181 research outputs found
Superconducting Electronic Devices
Contains reports on two research projects.Defense Advanced Research Projects Agency Contract MDA 972-90-C-002
Dynamical decoupling and dephasing in interacting two-level systems
We implement dynamical decoupling techniques to mitigate noise and enhance
the lifetime of an entangled state that is formed in a superconducting flux
qubit coupled to a microscopic two-level system. By rapidly changing the
qubit's transition frequency relative to the two-level system, we realize a
refocusing pulse that reduces dephasing due to fluctuations in the transition
frequencies, thereby improving the coherence time of the entangled state. The
coupling coherence is further enhanced when applying multiple refocusing
pulses, in agreement with our noise model. The results are applicable to
any two-qubit system with transverse coupling, and they highlight the potential
of decoupling techniques for improving two-qubit gate fidelities, an essential
prerequisite for implementing fault-tolerant quantum computing
Mach-Zehnder Interferometry in a Strongly Driven Superconducting Qubit
We demonstrate Mach-Zehnder-type interferometry in a superconducting flux
qubit. The qubit is a tunable artificial atom, whose ground and excited states
exhibit an avoided crossing. Strongly driving the qubit with harmonic
excitation sweeps it through the avoided crossing two times per period. As the
induced Landau-Zener transitions act as coherent beamsplitters, the accumulated
phase between transitions, which varies with microwave amplitude, results in
quantum interference fringes for n=1...20 photon transitions. The
generalization of optical Mach-Zehnder interferometry, performed in qubit phase
space, provides an alternative means to manipulate and characterize the qubit
in the strongly-driven regime.Comment: 14 pages, 6 figure
A tunable coupling scheme for implementing high-fidelity two-qubit gates
The prospect of computational hardware with quantum advantage relies
critically on the quality of quantum gate operations. Imperfect two-qubit gates
is a major bottleneck for achieving scalable quantum information processors.
Here, we propose a generalizable and extensible scheme for a two-qubit coupler
switch that controls the qubit-qubit coupling by modulating the coupler
frequency. Two-qubit gate operations can be implemented by operating the
coupler in the dispersive regime, which is non-invasive to the qubit states. We
investigate the performance of the scheme by simulating a universal two-qubit
gate on a superconducting quantum circuit, and find that errors from known
parasitic effects are strongly suppressed. The scheme is compatible with
existing high-coherence hardware, thereby promising a higher gate fidelity with
current technologies
Resonant Readout of a Persistent Current Qubit
We have implemented a resonant circuit that uses a SQUID as a flux-sensitive
Josephson inductor for qubit readout. In contrast to the conventional switching
current measurement that generates undesired quasi-particles when the SQUID
switches to the voltage state, our approach keeps the readout SQUID biased
along the supercurrent branch during the measurement. By incorporating the
SQUID inductor in a high-Q resonant circuit, we can distinguish the two flux
states of a niobium persistent-current (PC) qubit by observing a shift in the
resonant frequency of both the magnitude and the phase spectra. The readout
circuit was also characterized in the nonlinear regime to investigate its
potential use as a nonlinear amplifier.Comment: 4 pages, 2004 ASC Proceeding
Lower bounds in distributed computing
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 167-170).Distributed computing is the study of achieving cooperative behavior between independent computing processes with possibly conflicting goals. Distributed computing is ubiquitous in the Internet, wireless networks, multi-core and multi-processor computers, teams of mobile robots, etc. In this thesis, we study two fundamental distributed computing problems, clock synchronization and mutual exclusion. Our contributions are as follows. 1. We introduce the gradient clock synchronization (GCS) problem. As in traditional clock synchronization, a group of nodes in a bounded delay communication network try to synchronize their logical clocks, by reading their hardware clocks and exchanging messages. We say the distance between two nodes is the uncertainty in message delay between the nodes, and we say the clock skew between the nodes is their difference in logical clock values. GCS studies clock skew as a function of distance. We show that surprisingly, every clock synchronization algorithm exhibits some execution in which two nodes at distance one apart have Q( lo~gD clock skew, where D is the maximum distance between any pair of nodes. 2. We present an energy efficient and fault tolerant clock synchronization algorithm suitable for wireless networks. The algorithm synchronizes nodes to each other, as well as to real time. It satisfies a relaxed gradient property. That is, it guarantees that, using certain reasonable operating parameters, nearby nodes are well synchronized most of the time. 3. We study the mutual exclusion (mutex) problem, in which a set of processes in a shared memory system compete for exclusive access to a shared resource. We prove a tight Q(n log n) lower bound on the time for n processes to each access the resource once. .(cont.) Our novel proof technique is based on separately lower bounding the amount of information needed for solving mutex, and upper bounding the amount of information any mutex algorithm can acquire in each step. We hope that our results offer fresh ways of looking at classical problems, and point to interesting new open problemsby Rui Fan.Ph.D
Distinguishing coherent and thermal photon noise in a circuit QED system
In the cavity-QED architecture, photon number fluctuations from residual
cavity photons cause qubit dephasing due to the AC Stark effect. These unwanted
photons originate from a variety of sources, such as thermal radiation,
leftover measurement photons, and crosstalk. Using a capacitively-shunted flux
qubit coupled to a transmission line cavity, we demonstrate a method that
identifies and distinguishes coherent and thermal photons based on
noise-spectral reconstruction from time-domain spin-locking relaxometry. Using
these measurements, we attribute the limiting dephasing source in our system to
thermal photons, rather than coherent photons. By improving the cryogenic
attenuation on lines leading to the cavity, we successfully suppress residual
thermal photons and achieve -limited spin-echo decay time. The
spin-locking noise spectroscopy technique can readily be applied to other qubit
modalities for identifying general asymmetric non-classical noise spectra
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