Dynamics of magnetization at infinite temperature in a Heisenberg spin chain

Understanding universal aspects of quantum dynamics is an unresolved problem in statistical mechanics. In particular, the spin dynamics of the one-dimensional Heisenberg model were conjectured as to belong to the Kardar-Parisi-Zhang (KPZ) universality class based on the scaling of the infinite-temperature spin-spin correlation function. In a chain of 46 superconducting qubits, we studied the probability distribution of the magnetization transferred across the chain's center, [Formula: see text]. The first two moments of [Formula: see text] show superdiffusive behavior, a hallmark of KPZ universality. However, the third and fourth moments ruled out the KPZ conjecture and allow for evaluating other theories. Our results highlight the importance of studying higher moments in determining dynamic universality classes and provide insights into universal behavior in quantum systems.

Stable quantum-correlated many-body states through engineered dissipation

Engineered dissipative reservoirs have the potential to steer many-body quantum systems toward correlated steady states useful for quantum simulation of high-temperature superconductivity or quantum magnetism. Using up to 49 superconducting qubits, we prepared low-energy states of the transverse-field Ising model through coupling to dissipative auxiliary qubits. In one dimension, we observed long-range quantum correlations and a ground-state fidelity of 0.86 for 18 qubits at the critical point. In two dimensions, we found mutual information that extends beyond nearest neighbors. Lastly, by coupling the system to auxiliaries emulating reservoirs with different chemical potentials, we explored transport in the quantum Heisenberg model. Our results establish engineered dissipation as a scalable alternative to unitary evolution for preparing entangled many-body states on noisy quantum processors.

Optimizing quantum gates towards the scale of logical qubits

A foundational assumption of quantum error correction theory is that quantum gates can be scaled to large processors without exceeding the error-threshold for fault tolerance. Two major challenges that could become fundamental roadblocks are manufacturing high-performance quantum hardware and engineering a control system that can reach its performance limits. The control challenge of scaling quantum gates from small to large processors without degrading performance often maps to non-convex, high-constraint, and time-dynamic control optimization over an exponentially expanding configuration space. Here we report on a control optimization strategy that can scalably overcome the complexity of such problems. We demonstrate it by choreographing the frequency trajectories of 68 frequency-tunable superconducting qubits to execute single- and two-qubit gates while mitigating computational errors. When combined with a comprehensive model of physical errors across our processor, the strategy suppresses physical error rates by ~3.7× compared with the case of no optimization. Furthermore, it is projected to achieve a similar performance advantage on a distance-23 surface code logical qubit with 1057 physical qubits. Our control optimization strategy solves a generic scaling challenge in a way that can be adapted to a variety of quantum operations, algorithms, and computing architectures.

Measurement-induced entanglement and teleportation on a noisy quantum processor

Measurement has a special role in quantum theory1: by collapsing the wavefunction, it can enable phenomena such as teleportation2 and thereby alter the 'arrow of time' that constrains unitary evolution. When integrated in many-body dynamics, measurements can lead to emergent patterns of quantum information in space-time3-10 that go beyond the established paradigms for characterizing phases, either in or out of equilibrium11-13. For present-day noisy intermediate-scale quantum (NISQ) processors14, the experimental realization of such physics can be problematic because of hardware limitations and the stochastic nature of quantum measurement. Here we address these experimental challenges and study measurement-induced quantum information phases on up to 70 superconducting qubits. By leveraging the interchangeability of space and time, we use a duality mapping9,15-17 to avoid mid-circuit measurement and access different manifestations of the underlying phases, from entanglement scaling3,4 to measurement-induced teleportation18. We obtain finite-sized signatures of a phase transition with a decoding protocol that correlates the experimental measurement with classical simulation data. The phases display remarkably different sensitivity to noise, and we use this disparity to turn an inherent hardware limitation into a useful diagnostic. Our work demonstrates an approach to realizing measurement-induced physics at scales that are at the limits of current NISQ processors.

Non-Abelian braiding of graph vertices in a superconducting processor

Indistinguishability of particles is a fundamental principle of quantum mechanics¹. For all elementary and quasiparticles observed to date-including fermions, bosons and Abelian anyons-this principle guarantees that the braiding of identical particles leaves the system unchanged^2,3. However, in two spatial dimensions, an intriguing possibility exists: braiding of non-Abelian anyons causes rotations in a space of topologically degenerate wavefunctions^4-8. Hence, it can change the observables of the system without violating the principle of indistinguishability. Despite the well-developed mathematical description of non-Abelian anyons and numerous theoretical proposals^9-22, the experimental observation of their exchange statistics has remained elusive for decades. Controllable many-body quantum states generated on quantum processors offer another path for exploring these fundamental phenomena. Whereas efforts on conventional solid-state platforms typically involve Hamiltonian dynamics of quasiparticles, superconducting quantum processors allow for directly manipulating the many-body wavefunction by means of unitary gates. Building on predictions that stabilizer codes can host projective non-Abelian Ising anyons^9,10, we implement a generalized stabilizer code and unitary protocol²³ to create and braid them. This allows us to experimentally verify the fusion rules of the anyons and braid them to realize their statistics. We then study the prospect of using the anyons for quantum computation and use braiding to create an entangled state of anyons encoding three logical qubits. Our work provides new insights about non-Abelian braiding and, through the future inclusion of error correction to achieve topological protection, could open a path towards fault-tolerant quantum computing.

Formation of robust bound states of interacting microwave photons

Systems of correlated particles appear in many fields of modern science and represent some of the most intractable computational problems in nature. The computational challenge in these systems arises when interactions become comparable to other energy scales, which makes the state of each particle depend on all other particles1. The lack of general solutions for the three-body problem and acceptable theory for strongly correlated electrons shows that our understanding of correlated systems fades when the particle number or the interaction strength increases. One of the hallmarks of interacting systems is the formation of multiparticle bound states2-9. Here we develop a high-fidelity parameterizable fSim gate and implement the periodic quantum circuit of the spin-½ XXZ model in a ring of 24 superconducting qubits. We study the propagation of these excitations and observe their bound nature for up to five photons. We devise a phase-sensitive method for constructing the few-body spectrum of the bound states and extract their pseudo-charge by introducing a synthetic flux. By introducing interactions between the ring and additional qubits, we observe an unexpected resilience of the bound states to integrability breaking. This finding goes against the idea that bound states in non-integrable systems are unstable when their energies overlap with the continuum spectrum. Our work provides experimental evidence for bound states of interacting photons and discovers their stability beyond the integrability limit.

Noise-resilient edge modes on a chain of superconducting qubits

Inherent symmetry of a quantum system may protect its otherwise fragile states. Leveraging such protection requires testing its robustness against uncontrolled environmental interactions. Using 47 superconducting qubits, we implement the one-dimensional kicked Ising model, which exhibits nonlocal Majorana edge modes (MEMs) with ℤ 2 parity symmetry. We find that any multiqubit Pauli operator overlapping with the MEMs exhibits a uniform late-time decay rate comparable to single-qubit relaxation rates, irrespective of its size or composition. This characteristic allows us to accurately reconstruct the exponentially localized spatial profiles of the MEMs. Furthermore, the MEMs are found to be resilient against certain symmetry-breaking noise owing to a prethermalization mechanism. Our work elucidates the complex interplay between noise and symmetry-protected edge modes in a solid-state environment.

Small-world complex network generation on a digital quantum processor

Quantum cellular automata (QCA) evolve qubits in a quantum circuit depending only on the states of their neighborhoods and model how rich physical complexity can emerge from a simple set of underlying dynamical rules. The inability of classical computers to simulate large quantum systems hinders the elucidation of quantum cellular automata, but quantum computers offer an ideal simulation platform. Here, we experimentally realize QCA on a digital quantum processor, simulating a one-dimensional Goldilocks rule on chains of up to 23 superconducting qubits. We calculate calibrated and error-mitigated population dynamics and complex network measures, which indicate the formation of small-world mutual information networks. These networks decohere at fixed circuit depth independent of system size, the largest of which corresponding to 1,056 two-qubit gates. Such computations may enable the employment of QCA in applications like the simulation of strongly-correlated matter or beyond-classical computational demonstrations.

Realizing topologically ordered states on a quantum processor

[Figure: see text].

Time-Crystalline Eigenstate Order on a Quantum Processor

Quantum many-body systems display rich phase structure in their low-temperature equilibrium states¹. However, much of nature is not in thermal equilibrium. Remarkably, it was recently predicted that out-of-equilibrium systems can exhibit novel dynamical phases^2–8 that may otherwise be forbidden by equilibrium thermodynamics, a paradigmatic example being the discrete time crystal (DTC)^7,9–15. Concretely, dynamical phases can be defined in periodically driven many-body-localized (MBL) systems via the concept of eigenstate order^7,16,17. In eigenstate-ordered MBL phases, the entire many-body spectrum exhibits quantum correlations and long-range order, with characteristic signatures in late-time dynamics from all initial states. It is, however, challenging to experimentally distinguish such stable phases from transient phenomena, or from regimes in which the dynamics of a few select states can mask typical behaviour. Here we implement tunable controlled-phase (CPHASE) gates on an array of superconducting qubits to experimentally observe an MBL-DTC and demonstrate its characteristic spatiotemporal response for generic initial states^7,9,10. Our work employs a time-reversal protocol to quantify the impact of external decoherence, and leverages quantum typicality to circumvent the exponential cost of densely sampling the eigenspectrum. Furthermore, we locate the phase transition out of the DTC with an experimental finite-size analysis. These results establish a scalable approach to studying non-equilibrium phases of matter on quantum processors.

A study establishes a scalable approach to engineer and characterize a many-body-localized discrete time crystal phase on a superconducting quantum processor.

Information scrambling in quantum circuits

[Figure: see text].

Removing leakage-induced correlated errors in superconducting quantum error correction

Quantum computing can become scalable through error correction, but logical error rates only decrease with system size when physical errors are sufficiently uncorrelated. During computation, unused high energy levels of the qubits can become excited, creating leakage states that are long-lived and mobile. Particularly for superconducting transmon qubits, this leakage opens a path to errors that are correlated in space and time. Here, we report a reset protocol that returns a qubit to the ground state from all relevant higher level states. We test its performance with the bit-flip stabilizer code, a simplified version of the surface code for quantum error correction. We investigate the accumulation and dynamics of leakage during error correction. Using this protocol, we find lower rates of logical errors and an improved scaling and stability of error suppression with increasing qubit number. This demonstration provides a key step on the path towards scalable quantum computing.

Demonstrating a Continuous Set of Two-Qubit Gates for Near-Term Quantum Algorithms

Quantum algorithms offer a dramatic speedup for computational problems in material science and chemistry. However, any near-term realizations of these algorithms will need to be optimized to fit within the finite resources offered by existing noisy hardware. Here, taking advantage of the adjustable coupling of gmon qubits, we demonstrate a continuous two-qubit gate set that can provide a threefold reduction in circuit depth as compared to a standard decomposition. We implement two gate families: an imaginary swap-like (iSWAP-like) gate to attain an arbitrary swap angle, θ, and a controlled-phase gate that generates an arbitrary conditional phase, ϕ. Using one of each of these gates, we can perform an arbitrary two-qubit gate within the excitation-preserving subspace allowing for a complete implementation of the so-called Fermionic simulation (fSim) gate set. We benchmark the fidelity of the iSWAP-like and controlled-phase gate families as well as 525 other fSim gates spread evenly across the entire fSim(θ,ϕ) parameter space, achieving a purity-limited average two-qubit Pauli error of 3.8×10^{-3} per fSim gate.

Hartree-Fock on a superconducting qubit quantum computer

The simulation of fermionic systems is among the most anticipated applications of quantum computing. We performed several quantum simulations of chemistry with up to one dozen qubits, including modeling the isomerization mechanism of diazene. We also demonstrated error-mitigation strategies based on N-representability that dramatically improve the effective fidelity of our experiments. Our parameterized ansatz circuits realized the Givens rotation approach to noninteracting fermion evolution, which we variationally optimized to prepare the Hartree-Fock wave function. This ubiquitous algorithmic primitive is classically tractable to simulate yet still generates highly entangled states over the computational basis, which allowed us to assess the performance of our hardware and establish a foundation for scaling up correlated quantum chemistry simulations.

Use of Inductive, Problem-Based Clinical Reasoning Enhances Diagnostic Accuracy in Final-Year Veterinary Students

Despite tremendous progression in the medical field, levels of diagnostic error remain unacceptably high. Cognitive failures in clinical reasoning are believed to be the major contributor to diagnostic error. There is evidence in the literature that teaching problem-based, inductive reasoning has the potential to improve clinical reasoning skills. In this study, 47 final-year veterinary medicine students at the Royal Veterinary College (RVC) were presented with a complex small animal medicine case. The participants were divided into two groups, one of which received a prioritized problem list in addition to the history, physical exam, and diagnostic test results provided to both groups. The students' written approaches to the case were then analyzed and assigned a diagnostic accuracy score (DAS) and an inductive reasoning score (IRS). The IRS was based on a series of predetermined characteristics consistent with the inductive reasoning framework taught at the RVC. No significant difference was found between the DAS scores of each group, indicating that the provision of a prioritized problem list did not impact diagnostic accuracy. However, a significant positive correlation between the IRS and DAS was illustrated for both groups of students, suggesting increased use of inductive reasoning is associated with increased diagnostic accuracy. These results contribute to a body of research proposing that inductive, problem-based reasoning teaching delivered in an additive model, can enhance the clinical reasoning skills of students and reduce diagnostic error.

Diabatic Gates for Frequency-Tunable Superconducting Qubits

We demonstrate diabatic two-qubit gates with Pauli error rates down to 4.3(2)×10^{-3} in as fast as 18 ns using frequency-tunable superconducting qubits. This is achieved by synchronizing the entangling parameters with minima in the leakage channel. The synchronization shows a landscape in gate parameter space that agrees with model predictions and facilitates robust tune-up. We test both iswap-like and cphase gates with cross-entropy benchmarking. The presented approach can be extended to multibody operations as well.

Fluctuations of Energy-Relaxation Times in Superconducting Qubits

Superconducting qubits are an attractive platform for quantum computing since they have demonstrated high-fidelity quantum gates and extensibility to modest system sizes. Nonetheless, an outstanding challenge is stabilizing their energy-relaxation times, which can fluctuate unpredictably in frequency and time. Here, we use qubits as spectral and temporal probes of individual two-level-system defects to provide direct evidence that they are responsible for the largest fluctuations. This research lays the foundation for stabilizing qubit performance through calibration, design, and fabrication.

Spectroscopic signatures of localization with interacting photons in superconducting qubits

Quantized eigenenergies and their associated wave functions provide extensive information for predicting the physics of quantum many-body systems. Using a chain of nine superconducting qubits, we implement a technique for resolving the energy levels of interacting photons. We benchmark this method by capturing the main features of the intricate energy spectrum predicted for two-dimensional electrons in a magnetic field-the Hofstadter butterfly. We introduce disorder to study the statistics of the energy levels of the system as it undergoes the transition from a thermalized to a localized phase. Our work introduces a many-body spectroscopy technique to study quantum phases of matter.

Evaluation of the optimal standardized ileal digestible tryptophan:lysine ratio in lactating sow diets

Three hundred and fifteen primiparous and multiparous sows were evaluated in a study to determine the effect of standardized ileal digestible (SID) Trp:Lys ratio in lactating sow diets. Camborough sows (PIC USA, Hendersonville, TN) ranging from first parity to eighth parity were blocked by parity and randomly allotted to 1 of 4 experimental diets containing different levels of added L-Trp (0.006, 0.026, 0.045, and 0.064%, respectively) while soybean meal, 30% corn dried distiller's grain with solubles (DDGS), and L-Lys levels were held constant. The SID Lys level for the rations was 0.95% so that the SID Trp:Lys ratios were formulated to be 14, 16, 18, and 20%, respectively. All diets were formulated to have 3.2 Mcal ME/kg and to contain vitamins and minerals that exceeded NRC (1998) recommendations. Sows were fed twice a day with a Howema computerized feed system and were allowed a maximum intake (5.9 kg/d). Average daily feed intake had a tendency to be quadratically improved when the SID ratio was increased (5.11, 5.28, 5.24, 5.21 kg/d, P = 0.09). In addition, sow wean to estrus (6.71, 5.53, 5.58, 6.33, P < 0.02) was quadratically improved as SID Trp:Lys ratio increased. Percent of sows bred by 10 d (84.39, 90.82, 90.28, 90.61) was not linearly (P = 0.25) or quadratically (P = 0.40) improved. There was no difference in litter gain (2.44, 2.52, 2.60, 2.57 kg/d, P = 0.16). Based on a broken-line quadratic model, when sows are fed 30% DDGS, the SID Trp:Lys ratio of 17.6 is required for optimal sow average daily feed intake and 17.2 for wean to estrus interval.

Observation of Classical-Quantum Crossover of 1/f Flux Noise and Its Paramagnetic Temperature Dependence

By analyzing the dissipative dynamics of a tunable gap flux qubit, we extract both sides of its two-sided environmental flux noise spectral density over a range of frequencies around 2k_{B}T/h≈1  GHz, allowing for the observation of a classical-quantum crossover. Below the crossover point, the symmetric noise component follows a 1/f power law that matches the magnitude of the 1/f noise near 1 Hz. The antisymmetric component displays a 1/T dependence below 100 mK, providing dynamical evidence for a paramagnetic environment. Extrapolating the two-sided spectrum predicts the linewidth and reorganization energy of incoherent resonant tunneling between flux qubit wells.

Leaching losses from Kenyan maize cropland receiving different rates of nitrogen fertilizer

Meeting food security requirements in sub-Saharan Africa (SSA) will require increasing fertilizer use to improve crop yields, however excess fertilization can cause environmental and public health problems in surface and groundwater. Determining the threshold of reasonable fertilizer application in SSA requires an understanding of flow dynamics and nutrient transport in under-studied, tropical soils experiencing seasonal rainfall. We estimated leaching flux in Yala, Kenya on a maize field that received from 0 to 200 kg ha^-1 of nitrogen (N) fertilizer. Soil pore water concentration measurements during two growing seasons were coupled with results from a numerical fluid flow model to calculate the daily flux of nitrate-nitrogen (NO₃ ^--N). Modeled NO₃ ^--N losses to below 200 cm for 1 year ranged from 40 kg N ha^-1 year^-1 in the 75 kg N ha^-1 year^-1 treatment to 81 kg N ha^-1 year^-1 in the 200 kg N ha^-1 treatment. The highest soil pore water NO₃ ^--N concentrations and NO₃ ^--N leaching fluxes occurred on the highest N application plots, however there was a poor correlation between N application rate and NO₃ ^--N leaching for the remaining N application rates. The drought in the second study year resulted in higher pore water NO₃ ^--N concentrations, while NO₃ ^--N leaching was disproportionately smaller than the decrease in precipitation. The lack of a strong correlation between NO₃ ^--N leaching and N application rate, and a large decrease in flux between 120 and 200 cm suggest processes that influence NO₃ ^--N retention in soils below 200 cm will ultimately control NO₃ ^--N leaching at the watershed scale.-the daily flux of nitrate-nitrogen (NO₃ ^--N). The lack of a strong correlation between NO₃ ^--N leaching and N application rate, and a large decrease in flux between 120 and 200 cm suggest processes that influence NO₃ ^--N retention in soils below 200 cm will ultimately control NO₃ ^--N leaching at the watershed scale.

Measurement-Induced State Transitions in a Superconducting Qubit: Beyond the Rotating Wave Approximation

Many superconducting qubit systems use the dispersive interaction between the qubit and a coupled harmonic resonator to perform quantum state measurement. Previous works have found that such measurements can induce state transitions in the qubit if the number of photons in the resonator is too high. We investigate these transitions and find that they can push the qubit out of the two-level subspace, and that they show resonant behavior as a function of photon number. We develop a theory for these observations based on level crossings within the Jaynes-Cummings ladder, with transitions mediated by terms in the Hamiltonian that are typically ignored by the rotating wave approximation. We find that the most important of these terms comes from an unexpected broken symmetry in the qubit potential. We confirm the theory by measuring the photon occupation of the resonator when transitions occur while varying the detuning between the qubit and resonator.

Measuring and Suppressing Quantum State Leakage in a Superconducting Qubit

Leakage errors occur when a quantum system leaves the two-level qubit subspace. Reducing these errors is critically important for quantum error correction to be viable. To quantify leakage errors, we use randomized benchmarking in conjunction with measurement of the leakage population. We characterize single qubit gates in a superconducting qubit, and by refining our use of derivative reduction by adiabatic gate pulse shaping along with detuning of the pulses, we obtain gate errors consistently below 10^{-3} and leakage rates at the 10^{-5} level. With the control optimized, we find that a significant portion of the remaining leakage is due to incoherent heating of the qubit.

The feeding of dried distillers' grains with solubles to lactating sows

Three experiments were conducted to evaluate the feeding of dried distillers' grains with solubles (DDGS) in sow lactation diets. In Exp. 1, 168 multiparous sows (PIC, Camborough 22) were fed a 10% DDGS diet throughout gestation. Sows were randomly allotted to 1 of 4 corn-soybean meal lactation diets formulated to contain different levels of DDGS (0, 10, 20, and 30%, respectively). All diets were formulated to be isocaloric (3.46 Mcal ME/kg) and all other nutrients exceeded NRC (1998) nutrient recommendations. Sow ADFI was not different ( > 0.10) as DDGS level increased. Increasing DDGS resulted in a linear ( < 0.03) increase in sow weight gain (7.5, 11.3, 20.3, and 17.2 kg, respectively) and a reduction in wean-to-first-service interval (7.1, 5.2, 5.0, and 4.9 d, respectively). Increasing DDGS did not affect subsequent total born per litter (13.7, 12.8, 13.3, and 12.3, respectively; > 0.10). In Exp. 2 and 3, lactation diets consisted of corn and 20, 30, 40, or 50% DDGS. Diets were formulated at 3.25 Mcal ME/kg, 1.05% standardized ileal digestible lysine, and all other nutrients to exceed NRC (1998) nutrient recommendations. In both experiments, sows (PIC, Camborough) were fed 40% DDGS in gestation and allocated to a randomized complete block based on the parity of the sow at the time of entry into the farrowing house. In Exp. 2, 256 gilts and multiparous sows were fed the randomly assigned diets. As DDGS inclusion increased, sow feed intake (6.2, 6.2, 6.0, and 5.9 kg/d, respectively) and sow weight gain (10.5, 10.3, 8.2, and 6.2 kg, respectively) tended to linearly decrease ( < 0.06). Sow wean to estrus differed between 20 and 30% DDGS inclusion (4.9 vs. 6.9 d; < 0.01). Litter gain was not different (2.55, 2.53, 2.51, and 2.59 kg/d, respectively; > 0.10) as DDGS inclusion increased. In Exp. 3, 98 multiparous sows were randomly allotted to 1 of the 4 experimental diets during the summer months. Sow feed intake, sow weight gain, and litter gain were not different ( > 0.10) between treatments. The data suggest that feeding high levels of DDGS of 40 to 50% may reduce sow feed intake and litter performance. These results demonstrate that feeding up to 30% DDGS in lactation diets can be done without adversely influencing sow or litter performance.

Digital quantum simulation of fermionic models with a superconducting circuit

One of the key applications of quantum information is simulating nature. Fermions are ubiquitous in nature, appearing in condensed matter systems, chemistry and high energy physics. However, universally simulating their interactions is arguably one of the largest challenges, because of the difficulties arising from anticommutativity. Here we use digital methods to construct the required arbitrary interactions, and perform quantum simulation of up to four fermionic modes with a superconducting quantum circuit. We employ in excess of 300 quantum logic gates, and reach fidelities that are consistent with a simple model of uncorrelated errors. The presented approach is in principle scalable to a larger number of modes, and arbitrary spatial dimensions.

State preservation by repetitive error detection in a superconducting quantum circuit

Quantum computing becomes viable when a quantum state can be protected from environment-induced error. If quantum bits (qubits) are sufficiently reliable, errors are sparse and quantum error correction (QEC) is capable of identifying and correcting them. Adding more qubits improves the preservation of states by guaranteeing that increasingly larger clusters of errors will not cause logical failure-a key requirement for large-scale systems. Using QEC to extend the qubit lifetime remains one of the outstanding experimental challenges in quantum computing. Here we report the protection of classical states from environmental bit-flip errors and demonstrate the suppression of these errors with increasing system size. We use a linear array of nine qubits, which is a natural step towards the two-dimensional surface code QEC scheme, and track errors as they occur by repeatedly performing projective quantum non-demolition parity measurements. Relative to a single physical qubit, we reduce the failure rate in retrieving an input state by a factor of 2.7 when using five of our nine qubits and by a factor of 8.5 when using all nine qubits after eight cycles. Additionally, we tomographically verify preservation of the non-classical Greenberger-Horne-Zeilinger state. The successful suppression of environment-induced errors will motivate further research into the many challenges associated with building a large-scale superconducting quantum computer.

Qubit Architecture with High Coherence and Fast Tunable Coupling

We introduce a superconducting qubit architecture that combines high-coherence qubits and tunable qubit-qubit coupling. With the ability to set the coupling to zero, we demonstrate that this architecture is protected from the frequency crowding problems that arise from fixed coupling. More importantly, the coupling can be tuned dynamically with nanosecond resolution, making this architecture a versatile platform with applications ranging from quantum logic gates to quantum simulation. We illustrate the advantages of dynamical coupling by implementing a novel adiabatic controlled-z gate, with a speed approaching that of single-qubit gates. Integrating coherence and scalable control, the introduced qubit architecture provides a promising path towards large-scale quantum computation and simulation.

Optimal quantum control using randomized benchmarking

We present a method for optimizing quantum control in experimental systems, using a subset of randomized benchmarking measurements to rapidly infer error. This is demonstrated to improve single- and two-qubit gates, minimize gate bleedthrough, where a gate mechanism can cause errors on subsequent gates, and identify control crosstalk in superconducting qubits. This method is able to correct parameters so that control errors no longer dominate and is suitable for automated and closed-loop optimization of experimental systems.

Fast accurate state measurement with superconducting qubits

Faster and more accurate state measurement is required for progress in superconducting qubit experiments with greater numbers of qubits and advanced techniques such as feedback. We have designed a multiplexed measurement system with a bandpass filter that allows fast measurement without increasing environmental damping of the qubits. We use this to demonstrate simultaneous measurement of four qubits on a single superconducting integrated circuit, the fastest of which can be measured to 99.8% accuracy in 140 ns. This accuracy and speed is suitable for advanced multiqubit experiments including surface-code error correction.

Fast accurate state measurement with superconducting qubits

Faster and more accurate state measurement is required for progress in superconducting qubit experiments with greater numbers of qubits and advanced techniques such as feedback. We have designed a multiplexed measurement system with a bandpass filter that allows fast measurement without increasing environmental damping of the qubits. We use this to demonstrate simultaneous measurement of four qubits on a single superconducting integrated circuit, the fastest of which can be measured to 99.8% accuracy in 140 ns. This accuracy and speed is suitable for advanced multiqubit experiments including surface-code error correction.

Coherent Josephson qubit suitable for scalable quantum integrated circuits

We demonstrate a planar, tunable superconducting qubit with energy relaxation times up to 44 μs. This is achieved by using a geometry designed to both minimize radiative loss and reduce coupling to materials-related defects. At these levels of coherence, we find a fine structure in the qubit energy lifetime as a function of frequency, indicating the presence of a sparse population of incoherent, weakly coupled two-level defects. We elucidate this defect physics by experimentally varying the geometry and by a model analysis. Our "Xmon" qubit combines facile fabrication, straightforward connectivity, fast control, and long coherence, opening a viable route to constructing a chip-based quantum computer.

Excitation of superconducting qubits from hot nonequilibrium quasiparticles

Superconducting qubits probe environmental defects such as nonequilibrium quasiparticles, an important source of decoherence. We show that "hot" nonequilibrium quasiparticles, with energies above the superconducting gap, affect qubits differently from quasiparticles at the gap, implying qubits can probe the dynamic quasiparticle energy distribution. For hot quasiparticles, we predict a non-negligible increase in the qubit excited state probability Pe. By injecting hot quasiparticles into a qubit, we experimentally measure an increase of Pe in semiquantitative agreement with the model and rule out the typically assumed thermal distribution.

Catch and release of microwave photon states

We demonstrate a superconducting resonator with variable coupling to a measurement transmission line. The resonator coupling can be adjusted through zero to a photon emission rate 1000 times the intrinsic resonator decay rate. We demonstrate the catch and release of photons in the resonator, as well as control of nonclassical Fock states. We also demonstrate the dynamical control of the release waveform of photons from the resonator, a key functionality that will enable high-fidelity quantum state transfer between distant resonators or qubits.

Carnitine deficiency presenting as familial cardiomyopathy: a treatable defect in carnitine transport

We studied a boy who presented at age 3 1/2 years with cardiomegaly, a distinctive electrocardiogram, and a history of a brother dying with cardiomyopathy. From age 3 1/2 to 5 years, cardiac disease progressed, resulting in intractable congestive heart failure. Skeletal muscle weakness developed and a muscle biopsy showed lipid myopathy. Muscle and plasma carnitine were reduced to 2 and 10% of the normal mean values, respectively. Therapy with L-carnitine (174 mg/kg/da) was begun at age 5 1/2 years and continued to the present (age 6 1/2 years). The cardiac disease has resolved and the muscle strength has returned to normal. Plasma carnitine concentrations have risen to the low-normal range, while urinary carnitine excretion has increased to values which are 30 times normal. The renal clearance of carnitine exceeds normal at all plasma concentrations and plasma carnitine values do not change acutely after an oral carnitine load. These results suggest that there is a distinct form of carnitine deficiency which presents as cardiomyopathy and may be successfully treated with L-carnitine. A defect in renal and possibly gastrointestinal transport of carnitine is a likely cause of this patient's disorder.

Permanent cardiac pacemakers in children: technical considerations

Placement of permanent cardiac pacemakers in children presents technical problems that are not encountered in the adult. Problems unique to pacemaker implantation in children are related to the patient's size, the relative bulkiness of pulse generators, the lack of subcutaneous tissue, and the child's growth and long life expectancy. Based on our experience with implantation of 27 permanent cardiac pacemakers in 13 children, we have found that the use of small pulse generators, placement of epicardial leads, insertion of properitoneal pulse generators, and use of recharabeable pacemakers are satisfactory methods in children.