Spin effects in single-electron transport through carbon nanotube quantum dots

We investigate the total spin in an individual single-wall carbon nanotube quantum dot with various numbers of electrons in a shell by using the ratio of the saturation currents of the first steps of Coulomb staircases for positive and negative biases. The current ratio reflects the total-spin transition that is increased or decreased when the dot is connected to strongly asymmetric tunnel barriers. Our results indicate that total spin states with and without magnetic fields can be traced by this method.Comment: 5pages, 5figures, accepted for publication in Phys. Rev.

デジタルオーディオプレーヤの操作性に関する基礎的研究

Until digital audio players appeared the capacity of mobile audio players was only about ten songs. Now digital audio players have a capacity of thousands of songs. Searching among those thousands of songs for the one you want to hear can, however, be inconvenient. The authors, hoping to identify aspects of digital audio players that need to be improved, performed experiments on the usability of 3 different digital audio players using protocols and interviews. Ten subjects performed the same set of tasks using the 3 different players and their procedures and answers to the interviews were analyzed. This revealed the following 5 problems with the digital players.1 The function and the rapidity of response did not always match the users\u27 intentions.2 The organization of the tunes could not be understood easily.3 The large number of tunes caused the users to feel stress.4 The display of letters was hard to read.5 Problems occurred when the subjects attempted to manipulate the players in the same way they had manipulated players they had used previously but which did not work with the new players

Nonclassicality of open circuit QED systems in the deep-strong coupling regime

We investigate theoretically how the ground state of a qubit-resonator system in the deep-strong coupling (DSC) regime is affected by the coupling to an environment. We employ as a variational ansatz for the ground state of the qubit-resonator-environment system a superposition of coherent states displaced in qubit-state-dependent directions. We show that the reduced density matrix of the qubit-resonator system strongly depends on how the system is coupled to the environment, i.e., capacitive or inductive, because of the broken rotational symmetry of the eigenstates of the DSC system in the resonator phase space. When the resonator couples to the qubit and the environment in different ways (for instance, one is inductive and the other is capacitive), the system is almost unaffected by the resonator-waveguide coupling. In contrast, when the two couplings are of the same type (for instance, both are inductive), by increasing the resonator-waveguide coupling strength, the average number of virtual photons increases and the quantum superposition realized in the qubit-resonator entangled ground state is partially degraded. Since the superposition becomes more fragile with increasing the qubit-resonator coupling, there exists an optimal coupling strength to maximize the nonclassicality of the qubit-resonator system.Comment: 24 pages, 11 figure

Extremely large Lamb shift in a deep-strongly coupled circuit QED system with a multimode resonator

We report experimental and theoretical results on the extremely large Lamb shift in a multimode circuit quantum electrodynamics (QED) system in the deep-strong coupling (DSC) regime, where the qubit-resonator coupling strength is comparable to or larger than the qubit and resonator frequencies. The system comprises a superconducting flux qubit (FQ) and a quarter-wavelength coplanar waveguide resonator ($\lambda/4$ CPWR) that are coupled inductively through a shared edge that contains a Josephson junction to achieve the DSC regime. Spectroscopy is performed around the frequency of the fundamental mode of the CPWR, and the spectrum is fitted by the single-mode quantum Rabi Hamiltonian to obtain the system parameters. Since the qubit is also coupled to a large number of higher modes in the resonator, the single-mode fitting does not provide the bare qubit energy but a value that incorporates the renormalization from all the other modes. We derive theoretical formulas for the Lamb shift in the multimode resonator system. As shown in previous studies, there is a cut-off frequency $\omega_{\rm{cutoff}}$ for the coupling between the FQ and the modes in the CPWR, where the coupling grows as $\sqrt{\omega_n}$ for $\omega_n/\omega_{\rm{cutoff}}\ll 1$ and decreases as $1/\sqrt{\omega_n}$ for $\omega_n/\omega_{\rm{cutoff}}\gg 1$. Here $\omega_n$ is the frequency of the $n$th mode. The cut-off effect occurs because the qubit acts as an obstacle for the current in the resonator, which suppresses the current of the modes above $\omega_{\rm{cutoff}}$ at the location of the qubit and results in a reduced coupling strength. Using our observed spectrum and theoretical formulas, we estimate that the Lamb shift from the fundamental mode is 82.3\% and the total Lamb shift from all the modes is 96.5\%.Comment: 16 pages, 4 figure

The relationship between vocabulary and English standardized tests

　In foreign language learning, there is no question as to the importance of having enough vocabulary in the target language. Then, how much vocabulary is necessary? Schmitt (2000) mentions that acquiring 2,000 word families seems to be the most commonly cited initial goal for second language learners, and Nation (2001) claims that around 2,000 word families can provide over 95% coverage of spoken language. However, there are few empirical studies about how much vocabulary is necessary for listening comprehension. Imai et al. (2005) suggest that a 3,000 word-level vocabulary could be a threshold for Japanese EFL learners to be able to comprehend authentic listening materials. Yoshimura et al. (2005) report that learners\u27 listening ability and their vocabulary knowledge are related.　This research tried to see if having students look up 2,000 base words or academic words in a dictionary is an effective teaching strategy to improve their scores on standardized English tests HTOEIC and CELTI and a Vocabulary Levels Test (VLT). The result showed that there was no significant difference in TOEIC and CELT scores between the group checking 2,000 base words and the group checking academic words. Next, the scores of two groups, namely the group whose TOEIC score rose and the group whose TOEIC score fell, were extracted and analyzed to see the vocabulary levels of each group at the beginning of the course and at the term-end of the course. The results revealed that the group whose TOEIC scores rose improved their 3,000 word-level scores most along with their rising CELT scores, but the group whose TOEIC scores fell had not obtained even a 2,000 word-level vocabulary at the early stage of the course work. These results suggest that it is important for EFL learners to master the 2,000 word-level vocabulary as quickly as possible and then to learn the words in the 3,000 word-level.論文 (Article

Hamiltonian of a flux qubit-LC oscillator circuit in the deep–strong-coupling regime

We derive the Hamiltonian of a superconducting circuit that comprises a single-Josephson-junction flux qubit inductively coupled to an LC oscillator, and we compare the derived circuit Hamiltonian with the quantum Rabi Hamiltonian, which describes a two-level system coupled to a harmonic oscillator. We show that there is a simple, intuitive correspondence between the circuit Hamiltonian and the quantum Rabi Hamiltonian. While there is an overall shift of the entire spectrum, the energy level structure of the circuit Hamiltonian up to the seventh excited states can still be fitted well by the quantum Rabi Hamiltonian even in the case where the coupling strength is larger than the frequencies of the qubit and the oscillator, i.e., when the qubit-oscillator circuit is in the deep–strong-coupling regime. We also show that although the circuit Hamiltonian can be transformed via a unitary transformation to a Hamiltonian containing a capacitive coupling term, the resulting circuit Hamiltonian cannot be approximated by the variant of the quantum Rabi Hamiltonian that is obtained using an analogous procedure for mapping the circuit variables onto Pauli and harmonic oscillator operators, even for relatively weak coupling. This difference between the flux and charge gauges follows from the properties of the qubit Hamiltonian eigenstates