Impact of measurement backaction on nuclear spin qubits in silicon

Phosphorus donor nuclear spins in silicon couple weakly to the environment making them promising candidates for high-fidelity qubits. The state of a donor nuclear spin qubit can be manipulated and read out using its hyperfine interaction with the electron confined by the donor potential. Here we use a master equation-based approach to investigate how the backaction from this electron-mediated measurement affects the lifetimes of single and multi-donor qubits. We analyze this process as a function of electric and magnetic fields, and hyperfine interaction strength. Apart from single nuclear spin flips, we identify an additional measurement-related mechanism, the nuclear spin flip-flop, which is specific to multi-donor qubits. Although this flip-flop mechanism reduces qubit lifetimes, we show that it can be effectively suppressed by the hyperfine Stark shift. We show that using atomic precision donor placement and engineered Stark shift, we can minimize the measurement backaction in multi-donor qubits, achieving larger nuclear spin lifetimes than single donor qubits

Final report on the CCPR Key Comparison CCPR-K3.2014 Luminous Intensity

Main text The metrological equivalence of national measurement standards in the field of photometry and radiometry is determined by a set of key comparisons chosen and organised by the Consultative Committee of Photometry and Radiometry (CCPR) of the Comité international des poids et mesures (CIPM), working closely with the Regional Metrology Organisations (RMOs). In September 2009 the CCPR decided that a second round of the key comparison K3 Luminous Intensity be commenced. The National Research Council of Canada (NRC) was chosen to pilot this comparison. A total of 12 participants were selected from the three RMO group members: EURAMET&COOMET (6: IO-CSIC, LNE-CNAM, METAS, NPL, PTB, VNIIOFI), APMP&AFRIMETS (4: NMISA, NIM, NMIA, NMIJ), and SIM (2: NIST, NRC). The comparison was organised as a star comparison (NMI-Pilot-NMI) using incandescent standard lamps supplied by each NMI (National Metrology Institute) as the travelling comparison artifact. This report describes the comparison organisation (Section 2), the measurement methods and uncertainties achieved at all the participants and at the pilot (Sections 3 and 4), and the method for analysis and the results of the comparison according to this method (Section 4). It includes a comparison of the results of this comparison with the 1999 first round key comparison (Section 5). Section 6 presents a summary of the comparison. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/. The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA)

A view through the clouds of imprinting.

The purpose of this review is to examine whether our current knowledge of the higher order control of gene expression and nuclear organization can help us understand the mechanisms of genomic imprinting. Imprinting involves the inheritance of a silenced allele of a gene through either a paternal or maternal germline. We have approached the problem of imprinting using a model based on the dynamic attachment of chromatin loops to immobilized RNA polymerases and control elements. We have combined the information from different experimental approaches, examining primarily the IGF2-H19 locus, in an attempt to simplify the complexity of the imprinting data that has accumulated. It is hoped that a unified model may generate predictions amenable to experimental testing and contribute to the interpretation of future experiments