Tunable Coupling Architecture for Fixed-Frequency Transmon Superconducting Qubits.

Implementation of high-fidelity 2-qubit operations is a key ingredient for scalable quantum error correction. In superconducting qubit architectures, tunable buses have been explored as a means to higher-fidelity gates. However, these buses introduce new pathways for leakage. Here we present a modified tunable bus architecture appropriate for fixed-frequency qubits in which the adiabaticity restrictions on gate speed are reduced. We characterize this coupler on a range of 2-qubit devices, achieving a maximum gate fidelity of 99.85%. We further show the calibration is stable over one day.

High-coherence hybrid superconducting qubit.

We report quantum coherence measurements of a superconducting qubit whose design is a hybrid of several existing types. Excellent coherence times are found: T2∼T1∼1.5 µs. The topology of the qubit is that of a traditional three-junction flux qubit, but it has a large shunting capacitance, and the ratio of the junction critical currents is chosen so that the qubit potential has a single-well form. The qubit has a sizable nonlinearity, but its sign is reversed compared with most other popular qubit designs. The qubit is read out dispersively using a high-Q resonator in a λ/2 configuration.

[Estimation of biological tissue conductivity with contact-free magnetic impedance measurements]. / Bestimmung der komplexen elektrischen Leitfähigkeit biologischen Gewebes mittels kontaktloser Magnetimpedanzmessung.

At present, there are several methods that utilize electrical conductivity of biological tissue, such as biological impedance spectroscopy (BIS). Because these techniques use conductivity values for further analysis (e.g., body water distribution, etc.), accuracy of conductivity measurement is crucial. Traditionally, most impedance-based techniques rely on conductive interaction between tissue and external electrical measurement devices. Thus, electrode properties can influence the results of conductivity measurements. In this study, a contact-free measurement technique is presented, which is based on magnetic induction of eddy currents and measurement of the tiny reinduced voltages in external measurement coils. Our results indicate that it is principally possible to determine conductivity of biological tissue with this technique.

Quantum information storage using tunable flux qubits.

We present details and results for a superconducting quantum bit (qubit) design in which a tunable flux qubit is coupled strongly to a transmission line. Quantum information storage in the transmission line is demonstrated with a dephasing time of T(2)∼ 2.5 µs. However, energy lifetimes of the qubit are found to be short (∼ 10 ns) and not consistent with predictions. Several design and material changes do not affect qubit coherence times. In order to determine the cause of these short coherence times, we fabricated standard flux qubits based on a design which was previously successfully used by others. Initial results show significantly improved coherence times, possibly implicating losses associated with the large size of our qubit.

[Non-contact monitoring of heart and lung activity using magnetic induction measurement in a neonatal animal model]. / Kontaktlose Uberwachung von Atemtätigkeit und Herzaktion mittels magnetischer Bioimpedanzmessung in einem neonatalen Tiermodell.

BACKGROUND: Magnetic induction measurement (MIM) allows the identification of resistance in biologic tissues by alternating magnetic fields. These occur when well-conducting (blood) and poor-conducting matter (air) is moved through the thorax during heart and lung activity. As a result, allocation of the resistance changes and the total resistance of the thorax is shifted. By using coils, these changes can be registered in a non-contact manner and recorded. To date, this measuring principle was employed only in adult volunteers or in full-grown pigs. A neonatal animal model has not yet been described. The aim of this study was to test the hypothesis that non-contact monitoring of heart and lung activity using MIM in a porcine newborn piglet model can be applied in order to evaluate neonatal disorders of heart and lung activity in the future. MATERIALS AND METHODS: By using five coils (three measurement and two excitation coils), placed at the bottom of an experimental incubator, magnetic induction changes, depending on the heart and lung activity in 16 analgosedated piglets, were simultaneously measured and compared with pulse oximetry and airflow detection (flow resistance and pressure differential sensor) as reference signals. In addition, spontaneous breathing, including apnea, CPAP (continuous positive airway pressure to prevent end-expiratory alveolar collapse, flow 8 l/min; pressure 5 cm H(2)O), mechanical ventilation (inspiratory pressure 14 cm H(2)O; frequency 40/min) and high frequency oxygenation ventilation (HFOV, ventilation method in lung failure) (frequency 10 Hz, mean pressure 10 cm H(2)O, amplitude 1.5) were performed. Lung activity with MIM compared with the reference signal was estimated with a detection rate (%) of "correct registered lung activity". To quantify the analogy between MIM and reference signal for heart activity, the concordance correlation coefficient after Lin (95% confidence interval) and the Bland-Altman plot were calculated. RESULTS AND DISCUSSION: The detection rate for breathing [%] of MIM compared with the reference signal under CPAP was 88% [95% CI: (87.1%; 88.5%)], mechanical ventilation 91% [95% CI: (90.3%; 91.2%)] and under HFOV 95% [95% CI: (94.7%; 94.9%)]. For heart activity, during apnea the difference between MIM and reference signal was 1.1 bpm (+/-11.3 SD) in apnea and during HFOV 5.3 bpm (+/-26.4 SD). Under spontaneous breathing it was not possible to achieve a correlation. Owing to interference problems, registration of heart activity with MIM during simultaneous breathing activity (CPAP, conventional mechanical ventilation, HFOV) was insufficient. CONCLUSION: Non-contact monitoring of lung activity using MIM in a neonatal piglet model is possible under specific conditions. These results might be a basis for the development of non-invasive parameters in neonatology. It also provides the possibility of obtaining more information about the characteristics of lung activity of the newborn.

Multichannel simultaneous magnetic induction measurement system (MUSIMITOS).

Non-contact heart and lung activity monitoring would be a desirable supplement to conventional monitoring techniques. Based on the potential of non-contact magnetic induction measurements, requirements for an adequate monitoring system were estimated. This formed the basis for the development of the presented extendable multichannel simultaneous magnetic induction measurement system (MUSIMITOS). Special focus was given to the dynamic behaviour and simultaneous multichannel measurements, so that the system allows for up to 14 receiver coils working simultaneously at 6 excitation frequencies. Moreover, a real-time software concept for online signal processing visualization in combination with a fast software demodulation is presented. Finally, first steps towards a clinical application are pointed out and technical performance as well as first in vivo measurements are presented. This paper covers some aspects previously presented in Steffen and Leonhardt (2007 Proc. 13th Int. Conf. on Electrical Bioimpedance and the 8th Conf. on Electrical Impedance Tomography, Graz 2007).

State tomography of capacitively shunted phase qubits with high fidelity.

We introduce a new design concept for superconducting phase quantum bits (qubits) in which we explicitly separate the capacitive element from the Josephson tunnel junction for improved qubit performance. The number of two-level systems that couple to the qubit is thereby reduced by an order of magnitude and the measurement fidelity improves to 90%. This improved design enables the first demonstration of quantum state tomography with superconducting qubits using single-shot measurements.

Measurement of the entanglement of two superconducting qubits via state tomography.

Demonstration of quantum entanglement, a key resource in quantum computation arising from a nonclassical correlation of states, requires complete measurement of all states in varying bases. By using simultaneous measurement and state tomography, we demonstrated entanglement between two solid-state qubits. Single qubit operations and capacitive coupling between two super-conducting phase qubits were used to generate a Bell-type state. Full two-qubit tomography yielded a density matrix showing an entangled state with fidelity up to 87%. Our results demonstrate a high degree of unitary control of the system, indicating that larger implementations are within reach.

Coherent state evolution in a superconducting qubit from partial-collapse measurement.

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.

Decoherence in Josephson qubits from dielectric loss.

Dielectric loss from two-level states is shown to be a dominant decoherence source in superconducting quantum bits. Depending on the qubit design, dielectric loss from insulating materials or the tunnel junction can lead to short coherence times. We show that a variety of microwave and qubit measurements are well modeled by loss from resonant absorption of two-level defects. Our results demonstrate that this loss can be significantly reduced by using better dielectrics and fabricating junctions of small area . With a redesigned phase qubit employing low-loss dielectrics, the energy relaxation rate has been improved by a factor of 20, opening up the possibility of multiqubit gates and algorithms.

Simultaneous state measurement of coupled Josephson phase qubits.

One of the many challenges of building a scalable quantum computer is single-shot measurement of all the quantum bits (qubits). We have used simultaneous single-shot measurement of coupled Josephson phase qubits to directly probe interaction of the qubits in the time domain. The concept of measurement crosstalk is introduced, and we show that its effects are minimized by careful adjustment of the timing of the measurements. We observe the antiphase oscillation of the two-qubit 01 and 10 states, consistent with quantum mechanical entanglement of these states, thereby opening the possibility for full characterization of multiqubit gates and elementary quantum algorithms.

Observation of quantum oscillations between a Josephson phase qubit and a microscopic resonator using fast readout.

We have detected coherent quantum oscillations between Josephson phase qubits and critical-current fluctuators by implementing a new state readout technique that is an order of magnitude faster than previous methods. These results reveal a new aspect of the quantum behavior of Josephson junctions, and they demonstrate the means to measure two-qubit interactions in the time domain. The junction-fluctuator interaction also points to a possible mechanism for decoherence and reduced fidelity in superconducting qubits.

Test by NMR of the phase coherence of electromagnetically induced transparency.

Electromagnetically induced transparency is an effect observed in atomic systems, originating from quantum interference, in which electromagnetic transitions to and from a certain quantum state become suppressed. This dark state is also characterized by a quantum phase, relative to other states, which theoretically should stop evolving, but remain phase coherent, during transparency. We test this theoretical prediction using techniques developed for liquid-state nuclear magnetic resonance quantum computation, applied to a spin-7/2 nuclear spin system. A sequence of quantum operations is applied to create the dark state, and during transparency its phase evolution is measured relative to a reference state using Ramsey interferometry. Experimental measurements of the fringe visibility are in excellent agreement with theoretical expectations, taking into account measured decoherence rates.

Experimental implementation of an adiabatic quantum optimization algorithm.

We report the realization of a nuclear magnetic resonance computer with three quantum bits that simulates an adiabatic quantum optimization algorithm. Adiabatic quantum algorithms offer new insight into how quantum resources can be used to solve hard problems. This experiment uses a particularly well-suited three quantum bit molecule and was made possible by introducing a technique that encodes general instances of the given optimization problem into an easily applicable Hamiltonian. Our results indicate an optimal run time of the adiabatic algorithm that agrees well with the prediction of a simple decoherence model.