Fabrication and classical characterisation of an integrated optic controlled phase gate

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.

Manipulating photons in quantum photonic integrated circuits (QPICs) is essential for quantum communications and computation, demanding precise control over phase shifts and photon coupling. Despite the challenges arising from the low electro‐optic and thermo‐optic coefficients of glass waveguides, femtosecond laser direct writing (FsLDW) technology offers a rapid method for QPIC preparation. In this work, the use of auxiliary fields is proposed to reconfigure QPICs with FsLDW. Through adiabatic elimination, modulations of inter‐waveguide coupling are achieved, allowing for arbitrary, controllable beam‐splitting ratios and error correction in discrete quantum walking networks. The experiments also demonstrate full‐period phase shift modulation over 2π in a Mach‐Zehnder interferometer by employing an adjustable auxiliary field. Particularly, high‐quality two‐photon quantum interference and high‐fidelity quantum logic function conversion between a controlled‐NOT gate and a controlled‐PHASE gate are exhibited. This novel approach provides a promising mechanism for modulating the phase and coupling between photons in 3D and large‐scale QPICs.

We report on the inscription of optical waveguides into crystalline bismuth germanate (Bi4Ge3O12, BGO) via the femtosecond laser direct-write (FLDW) technique. We found that by utilizing femtosecond laser pulses at Megahertz (MHz) repetition rates, a uniquely different fabrication regime can be exploited. In this paper, we show that cumulative heating effects can initiate a local transformation of the crystalline structure into an amorphous (glass-like) state that is characterized by an increased refractive index. We compare and contrast this novel, type-I modification based waveguide inscription regime with the previously reported fabrication of type-II damage/stress field structures in BGO and present measurements that indicate that the femtosecond laser writing process unavoidably causes a reduction in the electro-optic coefficient in the waveguides as compared to the bulk material. We discuss the potential of this technique for the fabrication of advanced sensor arrays for high-energy radiation detection and voltage sensing applications.

Flexible ultraviolet (UV) photodetectors (PDs) have gained increasing demand because of their widespread applications in wearable devices. However, difficulties associated with complicated fabrication technologies significantly limit their scope of application. Herein, via the development of a femtosecond laser direct writing (FsLDW) strategy, silicon carbide (SiC) nanoparticles are found to be assembled in a single microwire within 30 s. The surface of the deposited SiC microwire presents a three-dimensional porous structure, which is conducive to improving the responsivity of the device. The responsivity of a SiC-based microwire PD to UV light at 365 nm is found to be 55.89 A W-1 at a 1 V bias. The as-fabricated SiC microwire PDs on a glass substrate exhibit thermal stability at 350 °C, and the response speed of the PDs becomes notably faster at high temperatures, suggesting their promising applications in harsh conditions. Due to the low-temperature processing characteristics of this process, they can be prepared not only on glass substrates, but also on thermosensitive polymer substrates without an extra transfer process. Moreover, the SiC microwires prepared via FsLDW are directly deposited on the flexible substrate, and the prepared flexible SiC-based PDs can still work stably after being bent 2000 times. This research unveils a feasible way to fabricate a PD with excellent thermal stability and mechanical flexibility.

Recent investigations demonstrate the huge potential of Femtosecond Laser Direct Writing (FLDW) to create Computer Generated Holograms (CGHs) [1]. The main advantages compared to common lithographic methods are the single fabrication step required and the large variety of applicable materials [2]. In addition the created elements are directly embedded into the bulk, leading to a better resistance against external influences and extensive opportunities for the integration with other photonic devices [3]. Moreover the FLDW is capable of creating multiple separated structures within the same fabrication process. As demonstrated in [4] this enables multiplexing of spatial and spectral information within a single element.

We reported on a new method for fabricating sapphire fiber Bragg gratings (SFBGs) using a femtosecond laser direct writing technique, i.e., line-by-line (LbL) scanning technique and Point-by-Point (PbP) method. At first, we fabricated a SFBG by using LbL scanning technique, and this SFBG exhibits a reflectivity of 6.3%. Moreover, we fabricated multi-layer SFBGs using this method, and the reflectivity was enhanced to 34.1 %. Furthermore, we proposed a filamentation process to enlarge the area of refractive index modulations (RIMs). An SFBG with a reflectivity of 2.3 % could be created by using PbP method, and the fabrication time only requires ~ 1.1 s. An SFBG array including five SFBGs was constructed successfully. In addition, we used an inert gas-sealed packaging method to suppress the lossy spots formed on surface of sapphire fiber after a high-temperature test. The long-term thermal stability of the SFBG was greatly enhanced. As a result, such an SFBG can be applied in many fields, including metallurgical, power, and aviation industries.

In this Letter, we propose a type of continuous phase vortex gratings (CPVGs), which are able to generate a series of vortex beams with equal or proportional diffracted energies and different topological charges (TCs). A set of CPVGs with dimensions (60 × 60 × 1.1 μm3) are directly designed by mathematical equations, which avoids the use of iterative algorithms. The CPVGs are fabricated by femtosecond laser direct writing (FsLDW) with photoresist, and the experimentally generated vortex beams are in good agreement with the theoretical designs, exhibiting high optical efficiencies. In addition, we realized a CPVG onto the tip of a composite fiber for integrated optical systems. Our work paves the way for applications in optical communications, optical manipulations, and high-performance integrated optics.

Universal quantum computing relies on high-fidelity entangling operations. Here, we demonstrate that four coupled qubits can operate as a quantum gate, where two qubits control the operation on two target qubits (a four-qubit gate). This configuration can implement four different controlled two-qubit gates: two different entangling swap and phase operations, a phase operation distinguishing states of different parity, and the identity operation (idle quantum gate), where the choice of gate is set by the state of the control qubits. The device exploits quantum interference to control the operation on the target qubits by coupling them to each other via the control qubits. By connecting several four-qubit devices in a two-dimensional lattice, one can achieve a highly connected quantum computer. We consider an implementation of the four-qubit gate with superconducting qubits, using capacitively coupled qubits arranged in a diamond-shaped architecture.

Unlike classical digital computers in which a bit can represent either 1 or 0 at any time, quantum computers use a two-level system, i.e., a qubit, to implement logical operations based on quantum mechanical laws, which can represent both values at once. Owing to the superposition property of qubits, quantum computers have natural parallel processing advantages and thus have potential to exceed the computational efficiency of classical computers for particular tasks. Quantum logic gates are the generalization of classical logic gates in computational networks. It has been proved that two-qubit quantum gates together with one-qubit quantum gates are adequate for constructing networks with any possible quantum computational property. Directional couplers are the most critical elementsfor constructing the quantum gates. In recent years, photonic quantum technologies have emerged as a promising experimental platform for quantum computing. Single photons have robust noise resistance, long coherence time, high transmission speed and great compatibility with other systems. They can be easily manipulated and encoded in any of several degrees of freedom, for example, polarization, path, spatial mode or time bin. Optical waveguide technology enables the realizing of complex optical schemes comprised of many elements with desired scalability, stability and miniaturization. Femtosecond laser direct writing of waveguide has been adopted as a powerful tool for integrated quantum photonics with characteristics of rapidness, cost-effectiveness, mask-less and single-step process. In particular, it has the ability to build arbitrary three-dimensional circuits directly inside bulk materials, which is impossible to achieve with conventional lithography. In this article we review the femtosecond laser writing and quantum characterization of directional coupler and important one-qubit and two-qubit optical quantum logic gates, such as Hadamard gate, Pauli-X gate, controlled-NOT gate, and controlled-Phase gate. The qubits in these gates are usually encoded through optical paths or polarizations of photons. The key to the realization of polarization-encoded one-qubit gates is to achieve flexible wave-plate operations, which is described in detail. Controlled-NOT gate and controlled-phase gate are the most crucial two-qubit gates in the linear optics computation and sometimes they can be converted into each other by adding some one-qubit gates or special superposition states. Many different kinds of waveguide circuits have been used to implement these two-qubit gates. The outlook and challenges for the femtosecond laser writing of three-qubit gates, such as Toffoli gate and Fredkin gate, are briefly introduced.

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.

The qubit state is represented by a coherent superposition of its orthonormal basis states (|0〉 and |1〉). The probability distribution of a qubit is a discrete probability distribution of the qubits probability amplitudes. Setting a custom probability amplitudes allows a qubit to evolve to any desired state. In this paper a controlled n-root of X gate is simulated on a real quantum processor using IBMs Quantum Experience platform. A custom quantum gate could be constructed where the only inputs are the control and target qubits and the desired probability distribution for the target qubit.

We present the fabrication of three-dimensional waveguide structures indide fused silica using the femtosecond laser direct-writing technique. The guiding properties of a produced 1x3 splitter are studied in detail. By appropriate choice of the processing parameters a multimode entrance port can be generated which allows to vary the splitting ratio by changing the input coupling. The processing requirements for such splitter devices will be discussed.

Three-dimensional photonic waveguide devices are fabricated in glass by use of femtosecond pulses from an extended-cavity laser oscillator. Three-dimensional devices, including a symmetric three-waveguide directional coupler and a three-dimensional microring resonator, are fabricated and tested. Waveguides can be fabricated at depths of approximately 1 mm inside a glass substrate, thus demonstrating the capability of achieving dramatic increases in device density. These results demonstrate the potential to fabricate new classes of devices that are not possible in two dimensions.

Controlled-phase (CP) gates with an arbitrary phase, which play an irreplaceable role in quantum computation and quantum information processing, are constructed between two neutral-atom qubits by using the Rydberg antiblockade regime. The phases needed for the CP gates are induced by the collective excitation of the pair of atoms. Numerical simulations show that high-fidelity CP gates can be obtained. In addition, the optimization of the scheme is given to enhance the robustness against the control error. As applications, we study in detail the preparation of n-qubit Knill–Laflamme–Milburn (KLM) states and a cluster state based on the Rydberg CP gates. The high-fidelity KLM states and cluster state can be obtained with the current physical parameters.

The controlled phase gate is one of the most important logic gates in the quantum computation field. In this paper, we proposed a protocol for implementing the two-qubit controlled phase gates with the help of cross-Kerr nonlinearity, optical elements and the conventional photon detectors, which are feasible with existing experimental technology. The protocol also can be applied to implement the controlled phase gates of many different atomic and photonic degrees of freedom with successful probability of 100%, that is, our protocol is efficient and flexible.