Nonclassical features of entangled coherent states

Nonclassical features of entangled coherent states (two-mode superposition coherent states) based on two coherent states shifted in phase by π/2 are discussed. Analysis of Cauchy–Schwartz inequality, two-mode quadrature squeezing, oscillatory and sub-Poissonian photon statistics show that nonclassicality exists for these states. Furthermore, it is also observed that special states have remarkably strong nonclassical properties than the entangled coherent states based on famous even–odd coherent states.     

Generation of motional entangled coherent state in an optomechanical system in the single photon strong coupling regime

The single-photon strong coupling in the deep-resolved sideband of the optomechanical system induces photon blockade (PB) effect. For the PB cavity, an initial mechanical coherent state evolves into superposition of phonon cat states entangled with the cavity Fock states. Measurement of the cavity photon number states produces phonon even and odd cat states. The information leakage effect of two photon states on the fidelity of cat states is calculated, it is shown that for low average phonon number this effect is negligible and decreases by increasing the two photon cavity state. The Lindblad equation is solved numerically to obtain the environmental effects on the fidelity of cat states.     

Optical protocol for quantum state sharing of superposed coherent state

We propose an optical protocol for quantum state sharing of superposed coherent state in terms optical elements. Our protocol can realize a near-complete quantum state sharing of a superposed coherent state with arbitrary coeficients. The realization of this protocol is appealing due to the fact that the quantum state of light is robust against the decoherence and photons are ideal carriers for transmitting quantum information over long distances. This protocol can also be generalized to the multiparty system.     

Entanglement of a new type of two-mode photon-added entangled coherent state

We introduce a new entangled state, which is composed of two photon-added coherent states. We discuss the entanglement of this state by using several sufficient entanglement criteria, such as higher-order entanglement criterion, EPR criterion, SU(1,1) algebra and Cauchy–Schwarz inequality. These criteria reflect some entanglement effects of this new state, but fail to find the degree of entanglement directly. Thus, we use the covariance to measure the entanglement in this state. Our findings show that the degree of entanglement decreases with the increasing of photon-number and the amplitude of the coherent states. One interesting result is that the new entangled state becomes a maximally entangled state when the photon-added number is 1 and the amplitude of the coherent states is zero, which is just one of the four Bell states.     

Measurement-device-independent quantum key distribution with odd coherent state

Measurement-device-independent quantum key distribution (MDI-QKD) is immune to all the detection attacks, thus when it is combined with the decoy-state method, the final key rate can be obtained by estimating the gain and quantum bit error rate for various input photon numbers. In this paper, we propose to perform MDI-QKD with odd coherent state (OCS) and compare the results with weak coherent source scenario. Our simulation indicates that both the secure key rate and transmission distance can be improved evidently with OCS owing to the lower probability of multi-photon events of the OCS. Furthermore, we apply the finite key analysis to the decoy-state MDI-QKD with OCS and obtain a practical key rate.     

从相干态观点看中子在旋转磁场中的几何相位

运用相干态方法，本文证明了中子在旋转磁场中的几何相位是粒子态矢量在态空间中扫过的立体角的一半，同时提供了一种几何相位在周期演化中的简单算法。     

Analyzing dissipation effects on the non-classicality witnessing by a qubit via the entangled state representation

ABSTRACT

In this paper, we study detection of the state non-classicality for a quantum harmonic oscillator by a qubit in the presence of dissipation effects. We show that dissipation can enhance the effectiveness of the method in case of using the corrected form of the related nonclassicality witness. Such an improvement is attributed to the fact that dissipation leads to probing a surface, instead of a curve, of the complex plane for non-classicality condition on normally-ordered characteristic function.     

Controlling entanglement dynamics of two qubits coupled to a common reservoir via a coherent field

We investigate the time evolution of entanglement between two two-level atoms which are coupled to a common multimode electromagnetic reservoir and simultaneously driven by a coherent field. We find that the entanglement can always be created and maintained with a moderate intensity of the driving field during the track of approaching steady entangled states when both atoms are initially in their ground states and the reservoir is in the vacuum state or the squeezed vacuum state. We also show that the steady-state entanglement between the atoms can be enhanced by use of the coherent field when the reservoir is in the weakly squeezed vacuum state. More interestingly, in the squeezed reservoir case, the sudden death period in the time evolution of the entanglement can be removed by use of the coherent driving field.     

New relation between input and output fields based on Collins diffraction formula and the Wigner function in entangled state representation

Based on the correspondence between the Collins diffraction formula (optical Fresnel transform) and the transform matrix element of a three-parameter two-mode squeezing operator in the entangled state representation 1 Fan, H-Y and Lu, H-L. 2006. Opt. Lett., 31: 2622–2624.  [Google Scholar] we further explore the relationship between output field intensity determined by the Collins formula and the input field's probability distribution along an infinitely thin phase space strip, both in the spatial domain and the frequency domain. The entangled Wigner function is introduced for recapitulating the result.     

Adaption of Collins formula to fractional Fourier transform studied in the entangled state representation of quantum optics

Adaption of Collins diffraction formula to the fractional Fourier transform in complex form (FFT-C) is demonstrated in entangled state |η? representation of quantum optics. The additivity property of FFT-C corresponding to Collins formula has been proved by using group multiplication of the Fresnel operator and the quantum mechanical version of the Collins formula.     

Formation of synthetic structures with micron size silica beads using optical tweezer

We have demonstrated that micron-size silica beads can be arranged in desired synthetic structure using an optical tweezer in a saline buffered solution. In 3.1 µm silica and water solution we added the right proportion of antistick glycol and NaCl to form a solution in which silica beads brought close to one another can bind by an adhesive electrostatic force without drifting away due to their thermal energy. Then by trapping and dragging one bead at a time using an optical tweezer, we have arranged the silica beads in two-dimensional structures.     

Overlapping single photons on coherent states with two independent laser sources: a proposal

Some very interesting pure non-Gaussian states in quantum optics have already been produced experimentally (with one pulsed laser): the single-photon-added coherent states, among others. Important interference phenomena by superposing beams from two independent masers or lasers have been investigated earlier experimentally and theoretically. By pursuing on both subjects altogether, we propose a possible new experiment to generate single-photon-added coherent states, by employing two independent laser sources, both in continuous regime and having approximately equal frequencies and coherence times t_c: we expect that such generations could occur during times of the order of t_c/3 (possibly, a bit shorter). This expectation follows from a fully quantized multimode analysis of the temporal mode structure, which extends previous studies of parametric down conversion and balanced homodyne detection     

Fast preparation of entangled states of two qutrits in cavity or circuit QED

We present a scheme for generating entangled states of two qutrits, by using a 4-level system and a 3-level system coupled to a single cavity. Because of only employing resonant interactions, the entangled state can be fast created. This scheme is easy to implement in experiments since only a single cavity is needed and none of measurement, auxiliary system and identical system-cavity coupling constant is required. Furthermore, two-qutrit partially or maximally entangled states can be created by using this scheme. The proposal is quite general, which can be used to prepare entangled states of two qutrits in various physical systems, such as two natural or artificial atoms of the Λ-type or Δ-type three lowest levels, coupled to an optical or microwave cavity.     

Composite coherence vortices in coherent and incoherent superpositions of two off-axis partially coherent vortex beams

The composite coherence vortices by coherently and incoherently superimposing two parallel, off-axis partially coherent vortex beams and their evolution in free space are studied. It is shown that the superposition scheme, off-axis distance, coherence parameter and propagation distance affect the position and number of composite coherence vortices. The motion, creation and annihilation of composite coherence vortices appear by varying the off-axis distance, coherence parameter and propagation distance. The coherent and incoherent superpositions result in the different position and number of composite coherence vortices and their different evolution behavior in the coherent limit.     

Quantum state measurement of any arbitrary entangled field-state via Ramsey interferometry

Multipartite entangled states are the key resource and play a crucial role in latest applications of quantum mechanics. We propose a scheme for the measurement of quantum state of multimode entangled field state trapped in multiple cavities. The scheme is based on the measurement of photon statistics of the displaced entangled field state in Ramsey type set-up. In this set-up, the atoms undergo a dispersive phase shift when they pass through the off-resonant entangled field in cavities. By measuring the internal states of the atoms, the photon statistics and the Wigner function can be reconstructed.     

Transferring the spatial state of an entangled photon pair to a single photon by photon detection

We show theoretically that the spatial state of entangled photons generated by parametric down-conversion can be transferred to the spatial state of an idler photon by signal photon detection. This study considered the general condition with an arbitrary pump field profile and the detection of a signal photon at an arbitrary distance from a nonlinear crystal where the entangled photons are generated. Upon the detection of a signal photon, the two-photon state function of the entangled state can be transferred to a single-photon state function of the idler field due to the EPR type correlation between the signal and idler fields. The spatial state of the idler field contains more information on the original two-photon state.     

Estimation of nonclassical properties of multiphoton coherent states from optical tomograms

We have examined both single and entangled two-mode multiphoton coherent states and shown how the ‘Janus-faced’ properties between two partner states are mirrored in appropriate tomograms. Entropic squeezing, quadrature squeezing and higher-order squeezing properties for a wide range of nonclassical states are estimated directly from tomograms. We have demonstrated how squeezing properties of two-mode entangled states produced at the output port of a quantum beamsplitter are sensitive to the relative phase between the reflected and transmitted fields. This feature allows for the possibility of tuning the relative phase to enhance squeezing properties of the state. Finally, we have examined the manner in which decoherence affects squeezing and the changes in the optical tomogram of the state due to interaction with the environment.