Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N33: Quantum Measurement |
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Sponsoring Units: GQI Chair: Girish S. Agarwal, Oklahoma State University Room: Colorado Convention Center 403 |
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N33.00001: Simple multi-parameter unitary estimation Sergio Boixo, Rolando Somma We consider multi-parameter estimation of a general unitary operation acting on a set of qubits. We show a simple quantum circuit that estimates operators at the optimal Heisenberg limit, i.e., achieving a sensitivity for determining the parameters that scales as $1/N$, where $N$ is the number of times the unknown unitary is applied. The circuit makes use of one extra qubit (ancilla) which is initially prepared in a pure state, while the system qubits are initially prepared in the totally mixed state. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N33.00002: Quantum State Detection through Repetitive Mapping D. B. Hume, T. Rosenband, J. C. Bergquist, D. J. Wineland State detection plays an important role in quantum information processing and quantum-limited metrology. In some cases the quantum system of interest can only be detected with poor efficiency. One approach to overcoming this limitation is to couple the primary quantum system to an ancillary quantum system used for measurement [1]. The measurement process consists of mapping the primary state to the ancilla followed by ancilla detection. If this can be done without affecting the projected populations of the primary system, the measurement may be repeated. In this case, detection fidelity can be significantly higher than both the fidelity of state transfer and the intrinsic measurement fidelity of the ancillary system. Using two ions as the primary and ancillary systems ($^{27}$Al$^{+ }$and $^{9}$Be$^{+}$ respectively) held in a harmonic trap, we demonstrate near unit fidelity measurement despite imperfect information transfer and ancilla detection. \newline \newline [1] P.O. Schmidt, et. al. Science 309 749 (2005) [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N33.00003: Examination of the Charge Quantum in a Single-Electron Pump Mark W. Keller, Neil M. Zimmerman, Ali L. Eichenberger In single-electron tunneling (SET) circuits, charge moves in discrete quanta that are generally assumed to carry a charge of exactly $e$, the free electron charge. To the extent that this is true, SET devices have an important role to play in fundamental metrology by providing a solid-state current source that is directly linked to a fundamental constant of nature. But is the SET charge quantum in fact exactly $e$? We discuss why this is not a trivial question and present an experimental answer to the question: by placing a known number of SET charge quanta onto a known capacitor, and by measuring the resulting voltage across the capacitor using a Josephson voltage standard, we compare the SET charge quantum to $e$. We find that the SET charge quantum is equal to $e$ within a relative standard uncertainty of 1 part in $10^6$, a constraint that is $\sim 100$ times smaller than the best previous result. This measurement is expected to reach an uncertainty $\sim 3$ parts in $10^7$ in the near future, at which point it will also give useful information on possible corrections to the Josephson constant, $K_J = 2e/h$. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N33.00004: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N33.00005: Radio frequency operation of a quantum point contact charge detector Madhu Thalakulam, A.J. Rimberg, L.N. Pfeiffer, K. W. West Quantum point contact (QPC) charge detectors are sensitive electrometers, and their ease of fabrication and integration into semiconductor-based qubit systems makes them an attractive candidate as a readout device for spin or charge based qubits in quantum dots. Nevertheless, QPC performance to date has been limited by relatively low operational speeds and 1/f noise. Here we report the operation of a QPC charge sensor realized in an GaAs/AlGaAs two dimensional electron gas at radio- frequencies (RF-QPC), in a mode analogous to rf operation of the single electron transistor [1]. For a typical QPC detector coupled to a quantum dot (QD), a charge oscillation of one electron in the QD corresponds to a change in the QPC conductance of 1-3 percent. We simulate these operating conditions by applying a small ac voltage to the QPC gate to cause a similar change in the zero bias QPC conductance. When operated this way the signal to noise ratio of the RF-QPC is about 30dB, which corresponds to a charge sensitivity of about $7x10^{-4}e/\sqrt{Hz}$ referred to the dot charge. The operational characteristics of the RF-QPC at 4.2K also will be discussed. [1] R. J. Schoelkopf et al., Science \b {280}, 1238–1242 (1998). [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N33.00006: Radio-frequency measurement of an asymmetric single electron transistor Zhongqing Ji, Weiwei Xue, A.J. Rimberg Since the invention of the radio-frequency single-electron transistor (RF-SET) by Schoelkopf et al.,[1] most measurements have focused on the symmetric single electron transistor. It has been shown, however, that the symmetric SET has a rather low measurement efficiency in its normal working regime.[2][3] Recently, it has been pointed out that an asymmetric SET can be considerably more efficient than a symmetric SET as a quantum amplifier. In this case the measurement efficiency of the asymmetric SET becomes similar to that of the quantum point contact (QPC) detector which can approach the quantum limit. We investigate the asymmetric SET by fabricating Al/AlO$_{x}$ SETs with junction areas 40x40 nm$^{2 }$and 40x80nm$^{2}$ and total resistance of about 25k$\Omega $. The results of RF and DC characterization of such asymmetric SETs will be discussed. [1] R. J. Schoelkopf, P. Wahlgren, A. A. Kozhevnikov, P. Delsing, D. E. Prober, Science, \textbf{280}, 1242 (1998). [2] A. N. Korotkov, Phys. Rev. B, \textbf{63}, 085312 (2001); \textbf{63}, 115403 (2001). [3] D. Mozyrsky, I. Martin, and M. B. Hastings, Phys. Rev. Lett., \textbf{92}, 018303 (2004). [4] S. A. Gurvitz and G. P. Berman, Phys. Rev. B, \textbf{72 }, 073303(2005). [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N33.00007: Spin Measurement in Quantum Electro-Mechanical Systems Dian Wahyu Utami, Jason Twamley, Hsi-Sheng Goan, Gerard J. Milburn Interests in spin measurement in solid state nanostructure has been growing in the last few years. The measurement of spin is particularly important in the realization of spin based solid state quantum computer proposals. Here we present our study on spin detection via a quantum electromechanical shuttle system using the example of an endohedral N@C60 that is placed in a magnetic gradient generated by a nearby nanomagnet. Using quantum optics methods, the currents across the system are found to be different for each of the different spin orientations. This is due to the different directional forces produced as a result of the interaction between each of the spin orientation to the magnetic gradient. The resulting force affects the steady state position of the island and thus modifies the system's conductance. We investigate the feasibility of the application of the system as a single spin measurement by looking at the current noise spectral density and investigating the measurement time required to distinguish the two currents for each of the spin states. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N33.00008: Laser cooling of diffraction limited size micromirrors Constanze Metzger, Ivan Favero, Khaled Karrai The prospect of realizing entangled quantum states between macroscopic objects and photons [1] has recently stimulated interest in laser-cooling schemes of macroscopic mechanical resonators [2-5]. We describe passive optical cooling of the Brownian motion of a cantilevered micromirror. Since the cantilever forms one mirror of a confocal Fabry-P\'{e}rot cavity, its mirror end has to be of the size of the optical wavelenght in order to ensure high reflectivity. In our setup, the mirror's size is 2.4$\mu $m and hence in the range of the diffraction limit for 1.3$\mu $m laser light. With its weigth of 11pg it represents the smallest mass cooled so far. The optically induced excitation regime was also explored, opening a path to optically driving nanostructures with high frequency resonances. [1] Marshall et al., Phys. Rev. Lett. \textbf{91}, 130401 (2003). [2] Metzger and Karrai, Nature \textbf{432}, 1002 (2004). [3] Gigan et al., Nature \textbf{444}, 67 (2006). [4] Arcizet et al., Nature \textbf{444}, 71 (2006). [5] Kleckner and Bouwmeester, Nature \textbf{444}, 75 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N33.00009: ABSTRACT HAS BEEN MOVED TO J32.00013 |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N33.00010: Quantum Undemolition: Undoing quantum measurement by erasing information Alexander Korotkov, Andrew Jordan Extensive research into controllable quantum systems and detectors has led to a reexamination of the very nature of quantum measurement in a condensed matter context. Quantum detectors used in recent experiments naturally give rise to weak quantum measurements, where the detector output is not perfectly correlated with the state of the measured system. According to textbook quantum measurements, wavefunction collapse of an unknown state is essentially an irreversible process; the measurement record is indelible. Contrary to this conventional wisdom, we will demonstrate how to undo a weak quantum measurement, showing that quantum information is written in pencil, not pen. The undoing procedure has a finite probability of success, and it is accompanied by a clear experimental indication of whether or not the undoing has been successful. Our proposed phenomenon can be experimentally realized using quantum dot (charge) or superconducting (phase) qubits. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N33.00011: ABSTRACT HAS BEEN MOVED TO U33.00015 |
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