fabrication technique. We exploit the fabricated circuit to implement a small instance of the boson-sampling experiments with up to three photons, which is one of the most promising approaches to realize phenomena hard to simulate with classical computers. We will further show how, by studying classical and quantum interference in many random multimode circuits, we may gain deeper insight into the bosonic coalescence phenomenon.

Taylor series error correction network for super-resolution of discretized partial differential equation solutions

A hybrid‐three‐level isolated dual‐active‐bridge (HTDAB) DC–DC converter is widely used for grid‐interactive power converters. However, a traditional extended‐phase‐shift (EPS) modulation scheme would cause voltage‐level‐disorder (VLD) problems in HDAB DC–DC converter system because the output voltage of HTDAB would be clamped at a high‐ or low‐level during power transmission when the inductor current changes directions. Thus, the zero voltage sequence of the HTDAB cannot be stably maintained under EPS control, which not only leads to a significant difference between the actual transmission power and the target output power but also seriously affects the stability of the system control. To address this limitation, the reasons leading to VLD are analysed in detail, that is, an inappropriate PWM switching sequence at the output zero voltage level would occur under EPS control. Then, a novel PWM plus phase shift (PPS) modulation strategy is proposed, which can eliminate VLD completely. Finally, an experimental prototype is built to verify the effectiveness of the proposed PPS modulation scheme. The experimental results demonstrate that the PPS modulation method proposed in this article can effectively eliminate level disturbances and improve system stability.

The integration of energy storage elements to solar photovoltaic (PV) systems provide a wide range of operational flexibilities. Traditional approaches use separate converter for PV and energy storage element, i.e. battery which cause significant conversion losses. This paper presents an isolated high frequency link multi-port converter based on triple active bridge (TAB) topology for PV integrated battery systems in residential applications. The conventional dual active bridge (DAB) is extended to establish the proposed high frequency link TAB topology. Limited soft-switching capability and high circulating current are common problems in this style of converters when there is a mismatch in the dc-link voltages. This is pronounced at lower powers with the single phase shift (SPS) modulation technique. To overcome these problems, this paper proposes an improved modulation and control method based on a quintuple phase shift modulation (QPSM) strategy. The QPSM modulation method produces 3-level modulated square wave bridge voltages. A mathematical model of the proposed high frequency linked multi-port converter with QPSM modulation is developed based on a harmonic analysis approach. The simulation and experimental results show that the QPSM scheme applied to the proposed multi-port converter provides better operation and improves the efficiency compared to the traditional SPS modulation scheme.

This article identifies how constant voltage load characteristics cause inductive power transfer systems to exhibit a poorly damped oscillation mode. When operated with pulse skipping strategies such as pulse density modulation (PDM), the skipped voltage pulses can excite this mode and cause severe oscillations that do not appear in systems with a constant resistance load. The critical mode is identified from a linearized state-space model of the system and two control approaches are proposed for attenuating the oscillations in current amplitude and power flow. First, the influence of the operating frequency on the critical eigenvalue is analyzed and it is shown how slightly off-resonant operation can increase the damping of the oscillation mode. Second, an active damping method based on sending current feedback control is studied. The active damping is based on phase shift modulation with limited phase shift angles applied to the PDM signal when oscillations are detected. The effectiveness and feasibility of the proposed methods are validated by simulations and experimental results from a small-scale laboratory prototype.

The inclusion of battery energy storage systems (BESSs) with PV is technically attractive and is capable of solving intermittency and voltage issues. From the economic perspective, it may allow a consumer to shift their PV generation to satisfy their household loads. The addition of a battery to a home solar system requires an additional power converter. The traditional method for interconnecting PV-battery systems uses at least two separate power converters which results in multistage power conversion for some power flows. This paper proposes an isolated dc-dc three port converter (TPC) based on a dual active bridge (DAB) configuration for the PV-battery system and presents small signal modeling and power flow control of the proposed TPC using a circuit averaging approach. The TPC will interface a PV, a rechargeable battery and a load. Two degrees of freedoms (DOFs) are required to control the instantaneous power of three ports simultaneously. As a result, two controllers are designed for port 2 and port 3 (i.e. PV and load) while port 1 (battery) is responsible for power balance. The phase shift modulation (PSM) technique is used to generate the phase shifted converter voltages to facilitate the power transfer between the ports. The simulation results show that the controllers are capable of independently regulating the instantaneous power flow between the three ports.