843 research outputs found
Learning trust
We examine the effects of different forms of feedback information on the performance of markets that suffer from moral hazard problems due to sequential exchange. As orthodox theory would predict, we find that providing buyers with information about sellers' trading history boosts market performance. More surprisingly, this beneficial effect of incentives for reputation building is considerably enhanced if sellers, too, can observe other sellers' trading history. This suggests that two-sided market transparency is an important ingredient for the design of well-functioning markets that are prone to moral hazard
Measuring out-of-time-order correlations and multiple quantum spectra in a trapped ion quantum magnet
Controllable arrays of ions and ultra-cold atoms can simulate complex
many-body phenomena and may provide insights into unsolved problems in modern
science. To this end, experimentally feasible protocols for quantifying the
buildup of quantum correlations and coherence are needed, as performing full
state tomography does not scale favorably with the number of particles. Here we
develop and experimentally demonstrate such a protocol, which uses time
reversal of the many-body dynamics to measure out-of-time-order correlation
functions (OTOCs) in a long-range Ising spin quantum simulator with more than
100 ions in a Penning trap. By measuring a family of OTOCs as a function of a
tunable parameter we obtain fine-grained information about the state of the
system encoded in the multiple quantum coherence spectrum, extract the quantum
state purity, and demonstrate the buildup of up to 8-body correlations. Future
applications of this protocol could enable studies of many-body localization,
quantum phase transitions, and tests of the holographic duality between quantum
and gravitational systems.Comment: main text: 7 pages, 4 figures; supplement: 9 pages, 4 figure
General Formalism for Evaluating the Impact of Phase Noise on Bloch Vector Rotations
Quantum manipulation protocols for quantum sensors and quantum computation
often require many single qubit rotations. However, the impact of phase noise
in the field that performs the qubit rotations is often neglected or treated
only for special cases. We present a general framework for calculating the
impact of phase noise on the state of a qubit, as described by its equivalent
Bloch vector. The analysis applies to any Bloch vector orientation, and any
rotation axis azimuthal angle for both a single pulse, and pulse sequences.
Experimental examples are presented for several special cases. We apply the
analysis to commonly used composite -pulse sequences: CORPSE, SCROFULOUS,
and BB1, used to suppress static amplitude and detuning errors, and also to
spin echo sequences. We expect the formalism presented will help guide the
development and evaluation of future quantum manipulation protocols.Comment: 12 pages, 6 figures, submitted to PR
Verification of a many-ion simulator of the Dicke model through slow quenches across a phase transition
We use a self-assembled two-dimensional Coulomb crystal of ions in
the presence of an external transverse field to engineer a simulator of the
Dicke Hamiltonian, an iconic model in quantum optics which features a quantum
phase transition between a superradiant/ferromagnetic and a normal/paramagnetic
phase. We experimentally implement slow quenches across the quantum critical
point and benchmark the dynamics and the performance of the simulator through
extensive theory-experiment comparisons which show excellent agreement. The
implementation of the Dicke model in fully controllable trapped ion arrays can
open a path for the generation of highly entangled states useful for enhanced
metrology and the observation of scrambling and quantum chaos in a many-body
system.Comment: 6 + 5 pages, 2 + 5 figures. arXiv admin note: substantial text
overlap with arXiv:1711.0739
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