4 research outputs found
Probing flux and charge noise with macroscopic resonant tunneling
We report on measurements of flux and charge noise in an rf-SQUID flux qubit
using macroscopic resonant tunneling (MRT). We measure rates of incoherent
tunneling from the lowest energy state in the initial well to the ground and
first excited states in the target well. The result of the measurement consists
of two peaks. The first peak corresponds to tunneling to the ground state of
the target well, and is dominated by flux noise. The second peak is due to
tunneling to the excited state and is wider due to an intrawell relaxation
process dominated by charge noise. We develop a theoretical model that allows
us to extract information about flux and charge noise within one experimental
setup. The model agrees very well with experimental data over a wide dynamic
range and provides parameters that characterize charge and flux noise.Comment: 11 pages, 5 figure
Quantum critical dynamics in a 5000-qubit programmable spin glass
Experiments on disordered alloys suggest that spin glasses can be brought
into low-energy states faster by annealing quantum fluctuations than by
conventional thermal annealing. Due to the importance of spin glasses as a
paradigmatic computational testbed, reproducing this phenomenon in a
programmable system has remained a central challenge in quantum optimization.
Here we achieve this goal by realizing quantum critical spin-glass dynamics on
thousands of qubits with a superconducting quantum annealer. We first
demonstrate quantitative agreement between quantum annealing and time-evolution
of the Schr\"odinger equation in small spin glasses. We then measure dynamics
in 3D spin glasses on thousands of qubits, where simulation of many-body
quantum dynamics is intractable. We extract critical exponents that clearly
distinguish quantum annealing from the slower stochastic dynamics of analogous
Monte Carlo algorithms, providing both theoretical and experimental support for
a scaling advantage in reducing energy as a function of annealing time
A fluorescence study of single trapped Ytterbium ions for quantum information applications
Trapped ions are one of the most promising candidates for quantum information applications. We describe an experimental setup using a linear rf Paul trap for confining 171Yb+ ions. In particular, the required lasers and lock setups are described. One of the key requirements for qubit manipulation is achieving high fidelity state-selective detection. A well established means to accomplish this is through the use of laser fluorescence. The fluorescence theory of 171Yb+, including the effect of coherent population trapping which can suppress the fluorescence, is documented together with the counter-acting effect of a magnetic field. In addition, the fluorescence theory of 174Yb+, which has simpler atomic structure, is also described for comparison. The resonance fluorescence behaviour of both isotopes is studied experimentally as a function of magnetic field, laser polarization, power and detuning. The experimental results for both isotopes agree with theoretical models, including the effect of coherent population trapping
Dynamics of Trapped Ions Near the Linear-Zigzag Structural Phase Transition
Laser-cooled ions held in a linear Paul trap with strong transverse confinement organize into a one-dimensional (1-D) linear crystal. If the transverse confinement is relaxed, the linear ion crystal undergoes a continuous, structural phase transition to a 2-D zigzag configuration. We study the dynamics near the critical point of the linear-zigzag transition. In the first part of this thesis, we study the spontaneous nucleation and dynamics of topological kink defects, formed as a result of a rapid quench across the linear-zigzag transition. The experimental results are compared to the Kibble-Zurek mechanism, which provides an intuitive model of defect formation and predicts a power-law scaling for the number of defectsformed as a function of transition quench rate. The second part of this thesis is focused on one of the key requirements for investigations of the near-transition dynamics in the quantum regime. To achieve an efficient ground state cooling of the zigzag vibrational mode, we demonstrate 3-D polarization-gradient cooling of strings of 1-4 trapped ions as an intermediate step between Doppler and sideband cooling, and study the polarization-gradient cooling rate and cooling limit as a function of the cooling beam intensity in and near the Lamb-Dicke regime. The results of this thesis pave the way towards our future experiments aimed at assessing the coherence time of a zigzag superposition state through measurements of tunneling oscillations