We demonstrate an efficient cross-phase modulation (XPM) based on a closed-loop double-Λ system. The property of the double-Λ medium can be controlled by changing the phases of the applied optical fields. This phase-dependent XPM scheme can achieve large phase modulations at low-light intensities without requiring cavities or tightly focusing of laser beams. With this scheme, we observe a π-level phase shift with two pulses both consisting of 8 photons in cold rubidium atoms. Such novel scheme provides a simple route to generate strong interactions between photons and may have potential applications in all-optical quantum signal processing.The realization of large cross-phase modulations (XPM) at low-light intensities, ultimately at the singlephoton level, is an important but challenging task in quantum information science [1][2][3]. To reach this goal, one often requires high-finesse cavities to enhance nonlinear interactions between photons [4,5]. However, cavitybased experiments require many compromises such as balancing cavity bandwidth and light-matter coupling strength, which remain technical difficulties. Another promising approach for generating strong photon-photon interaction is electromagnetically induced transparency (EIT) [6][7][8], but according to the theoretical predictions by Harris and coworkers, the cross-phase shift of the EITbased Kerr medium in free space has an upper limit of order 0.1 radians at the single-photon level [9]. To date, EIT-based XPM on the order of micro-radians per photon has been observed in cold atoms [10,11] and Rb-filled fiber system [12]. In recent years, to overcome this upper limit there have been many theoretical proposals and experimental studies on this subject including double slow-light schemes [13,14], stationary light schemes [15,16], cavity EIT schemes [17,18], or Rydberg EIT schemes [19][20][21][22][23][24]. Very recently, two research teams have overcame this upper limit and observed single-photon cross-phase shifts of π/3 and π by using cavity EIT [25] and Rydberg EIT [26], respectively. This is a great progress toward implementing a photonphoton gate.Here we report an experimental observation of a novel XPM scheme based on a phase-dependent double-Λ system. With this scheme, we observe a large cross-phase shift of 3.6±1.0 radians induced by a light pulse containing around 8 photons in cold rubidium atoms. This XPM scheme does not require cavities or Rydberg atoms, which provides a simple route to generate strong interactions between photons and obtain large cross-phase shifts per photon.In the present study, we investigate a closed-loop double-Λ XPM in a laser-cooled 87 Rb atomic system, as depicted in Fig. 1(a). Cold atomic gas with an optical depth of approximately 50 is produced in a dark spontaneous-force optical trap (SPOT) [27].A strong coupling field (Ω c denotes its Rabi frequency) drives the |2 ↔ |3 transition to create a transparent medium for a weak probe pulse (Ω p , |1 ↔ |3 ) through quantum interference. The coupling and probe fields form t...

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We report the first experimental demonstration of electromagnetically induced transparency (EIT) based cross-phase modulation (XPM) at few-hundred-photon levels. A phase shift of 0.005 rad of a probe pulse modulated by a signal pulse with an energy of 100 attojoules, equivalent to ~ 400 photons, was observed in a dark spontaneousforce optical trap. The experimental data show the single-photon-level XPM phase shift is 1 × 10-5 rad, which is in good agreement with the theoretical prediction. This work offers exciting prospects to the realization of EIT-based XPM scheme at the single-photon level and benefits experimental development in few-photon applications of EIT-based techniques for quantum optics and quantum information science.

We explore novel nanometer-scale gaps with different widths in palladium (Pd) thin-film strips using hydrogen absorption under high-pressure conditions and different temperatures. Both the experimental measurement and numerical calculation are conducted to examine the electron conduction properties of the newly proposed surface conduction electron-emitters (SCEs). It is shown that this novel structure exhibits a high emission efficiency, so that a low turn-on voltage of 40 V for an SCE with a 30 nm nanogap is obtained. A calibrated model is adopted to predict the effects of the emitter thickness and different material work functions on emission current with different width of nanogaps. It is found that the heightened thickness increases the emission current. However, it tends to saturate for smaller nanogaps. The decrement of work function is proportional to the increase in emission current, which is independent of the width of nanogap.

A nanometre scale gap (nanogap) structure in palladium strip fabricated by hydrogen absorption under high-pressure treatment was proposed and applied to the surface conduction electron emitter for flat panel displays. In this paper we demonstrate that the structure possesses different high field-emission efficiencies with low turn-on voltages and high focused capability, compared with the conventional type. An experimentally validated simulation is conducted to investigate the field-emission characteristics of the explored structure. It is observed the inclined sidewall and protrusion of this nanogap can enhance the local electric field and the focused capability and protect emission areas from being damaged by impurity ions during field-emission operation. This study benefits the advanced design of metallic electrodes in nanodevice technology for new types of electron sources and display applications.