[Show abstract][Hide abstract] ABSTRACT: We have grown epitaxial Rhenium (Re) (0001) films on α-Al2O3 (0001) substrates using sputter deposition in an ultra high vacuum system. We find that better epitaxy is achieved with DC rather than with RF sputtering. With DC sputtering, epitaxy is obtained with the substrate temperatures above 700 °C and deposition rates below 0.1 nm/s. The epitaxial Re films are typically composed of terraced hexagonal islands with screw dislocations, and island size gets larger with high temperature post-deposition annealing. The growth starts in a three dimensional mode but transforms into two dimensional mode as the film gets thicker. With a thin (∼2 nm) seed layer deposited at room temperature and annealed at a high temperature, the initial three dimensional growth can be suppressed. This results in larger islands when a thick film is grown at 850 °C on the seed layer. We also find that when a room temperature deposited Re film is annealed to higher temperatures, epitaxial features start to show up above ∼600 °C, but the film tends to be disordered.
[Show abstract][Hide abstract] ABSTRACT: At present, the performance of superconducting qubits is limited by decoher- ence. Strong decoherence of phase qubits is associated with spurious microwave resonators residing within the Josephson junction tunnel barrier (1). In this work, we investigate three different fabrication techniques for producing tun- nel junctions that vary the properties of the superconductor-insulator interface. Through experimental measurements, we characterize the junction and corre- sponding qubit quality. We find that there is a strong correlation between the morphology of oxidized base electrodes and the lowering of subgap currents in the junction I-V characteristics, while there is no noticeable improvement in the performance of fabricated phase qubits. Thus, "traditional" indicators of junc- tion performance may not be enough to determine qubit performance. However, truly crystalline insulating barriers may be the key to improving Josephson junc- tion based qubits.
[Show abstract][Hide abstract] ABSTRACT: Dielectric loss from two-level states is shown to be a dominant decoherence source in superconducting quantum bits. Depending on the qubit design, dielectric loss from insulating materials or the tunnel junction can lead to short coherence times. We show that a variety of microwave and qubit measurements are well modeled by loss from resonant absorption of two-level defects. Our results demonstrate that this loss can be significantly reduced by using better dielectrics and fabricating junctions of small area . With a redesigned phase qubit employing low-loss dielectrics, the energy relaxation rate has been improved by a factor of 20, opening up the possibility of multiqubit gates and algorithms.
Full-text · Article · Dec 2005 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: We have developed a two-step growth scheme for single-crystal Al2O3 tunnel barriers. The barriers are epitaxially grown on single-crystal rhenium (Re) base electrodes that are grown epitaxially on a sapphire substrate, while polycrystalline Al is used as the top electrode. We show that by first growing an amorphous aluminium (Al) oxide layer at room temperature and crystallizing it at a high temperature in oxygen environment, a morphologically intact single-crystal Al2O3 layer is obtained. Tunnel junctions fabricated from these trilayers show very low subgap leakage current. This single-crystal Al2O3 junction may open a new venue for coherent quantum devices.
[Show abstract][Hide abstract] ABSTRACT: One of the many challenges of building a scalable quantum computer is single-shot measurement of all the quantum bits (qubits).
We have used simultaneous single-shot measurement of coupled Josephson phase qubits to directly probe interaction of the qubits
in the time domain. The concept of measurement crosstalk is introduced, and we show that its effects are minimized by careful
adjustment of the timing of the measurements. We observe the antiphase oscillation of the two-qubit |01〉 and |10〉 states,
consistent with quantum mechanical entanglement of these states, thereby opening the possibility for full characterization
of multiqubit gates and elementary quantum algorithms.
[Show abstract][Hide abstract] ABSTRACT: We have detected coherent quantum oscillations between Josephson phase qubits and critical-current fluctuators by implementing a new state readout technique that is an order of magnitude faster than previous methods. These results reveal a new aspect of the quantum behavior of Josephson junctions, and they demonstrate the means to measure two-qubit interactions in the time domain. The junction-fluctuator interaction also points to a possible mechanism for decoherence and reduced fidelity in superconducting qubits.
Full-text · Article · Nov 2004 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: Although it is commonly believed that decoherence from quasiparticles can be estimated from the subgap quasiparticle resistance, this concept is in fact incorrect. We will review the theory of quasiparticle bound states and show how Josephson junctions in the zero voltage state, as appropriate for qubits, are strongly protected from quasiparticle dissipation because the qubit energies are lower than the energy gap. This protection is strong enough that even a microbridge junction may be used for a Josephson phase qubit. However, this analysis also indicates that quasiparticle trapping sites near the junction may produce decoherence, which possibly explains why Nb-based junctions have worse coherence than for Al.
[Show abstract][Hide abstract] ABSTRACT: Superconducting tunnel junction devices are promising candidates for constructing quantum bits (qubits) for quantum computation because of their inherently low dissipation and ease of scalability by microfabrication. Recently, the Josephson phase qubit has been characterized spectroscopically as having spurious microwave resonators that couple to the qubit and act as a dominant source of decoherence. While the origin of these spurious resonances remains unknown, experimental evidence points to the material system of the tunnel barrier. Here, we focus on our materials research aimed at elucidating and eliminating these spurious resonators. In particular, we have studied the use of high quality Al films epitaxially grown on Si(111) as the base electrode of the tunnel junction. During each step in the Al/AlOx/Al trilayer growth, we have investigated the structure in situ by AES, AED and LEED. While tunnel junctions fabricated with these epitaxial base electrodes prove to be of non-uniform oxide thickness and too thin, I-V characteristics have shown a lowering of subgap currents by a factor of two. Transport measurements will be correlated with morphological structure for a number of devices fabricated with various degrees of crystalline quality.
[Show abstract][Hide abstract] ABSTRACT: The current-biased Josephson junction is an attractive candidate for a quantum bit. Recent results suggest that decoherence is dominated by quantum mechanical coupling of the junction to resonant defects in the oxide tunnel barrier. Progress in the field of superconducting qubits will therefore require deep understanding of materials issues and exploration of novel techniques for junction growth and fabrication. Here we present preliminary results on the fabrication and characterization of Al-AlOx-Al trilayer tunnel junctions for qubits. The trilayer junctions display subgap currents which are an order of magnitude lower than those for junctions fabricated with an ion-mill clean of the base electrode, suggesting a larger number of conduction channels with a relatively low transmission probability. When operated as qubits, the trilayer junctions show smaller resonant splittings in cw spectroscopy, and coherence times in excess of 80 ns. Finally, we describe ongoing efforts to fabricate qubits from UHV-grown trilayers which involve crystalline electrodes and crystalline tunnel barriers.