We show that a nonlinear phase shift of π can be obtained by using a single two level atom in a one sided cavity with negligible losses. This result implies that the use of a one sided cavity can significantly improve the π/18 phase shift previously observed by Turchette et al. [Phys. Rev. Lett. 75, 4710 (1995)].One of the most significant achievements in the field of quantum optics is the realization of strong nonlinear effects by enhancing the coupling between single atoms and the light field. In particular, the possibility of obtaining large conditional phase shifts has attracted much attention because of its potential usefulness in the realization of phase gates for optical quantum computation and similar manipulations of quantum states at the few photon level [1,2,3,4,5,6]. In order to optimize controlled phase shifts, it is desirable to avoid losses while moving close to the resonance of the two level atom causing the nonlinear phase shift. In the experiment by Turchette et al. [2], the nonlinearity of the atom was observed in the phase change of the light transmitted through the atom-cavity system. However, the transmission of a two sided cavity at the atomic resonance is very low, so the experiment was conducted at frequencies significantly detuned from this resonance. As a result, the phase shift observed was limited to only about π/18. In order to improve this phase shift, it is necessary to move closer to resonance while avoiding dissipative losses. In this paper, we therefore propose the use of a one sided cavity with negligible losses to non-cavity modes. In such a geometry, the total reflection is always close to one and all dissipation is suppressed. The nonlinearity then has the maximal effect on the phase of the light field while leaving the intensity unchanged. This makes it possible to realize a nonlinear phase shift of π at the atomic resonance. Figure 1 shows an illustration of this dissipation free setup. Effectively, the cavity confines the light field interacting with the two level atom to a single beam with a well defined transversal profile. The suppression of losses to non-cavity modes can be achieved by covering a large solid angle of emission with the confocal cavity mirrors. Further improvements may be possible by using dielectric materials, e.g. in a photonic crystal geometry.The most simple case of this dissipation free setup is obtained in the bad cavity regime, where the cavity lifetime is so short that the cavity field can be adiabatically eliminated. In terms of the conventional cavity quantum electrodynamics parameters this regime is characterized by κ ≫ g, where κ is the cavity damping rate and g is the dipole coupling between the atom and the cavity. The effective dipole damping rate caused by emissions through the cavity

The nonlinear photon-photon interaction mediated by a single two-level atom is studied theoretically based on a one-dimensional model of the field-atom interaction. This model allows us to determine the effects of an atomic nonlinearity on the spatiotemporal coherence of a two-photon state. Specifically, the complete twophoton output wave function can be obtained for any two-photon input wave function. It is shown that the quantum interference between the components of the output state associated with different interaction processes causes bunching and antibunching in the two-photon statistics. This theory may be useful for various applications in photon manipulation, e.g., quantum information processing using photonic qubits, quantum nondemolition measurements, and the generation of entangled photons.

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This paper presents a 28-GHz CMOS four-element phased-array transceiver chip for the fifth-generation mobile network (5G) new radio (NR). The proposed transceiver is based on the local-oscillator (LO) phase-shifting architecture, and it achieves quasi-continuous phase tuning with less than 0.2-dB radio frequency (RF) gain variation and 0.3 • phase error. Accurate beam control with suppressed sidelobe level during beam steering could be supported by this work. At 28 GHz, a single-element transmitter-mode output P 1 dB of 15.7 dBm and a receiver-mode noise figure (NF) of 4.1 dB are achieved. The eight-element transceiver modules developed in this work are capable of scanning the beam from −50 • to +50 • with less than −9-dB sidelobe level. A saturated equivalent isotropic radiated power (EIRP) of 39.8 dBm is achieved at 0 • scan. In a 5-m overthe-air measurement, the proposed module demonstrates the first 512 quadrature amplitude modulation (QAM) constellation in the 28-GHz band. A data stream of 6.4 Gb/s in 256-QAM could be supported within a beam angle of ±50 • . The achieved maximum data rate is 15 Gb/s in 64-QAM. The proposed transceiver chip consumes 1.2 W/chip in transmitter mode and 0.59 W/chip in receiver mode.

We investigate the single-mode operation of a quantum optical nonlinear phase-shift gate implemented by a single two-level atom in one-dimensional free space. Since the single-mode property of the input photons at the atom is not preserved in the interaction at the atom, we analyze the efficiency of single-mode operation that can still be achieved. We show how the input pulse shape can be optimized to obtain high efficiencies for the nonlinear single-mode operation. With this analysis, we obtain an optimal single-mode transmittance per photon of 78% for the successful nonlinear phase-shift operation.