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Office:1365 Physics (301) 405-6130 lobb@squid.umd.edu

I. Quantum Computation using superconducting devices .

This project involves a collaboration with and Profs. J. R. Anderson , A. J. Dragt , and F. Wellstood , postdoctoral research associate Dr. Rupert Lewis, and Professor R. Ramos at Drexel University . Our group was the first to propose using an individual current biased Josephson junction as a qubit and we and other groups have now made significant progress towards its realization. Our group's most significant recent accomplishments include:

A. Reported detailed calculations of the spectra of two-coupled qubits (prior to the experimental work) and also proposed schemes for building quantum gates in this system that can have a high-fidelity and low-leakage. The spectroscopic calculations were reported in Phys. Rev. Letters.

B. Spectroscopic measurements of the quantum energy levels in two coupled Josephson junction qubits . We see an avoided energy level crossing when the energy levels of the two qubits are brought into coincidence. We interpret the results as providing evidence for the existence of quantum states of the coupled system that theory tells us are entangled. We also reported spectroscopic coherence times and evidence for escape rate broadening of the transitions. Although the measured coherence times are short, just a few ns due to low-frequency current noise, the times are not shorter than in the uncoupled devices. Another remarkable aspect of this result is that the qubits were separated by a distance of almost 1 mm. This work was published in Science .

C. Reported spectroscopic measurements of the energy levels in two junction qubits that are coupled to an LC (inductor-capacitor) resonator . In this three-body system, we see a triple avoided crossing which theory indicates involves a superposition of states of the three different qubits. Detailed calculations of the full three-body quantum system reveal good agreement with the data. The spectra also show evidence for resonant coupling between the two junctions, mediated by the LC resonator. A preprint of this work is available on CondMat , and the article itself can be found at Phys. Rev. Letters.

D. Developed a robust technique for measuring the relaxation time T 1 in a junction qubit. This work was published in PRB Rapid Communications .

E. Analyzed sources and effects of de-coherence in a current-biased josephson junction qubit . This work was published in Phys. Rev. B .

 

II. Phase Transitions in Superconductors

The hallmark of superconductivity, and the basis of its name, is zero resistance for temperature T below a transition temperature T c . Since the discovery of superconductivity, much research has been done to understand the limits of this zero-resistance state. It is known, for example, that a magnetic field greater than the critical field H c (T) will destroy superconductivity in a type-I superconductor, as will a current density greater than the critical current density J c (T).

One other property that determines whether the resistance goes to zero, or is, perhaps, just very small, is the spatial dimensionality of the system, D. In zero magnetic field, and for small currents, we know that the resistance is not strictly zero for one-dimensional systems. The resistance is zero for D=3, and for D=2, some systems do not have zero resistance, while others will. Interestingly, the two-dimensional superconductors that have zero resistance also have zero critical current.

At any non-zero temperature, fluctuations occur because a system can borrow an energy kT from its environment. For T< T c , this makes it possible to temporarily increase the energy of a small volume of superconductor, perhaps enough to drive the small volume into the normal state. The effects of such fluctuations may be relatively benign, perhaps weakening superconductivity without destroying it. Under the right circumstances, however, fluctuations can destroy the superconducting state.

The discovery of high-temperature superconductors brought about renewed interest in fluctuations and the superconducting phase transition [1] . At Maryland , we studied single-unit-cell films of YBa 2 Cu 3 O 7- d in zero magnetic field [2] . These samples are presumably as two-dimensional as you can get, yet we showed that they do not become superconducting. The resistance of these unit-cell thick films remained non-zero (but very small) to very low temperatures.

We next studied thick (D=3) samples in a magnetic field, because there were a very large number of recent theoretical and experimental papers on the topic, with theory and experiment agreeing that a new type of phase transition (depending on the type of pining in the sample, a vortex-glass [3] or a Bose-glass [4] transition) occurred in a field. Our experimental results in magnetic field were very similar to other people's results. When analyzing our data, however, our conclusions were not in agreement with most others': The resistance did not go to zero, suggesting that the superconducting phase transition does not occur in magnetic field for D=3 [5] .

Building on the results obtained in magnetic field, we next did D=3 zero field experiments. The results were disturbing: The zero-field results were very similar to the non-zero field results. In particular, the resistance did not appear to be going to zero in the manner expected as temperature was lowered. We were able to show that our samples were not sufficiently three dimensional: Even in very thick films (0.32 m m), the experiment's length scales were limited by the film thickness. Thus, sufficiently close to T c , the samples become two-dimensional, and the phase transition is interrupted [6].

REFERENCES

1. C. J. Lobb, Phys. Rev. B 36 , 3930 (1987).

2. J. M. Repaci, C. Kwon, Qi Li, Xiguang Jiang, T. Venkatesan,
R. E. Glover III, and C. J. Lobb, Phys. Rev. B 54, R9674 (1996).

3. D. S. Fisher, M. P. A. Fisher, and D. A. Huse, Phys. Rev. B 43 , 130 (1991), David A. Huse, Matthew P. A. Fisher, and Daniel S. Fisher, Nature 358 , 553 (1992).

4. D. R. Nelson and V. M. Vinokur, Phys. Rev. Lett. 68 , 2398 (1992).

5. D. R. Strachan, M. C. Sullivan, P. Fournier, S. P. Pai, T. Venkatesan, and C. J. Lobb, Phys. Rev. Lett. 87 , 067007 (2001).

6. M. C. Sullivan, D. R. Strachan, T. Frederiksen, R. A. Ott, M. Lilly, and C. J. Lobb, Phys. Rev. B 69 , 214524 (2004).

 

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Center for Superconductivity Research, University of Maryland, College Park, MD 20742-4111
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