Jaw-Shen Tsai

Charge Echo in a Cooper-Pair Box

A spin-echo-type technique is applied to an artificial two-level system that utilizes a charge degree of freedom in a small superconducting electrode. Gate-voltage pulses are used to produce the necessary pulse sequence in order to eliminate the inhomogeneity effect in the time-ensemble measurement and to obtain refocused echo signals. Comparison of the decay time of the observed echo signal with an estimated decoherence time suggests that low-frequency energy-level fluctuations due to the 1/f charge noise dominate the dephasing in the system.

Nonmagnetic semiconductors as read-head sensors for ultra-high-density magnetic recording

A mesoscopic nonmagnetic magnetoresistive read-head sensor based on the recently reported extraordinary magnetoresistance (EMR) effect has been fabricated from a narrow-gap Si-doped InSb quantum well. The sensor has a conservatively estimated areal-density of 116 Gb/in.2 with a 300 K EMR of 6% and a current sensitivity of 147 Ω/T at a relevant field of 0.05 T and a bias of 0.27 T. Because this sensor is not subject to magnetic noise, which limits conventional sensors to areal densities of order 100 Gb/in.2, it opens a pathway to ultra-high-density recording at areal densities of order 1 Tb/in.2.

Decoherence of Flux Qubits due to 1 / f Flux Noise

We have investigated decoherence in Josephson-junction flux qubits. Based on the measurements of decoherence at various bias conditions, we discriminate contributions of different noise sources. We present a Gaussian decay function extracted from the echo signal as evidence of dephasing due to 1/f flux noise whose spectral density is evaluated to be about (10(-6)Phi0)2/Hz at 1 Hz. We also demonstrate that, at an optimal bias condition where the noise sources are well decoupled, the coherence observed in the echo measurement is limited mainly by energy relaxation of the qubit.

Quantum Noise in the Josephson Charge Qubit

We study decoherence of the Josephson charge qubit by measuring energy relaxation and dephasing with help of the single-shot readout. We found that the dominant energy relaxation process is a spontaneous emission induced by quantum noise coupled to the charge degree of freedom. Spectral density of the noise at high frequencies is roughly proportional to the qubit excitation energy.

Room-temperature Al single-electron transistor made by electron-beam lithography

We present a lithographically made Al single-electron transistor that shows gate modulation at room temperature. The temperature dependence of the modulation agrees with the orthodox theory, however, energy-level quantization in a tiny metallic island affects the device characteristics below 30 K. The charge-equivalent noise of the device at 300 K was measured to be ∼4×10−2u200ae/Hz1/2 at 1 Hz and is expected to be 1000 times lower in the white-noise regime at higher frequencies.

Scalable quantum computing with Josephson charge qubits.

A goal of quantum information technology is to control the quantum state of a system, including its preparation, manipulation, and measurement. However, scalability to many qubits and controlled con-nectivity between any selected qubits are two of the major stumbling blocks to achieve quantum com-puting (QC). Here we propose an experimental method, using Josephson charge qubits, to efficiently solve these two central problems. The proposed QC architecture is scalable since any two charge qubits can be effectively coupled by an experimentally accessible inductance. More importantly, we formulate an efficient and realizable QC scheme that requires only one (instead of two or more) two-bit operation to implement conditional gates.

Controllable manipulation and entanglement of macroscopic quantum states in coupled charge qubits

Quantum-mechanical systems can exploit the fundamental properties of superposition and entanglement to process information in an efficient and powerful way that no classical device can do. Recently, Josephson-junction circuits have received renewed attention because these may be used as qubits in a quantum computer. 1 Based on the charge and phase degrees of freedom in Josephson-junction devices, charge 2,3 and phase qubits 4‐6 have been developed. Also, a type of solid-state qubit can be realized in a large-area current-biased Josephson junction. 7,8

Parallel pumping of electrons

We present the simultaneous operation of ten single-electron turnstiles leading to one order of magnitude increase in current level up to 100 pA. Our analysis of device uniformity and background charge stability implies that the parallelization can be made without compromising the strict requirements of accuracy and current level set by quantum metrology. In addition, we discuss how offset charge instability limits the integration scale of single-electron turnstiles.

Controllable coupling between flux qubits.

We propose an experimentally realizable method to control the coupling between two flux qubits. In our proposal, the bias fluxes are always fixed for these two inductively coupled qubits. The detuning of these two qubits can be initially chosen to be sufficiently large, so that their initial interbit coupling is almost negligible. When a variable frequency or time-dependent magnetic flux (TDMF) is applied to one of the qubits, a well-chosen frequency of the TDMF can be used to compensate the initial detuning and to couple two qubits. This proposed method avoids fast changes of either qubit frequencies or the amplitudes of the bias magnetic fluxes through the qubit loops, and also offers a remarkable way to implement any logic gate, as well as tomographically measure flux qubit states.

100-K Operation of Al-Based Single-Electron Transistors

We have made Al-based single-electron transistors with an artificially fabricated 20-nm island electrode by utilizing standard electron-beam lithography and three-angle shadow evaporation. A periodic gate-voltage dependence of current at above 100 K is demonstrated with this device. In addition, we increased the charging energy about 20% by using anodization.