他の研究室のスタッフ・学生の方々の来聴も歓迎し ます
事前申し込みなどは一切不要です が，他に何か問い合わせした い方 は shmzアットマークas2.c.u-tokyo.ac.jpまで。
セミナー会場へのアクセスは、こ こを ご覧下さい。
Open to all researchers and students!
No application is necessary.
last modified: 08/16/2019 11:29:55
2010/10/7 (Th) 13:45-17:30
Room 129, ground floor, building 16, Komaba I Campus, the University of Tokyo.
13:45-14:45 Prof. Tobias Brandes (Technische Universitat Berlin, Institut fur Theoretische Physik)
Feedback Control of Quantum Transport
Monitoring quantum objects during their time evolution usually introduces extra noise, but it can also compensate backaction effects and be used for recycling information in order to control the system dynamics. This is also of interest for various NEMS applications.
In this talk, I will discuss electronic fluctuations which have become a major tool for probing quantum coherence, interactions, and dissipation effects in quantum transport through nanoscale structures. The random tunnelling of electrons in quantum transport is described by the full counting statistics (FCS) of transferred charges. In analogy to equilibrium thermodynamics where, e.g., the cumulants of the particle number distribution in the grand canonical ensemble are proportional to the volume, FCS cumulants in stationary transport linearly increase in time (exceptions are possible). All quantum transport devices thus have to deal with a stochastic element that can become a major obstacle when very regular currents are required.
Here, I show that this situation changes by `freezing' the cumulants in time, if one applies feedback (closed loop) control to quantum transport. I propose a scheme where a time-dependent signal $q_n(t)$ is used to continuously adjust system parameters such as tunnel rates or energy levels. Here, $q_n(t) \equiv I_{0}t -n$ is an error charge determined from the ideal `target' current $I_{0}$ and the total charge $n$ that has been collected in (or flown out of) a reservoir during the measurement (e.g., by a nearby quantum point contact detector) up to time $t$. The error charge determines whether to speed up or slow down the transport process -- a form of feedback that is analogous to the centrifugal governor used, e.g., in thermo-mechanic machines like the steam engine. The feedback scheme generates a new kind of FCS that can not be obtained via ordinary transport. This is analogous to feedback control in quantum optics, where an in-loop photocurrent was used in order to alter the photon statistics of a light beam.
(14:45-15:00 break)
15:00-16:00 Akira Shimizu (Department of Basic Science, Univ. Tokyo)
Universal properties of response functions of nonequilibrium steady states
Nonequilibrium statistical mechanics has been attracting much attention for long years. It was established in the `linear nonequilibrium regime,' which is close to equilibrium. This regime can be described by the `linear response theory,' which treats response of equilibrium states to weak external forces. The linear response theory yields many universal properties, which form a core of statistical mechanics in the linear nonequilibrium regime. In contrast, the `nonlinear nonequilibrium regime,' which is not close to equilibrium, is only poorly understood.
In this talk, I generalize the linear response theory to the nonlinear nonequilibrium regime [1,2]. Specifically, I discuss linear response of nonequilibrium steady states (NESSs) far from equilibrium. Among the universal properties that existed in the linear nonequilibrium regime, some are lost in the nonlinear nonequilibrium regime. However, the others survives if appropriately generalized.
I further generalize the theory to nonlinear response functions of NESSs [1,2]. Universal properties, which hold in diverse physical systems, are also found for 2nd and higher-order response functions of NESSs.
These universal properties of response functions of NESSs are illustrated for nonlinear optical materials and nonlinear electrical conductors. We have obtained remarkable results. For example, the integral of differential conductivity over frequencies is independent of the degree of nonequilibrium.
References:
[1] A. Shimizu and T. Yuge, J. Phys. Soc. Jpn. 79 (2010) 013002.
[2] A. Shimizu, to appear in J. Phys. Soc. Jpn.; arXiv:1007.4376.
(16:00-16:30 break)
16:30-17:00 Kazuki Koshino (College of Liberal Arts and Sciences, Tokyo Medical and Dental Univ.)
Deterministic photon-photon root-SWAP gate using a Lambda system
We theoretically present a method to realize a deterministic photon-photon root-SWAP gate using a three-level Lambda system interacting with single photons in reflection geometry. The Lambda system is used completely passively as a temporary memory for a photonic qubit; the initial state of the Lambda system may be arbitrary, and active control by auxiliary fields is unnecessary throughout the gate operations. These distinct merits make this entangling gate suitable for deterministic and scalable quantum computation.
Reference:
Phys. Rev. A 82, 010301(R) (2010)
http://physics.aps.org/synopsis-for/10.1103/PhysRevA.82.010301
17:00-17:30 Tatsuro Yuge (Institute for International Advanced Interdisciplinary Research, Tohoku Univ.)
Measurement of bath spectrum by multiple pulse sequence in NMR
In recent years there have been some reports of NMR experiments [1] that decoherence is suppressed by applying a sequence of radio-frequency pulses. This is qualitatively explained by the dynamical decoupling [2]. In this work we analyze the spin-boson model [3] with a pulse sequence to compare the theory and the experiment quantitatively. We find that the long-time behavior of the decay curve of the coherence provides the information of the boson bath spectrum. We propose a form of the bath spectrum to fit the experimental data and analyze the results for 75As in GaAs, 29Si in silicon and 23Na in NaCl.
[1] S. Watanabe and S. Sasaki, Jpn. J. Appl. Phys. vol. 42 (2003) L1350.
[2] L. Viola and S. Lloyd, Phys. Rev. A vol. 58 (1998) 2733.
[3] A. J. Leggett et. al., Rev. Mod. Phys. vol. 59 (1987) 1.
18:00- Banquet at Shibuya or Shimo-kitazawa