Observation of non-contact Casimir friction

Quantum mechanics predicts the occurrence of random electromagnetic field fluctuations, or virtual photons, in vacuum. The exchange of virtual photons between two bodies in relative motion could lead to non-contact quantum vacuum friction or Casimir friction. Despite its theoretical significance, the non-contact Casimir frictional force has not been observed and its theoretical predictions have varied widely. In this work, we report the first measurement of the non-contact Casimir frictional force between two moving bodies. By employing two mechanical oscillators with resonant frequencies far lower than those in Lorentz models of electrons in dielectric materials, we have amplified the Casimir frictional force at low relative velocities by several orders of magnitude. We directly measure the non-contact Casimir frictional force between the two oscillators and show its linear dependence on velocity, proving the dissipative nature of Casimir friction. This advancement marks a pivotal contribution to the field of dissipative quantum electrodynamics and enhances our understanding of friction at the nanoscale

Optically Levitated Nanodumbbell Torsion Balance and GHz Nanomechanical Rotor

Levitated optomechanics has great potentials in precision measurements, thermodynamics, macroscopic quantum mechanics and quantum sensing. Here we synthesize and optically levitate silica nanodumbbells in high vacuum. With a linearly polarized laser, we observe the torsional vibration of an optically levitated nanodumbbell in vacuum. The linearly-polarized optical tweezer provides a restoring torque to confine the orientation of the nanodumbbell, in analog to the torsion wire which provides restoring torque for suspended lead spheres in the Cavendish torsion balance. Our calculation shows its torque detection sensitivity can exceed that of the current state-of-the-art torsion balance by several orders. The levitated nanodumbbell torsion balance provides rare opportunities to observe the Casimir torque and probe the quantum nature of gravity as proposed recently. With a circularly-polarized laser, we drive a 170-nm-diameter nanodumbbell to rotate beyond 1~GHz, which is the fastest nanomechanical rotor realized to date. Our calculations show that smaller silica nanodumbbells can sustain rotation frequency beyond 10 GHz. Such ultrafast rotation may be used to study material properties and probe vacuum friction

Fat- and iron-corrected ADC to assess liver fibrosis in patients with chronic hepatitis B

PURPOSE:We aimed to evaluate the diagnostic performance of apparent diffusion coefficient (ADC) in assessing liver fibrosis after correcting for the effects of hepatic steatosis or iron deposition.METHODS:Seventy-three patients with chronic hepatitis B (CHB) were included in this retrospective study. The aspartate aminotransferase-to-platelet ratio index (APRI) was calculated for classification of the fibrosis grade. Significant fibrosis and cirrhosis were diagnosed with the APRI. The proton density fat fraction (PDFF), R2*, and ADC values were measured. The impact of the PDFF and R2* on the ADC was analyzed. The PDFF- and R2*-corrected ADC values (ADCPDFF and ADCR2*) were calculated according to linear regression equations. The diagnostic performance of uncorrected ADC (ADCu), ADCPDFF and ADCR2* in predicting significant fibrosis and cirrhosis was assessed, and the area under the curve (AUC) values were compared.RESULTS:Among the 73 patients in this study, the mean ADC was 0.866 ± 0.084×10-3 mm2/s, the mean R2* was 60.24 (42.77, 85.37) 1/s, and the mean PDFF was 2.90% (1.60%- 4.80%). The ADC was negatively correlated with the PDFF (r= -0.298, P = .010) and R2* (r = -0.457, P < .001). Linear regression analysis showed that the PDFF and R2* were independent factors of the ADC (β= -0.315, P = .007, R2= 0.099 and β= -0.493, P < .001, R2= 0.243, respectively). Compared with the uncorrected ADC (r= -0.307, P = .022), the correlation between the ADCPDFF and fibrosis grade increased (r= -0.513, P < .001), and the correlation between the ADCR2* and fibrosis grade decreased (r=-0.168, P = .215). The AUC of the ADCPDFF was significantly larger than that of the ADCu in the diagnosis of significant fibrosis and cirrhosis, which increased from 0.68 to 0.81 (P = .003) for predicting significant fibrosis and from 0.75 to 0.84 (P = .009) for predicting cirrhosis. The AUCs for the ADCR2* in the diagnosis of significant fibrosis and cirrhosis were both lower than that for the uncorrected ADC (P = .206 and P = .109, respectively).CONCLUSION:After correcting for the effects of steatosis, the diagnostic performance of the ADC for signifi-cant fibrosis and cirrhosis increased. The ADC corrected for the effects of steatosis may be more reliable for identifying liver fibrosis