2,432 research outputs found
Measurement-induced qubit state mixing in circuit QED from up-converted dephasing noise
We observe measurement-induced qubit state mixing in a transmon qubit
dispersively coupled to a planar readout cavity. Our results indicate that
dephasing noise at the qubit-readout detuning frequency is up-converted by
readout photons to cause spurious qubit state transitions, thus limiting the
nondemolition character of the readout. Furthermore, we use the qubit
transition rate as a tool to extract an equivalent flux noise spectral density
at f ~ 1 GHz and find agreement with values extrapolated from a
fit to the measured flux noise spectral density below 1 Hz.Comment: 5 pages, 4 figures. Final journal versio
Trust and privacy in distributed work groups
Proceedings of the 2nd International Workshop on Social Computing, Behavioral Modeling and PredictionTrust plays an important role in both group cooperation and economic exchange. As new technologies emerge for communication and exchange, established mechanisms of trust are disrupted or distorted, which can lead to the breakdown of cooperation or to increasing fraud in exchange. This paper examines whether and how personal privacy information about members of distributed work groups influences individuals' cooperation and privacy behavior in the group. Specifically, we examine whether people use others' privacy settings as signals of trustworthiness that affect group cooperation. In addition, we examine how individual privacy preferences relate to trustworthy behavior. Understanding how people interact with others in online settings, in particular when they have limited information, has important implications for geographically distributed groups enabled through new information technologies. In addition, understanding how people might use information gleaned from technology usage, such as personal privacy settings, particularly in the absence of other information, has implications for understanding many potential situations that arise in pervasively networked environments.Preprin
Stochastic simulations of conditional states of partially observed systems, quantum and classical
In a partially observed quantum or classical system the information that we
cannot access results in our description of the system becoming mixed even if
we have perfect initial knowledge. That is, if the system is quantum the
conditional state will be given by a state matrix and if classical
the conditional state will be given by a probability distribution
where is the result of the measurement. Thus to determine the evolution of
this conditional state under continuous-in-time monitoring requires an
expensive numerical calculation. In this paper we demonstrating a numerical
technique based on linear measurement theory that allows us to determine the
conditional state using only pure states. That is, our technique reduces the
problem size by a factor of , the number of basis states for the system.
Furthermore we show that our method can be applied to joint classical and
quantum systems as arises in modeling realistic measurement.Comment: 16 pages, 11 figure
Qubit-photon interactions in a cavity: Measurement induced dephasing and number splitting
We theoretically study measurement induced-dephasing of a superconducting
qubit in the circuit QED architecture and compare the results to those obtained
experimentally by Schuster {\it et al.}, [Phys. Rev. Lett. 94, 123602 (2005)].
Strong coupling of the qubit to the resonator leads to a significant ac-Stark
shift of the qubit transition frequency. As a result, quantum fluctuations in
the photon number populating the resonator cause dephasing of the qubit. We
find good agreement between the predicted line shape of the qubit spectrum and
the experimental results. Furthermore, in the strong dispersive limit, where
the Stark shift per photon is large compared to the cavity decay rate and the
qubit linewidth, we predict that the qubit spectrum will be split into multiple
peaks, with each peak corresponding to a different number of photons in the
cavity.Comment: 15 pages and 10 figures. Section IV revised. Author and references
added. Version with high resolution figures available at available at
http://www.physique.usherbrooke.ca/~ablais/publications.ht
Networks of nonlinear superconducting transmission line resonators
We investigate a network of coupled superconducting transmission line
resonators, each of them made nonlinear with a capacitively shunted Josephson
junction coupling to the odd flux modes of the resonator. The resulting
eigenmode spectrum shows anticrossings between the plasma mode of the shunted
junction and the odd resonator modes. Notably, we find that the combined device
can inherit the complete nonlinearity of the junction, allowing for a
description as a harmonic oscillator with a Kerr nonlinearity. Using a dc SQUID
instead of a single junction, the nonlinearity can be tuned between 10 kHz and
4 MHz while maintaining resonance frequencies of a few gigahertz for realistic
device parameters. An array of such nonlinear resonators can be considered a
scalable superconducting quantum simulator for a Bose-Hubbard Hamiltonian. The
device would be capable of accessing the strongly correlated regime and be
particularly well suited for investigating quantum many-body dynamics of
interacting particles under the influence of drive and dissipation.Comment: 18 pages, 3 figure
Dissipative dynamics of circuit-QED in the mesoscopic regime
We investigate the behavior of a circuit QED device when the resonator is
initially populated with a mesoscopic coherent field. The strong coupling
between the cavity and the qubit produces an entangled state involving
mesoscopic quasi-pointer states with respect to cavity dissipation. The overlap
of the associated field components results in collapse and revivals for the
Rabi oscillation. Although qubit relaxation and dephasing do not preserve these
states, a simple analytical description of the dissipative dynamics of the
circuit QED device including cavity relaxation as well as qubit dissipation is
obtained from the Monte-Carlo approach. Explicit predictions for the
spontaneous and induced Rabi oscillation signals are derived and sucessfully
compared with exact calculations. We show that these interesting effects could
be observed with a 10 photon field in forthcoming circuit QED experiments.Comment: 10 figures, 1 tabl
Resolving photon number states in a superconducting circuit
Electromagnetic signals are always composed of photons, though in the circuit
domain those signals are carried as voltages and currents on wires, and the
discreteness of the photon's energy is usually not evident. However, by
coupling a superconducting qubit to signals on a microwave transmission line,
it is possible to construct an integrated circuit where the presence or absence
of even a single photon can have a dramatic effect. This system is called
circuit quantum electrodynamics (QED) because it is the circuit equivalent of
the atom-photon interaction in cavity QED. Previously, circuit QED devices were
shown to reach the resonant strong coupling regime, where a single qubit can
absorb and re-emit a single photon many times. Here, we report a circuit QED
experiment which achieves the strong dispersive limit, a new regime of cavity
QED in which a single photon has a large effect on the qubit or atom without
ever being absorbed. The hallmark of this strong dispersive regime is that the
qubit transition can be resolved into a separate spectral line for each photon
number state of the microwave field. The strength of each line is a measure of
the probability to find the corresponding photon number in the cavity. This
effect has been used to distinguish between coherent and thermal fields and
could be used to create a photon statistics analyzer. Since no photons are
absorbed by this process, one should be able to generate non-classical states
of light by measurement and perform qubit-photon conditional logic, the basis
of a logic bus for a quantum computer.Comment: 6 pages, 4 figures, hi-res version at
http://www.eng.yale.edu/rslab/papers/numbersplitting_hires.pd
Non-Markovian stochastic Schr\"odinger equations: Generalization to real-valued noise using quantum measurement theory
Do stochastic Schr\"odinger equations, also known as unravelings, have a
physical interpretation? In the Markovian limit, where the system {\em on
average} obeys a master equation, the answer is yes. Markovian stochastic
Schr\"odinger equations generate quantum trajectories for the system state
conditioned on continuously monitoring the bath. For a given master equation,
there are many different unravelings, corresponding to different sorts of
measurement on the bath. In this paper we address the non-Markovian case, and
in particular the sort of stochastic \sch equation introduced by Strunz, Di\'
osi, and Gisin [Phys. Rev. Lett. 82, 1801 (1999)]. Using a quantum measurement
theory approach, we rederive their unraveling which involves complex-valued
Gaussian noise. We also derive an unraveling involving real-valued Gaussian
noise. We show that in the Markovian limit, these two unravelings correspond to
heterodyne and homodyne detection respectively. Although we use quantum
measurement theory to define these unravelings, we conclude that the stochastic
evolution of the system state is not a true quantum trajectory, as the identity
of the state through time is a fiction.Comment: 17 pages, 3 figure
Proposal for generating and detecting multi-qubit GHZ states in circuit QED
We propose methods for the preparation and entanglement detection of
multi-qubit GHZ states in circuit quantum electrodynamics. Using quantum
trajectory simulations appropriate for the situation of a weak continuous
measurement, we show that the joint dispersive readout of several qubits can be
utilized for the probabilistic production of high-fidelity GHZ states. When
employing a nonlinear filter on the recorded homodyne signal, the selected
states are found to exhibit values of the Bell-Mermin operator exceeding 2
under realistic conditions. We discuss the potential of the dispersive readout
to demonstrate a violation of the Mermin bound, and present a measurement
scheme avoiding the necessity for full detector tomography.Comment: 9 pages, 5 figure
- …