Photon blockade is a dynamical quantum-nonlinear effect in driven systems with an anharmonic energy ladder. For a single atom strongly coupled to an optical cavity, we show that atom driving gives a decisively larger optical nonlinearity than cavity driving. This enhances single-photon blockade and allows for the implementation of two-photon blockade where the absorption of two photons suppresses the absorption of further photons. As a signature, we report on three-photon antibunching with simultaneous two-photon bunching observed in the light emitted from the cavity. Our experiment constitutes a significant step towards multiphoton quantum-nonlinear optics.

We investigate phase shifts in the strong coupling regime of single-atom cavity quantum electrodynamics (QED). On the light transmitted through the system, we observe a phase shift associated with an antiresonance and show that both its frequency and width depend solely on the atom, despite the strong coupling to the cavity. This shift is optically controllable and reaches 140 • -the largest ever reported for a single emitter. Our result offers a new technique for the characterization of complex integrated quantum circuits.The strongly coupled atom-cavity system plays a central role in research on fundamental quantum optics. Important achievements to date include the creation of single photon sources [1,2] and non-classical microwave states [3,4], single-atom squeezing [5], the observation of novel photon statistics [6][7][8] and the nondestructive detection of microwave and optical photons [9,10]. More complex interacting systems based on this basic element are now attracting much attention in quantum information and simulation. Recent achievements in this direction include the coupling of a single qubit to two cavities [11], the interaction of multiple qubits with a single cavity bus [12,13], and the exchange of quantum states between single qubits in remote cavities [14,15]. Integrated quantum circuits are promising candidates for on-chip quantum computation [16][17][18][19][20] and large strongly coupled networks have been proposed for simulating quantum phase transitions [21][22][23]. However, in such strongly interacting systems, the couplings no longer represent merely a perturbation of the subsystem dynamics, necessitating a holistic analysis of the coupled system. This makes the characterization of strongly coupled quantum circuits a challenging task [24,25].In this Letter, we propose a new technique for characterizing complex quantum circuits, which emerges from an analysis of the phase of light transmitted through a strongly coupled single-atom-cavity system. In particular, we report on the observation of an antiresonant phase shift caused by destructive interference between the coherent drive and the field radiated by the atom. The signature of the antiresonance is a large negative phase shift which depends solely on the atom, despite the strong coupling to the resonator. This is in sharp contrast to the normal modes [26,27], which depend on properties of both atom and cavity as well as the coupling strength [28]. Our measurement paves the way for individual components of strongly interacting quantum systems to be characterized via measurements performed only on the overall coupled system.Previous work on phase spectroscopy in cavity QED has focused on the so-called "bad-cavity" limit in which the cavity decay rate exceeds the coupling strength, κ > ∼ g, and only modest phase shifts were observed [29,30]. Phase changes due to strongly coupled atoms were seen in Ref. [31], but the antiresonance phase shift was not observed. The presence of a transmission dip at the atomic frequency (associated with th...

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A novel type of nanolasers, which combines the advantages of photonic crystal lasers and microdisk lasers, has been demonstrated based on InAlGaAs/InGaAs quantum wells using pulsed optical pumping at room temperature. It incorporates the properties of small footprint, small mode volume, and submilliwatt threshold, and favors vertical emission. We believe that this type of laser acts as a promising candidate for highly-integrated on-chip nanolasers in applications for signal processing and index sensing.

We demonstrate a new feedback algorithm to cool a single neutral atom trapped inside a standingwave optical cavity. The algorithm is based on parametric modulation of the confining potential at twice the natural oscillation frequency of the atom, in combination with fast and repetitive atomic position measurements. The latter serve to continuously adjust the modulation phase to a value for which parametric excitation of the atomic motion is avoided. Cooling is limited by the measurement back action which decoheres the atomic motion after only a few oscillations. Nonetheless, applying this feedback scheme to a ∼ 5 kHz oscillation mode increases the average storage time of a single atom in the cavity by a factor of 60 to more than 2 seconds. In contrast to previous feedback schemes, our algorithm is also capable of cooling a much faster ∼ 500 kHz oscillation mode within just microseconds. This demonstrates that parametric cooling is a powerful technique that can be applied in all experiments where optical access is limited.