Quantum reservoir computing (QRC) exploits the information-processing capabilities of quantum systems to solve nontrivial temporal tasks, improving over their classical counterparts. Recent progress has shown the potential of QRC exploiting the enlarged Hilbert space, but real-time processing and the achievement of a quantum advantage with efficient use of resources are prominent challenges towards viable experimental realizations. In this work, we propose a photonic platform suitable for real-time QRC based on a physical ensemble of reservoirs in the form of identical optical pulses recirculating through a closed loop. While ideal operation achieves maximum capacities, statistical noise is shown to undermine any quantum improvement. We propose a strategy to overcome this limitation and sustain the QRC performance when the size of the system is scaled up. The protocol is conceived for experimental implementations to be viable with current technology.

In addition to the need for stable and precisely controllable qubits, quantum computers take advantage of good readout schemes. Superconducting qubit states can be inferred from the readout signal transmitted through a dispersively coupled resonator. This work proposes a readout classification method for superconducting qubits based on a neural network pretrained with an autoencoder approach. A neural network is pretrained with qubit readout signals as autoencoders in order to extract relevant features from the data set. Afterward, the pretrained-network inner-layer values are used to perform a classification of the inputs in a supervised manner. We demonstrate that this method can enhance classification performance, particularly for short- and long-time measurements where more traditional methods present inferior performance.

Unpaired electronic spins at diamond surfaces are ubiquitous and can lead to excess magnetic noise. They have been observed in several studies to date, but their exact chemical nature is still unknown. We propose a simple model to explain the existence and chemical stability of surface spins associated with the ${sp}^3$ dangling bond on the (100) diamond surface using density-functional theory. We find that the (111) facet, which is naturally generated at a step edge of (100) crystalline diamond surface, can sterically protect a spinful defect. Our study reveals a mechanism for annihilation of these surface spins upon annealing, consistent with recent experimental results. We also demonstrate that the Fermi-contact term in the hyperfine coupling is not negligible between the surface spins and the surrounding nuclear spins, and thus ab initio simulation can be used to devise a sensing protocol where the surface spins act as reporter spins to sense nuclear spins on the surface.

Vulnerabilities of components used in quantum-key-distribution systems affect its implementation security and must be taken into consideration during system development and security analysis. In this paper, we investigate transmission of fiber optical elements, which are commonly used in quantum-key-distribution systems for designing countermeasures against Trojan-horse attacks, in the $1500$–$2100$-$\mathrm{nm}$ range. As a result, we find loopholes in their transmission spectra that open possibilities for eavesdropping. We also consider a simple passive countermeasure based on the violation of total internal reflection in a single-mode fiber, that provides additional insertion losses of at least 60 dB for double-pass Trojan-horse probe pulses for wavelengths longer than 1830 nm.

Bilayer graphene is an attractive material that hosts a high-quality two-dimensional electron gas with a controllable band gap. By utilizing the band gap, electrical gate tuning of the carrier is possible and formation of nanostructures such as quantum dots has been reported. To probe the dynamics of the electronic states and realize applications for quantum bit devices, radio-frequency (rf) reflectometry, which enables high-speed electrical measurements, is important. Here we demonstrate rf reflectometry in bilayer graphene devices. We utilize a microscale graphite back-gate and an undoped $\mathrm{Si}$ substrate to reduce the parasitic capacitance, which degrades the rf reflectometry measurement. We measure the resonance properties including the matching condition of a tank circuit, which contains the bilayer graphene device. We construct a demodulated rf reflectometry setup and compare the result with dc measurement, and confirm their consistency. The wide-range frequency-dependent noise behavior is also analyzed. From the noise analysis, we calculate the readout error rate of our device for single-electron detection and demonstrate that a vertically integrated charge sensor has the potential to achieve a low error rate. We also measure Coulomb diamonds of quantum dots possibly formed by bubbles and confirm that rf reflectometry of quantum dots can be performed. This technique enables high-speed measurements of bilayer graphene quantum dots and contributes to the study of bilayer-graphene-based quantum devices by fast readout of the states.

The fabrication of superconducting circuits requires multiple deposition, etching, and cleaning steps, each possibly introducing material property changes and microscopic defects. In this work, we specifically investigate the process of argon milling, a potentially coherence-limiting step, using niobium and aluminum superconducting resonators as a proxy for the surface-limited behavior of qubits. We find that niobium microwave resonators exhibit an order of magnitude decrease in quality factors after surface argon milling, while aluminum resonators are resilient to the same process. Extensive analysis of the niobium surface shows no change in the suboxide composition due to argon milling, while two-tone spectroscopy measurements reveal an increase in two-level system electrical dipole moments, indicating a structurally altered niobium oxide. However, a short dry etch can fully recover the argon-milling-induced losses on niobium, offering a potential route towards state-of-the-art overlap Josephson junction qubits with niobium circuitry.

A high-efficiency spin-photon interface is an essential piece of quantum hardware necessary for various quantum technologies. Self-assembled ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{As}$ quantum dots have excellent optical properties: if embedded in an optical microcavity, they can show near-deterministic spin-photon entanglement and spin readout. To address the individual spin states, an external magnetic field is required, and a superconducting magnet is usually used. Here we show a compact cryogenically compatible $\mathrm{Sm}$-$\mathrm{Co}$ magnet design that delivers a 475-mT in-plane (Voigt-geometry) magnetic field at $5\mathrm{K}$, which is suitable for lifting the energy degeneracy of the electron spin states and trion transitions of a single ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{As}$ quantum dot. This quantum dot is embedded in a birefringent high-finesse optical microcavity that enables efficient collection of single photons emitted by the quantum dot. We demonstrate spin-state manipulation by addressing the trion transitions with a single laser field and two laser fields. The experimental data agree well with our model, which covers single- and two-laser cross-polarized resonance fluorescence, Purcell enhancement in a birefringent cavity, and variation of the laser powers.

Greenberger-Horne-Zeilinger and $W$ states feature genuine tripartite entanglement that cannot be converted into each other by local operations and classical communication. Here, we present a dissipative protocol for deterministic interconversion between Greenberger-Horne-Zeilinger and $W$ states of three neutral ${}^{87}\mathrm{Rb}$ atoms arranged in an equilateral triangle of a two-dimensional array. With three atomic levels and diagonal van der Waals interactions of Rydberg atoms, the interconversion between tripartite entangled states can be efficiently accomplished in the Floquet-Lindblad framework through the periodic optical pump and dissipation engineering. We evaluate the feasibility of the existing methodology using the experimental parameters accessible to current neutral-atom platforms. We find that our scheme is robust against typical noises, such as laser phase noise and geometric imperfections of the atom array. In addition, our scheme can integrate the Gaussian soft quantum control technique, which further reduces the overall conversion time and increases the resilience to timing errors and interatomic distance fluctuations. The high-fidelity and robust tripartite entanglement interconversion protocol provides a route to save physical resources and enhance the computational efficiency of quantum networks formed by neutral-atom arrays.

Doubly parametric quantum transducers (DPTs), such as electro-optomechanical devices, show promise as quantum interconnects between the optical and microwave domains, thereby enabling long-distance quantum networks between superconducting qubit systems. However, any transducer will inevitably introduce loss and noise that will degrade the performance of a quantum network. We explore how DPTs can be used to construct a network capable of distributing remote two-mode microwave entanglement over an optical link by comparing 14 different network topologies. The 14 topologies we analyze consist of combinations of different transducer operations, entangled resources, and entanglement-swapping measurements. For each topology, we derive a necessary and sufficient analytic threshold on DPT parameters that must be exceeded in order to distribute microwave-microwave entanglement. We find that the thresholds are dependent on the given network topology, along with the available entanglement resources and measurement capabilities. In the high-optical-loss limit, which is relevant to realistic networks, we find that down-conversion of each half of an optical two-mode squeezed vacuum state is the most robust topology. Finally, using currently achievable experimental capabilities, we find the encouraging result that several of these topologies could produce microwave-microwave entanglement. However, most of these topologies cannot work given current transducer performance, which demonstrates the importance of thoroughly analyzing all possible networks.

Simulating quantum many-body systems is a highly demanding task since the required resources grow exponentially with the dimension of the system. In the case of fermionic systems, this is even harder since nonlocal interactions emerge due to the antisymmetric character of the fermionic wave function. Here, we introduce a digital-analog quantum algorithm to simulate a wide class of fermionic Hamiltonians including the paradigmatic one-dimensional Fermi-Hubbard model. These digital-analog methods allow quantum algorithms to run beyond digital versions via an efficient use of coherence time. Furthermore, we exemplify our techniques with a low-connected architecture for realistic digital-analog implementations of specific fermionic models.

We measure electron- and nuclear-spin transition frequencies in the ground state of nitrogen-vacancy (N-$V$) centers in diamond for two nitrogen isotopes (${}^{14}\mathrm{N}$-$V$ and ${}^{15}\mathrm{N}$-$V$) over temperatures ranging from 77 to 400 K. Measurements are performed using Ramsey interferometry and direct optical readout of the nuclear and electron spins. We extract coupling parameters $Q$ (for ${}^{14}\mathrm{N}$-$V$), $D$, $A_{\parallel }$, $A_{\perp }$, and ${\gamma }_e/{\gamma }_n$, and their temperature dependences for both isotopes. The temperature dependences of the nuclear-spin transitions within the $m_s=0$ spin manifold near room temperature are found to be 0.52(1) ppm/K for ${}^{14}\mathrm{N}$-$V$ ($|m_I=-1⟩\leftrightarrow |m_I=+1⟩$) and $-1.1(1)$ ppm/K for ${}^{15}\mathrm{N}$-$V$ ($|m_I=-1/2⟩\leftrightarrow |m_I=+1/2⟩$). An isotopic shift in the zero-field splitting parameter $D$ between ${}^{14}\mathrm{N}$-$V$ and ${}^{15}\mathrm{N}$-$V$ is measured to be $\sim 120$ kHz. Residual transverse magnetic fields are observed to shift the nuclear-spin transition frequencies, especially for ${}^{15}\mathrm{N}$-$V$. We have precisely determined the set of parameters relevant for the development of nuclear-spin-based diamond quantum sensors with greatly reduced sensitivity to environmental factors.

To successfully execute large-scale algorithms, a quantum computer will need to perform its elementary operations near perfectly. This is a fundamental challenge since all physical qubits suffer a considerable level of noise. Moreover, real systems are likely to have a finite yield, i.e., some nonzero proportion of the components in a complex device may be irredeemably broken at the fabrication stage. We present a threshold theorem showing that an arbitrarily large quantum computation can be completed with a vanishing probability of failure using a two-dimensional array of noisy qubits with a finite density of fabrication defects. To complete our proof we introduce a robust protocol to measure high-weight stabilizers to compensate for large regions of inactive qubits. We obtain our result using a surface-code architecture. Our approach is therefore readily compatible with ongoing experimental efforts to build a large-scale quantum computer.

Quantum simulation on noisy intermediate-scale quantum devices is severely limited by short qubit coherence times. A variational pulse-shaping algorithm known as ctrl-VQE was recently proposed to address this issue by eliminating the need for parameterized quantum circuits, which lead to long state preparation times. Here, we find the fastest possible pulses that prepare target molecular wave functions for a given device Hamiltonian describing coupled transmon qubits using simulations. We find that the resulting time-optimal pulses develop a bang-bang form consistent with Pontryagin’s maximum principle. We further investigate how the minimal state preparation time is impacted by the number of energy levels active in the transmon simulations. We find that leakage outside the computational subspace (something that is usually considered problematic) speeds up the state preparation, further reducing device coherence-time demands. Our analysis reveals that this speedup is due to both an enlarged solution space of target wave functions and the appearance of additional channels connecting initial and target states.

We propose a quantum charging scheme fueled by measurements on ancillary qubits serving as disposable chargers. A stream of identical qubits are sequentially coupled to a quantum battery of $N+1$ levels and measured by projective operations after joint unitary evolutions of optimized intervals. If charger qubits are prepared in excited state and measured on ground state, then their excitations (energy) can be near perfectly transferred to battery by iteratively updating the optimized measurement intervals. Starting from its ground state, the battery could be constantly charged to an even higher energy level. Starting from a thermal state, the battery could also achieve a near-unit ratio of ergotropy and energy through less than $N$ measurements, when a population inversion is realized by measurements. If charger qubits are prepared in ground state and measured on excited state, useful work extracted by measurements alone could transform the battery from a thermal state to a high-ergotropy state before the success probability vanishes. Our operations in charging are more efficient than those without measurements and do not invoke the initial coherence in both battery and chargers. Particularly, our finding features quantum measurement in shaping nonequilibrium systems.

Predictive design and optimization methods for controlled quantum systems depend on the accuracy of the system model. Any distortion of the input fields in an experimental platform alters the model accuracy and eventually disturbs the predicted dynamics. These distortions can be nonlinear with a strong frequency dependence so that the field interacting with the microscopic quantum system has limited resemblance to the input signal. We present an effective method for estimating these distortions, which is suitable for nonlinear transfer functions of arbitrary lengths and magnitudes provided the available training data has enough spectral components. Using a quadratic estimation, we have successfully tested our approach for a numerical example of a single Rydberg atom system. The transfer function estimated from the presented method is incorporated into an open-loop control optimization algorithm allowing for high-fidelity operations in quantum experiments.

Fluctuations of the qubit frequencies are one of the major problems to overcome on the way to scalable quantum computers. Of particular importance are fluctuations with a correlation time that exceeds the decoherence time due to decay and dephasing by fast processes. The statistics of the fluctuations can be characterized by measuring the correlators of the outcomes of periodically repeated Ramsey measurements. We describe qubit dynamics during repeated measurements and evaluate the two-time correlator for noise from two-level systems and two- and three-time correlators for Gaussian noise. The explicit expressions for the correlators are compared with simulations. We find that, even though the three-time correlators for noise from two-level systems and for Gaussian noise are generally significantly different, already ten two-level systems can mimic Gaussian noise provided they are symmetric. This is not the case for asymmetric systems.