YBa 2 Cu 3 O 7 24 • (30 •) bicrystal grain boundary junctions (GBJs), shunted with 60 nm (20 nm) thick Au, were fabricated by focused ion beam milling with widths 80 nm w 7.8 μm. At 4.2 K we find critical current densities j c in the 10 5 A cm −2 range (without a clear dependence on w) and an increase in resistance times junction area ρ n with an approximate scaling ρ n ∝ w 1/2. For the narrowest GBJs j c ρ n = I c R n ≈ 100 μV (with critical current I c and junction resistance R n), which is promising for the realization of sensitive nanoSQUIDs for the detection of small spin systems. We demonstrate that our fabrication process allows the realization of sensitive nanoscale dc SQUIDs; for a SQUID with w ≈ 100 nm wide GBJs we find an rms magnetic flux noise spectral density of S 1/2 ≈ 4 μ 0 Hz −1/2 in the white noise limit. We also derive an expression for the spin sensitivity S 1/2 μ , which depends on S 1/2 , on the location and orientation of the magnetic moment of a magnetic particle to be detected by the SQUID, and on the SQUID geometry. For the unoptimized SQUIDs presented here, we estimate S 1/2 μ = 390 μ B Hz −1/2 , which could be further improved by at least an order of magnitude.

We propose to couple a trapped single electron to superconducting structures located at a variable distance from the electron. The electron is captured in a cryogenic Penning trap using electric fields and a static magnetic field in the Tesla range. Measurements on the electron will allow investigating the properties of the superconductor such as vortex structure, damping and decoherence. We propose to couple a superconducting microwave resonator to the electron in order to realize a circuit QED-like experiment, as well as to couple superconducting Josephson junctions or superconducting quantum interferometers (SQUIDs) to the electron. The electron may also be coupled to a vortex which is situated in a double well potential, realized by nearby pinning centers in the superconductor, acting as a quantum mechanical two level system that can be controlled by a transport current tilting the double well potential. When the vortex is trapped in the interferometer arms of a SQUID, this would allow its detection both by the SQUID and by the electron.

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Strained YBa2Cu3O7−δ (YBCO) films of greater than the critical thickness are grown on LaAlO3 (100) substrates by the method of laser ablation. The strains are frozen in the films by rapid cooling after deposition. These films have an unusual temperature dependence of the critical current density: a characteristic minimum is observed in the temperature interval 55–57 K. The critical current density decreases from 106A∕cm2 at 77 K to 104A∕cm2 and lower. The films are stable against thermocycling from 300 K to the boiling point of liquid nitrogen. It is concluded on the basis of a scanning tunneling microscope study that a macroscopic structuring of the films occurs, with the formation of strain domains. The size of the domains decreases with increasing degree of strain of the films and is, on average, from 1 to 2.4μm. The penetration of magnetic field into a film with frozen strain differs from the penetration into single-crystal and granular films; this also suggests a macroscopic structuring in the strained films.

YBa 2 Cu 3 O 7 24 • (30 • ) bicrystal grain boundary junctions (GBJs), shunted with 60 nm (20 nm) thick Au, were fabricated by focused ion beam milling with widths 80 nm ≤ w ≤ 7.8 µm. At 4.2 K we find critical current densities j c in the 10 5 A/cm 2 range (without a clear dependence on w) and an increase in resistance times junction area ρ with an approximate scaling ρ ∝ w 1/2 . For the narrowest GBJs j c ρ ≈ 100 µV, which is promising for the realization of sensitive nanoSQUIDs for the detection of small spin systems. We demonstrate that our fabrication process allows the realization of sensitive nanoscale dc SQUIDs; for a SQUID with w ≈ 100 nm wide GBJs we find an rms magnetic flux noise spectral density of S 1/2 Φ ≈ 4 µΦ 0 /Hz 1/2 in the white noise limit. We also derive an expression for the spin sensitivity S 1/2 µ , which depends on S 1/2 Φ , on the location and orientation of the magnetic moment of a magnetic particle to be detected by the SQUID, and on the SQUID geometry. For the not optimized SQUIDs presented here, we estimate S 1/2 µ = 390 µ B / √ Hz, which could be further improved by at least an order of magnitude.