Wave-based computing has attracted extensive attention recently due to the benefits of parallel processing. In particular, several acoustic wave computing devices have been demonstrated to carry out classical algorithms and mathematical operations. Here, we extend acoustic wave computing to simulate a quantum algorithm, by proposing an integrated acoustic gradient metasurface system supporting spoof surface acoustic waves to implement the Grover quantum search algorithm. We show that this integrated metadevice can achieve a designed subdiffraction and transmission phase, which can be used to simulate operations used in a quantum algorithm, such as the Hadamard transformation and the inverse about the average. Numerical simulations demonstrate promising searching abilities of this device, including a quadratic speedup over classical algorithms and subwavelength searching accuracy. We anticipate that our results will inspire alternative design schemes for on-chip integrated metadevices for more quantum-inspired acoustic analog computations.

Protecting superconducting qubits from low-frequency noise is essential for advancing superconducting quantum computation. Based on the application of a periodic drive field, we develop a protocol for engineering dynamical sweet spots, which reduce the susceptibility of a qubit to low-frequency noise. Using the framework of Floquet theory, we prove rigorously that there are manifolds of dynamical sweet spots marked by extrema in the quasienergy differences of the driven qubit. In particular, for the example of fluxonium biased slightly away from half a flux quantum, we predict an enhancement of pure dephasing by 3 orders of magnitude. Employing the Floquet eigenstates as the computational basis, we show that high-fidelity single- and two-qubit gates can be implemented while maintaining dynamical sweet-spot operation. We further confirm that qubit readout can be performed by adiabatically mapping the Floquet states back to the static qubit states, and subsequently applying standard measurement techniques. Our work provides an intuitive tool to encode quantum information in robust, time-dependent states, and may be extended to alternative architectures for quantum-information processing.

Trends in photonic quantum information follow closely the technical progress in classical optics and telecommunications. In this regard, advances in multiplexing optical communications channels have also been pursued for the generation of multidimensional quantum states (qudits), since their use is advantageous for several quantum information tasks. One current path leading in this direction is through the use of space-division multiplexing multicore optical fibers, which provides a platform for efficiently controlling path-encoded qudit states. Here, we report on a parametric down-conversion source of entangled qudits that is fully based on (and therefore compatible with) state-of-the-art multicore-fiber technology. The source design uses modern multicore-fiber beam splitters to prepare the pump-laser beam as well as measure the generated entangled state, achieving high spectral brightness while providing a stable architecture. In addition, it can be readily used with any core geometry, which is crucial since widespread standards for multicore fibers in telecommunications have yet to be established. Our source represents a step toward the compatibility of quantum communications with the next-generation optical networks.

The nitrogen-vacancy (N-$V$) center in diamond has been established as a prime building block for quantum networks. However, scaling beyond a few network nodes is currently limited by low spin-photon entanglement rates, resulting from the N-$V$ center’s low probability of coherent photon emission and collection. Integration into a cavity can boost both values via the Purcell effect, but poor optical coherence of near-surface N-$V$ centers has so far prevented their resonant optical control, as would be required for entanglement generation. Here, we overcome this challenge, and demonstrate resonant addressing of individual, fiber-cavity-coupled N-$V$ centers, and collection of their Purcell-enhanced coherent photon emission. Utilizing off-resonant and resonant addressing protocols, we extract an enhancement of the zero-phonon line emission by a factor of up to 4, consistent with a detailed theoretical model. This model predicts that the probability of coherent photon detection per optical excitation can be increased to 10% for realistic parameters—an improvement over state-of-the art solid immersion lens collection systems by 2 orders of magnitude. The resonant operation of an improved optical interface for single coherent quantum emitters in a closed-cycle cryogenic system at $T\sim 4$ K is an important result towards extensive quantum networks with long coherence.

At the fundamental level, quantum communication is ultimately limited by noise. For instance, quantum signals cannot be amplified without the introduction of noise in the amplified states. Furthermore, photon loss reduces the signal-to-noise ratio, accentuating the effect of noise. Thus, most of the efforts in quantum communications have been directed towards overcoming noise to achieve longer communication distances, larger secret key rates, or to operate in noisier environmental conditions. Here, we propose and experimentally demonstrate a platform for quantum communication based on ultrafast optical techniques. In particular, our scheme enables the experimental realization of high rates and quantum signal filtering approaching a single spectrotemporal mode, resulting in a dramatic reduction in channel noise. By experimentally realizing a 1-ps optically induced temporal gate, we show that ultrafast time filtering can result in an improvement in noise tolerance by a factor of up to 1200 compared to a 2-ns electronic filter, enabling daytime quantum key distribution or quantum communication in bright fibers.

We report the development of a scanning force microscope based on an ultrasensitive silicon nitride membrane optomechanical transducer. Our development is made possible by inverting the standard microscope geometry—in our instrument, the substrate is vibrating and the scanning tip is at rest. We present topography images of samples placed on the membrane surface. Our measurements demonstrate that the membrane retains an excellent force sensitivity when loaded with samples and in the presence of a scanning tip. We discuss the prospects and limitations of our instrument as a quantum-limited force sensor and imaging tool.

We theoretically investigate the generation of two entangled beams of light in the process of single-pass type-I noncollinear frequency-degenerate parametric down-conversion with an ultrashort pulsed pump. We find the spatiotemporal squeezing eigenmodes and the corresponding squeezing eigenvalues of the generated field both numerically and analytically. The analytical solution is obtained by modeling the joint spectral amplitude of the field by a Gaussian function in curvilinear coordinates. We show that this method is highly efficient and is in good agreement with the numerical solution. We also reveal that, when the total bandwidth of the generated beams is sufficiently high, the modal functions cannot be factored into spatial and temporal parts, but exhibit a spatiotemporal coupling, whose strength can be increased by shortening the pump.

Acoustic devices play an important role in classical information processing. The slower speed and lower losses of mechanical waves enable compact and efficient elements for delaying, filtering, and storing of electric signals at radio and microwave frequencies. Discovering ways of better controlling the propagation of phonons on a chip is an important step towards enabling larger-scale phononic circuits and systems. We present a platform, inspired by decades of advances in integrated photonics, that utilizes the strong piezoelectric effect in a thin film of lithium niobate on sapphire to excite guided acoustic waves immune from leakage into the bulk due to the phononic analogue of index guiding. We demonstrate an efficient transducer matched to $50\mathrm{\Omega }$ and guiding within a $1\mu \mathrm{m}$ wide mechanical waveguide as key building blocks of this platform. Putting these components together, we realize acoustic delay lines, racetrack resonators, and meander line waveguides for sensing applications. To evaluate the promise of this platform for emerging quantum technologies, we characterize losses at low temperature and measure quality factors on the order of 50 000 at $4\mathrm{K}$. Finally, we demonstrate phononic four-wave mixing in these circuits and measure the nonlinear coefficients to provide estimates of the power needed for relevant parametric processes.

Hybrid superconducting circuits, which integrate nonsuperconducting elements into a circuit quantum electrodynamics (cQED) architecture, expand the possible applications of cQED. Building hybrid circuits that work in large magnetic fields presents even further possibilities, such as the probing of spin-polarized Andreev bound states and the investigation of topological superconductivity. Here we present a magnetic-field compatible hybrid fluxonium with an electrostatically tuned semiconducting nanowire as its nonlinear element. We operate the fluxonium in magnetic fields up to 1 T and use it to observe the ${\phi }_0$-Josephson effect. This combination of gate tunability and field compatibility opens avenues for the control of spin-polarized phenomena using superconducting circuits and enables the use of the fluxonium as a readout device for topological qubits.

In a superconducting quantum processor with nearest-neighbor coupling, the dispersive interaction between adjacent qubits can result in an effective next-nearest-neighbor coupling whose strength depends on the state of the intermediary qubit. Here, we theoretically explore the possibility of engineering this next-nearest-neighbor coupling to implement controlled two-qubit operations where the intermediary qubit controls the operation on the next-nearest-neighbor pair of qubits. In particular, in a system comprising two types of superconducting qubits with anharmonicities of opposite sign arranged in an -$A$-$B$-$A$- pattern, where the unwanted static $ZZ$ coupling between adjacent qubits could be heavily suppressed, a switchable coupling between the next-nearest-neighbor qubits can be achieved via the intermediary qubit, the qubit state of which functions as an on-off switch for this coupling. Therefore, depending on the adopted activating scheme, various controlled two-qubit operations such as the controlled-$i$swap gate can be realized, potentially enabling circuit depth reductions as to a standard decomposition approach for implementing generic quantum algorithms.

In magnetometry using optically detected magnetic resonance of nitrogen vacancy (N-$V^{-}$) centers, we demonstrate speedup of more than 1 order of magnitude with a sequential Bayesian experiment design as compared with conventional frequency-swept measurements. The N-$V^{-}$ center is an excellent platform for magnetometry, with potential spatial resolution down to a few nanometers and demonstrated single-defect sensitivity down to nanoteslas per square root hertz. The N-$V^{-}$ center is a quantum defect with spin S = 1 and coherence time up to several milliseconds at room temperature. Zeeman splitting of the N-$V^{-}$ energy levels allows detection of the magnetic field via photoluminescence. We compare conventional N-$V^{-}$-center photoluminescence measurements that use predetermined sweeps of the microwave frequency with measurements using a Bayesian-inference method. In sequential Bayesian experiment design, the settings with maximum utility are chosen for each measurement in real time on the basis of the accumulated experimental data. Using this method, we observe an order of magnitude decrease in the N-$V^{-}$-center-magnetometry measurement time necessary to achieve a set precision.

We use the quasienergy structure emerging in a periodically driven fluxonium superconducting circuit to encode quantum information with dynamically induced flux-insensitive sweet spots. The framework of Floquet theory provides an intuitive description of these high-coherence working points located away from the half-flux symmetry point of the undriven qubit. This approach offers flexibility in choosing the flux bias point and the energy of the logical qubit states as shown in Huang et al. [arXiv:2004.12458 (2020)]. We characterize the response of the system to noise in the modulation amplitude and dc flux bias, and experimentally demonstrate an optimal working point that is simultaneously insensitive against fluctuations in both. We observe a 40-fold enhancement of the qubit coherence times measured with Ramsey-type interferometry at the dynamical sweet spot compared with static operation at the same bias point.

Superinductors have a characteristic impedance exceeding the resistance quantum $R_Q\approx 6.45\mathrm{k}\mathrm{\Omega }$, which leads to a suppression of ground-state charge fluctuations. Applications include the realization of hardware-protected qubits for fault-tolerant quantum computing, improved coupling to small-dipole-moment objects, and the definition of a new quantum-metrology standard for the ampere. In this work, we refute the widespread notion that superinductors can only be implemented based on kinetic inductance, i.e., using disordered superconductors or Josephson-junction arrays. We present the modeling, fabrication, and characterization of 104 planar aluminum-coil resonators with a characteristic impedance up to 30.9 $\mathrm{k}\mathrm{\Omega }$ at 5.6 GHz and a capacitance down to $\le 1$ fF, with low loss and a power handling reaching ${10}^8$ intracavity photons. Geometric superinductors are free of uncontrolled tunneling events and offer high reproducibility, linearity, and the ability to couple magnetically—properties that significantly broaden the scope of future quantum circuits.

A challenge in building large-scale superconducting quantum processors is to find the right balance between coherence, qubit addressability, qubit-qubit coupling strength, circuit complexity, and the number of required control lines. Leading all-microwave approaches for coupling two qubits require comparatively few control lines and benefit from high coherence but suffer from frequency crowding and limited addressability in multiqubit settings. Here, we overcome these limitations by realizing an all-microwave controlled-phase gate between two transversely coupled transmon qubits that are far detuned compared to the qubit anharmonicity. The gate is activated by applying a single strong microwave tone to one of the qubits, inducing a coupling between the two-qubit $|f,g⟩$ and $|g,e⟩$ states, with $|g⟩$, $|e⟩$, and $|f⟩$ denoting the lowest energy states of a transmon qubit. Interleaved randomized benchmarking yields a gate fidelity of $97.5\pm 0.3\mathrm{\%}$ at a gate duration of 126 ns, with the dominant error source being decoherence. We model the gate in presence of the strong drive field using Floquet theory and find good agreement with our data. Our gate constitutes a promising alternative to present two-qubit gates and could have hardware scaling advantages in large-scale quantum processors as it requires neither additional drive lines nor tunable couplers.

Detectors of propagating microwave photons have recently been realized using superconducting circuits. However, a number-resolved photocounter is still missing. In this article, we demonstrate a single-shot counter for propagating microwave photons that can resolve up to three photons. It is based on a pumped Josephson ring modulator that can catch an arbitrary propagating mode by frequency conversion and store its quantum state in a stationary memory mode. A transmon qubit then counts the number of photons in the memory mode using a series of binary questions. Using measurement-based feedback, the number of questions is minimal and scales logarithmically with the maximal number of photons. The detector features a detection efficiency of $0.96\pm 0.04$, and a dark-count probability of $0.030\pm 0.002$ for an average dead time of $4.5\mu \mathrm{s}$. To maximize its performance, the device is first used as an in situ waveform detector from which an optimal pump is computed and applied. Depending on the number of incoming photons, the detector succeeds with a probability that ranges from $54\pm 2\mathrm{ to }99\mathrm{\%}$.

Single-photon detectors and bolometers represent the bridge between different topics in science, such as quantum computation, astronomy, particle physics, and biology. Nowadays, superconducting bolometers and calorimeters are the most-sensitive detectors in the terahertz and subterahertz bands. Here, we propose and demonstrate a Josephson escape sensor (JES) that could find natural application in astrophysics. The JES is composed of a fully superconducting one-dimensional Josephson junction, whose resistance-versus-temperature characteristics can be precisely controlled by a bias current. Therefore, differently from traditional superconducting detectors, the JES sensitivity and working temperature can be in situ simply and finely tuned depending on the application requirements. A JES bolometer is expected to show an intrinsic thermal-fluctuation-noise noise-equivalent power on the order of ${10}^{-25}{\mathrm{W/Hz}}^{1/2}$, while a JES calorimeter could provide a frequency resolution of about 2 GHz, as deduced from the experimental data. In addition, the sensor can operate at the critical temperature (i.e., working as a conventional transition-edge sensor), with a noise-equivalent power of approximately $6\times {10}^{-20}{\mathrm{W/Hz}}^{1/2}$ and a frequency resolution of approximately $100$ GHz.

Circuit quantum electrodynamics (QED) employs superconducting microwave resonators as quantum buses. In circuit QED with semiconductor quantum-dot-based qubits, increasing the resonator impedance is desirable as it enhances the coupling to the typically small charge dipole moment of these qubits. However, the gate electrodes necessary to form quantum dots in the vicinity of a resonator inadvertently lead to a parasitic port through which microwave photons can leak, thereby reducing the quality factor of the resonator. This is particularly the case for high-impedance resonators, as the ratio of their total capacitance over the parasitic port capacitance is smaller, leading to larger microwave leakage than for 50-$\mathrm{\Omega }$ resonators. Here, we introduce an implementation of on-chip filters to suppress the microwave leakage. The filters comprise a high-kinetic-inductance nanowire inductor and a thin-film capacitor. The filter has a small footprint and can be placed close to the resonator, confining microwaves to a small area of the chip. The inductance and capacitance of the filter elements can be varied over a wider range of values than their typical spiral inductor and interdigitated capacitor counterparts. We demonstrate that the total linewidth of a 6.4 GHz and approximately 3-$\mathrm{k}\mathrm{\Omega }$ resonator can be improved down to 540 kHz using these filters.

The recent development of arrays of quantum dots in semiconductor nanostructures highlights the progress of quantum devices toward a large scale. However, how to realize such arrays on a scalable platform such as silicon is still an open question. One of the main challenges lies in the detection of charges within the array. It is a prerequisite to initialize a desired charge state and read out spins through spin-to-charge conversion mechanisms. In this work, we use two methods based on either a single-lead charge detector or a reprogrammable single-electron transistor. By these methods, we study the charge dynamics and sensitivity by performing single-shot detection of the charge. Finally, we can probe the charge stability at any node of a linear array and assess the Coulomb disorder in the structure. We find an electrochemical potential fluctuation induced by charge noise comparable to that reported in other silicon quantum dots.

The combination of machine learning and quantum computing has emerged as a promising approach for addressing previously untenable problems. Reservoir computing is an efficient learning paradigm that utilizes nonlinear dynamical systems for temporal information processing, i.e., processing of input sequences to produce output sequences. Here we propose quantum reservoir computing that harnesses complex dissipative quantum dynamics. Our class of quantum reservoirs is universal, in that any nonlinear fading memory map can be approximated arbitrarily closely and uniformly over all inputs by a quantum reservoir from this class. We describe a subclass of the universal class that is readily implementable using quantum gates native to current noisy gate-model quantum computers. Proof-of-principle experiments on remotely accessed cloud-based superconducting quantum computers demonstrate that small and noisy quantum reservoirs can tackle high-order nonlinear temporal tasks. Our theoretical and experimental results pave the path for attractive temporal processing applications of near-term gate-model quantum computers of increasing fidelity but without quantum error correction, signifying the potential of these devices for wider applications including neural modeling, speech recognition, and natural language processing, going beyond static classification and regression tasks.

Superconducting circuits are at the forefront of quantum-computing technology because of the unparalleled combination of good coherence, fast gates, and flexibility in design parameters. The majority of experiments demonstrating small quantum algorithms in the superconducting architecture have used transmon qubits, which are coupled via capacitors or microwave cavities. Apart from coherence and gate fidelity, two important factors that currently limit the performance of a superconducting processor are nearest-neighbor interqubit coupling (in one or two dimensions) and being restricted to two-qubit entangling gates only. In this work, we present a programmable three-qubit processor circuit, nicknamed “trimon,” with strong all-to-all coupling and access to native three-qubit gates. We benchmark our processor by implementing a three-qubit version of various algorithms, namely Deutsch-Jozsa, Bernstein-Vazirani, Grover’s search and the quantum Fourier transform. In particular, we note that the native three-qubit controlled operations enable our ancilla-free implementation of Grover’s algorithm to outperform previous demonstrations. Our results clearly show the advantage of having native three-qubit gates, and that can play a crucial role in improving the performance of larger systems having the trimon as a building block.