High-accuracy measurement of atomic polarizability in an optical lattice clock

Presently, the Stark effect contributes the largest source of uncertainty in a ytterbium optical atomic clock through blackbody radiation. By employing an ultracold, trapped atomic ensemble and high stability optical clock, we characterize the quadratic Stark effect with unprecedented precision. We report the ytterbium optical clock's sensitivity to electric fields (such as blackbody radiation) as the differential static polarizability of the ground and excited clock levels α(clock) = 36.2612(7) kHz?(kV/cm)(-2). The clock's uncertainty due to room temperature blackbody radiation is reduced by an order of magnitude to 3×10(-17).     

Einstein-Hopf drag,Doppler shift of thermal radiation and blackbody drag: Three perspectives on quantum friction

The thermal friction force acting on an atom moving relative to a thermal photon bath has recently been calculated on the basis of the fluctuation-dissipation theorem. The thermal fluctuations of the electromagnetic field give rise to a drag force on an atom provided one allows for dissipation of the field energy via spontaneous emission. The drag force exists if the atomic polarizability has a nonvanishing imaginary part. Here, we explore alternative derivations. The damping of the motion of a simple harmonic oscillator is described by radiative reaction theory (result of Einstein and Hopf), taking into account the known stochastic fluctuations of the electromagnetic field. Describing the excitations of the atom as an ensemble of damped harmonic oscillators, we identify the previously found expressions as generalizations of the Einstein-Hopf result. In addition, we present a simple explanation for blackbody friction in terms of a Doppler shift of the thermal radiation in the inertial frame of the moving atom: The atom absorbs blue-shifted photons from the front and radiates off energy in all directions, thereby losing energy. The original plus the two alternative derivations provide for additional confirmation of an intriguing quantum friction effect, and leave no doubt regarding its existence.     

锶原子光晶格钟黑体辐射频移评估

在中性原子光晶格钟的系统不确定度评估中,通常黑体辐射引起的频移是最大的一项.黑体辐射频移主要受周围环境温度的影响.针对国家授时中心的锶原子光晶格钟实验系统,通过理论分析、腔体表面温度的测量和软件模拟相结合的方法,评估了锶原子光晶格钟黑体辐射频移的修正量和不确定度.其中主要分析了锶原子炉、蓝宝石加热窗口、透过窗口片进入到真空腔体内的室温以及Zeeman减速装置对原子团处的热辐射引起的黑体辐射频移.在真空腔体外表面设置了5个测温点,利用校准过的铂电阻温度传感器监测真空腔体外表面的温度变化,用SolidWorks绘图软件建立腔体模型,通过有限元分析软件模拟出在真空腔体温度变化0.72 K时,原子团所处位置温度的波动为0.34 K.最终得到黑体辐射频移总的修正量为-2.13(1) Hz,不确定度为2.4×10~(-17).     

Atomic clocks with suppressed blackbody radiation shift

We develop a concept of atomic clocks where the blackbody radiation shift and its fluctuations can be suppressed by 1-3 orders of magnitude independent of the environmental temperature. The suppression is based on the fact that in a system with two accessible clock transitions (with frequencies ν1 and ν2) which are exposed to the same thermal environment, there exists a "synthetic" frequency ν(syn) ∝ (ν1 - ε12ν2) largely immune to the blackbody radiation shift. For example, in the case of 171Yb+ it is possible to create a synthetic-frequency-based clock in which the fractional blackbody radiation shift can be suppressed to the level of 10(-18) in a broad interval near room temperature (300±15 K). We also propose a realization of our method with the use of an optical frequency comb generator stabilized to both frequencies ν1 and ν2, where the frequency ν(syn) is generated as one of the components of the comb spectrum.     

Sudden Vanishing of Atomic Dipole-Squeezing in a Kerr Nonlinear Blackbody

The dynamics of atomic dipole momentum and atomic dipole-squeezing effect are investigated inside a Kerr nonlinear blackbody. It is found that in a Kerr nonlinear blackbody, the atomic dipole momentum and its squeezing effect are heavily dependent on the Kerr nonlinear coefficient. It is also found that below a transition temperature T _c , the dipole momentum and its squeezing effect in a Kerr nonlinear blackbody can vanish much faster with time than those in a normal blackbody. Above T _c , the Kerr nonlinear blackbody becomes a normal blackbody, and the dynamics of the atomic dipole momentum and its squeezing effect behaves as those in normal blackbody radiation. The physical origin of the sudden-vanishing phenomenon in a Kerr nonlinear blackbody is also discussed.     

Spectral Energy Density in a Kerr Nonlinear Blackbody

In a Kerr nonlinear blackbody, bare photons with opposite wave vectors and helicities are bound into pairsand unpaired photons are transformed into a new kind of quasiparticle, the nonpolariton. The nonpolariton system constitutes free thermal radiation in the blackbody. The present paper investigates the spectral energy density of the thermal radiation. It is found that the spectral energy density of a Kerr nonlinear blackbody is larger than that of a normal blackbody.     

Spectral Energy Density in a Kerr Nonlinear Blackbody

In a Kerr nonlinear blackbody, bare photons with opposite wave vectors and helicities are bound into pairs and unpaired photons are transformed into a new kind of quasiparticle, the nonpolariton. The nonpolariton system constitutes free thermal radiation in the blackbody. The present paper investigates the spectral energy density of the thermal radiation. It is found that the spectral energy density of a Kerr nonlinear blackbody is larger than that of a normal blackbody.     

Size effect of two-dimensional thermal radiation

Properties of two-dimensional thermal radiation are investigated as a function of sample size and temperature. The two-dimensional thermal radiation is different from two-dimensional blackbody radiation when the size of the sample becomes small due to the uncertainty principle, which shows the allowed minimum energy can?t be neglected. The energy density is shown as a function of sample size at a constant temperature. The energy density is also shown as a function of temperature at a constant size. It is shown that our prediction can be measured from the thermal radiation of graphene.     

Multiphoton resonant transitions in atomic Ba in the presence of additional strong off-resonant radiation

The multiphoton transitions in atomic Ba are experimentally studied in the presence of strong off-resonant radiation that generates additional atomic polarization in the ground state. It is demonstrated that the additionally induced polarization of the Ba atoms in the ground state leads to a higher probability of the multiphoton transitions from this state.     

Thermodynamics of Phase Transitions of a Kerr Nonlinear Blackbody

We study the thermodynamics of phase transitions of a blackbody whose interior is filled by a Kerr nonlinear crystal. There is a transition temperature To, above which the Kerr nonlinear blackbody is in the normal thermal radiation state, and below which it is in the squeezed thermal radiation state. At To, the Gibbs free energy of the two phases is continuous but the entropy density of the two phases is discontinuous. Hence, there is a jump in the entropy density and this leads to a latent heat density. The photon system undergoes a first-order phase transition from the normal to the squeezed thermal radiation state.     

Photo- and thermodesorption of helium on Pt(111)

In a detailed study of thermal desorption of monolayers of both 4He and 3He adsorbed on Pt(111) (binding energy about 9 meV), we have observed photodesorption induced by the blackbody radiation from a room temperature environment. This process proceeds independently of the thermal desorption. Theoretical treatments of both thermal and photodesorption are given and agree very well with the data in all important aspects. We conclude that the photodesorption is due to direct coupling of photons to the adsorbate.     

Blackbody-radiation-assisted laser cooling of molecular ions

The translational motion of molecular ions can be effectively cooled sympathetically to temperatures below 100 mK in ion traps through Coulomb interactions with laser-cooled atomic ions. The distribution of internal rovibrational states, however, gets in thermal equilibrium with the typically much higher temperature of the environment within tens of seconds. We consider a concept for rotational cooling of such internally hot, but translationally cold, heteronuclear diatomic molecular ions. The scheme relies on a combination of optical pumping from a few specific rotational levels into a "dark state" with redistribution of rotational populations mediated by blackbody radiation.     

Quasiprobability distribution in a Kerr-nonlinear blackbody

In a Kerr-nonlinear blackbody, bare photons with opposite wave vectors and helicities are bound into pairs and unpaired photons are transformed into a new kind of quasiparticle, the nonpolariton. The nonpolariton system constitutes free thermal radiation in the blackbody. In this paper, the quasiprobability (Q function) distribution of thermal radiation is investigated. Non-classical effects of quadrature squeezing have been observed. The structure of nonpolaritons is unsteady and governed by the temperature. The phase space of the photon system is considered and found that in the transition from the normal to the squeezed thermal radiation state, the phase symmetry of the photon system is spontaneously broken.     

Effective temperature of thermal radiation from non-uniform temperature distributions and nanoparticles

We study thermal radiation properties from non-uniform temperature distributions and nanoparticles, and define effective temperature. Conventionally, the temperature of a body is measured by fitting with the blackbody radiation spectrum, which assumes a uniform temperature throughout the body. We show the energy density of thermal radiation for non-uniform temperature distribution of the body and derive the effective temperature. Furthermore, the energy density of thermal radiation from nanoparticles is derived and the effective temperature of the body is shown to depend on the particle size.     

Laser-driven blackbody radiator with bistability

We report a particular construction of a laser-driven blackbody radiator with bistability mode based on efficient light-into-heat conversion of a rare earth system. The laser-induced thermal avalanche nonlinearity and the internal stimulative feedback mechanisms are revealed to interpret the typical S-pattern bistability. The standard blackbody radiation and the sizable bistability mode are experimentally demonstrated through ultrabroadband thermal spectra measurements of ZrO₂:Yb–Tm nanophase compounds. Such a noncontact, laser-driven scheme for microscale blackbody radiation has attractive applications for compact standard spectra source and broadband spectra switching in the on-chip all-optical systems.     

Many-body ionization in a frozen Rydberg gas

In a dense gas of 300 microK 85Rb atoms of n approximately 50 ionization occurs on a 100 ns time scale, far too fast to be explained by the motion of the atoms or photoionization by 300 K blackbody radiation. Rapid ionization is accompanied by spectral broadening, with the spectrum becoming continuous at n=88 at a density of 5x10(10)cm(-3). The atomic transitions broaden both smoothly and by the emergence of new features, which we attribute to multiple atom absorptions. We attribute the rapid ionization to a sequence of near resonant dipole-dipole transitions through virtual states in this intrinsically many-body system, culminating in the ionization of some of the atoms.     

Fluctuational electrodynamics in the presence of finite thermal sources

We present exact calculations of the spatial correlation of the blackbody radiation in the presence of spheres whose dimensions are smaller or comparable to the radiation wavelength. By going beyond the standard scalar coherence theory, we show that the spatial correlation function of a spherical thermal source is not universal but depends on the material properties of the source and exhibits near-field-induced features. Near-field effects are also manifested in the case of a linear chain of dielectric spheres where the correlation function probes the inhomogeneity of the chain. For this latter system we have established the conditions when the near-field effects cancel out and the correlation function takes the typical form of a conventional Lambertian source. For the case of a chain of metallic nanospheres, the increased spatial correlation of the far field leads to a directional thermal emission spectrum.     

Energy in a highly ordered universe

A new theory of particles proposed in an earlier paper is now applied to explain energy. Having earlier derived the Rydberg formula for atomic spectra without using the Pauli principle, the authors now derive the photoelectric effect, deflection of light by gravitation, and Planck's law for blackbody radiation without using Planck's assumption on energy quanta or Einstein's theory of general relativity.     

A comparison of different entransy flow definitions and entropy generation in thermal radiation optimization

In thermal radiation, taking heat flow as an extensive quantity and defining the potential as temperature T or the black body emissive power U will lead to two different definitions of radiation entransy flow and the corresponding principles for thermal radiation optimization. The two definitions of radiation entransy flow and the corresponding optimization prin ciples are compared in this paper. When the total heat flow is given, the optimization objectives of the extremum entransy dissipation principles （EEDPs） developed based on potentials T and U correspond to the minimum equivalent temperature difference and the minimum equivalent blackbody emissive power difference respectively. The physical meaning of the definition based on potential U is clearer than that based on potential T, but the latter one can be used for the coupled heat transfer optimization problem while the former one cannot. The extremum entropy generation principle （EEGP） for thermal radiation is also derived, which includes the minimum entropy generation principle for thermal radiation. When the radiation heat flow is prescribed, the EEGP reveals that the minimum entropy generation leads to the minimum equivalent thermodynamic potential difference, which is not the expected objective in heat transfer. Therefore, the minimum entropy generation is not always appropriate for thermal radiation optimization. Finally, three thermal radiation optimization examples are discussed, and the results show that the difference in optimization objective between the EEDPs and the EEGP leads to the difference between the optimization results. The EEDP based on potential T is more useful in practical application since its optimization objective is usually consistent with the expected one.