Explore the vast range of titles available.

Real-time quantum feedback based on weak measurement of the quantum state is used to stabilize the oscillation phase of a driven quantum bit. Winding the quantum clock By performing weak measurements of a quantum state, it is possible to slow the rate of collapse of its wavefunction, so that information about the quantum state can be gradually acquired. Such information can be used to continuously track and steer the quantum state using feedback. This paper reports quantum feedback control of a superconducting quantum bit (qubit) coupled to a microwave cavity. The qubit undergoes coherent oscillations that can be made to speed up, slow down or persist indefinitely. This ability to actively suppress decoherence could find many applications in quantum error correction, quantum-state stabilization and purification, entanglement generation and adaptive measurements. The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single (probabilistic) outcome. The timescale of this ‘instantaneous’ process can be stretched using weak measurements 1 , 2 , such that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback 3 , 4 , 5 , 6 , 7 , 8 . Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity 9 . A weak measurement of the qubit is implemented by probing the cavity with microwave photons, maintaining its average occupation at less than one photon. These photons are then directed to a high-bandwidth, quantum-noise-limited amplifier 10 , 11 , which allows real-time monitoring of the state of the cavity (and, hence, that of the qubit) with high fidelity. We demonstrate quantum feedback control by inhibiting the decay of Rabi oscillations, allowing them to persist indefinitely 12 . Such an ability permits the active suppression of decoherence and enables a method of quantum error correction based on weak continuous measurements 13 , 14 . Other applications include quantum state stabilization 4 , 7 , 15 , entanglement generation using measurement 16 , state purification 17 and adaptive measurements 18 , 19 .

The von Neumann architecture for a classical computer comprises a central processing unit and a memory holding instructions and data. We demonstrate a quantum central processing unit that exchanges data with a quantum random-access memory integrated on a chip, with instructions stored on a classical computer. We test our quantum machine by executing codes that involve seven quantum elements: Two superconducting qubits coupled through a quantum bus, two quantum memories, and two zeroing registers. Two vital algorithms for quantum computing are demonstrated, the quantum Fourier transform, with 66% process fidelity, and the three-qubit Toffoli-class OR phase gate, with 98% phase fidelity. Our results, in combination especially with longer qubit coherence, illustrate a potentially viable approach to factoring numbers and implementing simple quantum error correction codes.

A universal set of logic gates in a superconducting quantum circuit is shown to have gate fidelities at the threshold for fault-tolerant quantum computing by the surface code approach, in which the quantum bits are distributed in an array of planar topology and have only nearest-neighbour couplings. Error-free quantum computing in prospect Quantum computers can only work in practice if, like conventional computers, they are fault-tolerant. This means that a system has to be in place to detect any errors and correct them. For quantum error correction such a system involves entangling several quantum bits (qubits) with each other. In the so-called surface code error-correction architecture, qubits are placed in a lattice and are entangled with four nearest neighbours. Rami Barends et al . report the construction of such a surface code system with five qubits in a row made from superconducting devices. This system performs with fidelity that is at the threshold for quantum error correction, suggesting that error-free quantum computing should be possible. The platform lends itself to scaling up to larger numbers of qubits and two-dimensional architecture. A quantum computer can solve hard problems, such as prime factoring 1 , 2 , database searching 3 , 4 and quantum simulation 5 , at the cost of needing to protect fragile quantum states from error. Quantum error correction 6 provides this protection by distributing a logical state among many physical quantum bits (qubits) by means of quantum entanglement. Superconductivity is a useful phenomenon in this regard, because it allows the construction of large quantum circuits and is compatible with microfabrication. For superconducting qubits, the surface code approach to quantum computing 7 is a natural choice for error correction, because it uses only nearest-neighbour coupling and rapidly cycled entangling gates. The gate fidelity requirements are modest: the per-step fidelity threshold is only about 99 per cent. Here we demonstrate a universal set of logic gates in a superconducting multi-qubit processor, achieving an average single-qubit gate fidelity of 99.92 per cent and a two-qubit gate fidelity of up to 99.4 per cent. This places Josephson quantum computing at the fault-tolerance threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearest-neighbour coupling. As a further demonstration, we construct a five-qubit Greenberger–Horne–Zeilinger state 8 , 9 using the complete circuit and full set of gates. The results demonstrate that Josephson quantum computing is a high-fidelity technology, with a clear path to scaling up to large-scale, fault-tolerant quantum circuits.

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. Correlated errors coming from leakage out of the computational subspace are an obstacle to fault-tolerant superconducting circuits. Here, the authors use a multi-level reset protocol to improve the performances of a bit-flip error correcting code by reducing the magnitude of correlations.

Temporal lobe epilepsy (TLE) is a common chronic neurological disease in humans. A number of studies have demonstrated differential expression of miRNAs in the hippocampus of humans with TLE and in animal models of experimental epilepsy. However, the dissimilarities in experimental design have led to largely discordant results across these studies. Thus, a comprehensive comparison is required in order to better characterize miRNA profiles obtained in various post-status epilepticus (SE) models. We therefore created a database and performed a meta-analysis of differentially expressed miRNAs across 3 post-SE models of epileptogenesis (electrical stimulation, pilocarpine and kainic acid) and human TLE with hippocampal sclerosis (TLE-HS). The database includes data from 11 animal post-SE studies and 3 human TLE-HS studies. A total of 378 differentially expressed miRNAs were collected (274 up-regulated and 198 down-regulated) and analyzed with respect to the post-SE model, time point and animal species. We applied the novel robust rank aggregation method to identify consistently differentially expressed miRNAs across the profiles. It highlighted common and unique miRNAs at different stages of epileptogenesis. The pathway analysis revealed involvement of these miRNAs in key pathogenic pathways underlying epileptogenesis, including inflammation, gliosis and deregulation of the extracellular matrix.

Various solutions of the problem of approximation of high-order fractional transfer functions are considered and a technique for solving the stability problem for such functions is shown. The existing solutions of the problem of approximating function p α are compared with those proposed on the basis of the least mean square error criterion. As an example, the results of filter simulation using the proposed solution are presented.

Measurement is one of the fundamental building blocks of quantum-information processing systems. Partial measurement, where full wavefunction collapse is not the only outcome, provides a detailed test of the measurement process. We introduce quantum-state tomography in a superconducting qubit that exhibits high-fidelity single-shot measurement. For the two probabilistic outcomes of partial measurement, we find either a full collapse or a coherent yet nonunitary evolution of the state. This latter behavior explicitly confirms modern quantum-measurement theory and may prove important for error-correction algorithms in quantum computation.

High speed flash analog-to-digital converters are used in modern high-performance telecommunication systems. Furthermore, high effective resolution of such converter should be provided. The paper presents a design of a flash analog-to-digital converter with high effective number of bits. As an example, the 8-bit flash ADC in 180 nm CMOS has been implemented. Simulation confirms efficiency of proposed design. The effective resolution of presented design achieves 6.9 bits.

The control of oxygen and carbon dioxide concentrations in an airflow released from a reactor, in which a sample is heated, can be used to investigate the thermal stability of polymer materials. This approach, known as oxithermography, involves analyzing experimental data (oxithermograms), representing the variation in oxygen concentration decrease and carbon dioxide appearance in an airflow with changing temperature conditions. This method allows for monitoring the effect of fillers introduced into polymer compositions on their thermal stability. The application of oxithermography to studying oxidative thermostability is demonstrated using pure polypropylene and polypropylene with titanium dioxide admixtures as examples.

The paper presents a generalized technique for calculating a current-driven passive mixer at an arbitrary intermediate frequency, taking into account the complex input impedance of the current source and output load. The results of simulation in the Micro-Cap environment and comparison with the calculation are presented. The frequency dependences of the transfer impedance are considered.