Gaussian Thermal Operations and The Limits of Algorithmic Cooling.

The study of thermal operations allows one to investigate the ultimate possibilities of quantum states and of nanoscale thermal machines. Whilst fairly general, these results typically do not apply to continuous variable systems and do not take into account that, in many practically relevant settings, system-environment interactions are effectively bilinear. Here we tackle these issues by focusing on Gaussian quantum states and channels. We provide a complete characterization of the most general Gaussian thermal operation acting on an arbitrary number of bosonic modes, which turn out to be all embeddable in a Markovian dynamics, and derive necessary and sufficient conditions for state transformations under such operations in the single-mode case, encompassing states with nonzero coherence in the energy eigenbasis (i.e., squeezed states). Our analysis leads to a no-go result for the technologically relevant task of algorithmic cooling: We show that it is impossible to reduce the entropy of a system coupled to a Gaussian environment below its own or the environmental temperature, by means of a sequence of Gaussian thermal operations interspersed by arbitrary (even non-Gaussian) unitaries. These findings establish fundamental constraints on the usefulness of Gaussian resources for quantum thermodynamic processes.

Ethnic disparities in the oral health of three- to four-year-old children in East London.

AIM: To report ethnic differences related to caries experience among three- to four-year-old children living in three of the most deprived boroughs in the UK in Inner North East London: Tower Hamlets, Hackney and Newham. METHODS: This cross-sectional survey used a cluster sampling study design following the British Association for the Study of Community Dentistry protocol. Twenty nurseries from each borough were randomly selected and all three- to four-year-old children in selected nurseries were invited to participate (n = 2,434). Calibrated dentists examined children. Demographic information was obtained from schools. RESULTS: One thousand, two hundred and eighty-five children were examined in 60 nurseries (response rate = 52.8%). Twenty-four percent of three- to four-year-old children had caries experience (mean dmft = 0.92). Few children (2.1%) had filled teeth. Children living in Hackney had significantly lower dmft scores (mean = 0.63) than children living in Newham (mean = 1.06) and Tower Hamlets (mean = 1.06). White European (mean = 1.91), Bangladeshi (mean = 1.05) and Pakistani (mean = 1.11) children had a significantly higher number of untreated carious teeth than White British children (mean = 0.56). CONCLUSION: Preschool children from a White Eastern European, Bangladeshi and Pakistani background are likely to experience significantly poorer oral health than their White British counterparts. These findings have profound implications for commissioning dental services and oral health promotion.