arXiv subjects

K. Semba

Publications and source records attributed to K. Semba.

28 records · Page 2Linked to original sources

arXiv2006

Fabrication and properties of MgB2/AlOx/MgB2 tunnel junctions

Sandwich-type MgB2 Josephson tunnel junctions (MgB2/AlOx/MgB2) have been fabricated using as-grown MgB2 films formed by molecular beam epitaxy. The junctions exhibited substantial supercurrent and a well-defined superconducting gap (D=2.2~2.3mV). The superconducting gap voltage (D) agrees well with those of the smaller gap in the multi-gap scenario. The IcRN product is 0.4-1.3 mV at 4.2K, but approaches to 2.0 mV at 50 mK. The Ic has peculiar temperature dependence far from the Ambegaokar-Baratoff formula. It rapidly decreases with temperature, and disappears above T = 20 K, which is much lower than the gap closing temperature. Interface microstructure between AlOx and MgB2 were investigated using cross-sectional transmission electron microscopy to clarify the problems in our tunnel junctions. There are poor-crystalline MgB2 layers and/or amorphous Mg-B composite layers of a few nanometers between AlOx barrier and the upper MgB2 layer. The poor-crystalline upper Mg-B layers seem to behave as normal metal or deteriorated superconducting layers, which may be the principal reason for all non-idealities of our MgB2/AlOx/MgB2 junctions.

cond-mat.supr-con↗

arXiv2005

Dephasing of a superconducting qubit induced by photon noise

We have studied the dephasing of a superconducting flux-qubit coupled to a DC-SQUID based oscillator. By varying the bias conditions of both circuits we were able to tune their effective coupling strength. This allowed us to measure the effect of such a controllable and well-characterized environment on the qubit coherence. We can quantitatively account for our data with a simple model in which thermal fluctuations of the photon number in the oscillator are the limiting factor. In particular, we observe a strong reduction of the dephasing rate whenever the coupling is tuned to zero. At the optimal point we find a large spin-echo decay time of $4 μs$.

cond-mat.mes-hall↗

arXiv2005

Vacuum Rabi oscillations in a macroscopic superconducting qubit LC oscillator system

We have observed the coherent exchange of a single energy quantum between a flux qubit and a superconducting LC circuit acting as a quantum harmonic oscillator. The exchange of an energy quantum is known as the vacuum Rabi oscillations: the qubit is oscillating between the excited state and the ground state and the oscillator between the vacuum state and the first excited state. We have also obtained evidence of level quantization of the LC circuit by observing the change in the oscillation frequency when the LC circuit was not initially in the vacuum state.

cond-mat.supr-con↗

arXiv2005

Parametric control of a superconducting flux qubit

Parametric control of a superconducting flux qubit has been achieved by using two-frequency microwave pulses. We have observed Rabi oscillations stemming from parametric transitions between the qubit states when the sum of the two microwave frequencies or the difference between them matches the qubit Larmor frequency. We have also observed multi-photon Rabi oscillations corresponding to one- to four-photon resonances by applying single-frequency microwave pulses. The parametric control demonstrated in this work widens the frequency range of microwaves for controlling the qubit and offers a high quality testing ground for exploring nonlinear quantum phenomena.

cond-mat.mes-hall↗

arXiv2004

Relaxation and Dephasing in a Flux-qubit

We report detailed measurements of the relaxation and dephasing time in a flux-qubit measured by a switching DC SQUID. We studied their dependence on the two important circuit bias parameters: the externally applied magnetic flux and the bias current through the SQUID in two samples. We demonstrate two complementary strategies to protect the qubit from these decoherence sources. One consists in biasing the qubit so that its resonance frequency is stationary with respect to the control parameters ({\it optimal point}) ; the second consists in {\it decoupling} the qubit from current noise by chosing a proper bias current through the SQUID. At the decoupled optimal point, we measured long spin-echo decay times of up to $4 μs$.

cond-mat.mes-hall↗

arXiv2004

All-MgB2 Josephson tunnel junctions

Sandwich-type all-MgB2 Josephson tunnel junctions (MgB2/AlOx/MgB2) have been fabricated for the first time with as-grown MgB2 films formed by molecular beam epitaxy. The junctions exhibit substantial superconducting current (IcRN product ~ 0.8 mV at 4.2K), a well-defined superconducting gap (delta = ~2.3 mV), and clear Fraunhofer patterns. The superconducting gap voltage of delta agrees well with the smaller gap in the multi-gap scenario. The results demonstrate that MgB2 has great promise for superconducting electronics that can be operated at T ~ 20 K.

cond-mat.supr-con↗

arXiv2004

Coherent dynamics of a flux qubit coupled to a harmonic oscillator

In the emerging field of quantum computation and quantum information, superconducting devices are promising candidates for the implementation of solid-state quantum bits or qubits. Single-qubit operations, direct coupling between two qubits, and the realization of a quantum gate have been reported. However, complex manipulation of entangled states - such as the coupling of a two-level system to a quantum harmonic oscillator, as demonstrated in ion/atom-trap experiments or cavity quantum electrodynamics - has yet to be achieved for superconducting devices. Here we demonstrate entanglement between a superconducting flux qubit (a two-level system) and a superconducting quantum interference device (SQUID). The latter provides the measurement system for detecting the quantum states; it is also an effective inductance that, in parallel with an external shunt capacitance, acts as a harmonic oscillator. We achieve generation and control of the entangled state by performing microwave spectroscopy and detecting the resultant Rabi oscillations of the coupled system.

cond-mat.mes-hall↗

arXiv2004

A theoretical analysis of flux-qubit measurements with a dc-SQUID

The readout process of a superconducting flux-qubit is theoretically analyzed in terms of the quantum dynamics of a qubit-SQUID coupled system during measurement. The quantity directly observed by the measurement is the switching current (Isw) of the dc-SQUID placed around the qubit ring. In order to clarify the relation between the Isw and the qubit state, we calculated the time evolution of the density operator of the qubit-SQUID system while increasing the SQUID bias current until switching events occur and obtained the switching current distributions. This clarifies what information of the qubit is obtained by a dc-SQUID switching current measurement under specific conditions, for example, when the qubit eigenstate is a superposition of two different flux states.

cond-mat.supr-con↗

arXiv2004

Detection of a persistent-current qubit by resonant activation

We present the implementation of a new scheme to detect the quantum state of a persistent-current qubit. It relies on the dependency of the measuring Superconducting Quantum Interference Device (SQUID) plasma frequency on the qubit state, which we detect by resonant activation. With a measurement pulse of only 5ns, we observed Rabi oscillations with high visibility (65%).

cond-mat.mes-hall↗

arXiv2004

Multiphoton transitions in a macroscopic quantum two-state system

We have observed multiphoton transitions between two macroscopic quantum-mechanical superposition states formed by two opposite circulating currents in a superconducting loop with three Josephson junctions. Resonant peaks and dips of up to three-photon transitions were observed in spectroscopic measurements when the system was irradiated with a strong RF-photon field. The widths of the multiphoton absorption dips are shown to scale with the Bessel functions in agreement with theoretical predictions derived from the Bloch equation or from a spin-boson model.

cond-mat.mes-hall↗