Chip-based quantum communications

Quantum communications aim to share encryption keys between the transmitters and receivers governed by the laws of quantum mechanics.Integrated quantum photonics offers significant advantages of dense integration,high stability and scalability,which enables a vital platform for the implementation of quantum information processing and quantum communications.This article reviews recent experimental progress and advances in the development of integrated quantum photonic devices and systems for quantum communications and quantum networks.

The topological structure of the integral quantum Hall effect in magnetic semiconductor-superconductor hybrids

An unconventional integer quantum Hall regime was found in magnetic semiconductor-superconductor hybrids. By making use of the decomposition of the gauge potential on a U（1） principal fibre bundle over k-space, we study the topological structure of the integral Hall conductance. It is labeled by the Hopf index β and the Brouwer degree η. The Hall conductance topological current and its evolution is discussed.

Statistical evaluation of 571 GaAs quantum point contact transistors showing the 0.7 anomaly in quantized conductance using cryogenic on-chip multiplexing

Themass production and the practical number of cryogenic quantum devices producible in a single chip are limited to the number of electrical contact pads and wiring of the cryostat or dilution refrigerator.It is,therefore,beneficial to contrast themeasurements of hundreds of devices fabricated in a single chip in one cooldown process to promote the scalability,integrability,reliability,and reproducibility of quantum devices and to save evaluation time,cost and energy.Here,we used a cryogenic on-chip multiplexer architecture and investigated the statistics of the 0.7 anomaly observed on the first three plateaus of the quantized conductance of semiconductor quantum point contact(QPC)transistors.Our single chips contain 256 split gate field-effect QPC transistors(QFET)each,with two 16-branch multiplexed source-drain and gate pads,allowing individual transistors to be selected,addressed and controlled through an electrostatic gate voltage process.A total of 1280 quantum transistors with nano-scale dimensions are patterned in 5 different chips of GaAs heterostructures.From the measurements of 571 functioning QFETs taken at temperatures T[1.4 K and T[40 mK,it is found that the spontaneous polarisation model and Kondo effect do not fit our results.Furthermore,some of the features in our data largely agreed with van Hove model with short-range interactions.Our approach provides further insight into the quantum mechanical properties and microscopic origin of the 0.7 anomaly in QFETs,paving the way for the development of semiconducting quantum circuits and integrated cryogenic electronics,for scalable quantum logic control,readout,synthesis,and processing applications.

Bohr–Sommerfeld Levels for Quantum Completely Integrable Systems

In this paper we will show how the Bohr–Sommerfeld levels of a quantum completely integrable system can be computed modulo O(h^(∞))by an inductive procedure starting at stage zero with the Bohr–Sommerfeld levels of the corresponding classical completely integrable system.

Bethe ansatz solutions of the 1D extended Hubbard-model

We construct an integrable 1D extended Hubbard model within the framework of the quantum inverse scattering method.With the help of the nested algebraic Bethe ansatz method,the eigenvalue Hamiltonian problem is solved by a set of Bethe ansatz equations,whose solutions are supposed to give the correct energy spectrum.

Thermally reconfigurable quantum photonic circuits at telecom wavelength by femtosecond laser micromachining

The importance of integrated quantum photonics in the telecom band is based on the possibility of interfacing with the optical network infrastructure that was developed for classical communications.In this framework,femtosecond laser-written integrated photonic circuits,which have already been assessed for use in quantum information experiments in the 800-nm wavelength range,have great potential.In fact,these circuits,being written in glass,can be perfectly mode-matched at telecom wavelength to the in/out coupling fibers,which is a key requirement for a low-loss processing node in future quantum optical networks.In addition,for several applications,quantum photonic devices must be dynamically reconfigurable.Here,we experimentally demonstrate the high performance of femtosecond laser-written photonic circuits for use in quantum experiments in the telecom band,and we demonstrate the use of thermal shifters,which were also fabricated using the same femtosecond laser,to accurately tune such circuits.State-of-the-art manipulation of single-and two-photon states is demonstrated,with fringe visibilities greater than 95%.The results of this work open the way to the realization of reconfigurable quantum photonic circuits based on this technological platform.

A novel integrated quantum circuit for high-order W-state generation and its highly precise characterization

Multipartite entangled states like the W-class are of growing interest since they exhibit a variety of possible applications ranging from quantum computation to genuine random number generation. Here, we present a universal setup to generate high-order single photon W-states based on three-dimensional integrated-photonic waveguide struc- tures. Additionally, we present a novel method to charac- terize the device＇s unitary by means of classical light only.

Quantum dot lasers and integrated optoelectronics on silicon platform Invited Paper

Chip-scale integration of optoelectronic devices such as lasers, waveguides, and modulators on silicon is prevailing as a promising approach to realize future ultrahigh speed optical interconnects. We review recent progress of the direct epitaxy and fabrication of quantum dot （QD） lasers and integrated guided-wave devices on silicon. This approach involves the development of molecular beam epitaxial growth of self- organized QD lasers directly on silicon substrates and their monolithic integration with amorphous silicon waveguides and quantum well electroabsorption modulators. Additionally, we report a preliminary study of long-wavelength （〉 1.3 μm） QD lasers grown on silicon and integrated crystalline silicon waveguides using membrane transfer technology.

Stable single-mode operation of a distributed feedback quantum cascade laser integrated with a distributed Bragg reflector

We report an index-coupled distributed feedback quantum cascade laser by employing an equivalent phase shift(EPS) of quarter-wave integrated with a distributed Bragg reflector(DBR) at λ~5.03 μm. The EPS is fabricated through extending one sampling period by 50% in the center of a sampled Bragg grating. The key EPS and DBR pattern are fabricated by conventional holographic exposure combined with the optical photolithography technology, which leads to improved flexibility, repeatability, and cost-effectiveness. Stable single-mode emission can be obtained by changing the injection current or heat sink temperature even under the condition of large driving pulse width.

Effect of unbalanced and common losses in quantum photonic integrated circuits

Loss is inevitable for the optical system due to the absorption of materials, scattering caused by the defects, and surface roughness. In quantum optical circuits, the loss can not only reduce the intensity of the signal, but also affect the performance of quantum operations. In this work, we divide losses into unbalanced linear losses and shared common losses, and provide a detailed analysis on how loss affects the integrated linear optical quantum gates. It is found that the orthogonality of eigenmodes and the unitary phase relation of the coupled waveguide modes are destroyed by the loss. As a result, the fidelity of single-and two-qubit operations decreases significantly as the shared loss becomes comparable to the coupling strength. Our results are important for the investigation of large-scale photonic integrated quantum information processes.

Witten's D_4 Integrable Hierarchies Conjecture

The authors prove that the total descendant potential functions of the theory of Fan-Jarvis-Ruan-Witten for D4 with symmetry group J and for D4T with symmetry group Gmax, respectively, are both tau-functions of the D4 Kac-Wakimoto/Drinfeld-Sokolov hierarchy. This completes the proof, begun in the article by Fan-Jarvis-Ruan（2013）, of the Witten Integrable Hierarchies Conjecture for all simple（ADE） singularities.

Lower Bound for Quantum Integration Error on Anisotropic Sobolev Classes

We study the approximation of the integration of multivariate functions in the quantum model of computation. Using a new reduction approach we obtain a lower bound of the n-th minimal query error on anisotropic Sobolev class R（Wpr（[0, 1]d）） （r R＋d）. Then combining this result with our previous one we determine the optimal bound of n-th minimal query error for anisotropic Hblder- Nikolskii class R（H∞r（[0,1]d）） and Sobolev class R（W∞r（[0,1]d））. The results show that for these two types of classes the quantum algorithms give significant speed up over classical deterministic and randomized algorithms.

Highlighting photonics: looking into the next decade

Let there be light-to change the world we want to be!Over the past several decades,and ever since the birth of the first laser,mankind has witnessed the development of the science of light,as light-based technologies have revolutionarily changed our lives.Needless to say,photonics has now penetrated into many aspects of science and technology,turning into an important and dynamically changing field of increasing interdisciplinary interest.In this inaugural issue of eLight,we highlight a few emerging trends in photonics that we think are likely to have major impact at least in the upcoming decade,spanning from integrated quantum photonics and quantum computing,through topological/non-Hermitian photonics and topological insulator lasers,to AI-empowered nanophotonics and photonic machine learning.This Perspective is by no means an attempt to summarize all the latest advances in photonics,yet we wish our subjective vision could fuel inspiration and foster excitement in scientific research especially for young researchers who love the science of light.

Note on higher-point correlation functions of the TT or JT deformed CFTs

We investigate generic n-point correlation functions of conformal field theories(CFTs),with TT and JT deformations,in terms of the perturbative CFT approach.We systematically obtain the first order correction to the generic correlation functions of CFTs with TT or JT deformation.We compute the out of time ordered correlation function(OTOC)in the Ising model with TT or Jdeformation,which confirms that these deformations do not change the integrable property up to the first order level.

Excitation spectra of one-dimensional spin-1/2 Fermi gas with an attraction

Using an exact Bethe ansatz solution,we rigorously study excitation spectra of the spin-1/2 Fermi gas(called Yang–Gaudin model)with an attractive interaction.Elementary excitations of this model involve particle-hole excitation,hole excitation and adding particles in the Fermi seas of pairs and unpaired fermions.The gapped magnon excitations in the spin sector show a ferromagnetic coupling to the Fermi sea of the single fermions.By numerically and analytically solving the Bethe ansatz equations and the thermodynamic Bethe ansatz equations of this model,we obtain excitation energies for various polarizations in the phase of the Fulde–Ferrell–Larkin–Ovchinnikov-like state.For a small momentum(long-wavelength limit)and in the strong interaction regime,we analytically obtained their linear dispersions with curvature corrections,effective masses as well as velocities in particle-hole excitations of pairs and unpaired fermions.Such a type of particle-hole excitations display a novel separation of collective motions of bosonic modes within paired and unpaired fermions.Finally,we also discuss magnon excitations in the spin sector and the application of Bragg spectroscopy for testing such separated charge excitation modes of pairs and single fermions.