Harnessing quantum: progress towards real world applications of quantum technologies

Conveyor-mode spin shuttling using a two-tone travelling-wave potential demonstrates an order of magnitude better spin coherence than bucket-brigade shuttling, achieving spin shuttling over 10 μm in under 200 ns with 99.5% fidelity in an isotopically purified Si/SiGe heterostructure.

Barren plateaus are widely considered as one of the main limitations for variational quantum algorithms. This Review summarizes the latest understandings of barren plateaus, indicating its causes, architecture that will suffer from this phenomenon, and discusses strategies that can — and cannot — avoid it.

Networking remote superconducting quantum computers requires low-noise microwave-to-optical photon conversion. A transducer based on an integrated silicon electro-optomechanical resonator now achieves below one photon of added noise referred to the transducer input while operating continuously under laser drive.

A data interface that is based on complementary metal–oxide–semiconductor technology can provide wireless data transfer between cryogenic and room temperature, and minimize the heat-to-information transfer ratio.

Molecular approaches in quantum information science are highly promising, but the synthesis and scale-up of suitable covalently linked moieties represent major challenges. Here it is demonstrated that efficient spin mixing between photogenerated spin centres is possible through hydrogen bonds, advancing supramolecular chemistry as a valuable tool to address these challenges.

The Collision Clustering decoder is introduced, which requires few logical resources on field-programmable gate array hardware, and low power and area occupation on application-specific integrated circuit hardware, while being performant enough to keep up with the syndrome generation time of a quantum processing unit.

Although quantum computers are still in their infancy, their computational power is growing rapidly. This Perspective surveys and critiques the known ways to benchmark quantum computer performance, highlighting new challenges anticipated on the road to utility-scale quantum computing.

The integration of 1,024 independent silicon quantum dot devices with on-chip digital and analogue electronics, all of which operate below 1 K, allows characteristic data across the quantum dot array to be acquired and analysed in under 10 min.

Two below-threshold surface code memories on superconducting processors markedly reduce logical error rates, achieving high efficiency and real-time decoding, indicating potential for practical large-scale fault-tolerant quantum algorithms.

The ongoing quest in particle physics to discover fundamentally new phenomena necessitates the continuous development of algorithms and technologies. The authors propose a methodology based on quantum machine learning that can identify new phenomena in proton collision experiments, showing that it can outperform its classical counterparts when sufficient quantum computing resources are utilized.

Conventional superconducting flux qubits require a finely tuned magnetic field to operate, hindering their on-chip integration. Here, ferromagnetic Josephson junctions with a π-phase shift in the superconducting order parameter allow the realization of a flux qubit operating at zero magnetic field.

Amorphous aluminum oxide tunnel junctions are important for cryogenic and room temperature devices. Here, the authors demonstrate the use of alternating-bias-assisted annealing for transforming and tuning transmon qubit junctions, where giant increases in excess of 70% in the room temperature resistance can be achieved.

Quantum machine learning faces applicability challenges as quantum computers are needed for both training and evaluation of trained models. This study explores models that can be quantumly trained but classically evaluated, highlighting their limits compared to fully quantum models and their advantages over classical ones.

Quantum coherent control of single-photon-emitting defect spins have been reported in hexagonal boron nitride, revealing that spin coherence is mainly governed by coupling to a few proximal nuclei and can be prolonged by decoupling protocols.

The authors report the sweet-spot operation of germanium hole spin qubits, exploring the optimization of the external magnetic field orientation, the g-tensor and its electric tunability, and hyperfine interactions.

Silicon spin qubits are promising for the realisation of scalable quantum computing platforms but their coherence times in natural silicon are limited by the non-zero nuclear spin of the ²⁹Si isotope. Here, enriched ²⁸ Si down to 2.3 ppm residual ²⁹Si is obtained by focused ion beam implantation.

Quantum low-density parity-check codes are highly efficient in principle but challenging to implement in practice. This proposal shows that these codes could be implemented in the near term using recently demonstrated neutral-atom arrays.

A versatile cloud-accessible single-photon-based quantum computing machine is developed, which shows a six-photon sampling rate of 4 Hz over weeks. Heralded generation of a three-photon Greenberger–Horne–Zeilinger state—a key milestone toward measurement-based quantum computing—is implemented.

Ensuring high-fidelity quantum gates while increasing the number of qubits poses a great challenge. Here the authors present a scalable strategy for optimizing frequency trajectories as a form of error mitigation on a 68-qubit superconducting quantum processor, demonstrating high single- and two-qubit gate fidelities.

Quantum computers promise to efficiently predict the structure and behaviour of molecules. This Perspective explores how this could overcome existing challenges in computational drug discovery.

The kernel method in machine learning can be implemented on near-term quantum computers. A 27-qubit device has now been used to solve learning problems using kernels that have the potential to be practically useful.

A programmable quantum processor based on encoded logical qubits operating with up to 280 physical qubits is described, in which improvement of algorithmic performance using a variety of error-correction codes is enabled.

Our current understanding of the computational abilities of near-intermediate scale quantum (NISQ) computing devices is limited, in part due to the absence of a precise definition for this regime. Here, the authors formally define the NISQ realm and provide rigorous evidence that its capabilities are situated between the complexity classes BPP and BQP.

An efficient control strategy is designed for quantum dot arrays, drawing inspiration from classical semiconductor technology. A two-dimensional array of 16 semiconductor quantum dots is operated using only a few shared control lines.

The paper explores the possibility of creating ansatz wave-functions for the variational quantum eigensolver that are much more compact than ADAPT-VQE while achieving chemical accuracy. The proposed Overlap-ADAPT-VQE combined with the classical selected-CI approach can reach chemical accuracy for a stretched linear H6 chain using an ansatz with only 40 operators compared to more than 150 for ADAPT-VQE.

Quantum computers are expected to surpass classical computers and transform industries. This Review focuses on quantum computing for financial applications and provides a summary for physicists on potential advantages and limitations of quantum techniques, as well as challenges that physicists could help tackle.

Fusion gates are common operations in photonic quantum information platforms, where they are employed to create entanglement. Here, the authors propose a quantum computation scheme where the same measurements used to generate entanglement can also be used to achieve fault-tolerance leading to an increased tolerance to errors.

The power of quantum machine learning algorithms based on parametrised quantum circuits are still not fully understood. Here, the authors report rigorous bounds on the generalisation error in variational QML, confirming how known implementable models generalize well from an efficient amount of training data.

Laser cooling of molecules with more than six atoms is challenging, mainly due to vibrational loss to dark states. Now, taking a step towards the development of a ‘quantum functional group’, it has been shown that such vibrational loss in molecules like phenol can be greatly restricted by functionalizing with a Ca(I)–O unit, which may serve as a generic qubit moiety.

The s-orbital mixing into the spin-bearing d orbital associated with a molecular Lu(II) complex is shown to both reduce spin–orbit coupling and increase electron–nuclear hyperfine interactions, which substantially improves electron spin coherence. Combined with the potential to tune interactions through coordination chemistry, it makes this system attractive for quantum information applications.

Expectations for quantum machine learning are high, but there is currently a lack of rigorous results on which scenarios would actually exhibit a quantum advantage. Here, the authors show how to tell, for a given dataset, whether a quantum model would give any prediction advantage over a classical one.

In a quantum simulation of a (2+1)D lattice gauge theory using a superconducting quantum processor, the dynamics of strings reveal the transition from deconfined to confined excitations as the effective electric field is increased.

A quantum simulation of a (2 + 1)-dimensional lattice gauge theory is carried out on a quantum computer working with neutral atoms trapped by optical tweezers in a Kagome geometry.

Recently, spontaneous symmetry breaking was reported in 1D and 2D lattices with long-range interactions on analogue quantum simulators. Here, using a digital quantum annealing algorithm, Hu et al. observe this effect in a tree-like superconducting qubit lattice with short-range interactions at zero temperature.

Qubit-based simulations of gauge theories are challenging as gauge fields require high-dimensional encoding. Now a quantum electrodynamics model has been demonstrated using trapped-ion qudits, which encode information in multiple states of ions.

When performing quantum simulation, oftentimes the properties of interest are only a subset of the information contained in the entire state. Here, the authors devise a different type of quantum simulation, where they work with a compressed quantum state whose amplitudes are proportional to expected values of some specific observables of interest.

Bubble formation is a signal of false vacuum decay, in which a system transitions from a local energy minimum to a true vacuum. Now, simulations on a quantum annealer show how interactions between bubbles drive the long-time dynamics of this process.

Non-Hermitian skin effect, a phenomenon where eigenstates accumulate at the boundaries of a non-Hermitian system, has been observed in various platforms but primarily at the single particle level. Here the authors demonstrate the interplay of this effect with many-body physics on a superconducting quantum processor.

Arrays of superconducting transmon qubits can be used to study the Bose–Hubbard model. Synthetic electromagnetic fields have now been added to this analogue quantum simulation platform.

Recently, there have been proposals to extend the concept of time crystals to topological order. Here the authors observe a prethermal topologically ordered time crystal on a superconducting quantum processor, where discrete time-translation symmetry breaking manifests for nonlocal rather than local observables.

Analogue quantum simulators have looser requirements than digital ones, but rigorous results on their usefulness in the noisy case are few. Here, the authors conclude that analogue quantum simulators are robust to errors and can provide superpolynomial to exponential quantum advantage when used to compute relevant many-body observables.

Topological quantum states are essential resources in quantum error correction and quantum simulation but unitary quantum circuits for their preparation require extensive circuit depth. The authors demonstrate a constant-depth protocol to prepare topologically ordered states on a trapped-ion quantum computer using non-unitary operations.

Time reflection and refraction are experimentally observed in ultracold atoms. To this end, the time boundary is formed by imposing an abrupt change in the coupling strength of the atomic chain. Time boundary effects are robust against material disorder.

Geometric phase interference has been predicted to appear around conical intersections but has been experimentally illusive owing to competing effects in molecular systems. Now, this effect has been demonstrated in chains of trapped ions using state-of-the-art quantum simulation and read-out techniques.

Wavepacket dynamics around conical intersections are influenced by geometric phase, which can affect chemical reaction outcomes but has only been observed through indirect signatures. Now, by engineering a controllable conical intersection in a trapped-ion quantum simulator, the destructive wavepacket interference caused by a geometric phase has been observed.

Quantum simulations of the fundamental particles and forces of nature have a central role in understanding key static and dynamic quantum properties of matter. Motivations, techniques and future challenges for simulations of quantum fields are discussed, highlighting examples of early progress towards the dynamics of high-density, non-equilibrium systems of quarks, gluons and neutrinos.

The robust implementation of gauge fields coupled to dynamical matter in large-scale quantum simulators is limited by the ever-present gauge-breaking errors. The authors propose an experimentally suitable scheme combining two-body interactions with weak fields, demonstrating its robustness against gauge breaking errors and its flexibility in the study of various models with Z2 gauge symmetry.

Hall resistance quantization measurements in the quantum anomalous Hall effect regime on a device based on the magnetic topological insulator V-doped (Bi,Sb)₂Te₃ show that the system can provide a zero external magnetic field quantum standard of resistance.

A family of multi-qubit Rydberg quantum gates is developed and used to generate Schrödinger cat states in an optical clock, allowing improvement in frequency measurement precision by taking advantage of entanglement.

We demonstrate high-fidelity entangling gates, universal quantum operations, and ancilla-based read-out for ultranarrow optical transitions of neutral atoms in a tweezer clock platform.

A vacuum ultraviolet frequency comb is used to directly excite the narrow ²²⁹Th nuclear clock transition in a solid-state CaF₂ host material, marking the start of nuclear-based solid-state optical clocks.

The fabrication of a molecular quantum sensor on the tip of a scanning tunnelling microscope enables the detection of minute magnetic and electric fields of single atoms with sub-angstrom resolution.

Quantum sensing exploits properties of quantum systems to go beyond what is possible with traditional measurement techniques, hence opening exciting opportunities in both low-energy and high-energy particle physics experiments.

Bolometers can be used for qubit readout, offering a scalable, sensitive and high-speed alternative to current parametric-amplifier-based approaches.

The ability to characterize large and complex nuclear-spin networks could enable quantum applications, such as quantum simulations of many-body physics. Here the authors develop a high-resolution quantum-sensing method and use it to image a network of 50 nuclear spins surrounding a single NV center in diamond.

Noise is a fundamental obstacle to the stability of atomic optical clocks. An experiment now realizes the design of a spin-squeezed clock that improves interrogation times and enables direct comparisons of performance between different clocks.

Boundary time crystals are gaining attention due to their distinctive features like persistent oscillations at the thermodynamic limit. This work shows that the boundary time crystal phase transition can be exploited for quantum-enhanced sensitivity, which bridges many-body physics and quantum metrology and hence triggers broad interest in the condensed matter and quantum technology communities.

An emerging set of proposals seeks to use arrays of optomechanical sensors to detect weak distributed forces, for applications ranging from gravity-based subterranean imaging to dark matter searches. We propose an array of entanglement-enhanced optomechanical sensors to improve the broadband sensitivity of distributed force sensing.

Quantum sensors based on NV centers in diamond are well established, however the sensitivity of detection of high-frequency radio signals has been limited. Here the authors use nanoscale field-focusing to enhance sensitivity and demonstrate ranging for GHz radio signals in an interferometer set-up.

Hong–Ou–Mandel interference enables depth-resolved quantum imaging at very low light levels.

A twin-field quantum key distribution protocol based on optical coherence is deployed over a 254-kilometre commercial telecom network, demonstrating that coherence-based quantum communication can be aligned with existing telecommunication infrastructure.

Converting photons from one frequency range to another uses nonlinear effects that are often weak. Strong nonlinearities in rare-earth-ion-doped crystals have now been used to perform microwave-to-optical transduction at the single-photon level.

The so-called Green Machine is an old concept for a joint detection receiver that would allow superadditive optical communication capacity, but earlier designs are very hard to implement. Here, the authors propose a modified scheme and use it to demonstrate superadditive capacity with the BPSK Hadamard codewords.

A new quantum operating system architecture is described that is capable of executing applications on quantum networks in high-level software, which is a step towards bringing quantum network technology to society.

This Review introduces coherent light–matter interactions in solution-processed lead halide perovskite colloidal nanocrystals, discussing opportunities and challenges in the context of quantum information technologies.

Security proofs against general attacks are the ultimate goal of QKD. Here, the authors show how the Generalised Entropy Accumulation Theorem can be used, for some classes of QKD scenarios, to translate security proofs against collective attacks in the asymptotic regime into proofs against general attacks in the finite-size regime.

Colloidal quantum dots are a potential source of scalable single-photon emitters, but they typically exhibit broad emission linewidths. Proppe et al. show narrow-linewidth emission from heavy-metal-free InP/ZnSe/ZnS dots with coherence times of up to 250 ps.

Superconducting single-photon detectors are critical for quantum communication, fluorescence lifetime imaging and remote sensing, but commonly operate at very low temperatures. Now, high-temperature cuprate superconducting nanowires enable single-photon detection up to 25 K.

The first generation of global-scale quantum networks are expected to make extensive use of satellite-mediated channels. As a first step towards this goal, this manuscript proposes a full-scale architecture to implement the exchange of quantum information, taking us from use cases through to a detailed plan for the road ahead.

With the continuous development of metropolitan broadband and network, the need of secure and faster transmission also increases. The authors demonstrate a single-carrier four-state continuous-variable quantum key distribution (CVQKD) with sub-Gbps key rate within metropolitan area and secure transmissions up to 25 km

Twin-field (TF) quantum key distribution (QKD) over a secure distance of 833.8 km is demonstrated even in the finite-size regime. To this end, an optimized four-phase TF-QKD protocol and a high-speed low-noise TF-QKD system are developed.

The quantum aspect of soliton microcomb from an integrated silicon carbide microresonator is studied in several regimes — below threshold, above threshold and in the soliton regime — using a single-photon optical spectrum analyser for second-order photon correlation measurement.

Colour centres are a promising quantum information platform, but coherence degradation after integration in nanostructures has hindered scalability. Here, the authors show that waveguide-integrated V_Si centres in SiC maintain spin-optical coherences, enabling nuclear high-fidelity spin qubit operations.