Quantum information articles within Nature Communications

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  • Article
    | Open Access

    Performing quantum computing in the NISQ era requires reliable information on the gate noise characteristics and their performance benchmarks. Here, the authors show how to estimate the individual noise properties of any quantum process from the noisy eigenvalues of its corresponding quantum channel.

    • Yanwu Gu
    • , Wei-Feng Zhuang
    •  & Dong E. Liu

  • Article
    | Open Access

    Quantum theory allows for indefinite causal order, but experimental demonstrations of such scenarios have so far required trust in the internal functioning of the apparatus. Here, the authors point out a scenario where indefinite causal order could be certified in a device-independent way, if one excludes superluminal and retrocausal influences.

    • Tein van der Lugt
    • , Jonathan Barrett
    •  & Giulio Chiribella

  • Article
    | Open Access

    The beamsplitter operation is a key component for quantum information processing, but implementations in superconducting circuit-QED usually introduce additional decoherence. Here, the authors exploit the symmetry within a SQUID, driven in a purely differential manner, to realise clean BS operations between two SC cavity modes.

    • Yao Lu
    • , Aniket Maiti
    •  & Robert J. Schoelkopf

  • Article
    | Open Access

    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.

    • Tony Metger
    •  & Renato Renner

  • Article
    | Open Access

    In order to be practical, schemes for characterizing quantum operations should require the simplest possible gate sequences and measurements. Here, the authors show how random gate sequences and native measurements (followed by classical post-processing) are sufficient for estimating several gate set properties.

    • J. Helsen
    • , M. Ioannou
    •  & I. Roth

  • Article
    | Open Access

    Continuous-variable quantum networks are easier to implement than discrete-variable ones, but suffer from a lower teleportation fidelity. Here, the authors demonstrate a CV teleportation protocol exploiting heralded noiseless amplification to increase the fidelity, at the expense of probabilistic operation.

    • Jie Zhao
    • , Hao Jeng
    •  & Ping Koy Lam

  • Article
    | Open Access

    Hybrid quantum systems, such as superconducting qubits interacting with microwave photons in resonators, offer a rich platform for exploring fundamental physics. Wang et al. observe parity symmetry breaking in a probe qubit dispersively coupled to a resonator in the deep-strong coupling regime.

    • Shuai-Peng Wang
    • , Alessandro Ridolfo
    •  & J. Q. You

  • Article
    | Open Access

    The ability to perform nonlinear feedforward operations - that is, conditional operations controlled by nonlinear function of the measurement outcomes - is still a missing ingredient for measurement-based quantum computation. Here, the authors fill this gap using nonlinear electro-optical feedforward and non-Gaussian ancillary states.

    • Atsushi Sakaguchi
    • , Shunya Konno
    •  & Akira Furusawa

  • Article
    | Open Access

    Experimental studies about the trainability and generalization capacities of quantum neural networks are highly in need. Here, the authors implement a previously proposed parametrization and training scheme using a 6-qubit superconducting quantum processor.

    • Xiaoxuan Pan
    • , Zhide Lu
    •  & Luyan Sun

  • Article
    | Open Access

    Generalization - that is, the ability to extrapolate from training data to unseen data - is fundamental in machine learning, and thus also for quantum ML. Here, the authors show that QML algorithms are able to generalise the training they had on a specific distribution and learn over different distributions.

    • Matthias C. Caro
    • , Hsin-Yuan Huang
    •  & Zoë Holmes

  • Article
    | Open Access

    Hybrid quantum acoustic systems integrating qubits with phonons offer a novel platform for investigating open quantum systems. Kitzman et al. report control of superposition states of a transmon qubit under the effect of drive and dissipation by engineering its coupling to a bath of surface acoustic wave phonons.

    • J. M. Kitzman
    • , J. R. Lane
    •  & J. Pollanen

  • Article
    | Open Access

    The emergence of relaxation in unitarily evolving systems can be seen as a paradox, but not once the distinction between local and global dynamics is considered. Here, the authors use photons in an integrated optical interferometer to show that, for a system evolving unitarily on a global level, single-mode measurements converge to those of a thermal state.

    • F. H. B. Somhorst
    • , R. van der Meer
    •  & J. J. Renema

  • Article
    | Open Access

    There are different quantum algorithms developed for the security of current cryptographic concepts. Here the authors demonstrate a method to perform quantum-secured digital payments using unforgeable quantum cryptograms over an optical fiber link and verify the information-theoretic security.

    • Peter Schiansky
    • , Julia Kalb
    •  & Philip Walther

  • Article
    | Open Access

    As the size of quantum processors scales up, accurate characterization of errors due to various crosstalks between qubits becomes important. Here the authors use a novel benchmarking protocol to study single-gate fidelities in a 2x2 hole spin qubit array in germanium when qubits are driven simultaneously.

    • W. I. L. Lawrie
    • , M. Rimbach-Russ
    •  & M. Veldhorst

  • Article
    | Open Access

    Future single-photon-based quantum networks will require both reliable telecom single-photon sources and improvements in security analysis. Here, the authors show how to use quantum dots and difference frequency generation to perform long-distance QKD, also reducing secure key acquisition time thanks to improved analytical bounds.

    • Christopher L. Morrison
    • , Roberto G. Pousa
    •  & Alessandro Fedrizzi

  • Article
    | Open Access

    Out-of-time-ordered correlators of local operators can quantify information scrambling in quantum many-body systems, but they are not easily accessible in experiments. Here the authors show that their global versions can be used for the same purpose and has been measured in nuclear magnetic resonance experiments.

    • Tianci Zhou
    •  & Brian Swingle

  • Article
    | Open Access

    The boron vacancy center in hBN has been intensively studied, but its characterizations have remained limited. Here the authors achieve a 5-fold enhancement of coherence time using dynamical decoupling, which enables the direct estimation of defect concentration and its electric field susceptibility.

    • Ruotian Gong
    • , Guanghui He
    •  & Chong Zu

  • Article
    | Open Access

    Optically active defects in hBN are promising for quantum sensing and information applications, however, coherent control of a single defect has not been achieved so far. By using an efficient method to produce arrays of defects in hBN, Guo et al. isolate a new carbon-related defect and show its coherent control.

    • Nai-Jie Guo
    • , Song Li
    •  & Guang-Can Guo

  • Article
    | Open Access

    In quantum technologies, scalable ways to characterise errors in quantum hardware are highly needed. Here, the authors propose an approximate version of quantum process tomography based on tensor network representations of the processes and data-driven optimisation.

    • Giacomo Torlai
    • , Christopher J. Wood
    •  & Leandro Aolita

  • Article
    | Open Access

    Quantum technologies allow memory advantages in simulating stochastic processes, but a demonstration of this for non-Markovian processes (where the advantage would be stronger) has been missing so far. Here the authors fill this gap analytically and experimentally, using a single qubit memory to model non-Markovian processes.

    • Kang-Da Wu
    • , Chengran Yang
    •  & Thomas J. Elliott

  • Article
    | Open Access

    Encoding quantum information in qudits instead of qubits allows for several advantages, but scalable native entangling techniques would be needed. Here, the authors show how to use light-shift gates to perform entangling operations on trapped ion systems, with a calibration overhead which is independent on the qudit dimension.

    • Pavel Hrmo
    • , Benjamin Wilhelm
    •  & Martin Ringbauer

  • Article
    | Open Access

    It has been predicted that entanglement phase diagrams of Haar-measure random states can show interesting phenomenology, including entanglement phase transitions. Here, the authors confirm these predictions using up to 15 qubits in a fully-connected superconducting quantum processor.

    • Tong Liu
    • , Shang Liu
    •  & Heng Fan

  • Article
    | Open Access

    Quantum-enhanced versions of weak coin flipping (a cryptographic primitive where two mistrustful parties agree on a random bit while favouring opposite outcomes) have been proposed in the past but never realised. Here, the authors fill this gap by improving on a previous proposal and implementing it with single photons in a fibre-based setup.

    • Simon Neves
    • , Verena Yacoub
    •  & Eleni Diamanti

  • Article
    | Open Access

    Authors model programmable photonic circuits targeting universal unitaries and verify that a type of unit rotation operator has a heavy-tailed distribution. They suggest hardware pruning for random unitary and present design strategies for high fidelity and energy efficiency in large-scale quantum computations and photonic deep learning accelerators.

    • Sunkyu Yu
    •  & Namkyoo Park

  • Article
    | Open Access

    Recently there has been interest in exploring the coupling between magnons for use in information processing, however, this is hampered by the fact that such coupling is forbidden due to the different parity of the acoustic and optical magnons. Here, Comstock et al show that the interlayer Dzyaloshinskii–Moriya-Interaction in a layered hybrid antiferromagnet can allow for strong coupling between the acoustic and optical magnons, offering a pathway for magnon coherent information processing.

    • Andrew H. Comstock
    • , Chung-Tao Chou
    •  & Dali Sun

  • Article
    | Open Access

    Probing single rare earth ions is highly desirable for several quantum applications, but it is difficult due to low emission rates. Here, the authors demonstrate the detection and control of single Erbium ions emission using electro-optically active photonic crystal cavities patterned from thin-film lithium niobate.

    • Likai Yang
    • , Sihao Wang
    •  & Hong X. Tang

  • Article
    | Open Access

    Usually, observation of quantum interference in a non-local scenario (that is, when Alice’s measurement settings and Bob’s outcomes are space-like separated) relies on entanglement. Here, the authors experimentally show four-photon frustrated interference originating from the sources’ indistinguishability, without the need for entanglement.

    • Kaiyi Qian
    • , Kai Wang
    •  & Xiao-song Ma

  • Article
    | Open Access

    Quantum theory can describe scenarios with an indefinite causal order, but whether such processes could be witnessed in real scenarios by violating causal inequalities is still subject to debate. Here, the authors give an affirmative answer, showing that noncausal processes admit a description using the framework of time-delocalised subsystems.

    • Julian Wechs
    • , Cyril Branciard
    •  & Ognyan Oreshkov

  • Article
    | Open Access

    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.

    • Xiang-Dong Chen
    • , En-Hui Wang
    •  & Fang-Wen Sun

  • Article
    | Open Access

    Twin-field QKD should allow secure quantum communication with favourable rate-loss scaling, but requires interferometric implementations which are often impractical for long distances. Here, the authors show how to realise it without the need for closed interferometers.

    • Lai Zhou
    • , Jinping Lin
    •  & Zhiliang Yuan

  • Article
    | Open Access

    The triangle causal structure represents a departure from the usual Bell scenario, as it should allow to violate classical predictions without the need for external inputs setting the measurement bases. Here the authors realise this scenario using a photonic setup with three independent photon sources.

    • Emanuele Polino
    • , Davide Poderini
    •  & Fabio Sciarrino

  • Article
    | Open Access

    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.

    • Sara Bartolucci
    • , Patrick Birchall
    •  & Chris Sparrow

  • Article
    | Open Access

    Applications of solid-state qubits in large-scale quantum networks are limited by power and density constraints associated with microwave driving. Here the authors propose a programmable architecture based on diamond color centers driven by electric or strain fields for reduced cross-talk and power consumption.

    • Hanfeng Wang
    • , Matthew E. Trusheim
    •  & Dirk R. Englund

  • Article
    | Open Access

    Halide perovskites have a variety of attractive feature such as high quantum yield, and tunable optical properties, combined with easy fabrication. Here, Kirstein et al demonstrate spin-mode locking in CsPb(Cl0.56Br0.44)3 lead halide perovskite nanocrystals embedded in a fluorophosphate glass matrix, and a hole spin lifetime extending into the microsecond range.

    • E. Kirstein
    • , N. E. Kopteva
    •  & A. Greilich

  • Article
    | Open Access

    A possible route to scalability of trapped-ion-based quantum computing platforms is to connect multiple modules where ions can be shuttled across different registers. Here, the authors demonstrate fast and low-loss transfer of trapped ions between two microchip modules.

    • M. Akhtar
    • , F. Bonus
    •  & W. K. Hensinger

  • Article
    | Open Access

    Rigorous results about the real computational advantages of quantum machine learning are few. Here, the authors prove that a PROMISEBQP-complete problem can be expressed by variational quantum classifiers and quantum support vector machines, meaning that a quantum advantage can be achieved for all ML classification problems that cannot be classically solved in polynomial time.

    • Jonas Jäger
    •  & Roman V. Krems

  • Article
    | Open Access

    Comparing the capabilities of different quantum machine learning protocols is difficult. Here, the authors show that different learning models based on parametrized quantum circuits can all be seen as quantum linear models, thus driving general conclusions on their resource requirements and capabilities.

    • Sofiene Jerbi
    • , Lukas J. Fiderer
    •  & Vedran Dunjko

  • Article
    | Open Access

    Quantum random number generators should ideally rely on few assumptions, have high enough generation rates, and be cost-effective and easy to operate. Here, the authors show an untrusted-homodyne-based MDI scheme that does not rely on i.i.d. assumption and is secure against quantum side information.

    • Chao Wang
    • , Ignatius William Primaatmaja
    •  & Charles Lim

  • Article
    | Open Access

    Frequency-bin qubits get the best of time-bin and dual-rail encodings, but require external modulators and pulse shapers to build arbitrary states. Here, instead, the authors work directly on-chip by controlling the interference of biphoton amplitudes generated in multiple, coherently-pumped ring resonators.

    • Marco Clementi
    • , Federico Andrea Sabattoli
    •  & Daniele Bajoni

  • Article
    | Open Access

    Characterisation of quantum hardware requires clear indications on what can and cannot be learned about quantum noise. Here, the authors show how to characterise learnable degrees of freedom of a Clifford gate using tools from algebraic graph theory.

    • Senrui Chen
    • , Yunchao Liu
    •  & Liang Jiang

  • Article
    | Open Access

    Quantifying communication capabilities produced by sharing an entangled qubit pair is still a subject of debate. Here the authors show that there are communication tasks for which sharing an entangled pair gives higher power than sharing two classical bits, even when there is no entanglement in the measurements.

    • Amélie Piveteau
    • , Jef Pauwels
    •  & Armin Tavakoli

  • Article
    | Open Access

    Implementations of shallow quantum machine learning models are a promising application of near-term quantum computers, but rigorous results on their trainability are sparse. Here, the authors demonstrate settings where such models are untrainable.

    • Eric R. Anschuetz
    •  & Bobak T. Kiani

  • Article
    | Open Access

    Quantum-dot spin qubits in Si/SiGe quantum wells require a large and uniform valley splitting for robust operation and scalability. Here the authors introduce and characterize a new heterostructure with periodic oscillations of Ge atoms in the quantum well, which could enhance the valley splitting.

    • Thomas McJunkin
    • , Benjamin Harpt
    •  & M. A. Eriksson

  • Article
    | Open Access

    Applications of ultra-low-loss photonic circuitry in quantum photonics, in particular including triggered single photon sources, are rare. Here, the authors show how InAs quantum dot single photon sources can be integrated onto wafer-scale, CMOS compatible ultra-low loss silicon nitride photonic circuits.

    • Ashish Chanana
    • , Hugo Larocque
    •  & Marcelo Davanco

  • Article
    | Open Access

    Renormalisation group methods serve for finding analytic solutions, critical points and computing phase diagrams of many-body systems. Here the authors demonstrate that renormalisation group schemes can be constructed for undecidable many-body systems, giving rise to the types of renormalisation group flow which are strictly more unpredictable than chaotic flows.

    • James D. Watson
    • , Emilio Onorati
    •  & Toby S. Cubitt

  • Article
    | Open Access

    Interaction-free measurements typically use repeated interrogations of an object that suppress the coherent evolution of the system. Dogra et al. demonstrate in a superconducting circuit a novel protocol that employs coherent repeated interrogations, and show that it yields a higher detection probability.

    • Shruti Dogra
    • , John J. McCord
    •  & Gheorghe Sorin Paraoanu

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