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Drexel's first Superconducting Artificial Atom

http://www.physics.drexel.edu/~lowtemp/Drexel_artificial_atom.pdf

We have performed microwave spectroscopy measurements on an isolated Josephson junction "phase qubit" at 20 mK. The phase qubit is an artificial atom that was first proposed by Dr. Ramos* with collaborators in 2001 and is now one of the leading "qubits" being studied around the world. The series of peaks show the transition from ground state |0> to first excited state |1> of a device that is 10 microns x 10 microns in size. Because it is 100,000 times bigger than a real atom, we can connect wires to it and tune its quantum properties with macroscopic currents and voltages.

*Design for Effective Thermalization of Junctions for Quantum Coherence, R. C. Ramos, et al., IEEE Trans. On Appl. Supercond., vol. 11, pp. 998-1001 (2001).

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Quantum-to-Classical Crossover

http://www.physics.drexel.edu/~lowtemp/quantum_classical_crossover.pdf

We have systematically studied the quantum-to-classical crossover behavior of Josephson junctions. At base temperature, the escape rate enhancement peak represents the transition from ground state |0>→|1>, the first excited state. At higher temperatures, a second peak representing |1>→|2> quantum transitions appears since the |1> state is thermally populated. Close to the crossover temperature T_cr = hf/(2πkT), the two peaks broaden to create a step with an "elbow" at the classical plasma frequency ωp.

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Microwave Resonant Activation in MgB2 Junctions

http://www.physics.drexel.edu/~lowtemp/mgb2_data_thermal_activation_Ramos.pdf

We have made the first microwave resonant activation of MgB2 junctions. Below 1K, we can manipulate the classical resonator states of MgB2 junctions using both microwave power, frequency and current bias. We acknowledge Prof. Xi (Temple University) for providing high quality samples.

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Sub-Kelvin Differential Conductance of MgB2 Junctions

http://www.physics.drexel.edu/~lowtemp/mgb2_data_subgap_Ramos.pdf

We have made the first sub-Kelvin I-V and differential conductance measurements of MgB2 Josephson junctions showing sub-gap structure as predicted by H. J. Choi, D. Roundy, H. Sun, M. L. Cohen, and S. G. Louie, Nature 418, 758 (2002) .

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Graphene-based Quantum Devices.

http://www.physics.drexel.edu/~lowtemp/graphene_junctions.pdf

We are investigating macroscopic quantum effects in graphene-based quantum devices. Graphene is a single atomic layer of carbon that exhibits interesting properties including high mobilities and quantum coherence. We have fabricated 2-4-layer graphene which we have contacted with superconducting electrodes to study superconductivity, measure Josephson supercurrents and probe for quantum states at mK temperatures in these graphene junctions. We collaborate with the Mesomaterials and Nanomaterials groups in the College of Engineering at Drexel University.

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Laser- and Particle-induced Defects in Graphene.

http://www.physics.drexel.edu/~lowtemp/graphene_laser_induced_defects.pdf

One of our undergraduates, Gali Galwaduge, has collected new data measuring the impact of defects in single-layer graphene, which is otherwise a pristine honeycomb network of carbon atoms. Such defects contribute disorder in this highly-ordered two-dimensional crystal. Gali uses lasers and highly energetic alpha particles to produce defects and measures the changes in the peak heights and positions in Raman spectroscopy.

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Multiply-coupled Phase Qubits.

We are studying theoretically and experimentally how multiple Josephson phase qubits that are coupled to each other and networked in different topologies can be used to transmit quantum state information. We have simulated the resulting entangled quantum states, mapped their energy levels and are performing microwave spectroscopy and time-domain experiments of these coupled devices at 10-20 mK.

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Berry's Phase and Lasing in Phase Qubits.

Our undergraduate students are investigating how Berry's phase is acquired in quantum systems involving phase qubits and its role in topological quantum computing. We are also studying how lasing can occur in this type of qubit.