China

Superconducting qubits are a subject of intense research as a platform for scalable quantum computing. While transmon qubits have received a lot of attention, the less ubiquitous Fluxonium qubit has been shown to have long life-times and gates unlimited by level anharmonicity [1-2]. Despite this, little research has been put into studying multi-Fluxonium devices. Here, one of the difficulties is understanding how their rich level structure can make them prone to measurement photons inducing transitions out of the qubit subspace [3]. We present a systematic study of a system comprising two capacitively coupled Fluxonium qubits sharing the same read-out cavity. By tracking the state dependent transmission of the read-out pulse, we determine the transition rates Γij from state i to state j of the coupled system using a forward-backward analysis [4] to characterise these cavity induced transitions. By varying the flux bias of the system and the population of the cavity, we characterise the fidelity of this read-out.

[1] L.B. Nguyen et al., Phys. Rev. X 9 041041, 2019
[2] H. Zhang et al., arXiv:2002.10653, 2020
[3] D. Sank et al., Phys. Rev. Lett. 117 190503, 2016
[4] T. Rybarczyk et al., Physical Review A 91 062116, 2015


Quantum 2020

Cavity Induced State Transitions in a Coupled Fluxonium Qubit System

transition rate

forward-backward smoothing

quantum non-demolition

fluxonium

The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.


Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. The broad themes covered by Quantum 2020 included:

Quantum communication;
Quantum computing and quantum simulation;
Quantum sensing, metrology and imaging;
Quantum information theory, software and algorithms;
Quantum foundations;
Quantum technology in industry.
The four-day event featured talks from world-renowned scientists, interactive poster sessions and two exciting panel discussions on the themes of Quantum Technology in Industry and Quantum Technology Programmes Around the World.


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The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.

Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. 

poster

We study the interplay between entanglement and chaos in the time evolution of Bohmian trajectories of a two qubit system, made of coherent states of the quantum harmonic oscillator. This simple quantum system has very rich dynamics from a Bohmian standpoint, since with a proper choice of the physical
parameters one finds ordered (periodic or non-periodic) and chaotic trajectories. We provide strong numerical evidence that the chaotic trajectories of this system are ergodic regardless of the amount of the entanglement and study their impact on the dynamical approximation to Born’s rule when the initial configuration of the quantum particles does not satisfy it.

Chaos and entanglement in Bohmian Qubits

"Many-body bosonic Hamiltonians are a cornerstone of condensed-matter physics. Quantum simulators, that is, quantum-controlled atomic, optical, or solid-state experimental platforms, hold the promise to explore such models with a degree of precision and flexibility out of reach for real materials. For example, while common Hamiltonians emerging in condensed-matter preserve the number of particles, quantum simulators are intrinsically driven-dissipative systems in which processes that break particle-number conservation can be naturally induced either coherently or incoherently.

Here we show that such processes, and down-conversion or pair injection in particular, make the phase diagram of paradigmatic examples such as the Bose-Hubbard model much richer. Specifically, we show that in combination with two-particle loss, exotic superfluid steady states emerge [1] in which bosons condense on a closed ring in momentum space, a state conjectured to play an important role in open condensed-matter problems such as high-Tc superconductivity. In addition, we evidence the potential of pair injection for the development of supersolid regions in the phase diagram of the one-dimensional extended Bose-Hubbard model. We propose realistic implementations of our ideas in modern quantum-controlled platforms, with focus on arrays of superconducting circuits.

[1] Zijian Wang, Carlos Navarrete-Benlloch, and Zi Cai, Phys. Rev. Lett. 125, 115301 (2020).
"

Opportunities in bosonic quantum simulators subject to particle non-conserving processes

Machine Learning has wide applications in a broad range of subjects, including physics. Recent works have shown that neural networks can learn classical Hamiltonian mechanics. The results of these works motivate the following question: Can we endow neural networks with inductive biases coming from quantum mechanics and provide insights for quantum phenomena? In this work, we try answering these questions by investigating possible approximations for reconstructing the Hamiltonian of a quantum system given one of its wave–functions. Instead of handcrafting the Hamiltonian and a solution of the Schrödinger equation, we design neural networks that aim to learn it directly from our observations. We show that our method, termed Quantum Potential Neural Networks (QPNN), can learn potentials in an unsupervised manner with remarkable accuracy for a wide range of quantum systems, such as the quantum harmonic oscillator, particle in a box perturbed by an external potential, hydrogen atom, Pöschl–Teller potential, and a solitary wave system. Furthermore, in the case of a particle perturbed by an external force, we also learn the perturbed wave function in a joint end-to-end manner.

Learning Potentials of Quantum Systems Using Deep Neural Networks

This poster details some of the current initiatives at IOP Publishing (IOPP) aimed at improving the author and reviewer experience.

Our research suggests there are deficiencies in the peer review process, specifically linked to equality, diversity, transparency, quality and training. This is supported by academic literature on publishing and higher education. Many of these themes are the result of multiple historical and systemic issues.

In recent months IOPP have announced a mix of new and updated initiatives aimed at addressing some of these complex challenges. These include changes to the peer review workflow, the creation of a unique training programme, and publication of peer review reports with published articles.


Evolving Peer Review at IOPP: Diversity, Training and Transparency

Photonic graph states are underlying resources for one-way optical quantum computation, quantum error correction, fundamental testing of quantum mechanics, and quantum communication networks. Most existing works, however, are based on the spontaneous parametric down-conversion sources that intrinsically suffer from probabilistic generation and double pair components. Here, we create two important classes of graph states, a polarization-encoded four-photon Greenberger−Horne−Zeilinger (GHZ) state and a linear cluster state, by actively demultiplexing a deterministic single photon source from a semiconductor quantum dot embedded in a micropillar. A state fidelity of 0.790 ± 0.009 (0.763 ± 0.004) and a count rate of ~13Hz are observed for the four-photon GHZ (cluster) state. The results constitute a new route toward the multiphoton entanglement with deterministic single-photon sources.

Multiphoton Graph States from a Solid-State Single-Photon Source

Realizing strongly-correlated topological phases of ultracold gases is a central goal for ongoing experiments. And while fractional quantum Hall states could soon be implemented in small atomic ensembles, detecting their signatures in few-particle settings remains a fundamental challenge. In this work, we numerically analyze the center-of-mass Hall drift of a small ensemble of hardcore bosons, initially prepared in the ground state of the Harper-Hofstadter-Hubbard model. By extracting the Hall conductivity in a wide range of the magnetic flux, we identify an emergent Hall plateau compatible with a fractional Chern insulator state: the width of the plateau agrees with the spectral and topological properties of the prepared ground state, while the Hall conductivity approaches a fractional value determined by the many-body Chern number. A comparison with a direct application of Streda’s formula is also discussed. Our calculations suggest that fractional Chern insulators can be detected in cold-atom experiments, using available detection methods.

Ref: C. Repellin, J. Léonard, and N. Goldman, arXiv:2005.09689.

Hall drift of fractional Chern insulators in few-boson systems

"One of the promising technical solutions to improve qubit lifetimes is a logical qubit comprised of a Josephson junction located inside of a high Q-factor 3D superconducting cavity. Niobium superconducting cavities used in particle accelerators are able to reach quality factors of ~1e10, potentially enabling storage times approaching seconds. RadiaBeam is presently developing a 3D quarter-wave resonator (QWR) with shape optimized for operation in the quantum regime. This program includes several important steps: development of a QWR cavity with the optimal shape to increase the Q-factor by adopting surface treatment techniques from accelerating cavities; demonstration of the performance of a multi-qubit system with Josephson junctions, and improvement of fabrication techniques to reduce the complexity and costs of the system. The activities for most of the objectives are already ongoing. For example, we recently developed a resonator with the improved shape, machined it out of niobium, and tested it at a temperature of 10 mK. Other ongoing work includes the study of niobium surface treatments, and the use of advanced fabrication techniques such as additive manufacturing (AM), stamping, and hydro- or electro-hydraulic forming. In this paper, we will discuss the current and future fabrication activities of qubit cavities at RadiaBeam.
"
![](https://assets.underline.io/uploads/markdown_image/1/image/7f2315a9324b7f771d2b010b7639c22a.png)

Fabrication approaches to 3D superconducting qubit resonators with a high Q-factor

Alkali and Alkaline Earth Metal Salts of cyclic N4x– Anions, where x = 1, 2 and 3: a First-Principles Crystal Structure Prediction Sergey V. Bondarchuk A recent study of binary MgxNy systems proved that nature is much more inventive than any human imagination [1]. It was found that N4 species can be recovered to ambient conditions in the form of syn-N44– anions in Mg2N4. We have studied recoverability of other similar anions, including syn- and anti-N42–, and found that these are unstable at ambient conditions. Meanwhile, the cyclic form of N4 (tetrazete, c-N4), appeared to be dynamically, mechanically and thermally stable in crystalline form at ambient conditions, that follows from our first-principles crystal structure prediction and ab initio molecular dynamics simulations. Moreover, metal salts of c-N4x– with several alkali and alkaline earth metals are found to be stable in a wide range of oxidation states (x = 1, 2 and 3). Thus, crystal structures of the following salts were predicted and their properties were studied. Depending on both nature of the metal cation and stoichiometry, these demonstrate a wide scattering in properties, namely, band gap, bulk modulus, enthalpy of formation, etc. Thus, further study of such salts is of great importance. References: [1] Laniel D, Winkler B, Koemets E, Fedotenko T, Bykov M, Bykova E, Dubrovinsky L and Dubrovinskaia N 2019 Nat. Commun. 10 451.

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Cavity Induced State Transitions in a Coupled Fluxonium Qubit System

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