Classical-Nonclassical Polarity of Gaussian States.

Gaussian states with nonclassical properties such as squeezing and entanglement serve as crucial resources for quantum information processing. Accurately quantifying these properties within multimode Gaussian states has posed some challenges. To address this, we introduce a unified quantification: the "classical-nonclassical polarity," represented by P. For a single mode, a positive value of P captures the reduced minimum quadrature uncertainty below the vacuum noise, while a negative value represents an enlarged uncertainty due to classical mixtures. For multimode systems, a positive P indicates bipartite quantum entanglement. We show that the sum of the total classical-nonclassical polarity is conserved under arbitrary linear optical transformations for any two-mode and three-mode Gaussian states. For any pure multimode Gaussian state, the total classical-nonclassical polarity equals the sum of the mean photon number from single-mode squeezing and two-mode squeezing. Our results provide a new perspective on the quantitative relation between single-mode nonclassicality and entanglement, which may find applications in a unified resource theory of nonclassical features.

Graphene Plasmon Excitation with Ground-State Two-Level Quantum Emitters.

The transmission of a two-level quantum emitter in its ground state through a graphene nanosheet is investigated. The graphene plasmons (GPs) field distribution, especially the opposite orientations of the vertical electric field components on the two sides of the graphene nanosheet, produces a significant nonadiabatic process during the interaction between the emitter and the localized GPs. By taking into account the counterrotating terms, the excitation of the quantum emitter with simultaneous emission of a GP has a large probability. This happens for emitter speeds of about 10^{-4} times the speed of light. For accelerated emitters, the GPs exhibit thermal field photon distribution with a high temperature. As a consequence, this study provides a promising platform to observe the dynamical Casimir effect as well as a simulation of the Unruh effect.

Tunable and enhanced Goos-Hänchen shift via surface plasmon resonance assisted by a coherent medium.

We present a scheme for enhancing Goos-Hänchen shift of light beam that is reflected from a coherent atomic medium in the Kretschmann-Raether configuration. The complex permittivity of the medium can be coherently controlled and has significant influence on the surface plasmon resonance (SPR) at the metal-medium interface. By tuning the atomic absorption, the internal damping of SPR system can be modulated effectively, thereby leading to giant positive and negative lateral displacements. The refractive index of medium determines the SPR angle. Thus the peak position of the beam shift becomes tunable. As the optical response of the coherent medium depends on the intensity and detuning of the controlling fields, we are able to conveniently manipulate the magnitude, the sign, and the angular position of Goos-Hänchen shift peaks.

Exchange unknown quantum states with almost invisible photons.

We propose a quasi-counterfactual quantum swap gate for exchanging Alice's unknown photon state and Bob's unknown atomic state under the condition that only Alice's photon may appear in the transmission channel between Alice and Bob, while the probability of the existence of photon in the transmission channel is controllable and can tend to zero. Unlike standard counterfactual quantum communication protocols, quantum states exchange in present scenario is achieved by multiple phase operations, rather than multiple measurements. The total effect of those operations can be considered as a unitary time evolution operator. Therefore, the communication fidelity and efficiency of our protocol are always one if system imperfection and channel noise are not considered. Compared to standard counterfactual communication protocols, our protocol is easy to implement. We also show that it can be easily converted to a standard counterfactual one.

Quantum state protection in finite-temperature environment via quantum gates.

We propose a protocol to protect the quantum states and entanglements from finite-temperature thermal noise via quantum gates. Compared to the common protocols protecting the quantum states and entanglements by using weak measurements and their reversals, no time-consuming weak measurements are needed in the present protocol and consequently, it is much faster. We also discuss the possible implementation of the protocol in cavity QED system.

Deep subwavelength lithography via tunable terahertz plasmons.

A scheme to overcome diffraction limit in optical lithography via tunable plasmons is proposed. The plasmons are generated by a current-driven instability and are resonance amplified between the drain and source barriers of the transistor. A series of discrete deep subwavelength can be obtained by controlling the gate voltage. Thus, it is possible to realize lithography with a resolution over 1/100 vacuum wavelength and achieve arbitrary one-dimensional and even simple two-dimensional patterns. Our scheme works in the linear optics regime and is easy to be experimentally realized.

Polariton-Assisted Cooperativity of Molecules in Microcavities Monitored by Two-Dimensional Infrared Spectroscopy.

Molecular polaritons created by the strong coupling between matter and field in microcavities enable the control of molecular dynamical processes and optical response. Multidimensional infrared spectroscopy is proposed for monitoring the polariton-assisted cooperative properties. The response of molecules to local fluctuations is incorporated and the full dynamics is monitored through the time- and frequency-resolved multidimensional signal. The cooperativity against solvent-induced disorder and its connection to the localization of the vibrational excitations are predicted. New insights are provided for recent 2DIR experiments on vibrational polaritons.

Quantum Secure Group Communication.

We propose a quantum secure group communication protocol for the purpose of sharing the same message among multiple authorized users. Our protocol can remove the need for key management that is needed for the quantum network built on quantum key distribution. Comparing with the secure quantum network based on BB84, we show our protocol is more efficient and securer. Particularly, in the security analysis, we introduce a new way of attack, i.e., the counterfactual quantum attack, which can steal information by "invisible" photons. This invisible photon can reveal a single-photon detector in the photon path without triggering the detector. Moreover, the photon can identify phase operations applied to itself, thereby stealing information. To defeat this counterfactual quantum attack, we propose a quantum multi-user authorization system. It allows us to precisely control the communication time so that the attack can not be completed in time.

Measurement of deep-subwavelength emitter separation in a waveguide-QED system.

In the waveguide quantum electrodynamics (QED) system, emitter separation plays an important role for its functionality. Here, we present a method to measure the deep-subwavelength emitter separation in a waveguide-QED system. In this method, we can also determine the number of emitters within one diffraction-limited spot. In addition, we also show that ultrasmall emitter separation change can be detected in this system which may then be used as a waveguide-QED-based sensor to measure tiny local temperature/strain variation.

Tunable Goos-Hänchen shift from graphene ribbon array.

The Goos-Hänchen (GH) shift of light beam incident on graphene ribbon array is investigated by Green's function method. Due to the resonance effects of leaky surface plasmons on ribbons, the zeroth-order reflection field shows both giant positive and negative GH shifts. By tuning the graphene Fermi level, we can control the shift conveniently. This effect is important to graphene-based metasurface and electro-optical devices.

Counterintuitive dispersion violating Kramers-Kronig relations in gain slabs.

We demonstrate the counterintuitive dispersion effect that the peaks (dips) in the gain spectrum correspond to abnormal (normal) dispersion, contrary to the usual Kramers-Kronig point of view. This effect may also lead to two unique features: a broadband abnormal dispersion region and an observable Hartman effect. These results are explained in terms of interference and boundary effects. Finally, two experiments are proposed for the potential experimental verification.

Goos-Hänchen shifts of partially coherent light fields.

The Goos-Hänchen (GH) shift refers to a lateral displacement (from the path expected from geometrical optics) along an interface in totally internal reflection. This phenomenon results from a coherence effect. In order to bring to light the role of coherence, the reflection of partially coherent light fields was investigated within the framework of the theory of coherence. A formal expression for the GH shifts of partially coherent light fields is obtained in terms of Mercer's expansion. It is shown that both the spatial coherence and the beam width have an important effect on the GH shift, especially near the critical angles (such as totally reflection angle). These results are important to observe the GH shifts of the beams with imperfect coherence, like x-ray and matter-wave beams.

Ultrabroadband nonreciprocal transverse energy flow of light in linear passive photonic circuits.

Using a technique, analogous to coherent population trapping in an atomic system, we propose schemes to create transverse light propagation violating left-right symmetry in a photonic circuit consisting of three coupled waveguides. The frequency windows for the symmetry breaking of the left-right energy flow span over 80 nm. Our proposed system only uses linear passive optical materials and is easy to integrate on a chip.

Protocol for direct counterfactual quantum communication.

It has long been assumed in physics that for information to travel between two parties in empty space, "Alice" and "Bob," physical particles have to travel between them. Here, using the "chained" quantum Zeno effect, we show how, in the ideal asymptotic limit, information can be transferred between Alice and Bob without any physical particles traveling between them.

Time evolution of the Lamb shift.

The time evolution of the Lamb shift that accompanies the real photon emission is studied for the first time (to our knowledge). The investigation of the explicit time dependence of the Lamb shift becomes possible because the self-energy of the free electron, which is divergent, is subtracted from the Hamiltonian after a unitary transformation. The Lamb shift can then be separated into two parts: one is the time-independent shift due to the virtual photon exchange, and the other is the time-dependent shift due to the real photon emission. The time evolution depends on the nature of the coupling spectrum of the reservoir.

Quantum lithography beyond the diffraction limit via Rabi oscillations.

We propose a quantum optical method to do the subwavelength lithography. Our method is similar to the traditional lithography but adding a critical step before dissociating the chemical bound of the photoresist. The subwavelength pattern is achieved by inducing the multi-Rabi oscillation between the two atomic levels. The proposed method does not require multiphoton absorption and the entanglement of photons. It is expected to be realizable using current technology.

Uncertainty inequalities as entanglement criteria for negative partial-transpose States.

In this Letter, we show that the fulfillment of uncertainty relations is a sufficient criterion for a quantum-mechanically permissible state. We specifically construct two pseudospin observables for an arbitrary nonpositive Hermitian matrix whose uncertainty relation is violated. This method enables us to systematically derive separability conditions for all negative partial-transpose states in experimentally accessible forms. In particular, generalized entanglement criteria are derived from the Schrödinger-Robertson inequalities for bipartite continuous-variable states.

Time-bandwidth problem in room temperature slow light.

For many applications of slow or stopped light, the delay-time-bandwidth product is a fundamental issue. So far, however, slow-light demonstrations do not show a large delay-time-bandwidth product, especially in room temperature solids. Here we demonstrate that the use of artificial inhomogeneous broadening has the potential to solve this problem. A proof-of-principle experiment is done using slow light produced by two-beam coupling in a photorefractive crystal Ce:BaTiO3 where Bragg selection is used to provide the artificial inhomogeneity. Examples of how to generalize this concept for use with other room temperature slow-light solids are also given.