Keywords: quantum interference, multi-photon interference, Hong-Ou-Mandel effect, squeezed coherent state, quantum cryptography, measurement-device-independent quantum key distribution, quantum bit error rate AbstractDespite the relative simplicity of the traditional Hong-Ou-Mandel (HOM) interferometer setup, it is of fundamental interest in various quantum optics applications and quantum information technologies. In this article, multi-photon interference using the original HOM interferometer setup is analyzed. More specifically, for any photon number state with Gaussian spectral distribution entering the beam splitter, the general analytical solution is calculated. The result is then used to study the coincident probability for coherent sources (laser) and squeezed coherent state. We also look into the potential benefits of implementing the squeezed coherent state in discrete-variable Measurement-Device-Independent Quantum Key Distribution and find that by optimizing the squeezing parameter, the error rate can be significantly reduced. This in turn also enhances the secret key rate performance over the coherent state.When two identical single photons enter a 50:50 beam splitter, quantum interference of the photons causes both photons to exit at the same (but random) side of the beam splitter. This is known as the Hong-Ou-Mandel (HOM) effect. Experimentally, this is characterized by a dip in the coincidence rate [1], also referred to as the HOM dip. Coincidence here refers to mutual detection of photons at both sides or output modes of the beam splitter.The original purpose of HOM experiment is to characterize the temporal distinguishability of single photons, by accurate measurement of time interval and bandwidth of the photons, allowed by the HOM interferometer setup. An account of HOM effect for single photons with various choices of spectra, and also taking into account photon distinguishability, can be seen in [2]. To accomplish the same task of characterizing the temporal distinguishability for multi photons, however, the experimental setup is generally modified to include multiple beam splitters and photon detectors [3]. A number of multi-photon interference experiments with various setups can be found, for example, in [4][5][6][7][8][9][10][11]. A summary for some of these experiments with discussion of temporal distinguishability is given in [12]. The general results for multi-photon quantum interference using traditional HOM interferometer, specifically, for any photon number state with Gaussian spectral distribution, have not been discussed in the literature.Despite that, however, and despite the relative simplicity of the original HOM interferometer setup, it has proven to be useful and important in various applications, for example, in quantum interference of successive single photons from the same emitter [13][14][15][16], to determine the purity of photons emitted by quantum dot [13], in quantum interference between single photons emitted by independent atoms [17], to measure biphoton temporal wav...

The combined use of quantum channels can grant communication advantages in the form of enhancements to communication capacities. One such channel composition is the quantum switch, which implements a system with indefinite causal order by coherent control of the orderings of two or more quantum channels, resulting in enhanced communication capacities. Here, using the monogamy relation, we studied the flow of entanglement monotone in these quantum compositions of channels in the environmental representation. We implemented the two-party quantum switch in this framework, and demonstrated that non-Markovianity is the source of revival of the entanglement monotone in this setup. The possibility and amount of revival was shown to depend on the entangling capability of the channels, and the perfect activation of coherent information with entanglement-breaking channels was also replicated with the perfect revival of entanglement monotone. Additionally, we showed that a more general non-Markovian circuit can still grant enhancements to coherent information and Holevo quantity without the presence of indefinite causal order.

In this paper, we investigate the effect of initial squeezing on tripartite continuous-variable (CV) Hamiltonians based on real physical systems. The Hamiltonians are based on the three-mode magnons excited by an external field, the one-dimensional interconnected tripartite spring-like system, and two spin ensembles coupled to a cavity resonator in the nondispersive regime in the CV limit. We find that for the three-mode magnons and the spring-like system, implementation of initial squeezing invariably enhances the two-way and three-way entanglement, while for the spin-resonator system, it depends on the alignment of the spin ensembles relative to the resonator field.

We study properties of the symbolic sequences extracted from the fractals generated by the arc-fractal system introduced earlier by Huynh and Chew. The sequences consist of only a few symbols yet possess several nontrivial properties. First using an operator approach, we show that the sequences are not periodic, even though they are constructed from very simple rules. Second by employing the ϵ-machine approach developed by Crutchfield and Young, we measure the complexity and randomness of the sequences and show that they are indeed complex, i.e. neither periodic nor random, with the value of complexity measure being significant as compared to the known example of logistic map at the edge of chaos. The complexity and randomness of the sequences are then discussed in relation with the properties of associated fractal objects, such as their fractal dimension, symmetry and orientations of the arcs.

In this paper, we investigate entanglement of an experimental system of two nitrogen-vacancycenter ensembles which are initially squeezed under the one-axis twisting Hamiltonian. We take into account three scenarios in which initial squeezing and entanglement are mediated by phonons or photons: (a) the phonon-squeezed photon-entangled scenario, (b) the phonon-squeezed phononentangled scenario, and (c) the photon-squeezed photon-entangled scenario. For our investigation, we employ the Tavis-Cummings model, which includes dissipative decoherence of the collective spin ensemble, and analyze the system both for a relatively small number of spins and in the limit of a large number of spins using the approach of a quantum master equation. Although evidence in the literature on idealized coupled oscillator systems and coupled quantum kicked tops suggests that initial squeezing can enhance entanglement, we find that, in the realistic system studied in this paper, initial squeezing can improve entanglement overall when the field mode interacts in a particular manner with the two spin ensembles. Our analysis using the Holstein-Primakoff transformation and Wigner characteristic function in the rotating frame of reference shows that the entanglement enhancement is a subtle consequence of the way in which the dissipative decoherence rotates the state of the collective spin ensemble such that enhancement depends on the time-evolved rotated states between the presence and absence of initial squeezing.