High-fidelity initialization and control of electron and nuclear spins in a four-qubit register.

Single electron spins bound to multi-phosphorus nuclear spin registers in silicon have demonstrated fast (0.8 ns) two-qubit SWAP gates and long spin relaxation times (~30 s). In these spin registers, when the donors are ionized, the nuclear spins remain weakly coupled to their environment, allowing exceptionally long coherence times. When the electron is present, the hyperfine interaction allows coupling of the spin and charge degrees of freedom for fast qubit operation and control. Here we demonstrate the use of the hyperfine interaction to enact electric dipole spin resonance to realize high-fidelity ( F = 10 0 - 6 + 0 %) initialization of all the nuclear spins within a four-qubit nuclear spin register. By controllably initializing the nuclear spins to ⇓ ⇓ ⇓ , we achieve single-electron qubit gate fidelities of F = 99.78 ± 0.07% (Clifford gate fidelities of 99.58 ± 0.14%), above the fault-tolerant threshold for the surface code with a coherence time of T 2 * = 12 µ s .

Dressed-state amplification by a single superconducting qubit.

We demonstrate amplification of a microwave signal by a strongly driven two-level system in a coplanar waveguide resonator. The effect, similar to the dressed-state lasing known from quantum optics, is observed with a single quantum system formed by a persistent current (flux) qubit. The transmission through the resonator is enhanced when the Rabi frequency of the driven qubit is tuned into resonance with one of the resonator modes. Amplification as well as linewidth narrowing of a weak probe signal has been observed. The stimulated emission in the resonator has been studied by measuring the emission spectrum. We analyzed our system and found an excellent agreement between the experimental results and the theoretical predictions obtained in the dressed-state model.

Tuned transition from quantum to classical for macroscopic quantum states.

The boundary between the classical and quantum worlds has been intensely studied. It remains fascinating to explore how far the quantum concept can reach with use of specially fabricated elements. Here we employ a tunable flux qubit with basis states having persistent currents of 1 µA carried by a million pairs of electrons. By tuning the tunnel barrier between these states we see a crossover from quantum to classical. Released from nonequilibrium, the system exhibits spontaneous coherent oscillations. For high barriers the lifetime of the states increases dramatically while the tunneling period approaches the phase coherence time and the oscillations fade away.

Strong coupling of a quantum oscillator to a flux qubit at its symmetry point.

A flux qubit biased at its symmetry point shows a minimum in the energy splitting (the gap), providing protection against flux noise. We have fabricated a qubit of which the gap can be tuned fast and have coupled this qubit strongly to an LC oscillator. We show full spectroscopy of the qubit-oscillator system and generate vacuum Rabi oscillations. When the gap is made equal to the oscillator frequency ν(osc) we find the largest vacuum Rabi splitting of ∼0.1 ν(osc). Here being at resonance coincides with the optimal coherence of the symmetry point.

Genetic variation in a Tanzanian population of Glossina swynnertoni (Diptera: Glossinidae).

Adult Glossina swynnertoni Austen that emerged from puparia collected during 1989 and 1991 near Makuyuni, Tanzania, were examined by polyacrylamide gel electrophoresis. Fourteen of 17 enzymes were monomorphic. Midgut alkaline phosphatase (ALKPH), phosphoglucomutase (PGM), and glucose-6-phosphate isomerase (PGI) from the head and thorax were polymorphic. Banding patterns indicated that the locus for PGM was on the X chromosome and loci for ALKPH and PGI were autosomal. For the 17 loci studied, the mean heterozygosity per locus was 6.1 +/- 3.7% in the 1989 sample and 5.7 +/- 3.7% in the 1991 sample. The effective number of alleles per locus was 1.11 and 1.10 in these samples. This level of genetic variation was low compared with other populations of tsetse flies and indicated that the sample may have been drawn from a small inbred population or one that recently had gone through a genetic bottleneck.