Abstract In this short review article, we aim to provide physicists not working within the quantum computing community a hopefully easy-to-read introduction to the state of the art in the field, with minimal mathematics involved. In particular, we focus on what is termed the Noisy Intermediate Scale Quantum era of quantum computing. We describe how this is increasingly seen to be a distinct phase in the development of quantum computers, heralding an era where we have quantum computers that are capable of doing certain quantum computations in a limited fashion, and subject to certain constraints and noise. We further discuss the prominent algorithms that are believed to hold the most potential for this era, and also describe the competing physical platforms on which to build a quantum computer that have seen the most success so far. We then talk about the applications that are most feasible in the near-term, and finish off with a short discussion on the state of the field. We hope that as non-experts read this article, it will give context to the recent developments in quantum computers that have garnered much popular press, and help the community understand how to place such developments in the timeline of quantum computing. 

/pdf/nisq-computing-where-are-we-and-where-do-we-go-2xoj7rka.pdf

NISQ computing: where are we and where do we go?

Topological photonics provides a novel platform to explore topological physics beyond traditional electronic materials and stimulates promising applications in topologically protected light transport and lasers. Classical degrees of freedom such as polarizations and wavevectors are routinely used to synthesize topological light modes. Beyond the classical regime, inherent quantum nature of light gives birth to a wealth of fundamentally distinct topological states, which oﬀer topological protection in quantum information processing. Here we implement such experiments on topological states of quantized light in a superconducting circuit, on which three resonators are tunably coupled to a gmon qubit. We construct one and two-dimensional Fock-state lattices where topological transport of zero-energy states, strain induced pseudo-Landau levels, valley Hall eﬀect and Haldane chiral edge currents are demonstrated. Our study extends the topological states of light to the quantum regime, bridges topological phases of condensed matter physics with circuit quantum electrodynamics, and oﬀers a new freedom in controlling the quantum states of multiple resonators.

/pdf/coherent-control-of-quantum-topological-states-of-light-in-2nuholcs.pdf

Coherent control of quantum topological states of light in Fock-state lattices

0D nanomaterials with high efficiency of atom utilization possess extraordinary tunability over bulk materials. Precise reconstruction of atoms in a 0D nanoparticle toward tuning of crystalline phases and defects is highly desirable but remains a grand challenge. In this study, a crystallization rate-controlled strategy is reported to achieve controllable reconstruction of atoms in situ, which inducts a series of monodisperse 0D molybdenum carbide nanoparticles (MoxC NP) that anchor on a carbon matrix with adjustable crystalline phases and atom vacancies. Aberration-corrected transmission electron microscopy, electron paramagnetic resonance technique, density functional theory calculation, and electron holography jointly reveal the atomic reconstruction process and confirm its remarkable effects of optimizing the local electronic states and enhancing the heterointerface interactions. As a result, the optimized MoC/Mo2C heterostructure on the carbon matrix is shown to enable the promoted dielectric response and generate more than 90% absorption of lower-frequency microwaves (the current 5th-generation communication band). The control of atomic reconstruction may provide an effective pathway for unlocking tunable dielectric properties of 0D nanomaterials toward various technological applications. 

Atomic and Electronic Reconstruction in Defective 0D Molybdenum Carbide Heterostructure for Regulating Lower‐Frequency Microwaves

Individual electron spins in solids are promising candidates for quantum science and technology, where bottom-up assembly of a quantum device with atomically precise couplings has long been envisioned. Here, we realized atom-by-atom construction, coherent operations, and readout of coupled electron-spin qubits using a scanning tunneling microscope. To enable the coherent control of “remote” qubits that are outside of the tunnel junction, we complemented each electron spin with a local magnetic field gradient from a nearby single-atom magnet. Readout was achieved by using a sensor qubit in the tunnel junction and implementing pulsed double electron spin resonance. Fast single-, two-, and three-qubit operations were thereby demonstrated in an all-electrical fashion. Our angstrom-scale qubit platform may enable quantum functionalities using electron spin arrays built atom by atom on a surface. Description Editor’s summary Qubits implemented using solid-state spins are easy to manipulate, but such architectures remain tricky to scale up. Wang et al. used a scanning tunneling microscope (STM) to engineer an atomic-scale multi-qubit system consisting of a sensor qubit and up to two remote qubits. The sensor qubit, a titanium atom on a bilayer magnesium oxide film, was directly controlled by the STM tip, whereas the remote qubits were each placed next to an iron atom and read out using the sensor qubit. The researchers demonstrated single- and multi-qubit gates. The size and performance of the setup may be further improved by optimizing the choice of materials and spin species. —Jelena Stajic A scanning tunneling microscope was used to engineer a multi-qubit system consisting of a sensor and remote qubits.

An atomic-scale multi-qubit platform

Abstract Simulating quantum many-body systems is a key application for emerging quantum processors. While analog quantum simulation has already demonstrated quantum advantage, its digital counterpart has recently become the focus of intense research interest due to the availability of devices that aim to realize general-purpose quantum computers. In this perspective, we give a selective overview of the currently pursued approaches, review the advances in digital quantum simulation by comparing non-variational with variational approaches and identify hardware and algorithmic challenges. Based on this review, the question arises: What are the most promising problems that can be tackled with digital quantum simulation? We argue that problems of a qualitative nature are much more suitable for near-term devices then approaches aiming purely for a quantitative accuracy improvement. 

Quantum many-body simulations on digital quantum computers: State-of-the-art and future challenges

The realization of controllable fermionic quantum systems via quantum simulation is instrumental for exploring many of the most intriguing effects in condensed-matter physics1-3. Semiconductor quantum dots are particularly promising for quantum simulation as they can be engineered to achieve strong quantum correlations. However, although simulation of the Fermi-Hubbard model4 and Nagaoka ferromagnetism5 have been reported before, the simplest one-dimensional model of strongly correlated topological matter, the many-body Su-Schrieffer-Heeger (SSH) model6-11, has so far remained elusive-mostly owing to the challenge of precisely engineering long-range interactions between electrons to reproduce the chosen Hamiltonian. Here we show that for precision-placed atoms in silicon with strong Coulomb confinement, we can engineer a minimum of six all-epitaxial in-plane gates to tune the energy levels across a linear array of ten quantum dots to realize both the trivial and the topological phases of the many-body SSH model. The strong on-site energies (about 25 millielectronvolts) and the ability to engineer gates with subnanometre precision in a unique staggered design allow us to tune the ratio between intercell and intracell electron transport to observe clear signatures of a topological phase with two conductance peaks at quarter-filling, compared with the ten conductance peaks of the trivial phase. The demonstration of the SSH model in a fermionic system isomorphic to qubits showcases our highly controllable quantum system and its usefulness for future simulations of strongly interacting electrons. 

/pdf/engineering-topological-states-in-atom-based-semiconductor-2fc2ltom.pdf

Engineering topological states in atom-based semiconductor quantum dots

Phosphorus atoms in silicon offer a rich quantum computing platform where both nuclear and electron spins can be used to store and process quantum information. While individual control of electron and nuclear spins has been demonstrated, the interplay between them during qubit operations has been largely unexplored. This study investigates the use of exchange‐based operation between donor bound electron spins to probe the local magnetic fields experienced by the qubits with exquisite precision at the atomic scale. To achieve this, coherent exchange oscillations are performed between two electron spin qubits, where the left and right qubits are hosted by three and two phosphorus donors, respectively. The frequency spectrum of exchange oscillations shows quantized changes in the local magnetic fields at the qubit sites, corresponding to the different hyperfine coupling between the electron and each of the qubit‐hosting nuclear spins. This ability to sense the hyperfine fields of individual nuclear spins using the exchange interaction constitutes a unique metrology technique, which reveals the exact crystallographic arrangements of the phosphorus atoms in the silicon crystal for each qubit. The detailed knowledge obtained of the local magnetic environment can then be used to engineer hyperfine fields in multi‐donor qubits for high‐fidelity two‐qubit gates.

The Use of Exchange Coupled Atom Qubits as Atomic‐Scale Magnetic Field Sensors

https://www.nature.com/articles/s41534-022-00523-5.pdf

Impact of charge noise on electron exchange interactions in semiconductors

Corrigendum: Benchmarking high fidelity single-shot readout of semiconductor qubits (2019 New J. Phys. 21 063011)

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Engineering topological states in atom-based semiconductor quantum dots

The Use of Exchange Coupled Atom Qubits as Atomic‐Scale Magnetic Field Sensors

Impact of charge noise on electron exchange interactions in semiconductors

Impact of charge noise on electron exchange interactions in semiconductors

Corrigendum: Benchmarking high fidelity single-shot readout of semiconductor qubits (2019 New J. Phys. 21 063011)