Experimental determination of the Berry phase in a superconducting charge pump

We present the first measurements of the Berry phase in a superconducting Cooper pair pump. A fixed amount of Berry phase is accumulated to the quantum-mechanical ground state in each adiabatic pumping cycle, which is determined by measuring the charge passing through the device. The dynamic and geometric phases are identified and measured quantitatively from their different response when pumping in opposite directions. Our observations, in particular, the dependencies of the dynamic and geometric effects on the superconducting phase bias across the pump, agree with the basic theoretical model of coherent Cooper pair pumping.Comment: 4 pages, 3 figure

Quantum circuits with uniformly controlled one-qubit gates

Uniformly controlled one-qubit gates are quantum gates which can be represented as direct sums of two-dimensional unitary operators acting on a single qubit. We present a quantum gate array which implements any n-qubit gate of this type using at most 2^{n-1} - 1 controlled-NOT gates, 2^{n-1} one-qubit gates and a single diagonal n-qubit gate. The circuit is based on the so-called quantum multiplexor, for which we provide a modified construction. We illustrate the versatility of these gates by applying them to the decomposition of a general n-qubit gate and a local state preparation procedure. Moreover, we study their implementation using only nearest-neighbor gates. We give upper bounds for the one-qubit and controlled-NOT gate counts for all the aforementioned applications. In all four cases, the proposed circuit topologies either improve on or achieve the previously reported upper bounds for the gate counts. Thus, they provide the most efficient method for general gate decompositions currently known.Comment: 8 pages, 10 figures. v2 has simpler notation and sharpens some result

Suppression of 1/f noise in one-qubit systems

We investigate the generation of quantum operations for one-qubit systems under classical noise with 1/f^\alpha power spectrum, where 2>\alpha > 0. We present an efficient way to approximate the noise with a discrete multi-state Markovian fluctuator. With this method, the average temporal evolution of the qubit density matrix under 1/f^\alpha noise can be feasibly determined from recently derived deterministic master equations. We obtain qubit operations such as quantum memory and the NOT}gate to high fidelity by a gradient based optimization algorithm. For the NOT gate, the computed fidelities are qualitatively similar to those obtained earlier for random telegraph noise. In the case of quantum memory however, we observe a nonmonotonic dependency of the fidelity on the operation time, yielding a natural access rate of the memory.Comment: 8 pages, 7 figure

Stabilization and pumping of giant vortices in dilute Bose-Einstein condensates

Recently, it was shown that giant vortices with arbitrarily large quantum numbers can possibly be created in dilute Bose-Einstein condensates by cyclically pumping vorticity into the condensate. However, multiply quantized vortices are typically dynamically unstable in harmonically trapped nonrotated condensates, which poses a serious challenge to the vortex pump procedure. In this theoretical study, we investigate how the giant vortices can be stabilized by the application of a Gaussian potential peak along the vortex core. We find that achieving dynamical stability is feasible up to high quantum numbers. To demonstrate the efficiency of the stabilization method, we simulate the adiabatic creation of an unsplit 20-quantum vortex with the vortex pump.Comment: 8 pages, 6 figures; to be published in J. Low Temp. Phys., online publication available at http://dx.doi.org/10.1007/s10909-010-0216-

Geometric Phase Gates with Adiabatic Control in Electron Spin Resonance

High-fidelity quantum operations are a key requirement for fault-tolerant quantum information processing. In electron spin resonance, manipulation of the quantum spin is usually achieved with time-dependent microwave fields. In contrast to the conventional dynamic approach, adiabatic geometric phase operations are expected to be less sensitive to certain kinds of noise and field inhomogeneities. Here, we investigate such phase gates applied to electron spins both through simulations and experiments, showing that the adiabatic geometric phase gate is indeed inherently robust against inhomogeneity in the applied microwave field strength. While only little advantage is offered over error-correcting composite pulses for modest inhomogeneities <=10%, the adiabatic approach reveals its potential for situations where field inhomogeneities are unavoidably large

Modelling old-age retirement:an adaptive multi-outcome LAD-lasso regression approach

Abstract Using unique administrative register data, we investigate old-age retirement under the statutory pension scheme in Finland. The analysis is based on multi-outcome modelling of pensions and working lives together with a range of explanatory variables. An adaptive multi-outcome LAD-lasso regression method is applied to obtain estimates of earnings and socioeconomic factors affecting old-age retirement and to decide which of these variables should be included in our model. The proposed statistical technique produces robust and less biased regression coefficient estimates in the context of skewed outcome distributions and an excess number of zeros in some of the explanatory variables. The results underline the importance of late life course earnings and employment to the final amount of pension and reveal differences in pension outcomes across socioeconomic groups. We conclude that adaptive LAD-lasso regression is a promising statistical technique that could be usefully employed in studying various topics in the pension industry

System-environment correlations in qubit initialization and control

Abstract The impressive progress in fabricating and controlling superconducting devices for quantum information processing has reached a level where reliable theoretical predictions need to account for quantum correlations that are not captured by the conventional modeling of contemporary quantum computers. This applies particularly to the qubit initialization as the process which crucially limits typical operation times. Here, we employ numerically exact methods to study realistic implementations of a transmon qubit embedded in electromagnetic environments focusing on the most important system-reservoir correlation effects such as the Lamb shift and entanglement. For the qubit initialization we find a fundamental trade-off between speed and accuracy which sets intrinsic constraints in the optimization of future reset protocols. Instead, the fidelities of quantum logic gates can be sufficiently accurately predicted by standard treatments. Our results can be used to accurately predict the performance of specific setups and also to guide future experiments in probing low-temperature properties of qubit reservoirs

Tunable refrigerator for nonlinear quantum electric circuits

Abstract The emerging quantum technological applications call for fast and accurate initialization of the corresponding devices to low-entropy quantum states. To this end, we theoretically study a recently demonstrated quantum-circuit refrigerator in the case of nonlinear quantum electric circuits such as superconducting qubits. The maximum refrigeration rate of transmon and flux qubits is observed to be roughly an order of magnitude higher than that of usual linear resonators, increasing flexibility in the design. We find that for typical experimental parameters, the refrigerator is suitable for resetting different qubit types to fidelities above 99.99% in a few or a few tens of nanoseconds depending on the scenario. Thus the refrigerator appears to be a promising tool for quantum technology and for detailed studies of open quantum systems

Cardiac remodeling from middle age to senescence

Abstract Background: The data on cardiac remodeling outside the scope of myocardial infarction and heart failure are limited. Methods: A cohort of middle-aged hypertensive subjects with age- and gender-matched control subjects without hypertension (n = 1,045, aged 51 ± 6 years) were randomly selected for the OPERA study (Oulu Project Elucidating Risk of Atherosclerosis study). The majority of those who were still alive after more than 20 years of follow-up underwent thorough re-examinations. Results: Left ventricular mass index (LVMI) increased significantly from 106.5 ± 27.1 (mean ± SD) to 114.6 ± 29.1 g/m² (p &lt; 0.001), the thickness of the left ventricular posterior wall (LVPW) from 10.0 ± 1.8 to 10.6 ± 1.7 mm (p &lt; 0.001), fractional shortening (FS) from 35.0 ± 5.7 to 38.4 ± 7.2 % (p &lt; 0.001), and left atrial diameter (LAD) from 38.8 ± 5.2 to 39.4 ± 6.7 mm (p = 0.028) during the 20-year follow-up. After multivariate adjustments, hypertension treated with antihypertensive medication and male gender predicted a smaller increase in the thickness of LVPW (p = 0.017 to &lt;0.001). Baseline higher fasting plasma insulin level, larger intima media thickness of the carotid artery, greater height and antihypertensive medication (p = 0.046–0.002) predicted a smaller (less favorable) change of FS. The increase of LAD was associated with higher baseline diastolic blood pressure (p = 0.034) and greater height (p = 0.006). Conclusion: Aging from middle age to senescence increases the echocardiographic indexes of LVMI, LVPW thickness, FS and LAD. Several baseline factors are associated with these changes