Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W1: Superconducting Qubits: Advances in Single-Shot QND Readout |
Hide Abstracts |
Sponsoring Units: DCMP Chair: John Martinis, University of California, Santa Barbara Room: Ballroom A1 |
Thursday, March 24, 2011 11:15AM - 11:51AM |
W1.00001: DC-SQUID Quantum Non-Demolition Readout of Superconducting Flux Qubits Invited Speaker: Extracting state information from a quantum system is a central theme in quantum mechanics. As the process of state extraction by a detector implies system-detector entanglement, reverse action from the detector onto the quantum object can not be avoided. Consequently, detectors that minimise this back action are crucial. For superconducting flux qubits [1] commonly a DC-SQUID detector is used, either in an AC dispersive scheme or in a switching mode. The latter can be by AC bifurcation or by direct DC switching. The DC approach combines simplicity in use with complexity in dynamical behaviour. This complexity results from the fast Josephson phase dynamics and the significant generation of quasi-particles in the dissipative detector ON-state. This gave rise to the long-standing belief that it can not act as a ``good'' detector. This includes it to fail as a Quantum Non-Demolition (QND) detector, i.e. the preservation of the state of the quantum object after a state readout. In a recent experiment for relatively weak qubit-SQUID interaction strength [2] we investigated the detection properties of such a DC-switching SQUID, finding a remarkably good QND fidelity. This was achieved by shunting the SQUID by a low-value resistor, thus strongly suppressing the generation of quasi-particles. Also the detector ON-time was minimised to a few tens of ns using a nearby cryogenic amplifier. The QND-ness was obtained from measuring the correlation between two successive readouts, and found to reach 75{\%} QND fidelity. The weak qubit-detector interaction leads to a limited readout contrast. We will discuss the results as well as its consequences, including the potential for combining high contrast and good QND fidelity.\\[4pt] [1] J.E. Mooij et. al., Science \underline {\textbf{285}}, 1036 (1999) \\[0pt] [2] T. Picot et. al., Phys. Rev. Lett. \underline {\textbf{105}}, 040506 (2010) [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:27PM |
W1.00002: Novel approaches to high fidelity qubit state measurement in circuit quantum electrodynamics Invited Speaker: Qubit state measurement (`readout') in solid state systems is an open problem, which is currently the subject of intensive experimental and theoretical research. Achieving high fidelity in a single-shot measurement is an interesting quantum control problem, as well as an important component for the successful implementation of quantum information protocols. For superconducting qubits we can distinguish between linear dispersive and nonlinear methods, the latter relying on the bistability of a nonlinear resonator. In the context of circuit quantum electrodynamics, the transmon qubit is strongly coupled to a linear resonator and described by a generalized Jaynes-Cummings model (JCM) with external drive and dissipation. Recent novel approaches to achieve high-fidelity readout in the dispersive regime rely on the intrinsic nonlinearity of the JCM and its ultimate linearity in the high excitation regime. In the degenerate regime we rely on the photon blockade and precise transient dynamics of the system. This regime presents a theoretical challenge and the driven damped JCM model exhibits a dynamical phase transition. Another proposed approach extends the Josephson Bifurcation Amplifier and employs the dynamical effects of frequency chirping of the drive on the coupled qubit-resonator system. We will discuss the physics of these different regimes and describe the readout schemes which have been demonstrated by recent experiments and quantum simulations, as well as the role of quantum fluctuations and optimal control. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W1.00003: Observation of quantum jumps in a superconducting quantum bit Invited Speaker: Superconducting qubit technology has made great advances since the first demonstration of coherent oscillations more than 10 years ago. Coherence times have improved by several orders of magnitude and significant progress has been made in qubit state readout fidelity. However, a fast, high-fidelity, quantum non-demolition measurement scheme which is essential to implement quantum error correction has so far been missing. We demonstrate such a scheme for the first time where we continuously measure the state of a superconducting quantum bit using a fast, ultralow-noise parametric amplifier. This arrangement allows us to observe quantum jumps between the qubit states in real time. The key development enabling this experiment is the use of a low quality factor (Q), nonlinear resonator to implement a phase-sensitive parametric amplifier operating near the quantum limit. The nonlinear resonator was constructed using a two junction SQUID shunted with an on-chip capacitor. The SQUID allowed us to tune the operating band of the amplifier and the low Q provided us with a bandwidth greater than 10 MHz, sufficient to observe jumps in the qubit state in real time. I will briefly describe the operation of the parametric amplifier and discuss how it was used to measure the state of a transmon qubit in the circuit QED architecture. I will discuss measurement fidelity and the statistics of the quantum jumps. I will conclude by discussing the implications of this development for quantum information processing and further improvements to the measurement technique. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:39PM |
W1.00004: Probing the quantum fluctuations of a nonlinear resonator with a superconducting qubit Invited Speaker: Coupling a superconducting quantum bit to a superconducting resonator offers the opportunity to investigate the interaction between light and an atom in regimes hardly accessible otherwise [1]. Making the resonator nonlinear has enabled important recent progress in the readout of qubits. Indeed, when pumped by a microwave field of well-chosen amplitude and frequency, nonlinear resonators (NRs) provide parametric amplification close to the quantum limit. In other drive conditions, the intra-cavity field can take two stable values between which the resonator can switch stochastically. Both regimes have been shown to yield a high-fidelity qubit readout [2,3]. Qubits can also be used to obtain interesting insight into the physics of NRs. In this work we use a transmon qubit [4] coupled to such a pumped NR as a probe of its quantum fluctuations. The qubit-NR coupling is manifested by the appearance around the qubit spectral line of two sidebands that we interpret as processes in which the driven resonator fluctuations are effectively cooled down or heated with the assistance of the qubit. The ratio of the sidebands amplitudes gives thus a direct experimental access to the pumped NR effective temperature which is found to be in quantitative agreement with the theory, bringing a clear support to the quantum description of a driven nonlinear resonator [5]. \\[4pt] [1] A. Wallraf, D. I. Schuster, A. Blais, L. Frunzio, R.-S. Huang, J. Majer, S. Kumar, S. M. Girvin and R. J. Schoelkopf, Nature 431 162 (2004). \\[0pt] [2] R. Vijay, D.H. Slichter, I. Siddiqi, arxiv:cond-mat/1009.2969. \\[0pt] [3] F. Mallet, F.R. Ong, A. Palacios-Laloy, F. NGuyen, P. Bertet, D. Vion, and D. Esteve, Nature Physics 5, 791-795 (2009). \\[0pt] [4] J. Koch, T.M. Yu, J.M. Gambetta, A.A. Houck, D.I Schuster, J. Majer, A. Blais, M.H. Devoret, S.M. Girvin, and R.J. Schoelkopf, Phys. Rev. A 76, 042319 (2007). \\[0pt] [5] M. Marthaler and M.I. Dykman, PRA 73, 042108 (2006). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700