The electromagnetic field within an aperture in the wall of a blackbody cavity is a known example of a Lambertian source producing a far field which is unpolarized in all directions. In this work we show that in the electromagnetic context other Lambertian sources exist whose far fields, while obeying the cosine law for the radiant intensity, differ by their polarization states and degrees. For example, the far field may be azimuthally, radially, or circularly polarized, or the polarization state may vary depending on the direction. For specific Lambertian fields generated by quasihomogeneous sources it is possible to calculate explicitly the $3\times 3$ electric cross-spectral density matrix of the nonevanescent part of the source. This enables one to assess the source's spatial coherence and partial polarization properties. In all cases, the coherence length turns out to be roughly half a wavelength, whereas the polarization characteristics of the sources may differ significantly. Our results could find uses, for instance, in radiometry and photometry, lighting applications, and remote sensing.

We study the probe-field transmission in cavity quantum electrodynamics (cavity-QED) systems with a partially transmitting element (PTE), where the PTE is used to control and tune the amplitude of the weak probe field propagating along a single waveguide channel in the structure. We derive analytic formulas utilized to determine the transmission coefficient of the probe field within the framework of quantum optics. Using experimentally accessible parameters, it is clearly shown that the asymmetric Fano-resonance line shape can be formed and manipulated by means of the added PTE. Furthermore, we reveal that there exists superluminal light with large intensity transmission in the transport spectrum of the waveguide-coupled cavity-QED system. This superluminal-light propagation effect, which exhibits the anomalous phase shift and is characterized by the negative group delay, can be enhanced by properly choosing the system parameters. The obtained results may be used for designing switching, modulation, and sensing for nanophotonic applications and ultrafast on-chip signal processing in telecom applications.

The asymmetric photoabsorption line profile (Beutler-Fano profile) arising from two interacting channels has applications in atomic, molecular, and condensed matter physics. In this work, the angle-resolved and fully-rotationally-resolved Beutler-Fano profiles for the predissociation of ${\mathrm{H}}_2,{\mathrm{H}}_2+hv\to \mathrm{H}(1s)+\mathrm{H}(2s)/\mathrm{H}\left(2p\right)$, have been measured. It is found that the Beutler-Fano profiles display different shapes for the fragments H($2s,2p$) recoiling at the parallel and perpendicular directions relative to the polarization direction of the dissociation laser. Tuning the excitation photon energy from the resonance center to the lower energies within the Beutler-Fano profile, the branching ratios H($2s$)/[H($2s$)+H($2p$)] decrease, and the fragment angular distributions change in a trend indicating the presence of more components of direct dissociation channels in comparison with the predissociation channel.

We show that $n$ thermal fermionic alkaline-earth-metal atoms in a flat-bottom trap allow one to robustly implement a spin model displaying two symmetries: the $S_n$ symmetry that permutes atoms occupying different vibrational levels of the trap and the $\mathrm{SU}\left(N\right)$ symmetry associated with $N$ nuclear spin states. The symmetries make the model exactly solvable, which, in turn, enables the analytic study of dynamical processes such as spin diffusion in this $\mathrm{SU}(N$) system. We also show how to use this system to generate entangled states that allow for Heisenberg-limited metrology. This highly symmetric spin model should be experimentally realizable even when the vibrational levels are occupied according to a high-temperature thermal or an arbitrary nonthermal distribution.

We consider the problem of quantum-state tomography under the assumption that the state is pure, and more generally that its rank is bounded by a given value $r$. In this scenario two notions of informationally complete measurements emerge: rank-$r$-complete measurements and rank-$r$ strictly-complete measurements. Whereas in the first notion, a rank-$r$ state is uniquely identified from within the set of rank-$r$ states, in the second notion the same state is uniquely identified from within the set of all physical states, of any rank. We argue, therefore, that strictly-complete measurements are compatible with convex optimization, and we prove that they allow robust quantum-state estimation in the presence of experimental noise. We also show that rank-$r$ strictly-complete measurements are as efficient as rank-$r$-complete measurements. We construct examples of strictly-complete measurements and give a complete description of their structure in the context of matrix completion. Moreover, we numerically show that a few random bases form such measurements. We demonstrate the efficiency-robustness property for different strictly-complete measurements with numerical experiments. We thus conclude that only strictly-complete measurements are useful for practical tomography.

We develop a dynamical model to describe the operation of the nondeterministic noiseless linear amplifier (NLA) in the regime of continuous-mode inputs. We analyze the dynamics conditioned on the detection of photons and show that the amplification gain depends on detection times and on the temporal profile of the input state and the auxiliary single-photon state required by the NLA. We also show that the output amplified state inherits the pulse shape of the ancilla photon.

Between any two resonance lines for an atomic medium there exists a dispersionless wavelength where the ac Stark shift from the two resonances cancel and the real part of the resonant susceptibility is 0. We experimentally demonstrate the effects of this wavelength in a pump-probe experiment in high-density atomic vapor. A strong pump field excites the medium, in which we counterpropagate a broadband probe field. In a long cell, only at the dispersionless wavelength will scattering and self-focusing or defocusing not cause attenuation, allowing a spectrometer to read off the dispersionless wavelength. We perform a propagation experiment that shows the passband centered around the 790-nm dispersionless wavelength between the $D_1$ and the $D_2$ lines of rubidium.

Recently, a series of papers by Combes et al. [J. Combes, H. M. Wiseman, and K. Jacobs, Phys. Rev. Lett. 100, 160503 (2008); J. Combes, H. M. Wiseman, and A. J. Scott, Phys. Rev. A 81, 020301(R) (2010); J. Combes and H. M. Wiseman, Phys. Rev. X 1, 011012 (2011); J. Combes, A. Denney, and H. M. Wiseman, Phys. Rev. A 91, 022305 (2015)] has shown that it is possible to greatly improve the measurement rate of a register of qubits for given detector resources by means of a clever feedback control scheme. However, this speedup came at an exponential cost in terms of complexity and memory use. In this paper, I propose a simple efficient algorithm—exponentially more frugal in memory and less complex to implement—which is asymptotically as fast. I use extensively the implicit classicality of the situation to provide a slightly more straightforward interpretation of the results. I compute the speedup rates exactly in the case of the proposed model and in the case of the open-loop scheme of Combes et al. and prove that they indeed provide the same asymptotic speedup.

Einstein-Podolsky-Rosen (EPR) steering is a form of quantum nonlocality which is intermediate between entanglement and Bell nonlocality. EPR steering is a resource for quantum key distribution that is device independent on only one side in that it certifies bipartite entanglement when one party's device is not characterized while the other party's device is fully characterized. In this work, we introduce two types of genuine tripartite EPR steering, and derive two steering inequalities to detect them. In a semi-device-independent scenario where only the dimensions of two parties are assumed, the correlations which violate one of these inequalities also certify genuine tripartite entanglement. It is known that Alice can demonstrate bipartite EPR steering to Bob if and only if her measurement settings are incompatible. We demonstrate that quantum correlations can also detect tripartite EPR steering from Alice to Bob and Charlie, even if Charlie's measurement settings are compatible.

Absorption spectra of cold rubidium atoms in an integrating sphere under the influence of a diffuse laser field have been systematically investigated. A pronounced dispersionlike structure centered at the light-shifted pump frequency is observed with a subnatural linewidth. In particular, two clearly resolved absorption resonances on the $5S_{1/2}(F=2)\to 5P_{3/2}(F^{\prime }=3)$ transitions occur with variable probe beam intensity, which is consistent with our proposed theoretical model. Based on the two absorption resonances,we measure the dependence of light shifts, from which we can directly extract the effective Rabi frequency in a diffuse laser field, on the probe laser intensity, pump laser intensity, and pump laser detuning. Our work helps to identify the physical mechanisms behind these spectral features and is beneficial for studying the corresponding effect in a cold sample.

We experimentally and theoretically investigated the deprotonation process on nanosecond to microsecond timescales in ethylene and acetylene molecules following their double ionization by a strong femtosecond laser field. In our experiments we utilized coincidence detection with the reaction microscope technique. We found that both the lifetime of the long-lived ethylene dication leading to the delayed deprotonation and the relative channel strength of the delayed deprotonation compared to the prompt one have no evident dependence on the laser pulse duration and the laser peak intensity. Quantum chemical simulations suggest that the observed delayed fragmentation process originates from the tunneling from near-dissociation-threshold C–H stretch vibrational states on a dicationic electronic state. These vibrational states can be populated through strong field double-ionization-induced vibrational excitation on an electronically excited state in the case of ethylene, and through a spin-flip transition from electronically excited singlet states to the triplet ground state in the case of acetylene.

We theoretically show that the spin Hall effect arises in a Bose-Einstein condensate (BEC) of neutral atoms interacting via the magnetic dipole-dipole interactions (MDDIs). Since the MDDI couples the total spin angular momentum and the relative orbital angular momentum of two colliding atoms, it works as a spin-orbit coupling. Thus, when we prepare a BEC in a magnetic sublevel $m=0$, thermally and quantum-mechanically excited atoms in the $m=1$ and $-1$ states feel the Lorentz-like forces in the opposite directions. This is the origin for the emergence of the spin Hall effect. We define the mass-current and spin-current operators from the equations of continuity and calculate the spin Hall conductivity from the off-diagonal current-current correlation function within the Bogoliubov approximation. We find that the correction of the current operators due to the MDDI significantly contributes to the spin Hall conductivity. A possible experimental situation is also discussed.

The macroscopic surface electric susceptibility determines the linear optical properties of an insulating single-layer two-dimensional atomic crystal and can be expressed in terms of the microscopic polarizability of the atoms. We compute the local electric field acting on a single atom, for both the static and the dynamic case, as the superposition of the external applied electric field and the fields generated by the induced dipoles in the crystal. We find that, in the dynamic case, retardation effects dephase the local electric field with respect to the incident one. This explains why the Fresnel coefficients of a single-layer two-dimensional atomic crystal are intrinsically complex quantities, even when a null macroscopic surface conductivity is assumed.

We develop a classical microscopic model of a dielectric. The model features nonlinear interaction terms between polarizable dipoles and lattice vibrations. The lattice vibrations are found to act as a pseudoreservoir, giving broadband absorption of electromagnetic radiation without the addition of damping terms in the dynamics. The effective permittivity is calculated using a perturbative iteration method and is found to have the form associated with real dielectrics. Spatial dispersion is naturally included in the model and we also calculate the wave vector dependence of the permittivity.

We study strong-field molecular ionization using few- (four to ten) cycle laser pulses. Employing a supercontinuum light source, we are able to tune the optical laser wavelength (photon energy) over a range of $\sim 200$ nm (500 meV). We measure the photoelectron spectrum for a series of different molecules as a function of laser intensity, frequency, and bandwidth and illustrate how the ionization dynamics vary with these parameters. We find that multiphoton resonances and nonadiabatic dynamics (internal conversion) play an important role and result in ionization to different ionic continua. Interestingly, while nuclear dynamics can be “frozen” for sufficiently short laser pulses, we find that resonances strongly influence the photoelectron spectrum and final cationic state of the molecule regardless of pulse duration—even for pulses that are less than four cycles in duration.

We introduce a method to test the state-independent contextuality using a single copy of a system and without a state preparation. It can be implemented in realistic setups, like ion traps, or in other scenarios in which measurements are nondestructive. In particular, we show how to apply our idea to the Peres-Mermin state-independent contextuality test designed for nine observables of two spin-$1/2$ particles. Each measurement round is performed on a postmeasurement state from the previous round. The sequence of measurements can be described as a Markov chain, and we observe that after a few rounds the system arrives at a stationary state to which the contextuality of the system can be attributed. However, it has to be noted that it does not depend on the initial state of the system and therefore it is not a property of a particular state preparation. Finally, we show that our approach to test contextuality admits noisy measurements as it implies the self-correction of possible detection errors present in any real experiments.

Blind quantum computation (BQC) allows a client (Alice), who only possesses relatively poor quantum devices, to delegate universal quantum computation to a server (Bob) in such a way that Bob cannot know Alice's inputs, algorithm, and outputs. The quantum channel between Alice and Bob is noisy, and the loss over the long-distance quantum communication should also be taken into account. Here we propose to use decoherence-free subspace (DFS) to overcome the collective noise in the quantum channel for BQC, which we call DFS-BQC. We propose three variations of DFS-BQC protocols. One of them, a coherent-light-assisted DFS-BQC protocol, allows Alice to faithfully send the signal photons with a probability proportional to a transmission rate of the quantum channel. In all cases, we combine the ideas based on DFS and the Broadbent-Fitzsimons-Kashefi protocol, which is one of the BQC protocols, without degrading unconditional security. The proposed DFS-based schemes are generic and hence can be applied to other BQC protocols where Alice sends quantum states to Bob.

We discuss the relationships among various quantities of information during the process of an efficient quantum measurement, e.g., information gain, quantum loss, Holevo information, and coherent information. In particular, we give an uncertaintylike relation between information gain and coherent information. We also investigate the information gain by local measurements and quantum correlations in bipartite quantum systems. Moreover, we discuss two cases of information leak according to whether the observer of the environment possesses extra information about the measured system.

The process of elastic (Rayleigh) scattering of two x-ray free-electron laser (XFEL) photons by a free He atom is theoretically investigated. We obtain the absolute values and the forms of the triple differential scattering cross section. The main theoretical result is the highest probability of creation of scattered photons with energy $\hslash {\omega }_{\pm }\cong \hslash \omega \pm I_{1s}$ $(\hslash \omega$ is the energy of the incident XFEL photon, $I_{1s}$ is the energy of the ionization threshold of the $1s^2$ atomic shell). The probability of creation cooled $\left(<{\omega }_{-}\right)$ and hot $\left(>{\omega }_{+}\right)$ photons is smaller by many orders of magnitude, and is identically zero when the formal (nonphysical) energy of one of the scattered photons is $2\hslash \omega$.