Although two-dimensional monolayer transition-metal dichalcogenides reveal numerous unique features that are inaccessible in bulk materials, their intrinsic properties are often obscured by environmental effects. Among them, work function, which is the energy required to extract an electron from a material to vacuum, is one critical parameter in electronic/optoelectronic devices. Here, we report a large work function modulation in MoS2 via ambient gases. The work function was measured by an in situ Kelvin probe technique and further confirmed by ultraviolet photoemission spectroscopy and theoretical calculations. A measured work function of 4.04 eV in vacuum was converted to 4.47 eV with O2 exposure, which is comparable with a large variation in graphene. The homojunction diode by partially passivating a transistor reveals an ideal junction with an ideality factor of almost one and perfect electrical reversibility. The estimated depletion width obtained from photocurrent mapping was ∼200 nm, which is much narrower than bulk semiconductors.

Photonic crystal membranes (PCM) provide a versatile planar platform for on-chip implementations of photonic quantum circuits 1-3 . One prominent quantum element is a coupled system consisting of a nanocavity and a single quantum dot (QD) 4-7 which forms a fundamental building block for elaborate quantum information networks 8-10 and a cavity quantum electrodynamic (cQED) system controlled by single photons 3 . So far no fast tuning mechanism is available to achieve control within the system coherence time. Here we demonstrate dynamic tuning by monochromatic coherent acoustic phonons formed by a surface acoustic wave (SAW) with frequencies exceeding 1.7 gigahertz, one order of magnitude faster than alternative approaches 5-7 . We resolve a periodic modulation of the optical mode exceeding eight times its linewidth, preserving both the spatial mode profile and a high quality factor. Since PCMs confine photonic and phononic excitations 11,12 , coupling optical to acoustic frequencies, our technique opens ways towards coherent acoustic control of optomechanical crystals.In basic research SAWs found applications in the investigation of fundamental quantum effects in nanosystems 13-17 , the manipulation of photonic bandgap structures 18 , microcavity and surface plasmon polaritons 19-21 with frequencies spanning from a few megahertz up to a several gigahertz. We electrically generate SAWs by applying a short radio frequency (RF) voltage pulse to interdigital transducer electrodes (IDT) as shown schematically in Fig. 1 (a). As this pulse propagates across the PCM, it dynamically

We demonstrate fast nonlinear optical switching between two laser pulses with as few as 140 photons of pulse energy by utilizing strong coupling between a single quantum dot (QD) and a photonic crystal cavity. The cavity-QD coupling is modified by a detuned pump pulse, resulting in a modulation of the scattered and transmitted amplitude of a time synchronized probe pulse that is resonant with the QD. The temporal switching response is measured to be as fast as 120 ps, demonstrating the ability to perform optical switching on picosecond timescales.

Strong interactions between single spins and photons are essential for quantum networks and distributed quantum computation. They provide the necessary interface for entanglement distribution, non-destructive quantum measurements, and strong photon-photon interactions.Achieving spin-photon interactions in a solid-state device could enable compact chip-integrated quantum circuits operating at gigahertz bandwidths. Many theoretical works have suggested using spins embedded in nanophotonic structures to attain this high-speed interface. These proposals exploit strong light-matter interactions to implement a quantum switch, where the spin flips the state of the photon and a photon flips the spin-state. However, such a switch has not yet been realized using a solid-state spin system. Here, we report an experimental realization of a spin-photon quantum switch using a single solid-state spin embedded in a nanophotonic cavity.We show that the spin-state strongly modulates the cavity reflection coefficient, which conditionally flips the polarization state of a reflected photon on picosecond timescales. We also demonstrate the complementary effect where a single photon reflected from the cavity coherently rotates the spin. These strong spin-photon interactions open up a promising direction for solidstate implementations of high-speed quantum networks and on-chip quantum information processors using nanophotonic devices. In this article we report an experimental demonstration of a quantum phase switch using a single solid-state spin embedded in a nanophotonic cavity. We implement this switch using a spin trapped in a charged quantum dot that is strongly coupled to a photonic crystal defect cavity. We show that the switch applies a spin-dependent phase shift on a reflected photon that rotates its polarization state. We also demonstrate the complementary effect where a single reflected photon applies a  phase shift to one of the spin-states and thereby coherently rotates the spin. These results demonstrate that the quantum switch retains phase coherence, an essential requirement for quantum information applications. We demonstrate switching of photon wavepackets as short as 63 ps, which corresponds to a three orders of magnitude increase in bandwidth over atom-based quantum switches. Our work represents a critical step towards interconnecting multiple solidstate spins using photons for implementing quantum networks and distributed quantum information protocols. Figure 1a The quantum phase switch allows one qubit to conditionally switch the quantum state of the other qubit. We consider the case where the polarization state of the photon encodes quantum information. Since photonic crystal cavities have a single mode with a well-defined polarization, we can express the state of a photon incident on the cavity in the basis states x and y , which denote the polarization states oriented orthogonal and parallel to the cavity mode respectively. OPERATING PRINCIPLEThe quantum state of a right-circularly polarized incident p...

Single-photon switches and transistors generate strong photon-photon interactions that are essential for quantum circuits and networks. However, the deterministic control of an optical signal with a single photon requires strong interactions with a quantum memory, which has been challenging to achieve in a solid-state platform. We demonstrate a single-photon switch and transistor enabled by a solid-state quantum memory. Our device consists of a semiconductor spin qubit strongly coupled to a nanophotonic cavity. The spin qubit enables a single 63-picosecond gate photon to switch a signal field containing up to an average of 27.7 photons before the internal state of the device resets. Our results show that semiconductor nanophotonic devices can produce strong and controlled photon-photon interactions that could enable high-bandwidth photonic quantum information processing.

Single self-assembled InAs quantum dots embedded in GaAs photonic crystal defect cavities are a promising system for cavity quantum electrodynamics experiments and quantum information schemes. Achieving controllable coupling in these small mode volume devices is challenging due to the random nucleation locations of individual quantum dots. We have developed an all optical scheme for locating the position of single dots with sub-10 nm accuracy. Using this method, we are able to deterministically reach the strong coupling regime with a spatial positioning success rate of approximately 70%. This flexible method should be applicable to other microcavity architectures and emitter systems.

QDs are robust and spectrally narrow quantum emitters that have attracted significant interest as solid-state qubits. Various approaches have been pursued for storage and manipulation of quantum information in QDs. One approach has been to exploit neutral exciton transitions that can be controlled all-optically to enable both single qubit operations as well as two-qubit operations between distinguishable excitons in a QD 12 .More recently, major progress has been achieved in coherently manipulating highly 2 stable spin states of a charged QD, which promise significantly longer coherence times [13][14][15][16][17] .Another important property of QDs is that they can be coupled to optical nano-cavities in the strong coupling regime 18-21 where a QD can modify the cavity spectral response 22,23 , enabling novel applications such as ultra-fast low photon number optical switching [24][25][26] and single QD lasing 27 . Furthermore, the strong coupling regime can be exploited to interface these solid-state qubits with a flying photonic qubit through direct QD-photon quantum logic operations, as proposed in a number of theoretical works [6][7][8] . In order to realize this capability, three essential requirements must be met. First, the QD must possess two quantum states whose coherence time is long compared to the interaction time with the photonic qubit. Second, the qubit states of the QD must be coherently controllable. Finally, the qubit state of the QD must have a strong effect on the quantum state of the photon. Achieving these requirements in a solid-state photonic platform has remained an outstanding challenge.In this letter we demonstrate that a QD strongly coupled to an optical nanocavity can satisfy all of the above requirements, implementing a solid-state qubit in a cavity system that can perform quantum gates on a photon at picosecond timescales. We experimentally demonstrate a cNOT logic gate between the QD and a photonic qubit, which is a universal quantum operation that can serve as a general light-matter interface for remote entanglements and distributed quantum computation. Our device is composed of an indium arsenide (InAs) QD strongly coupled to a photonic crystal cavity. Fig. 1a illustrates the level structure of an InAs QD, which includes a ground state (|g) and two bright exciton states, labelled |+ and |-, representing the two anti-aligned spin configurations of the electron and hole. The optical transitions from the ground state to the two bright excitons, denoted  + and  -, exhibit right and left circularly polarized emission respectively at high magnetic field. For all measurements performed in this work the biexciton transition is significantly detuned and can therefore be ignored, enabling the QD to be treated as a three-level system. By applying a magnetic field in the sample growth direction (Faraday configuration), the  + transition can be tuned on resonance with the cavity while the  -transition remains detuned 28 . In this configuration, 3 states |g and |- are the qubit stat...

We study the coupling between a photonic crystal cavity and an off-resonant quantum dot under resonant excitation of the cavity or the quantum dot. Linewidths of the quantum dot and the cavity as a function of the excitation laser power are measured. We show that the linewidth of the quantum dot, measured by observing the cavity emission, is significantly broadened compared to the theoretical estimate. This indicates additional incoherent coupling between the quantum dot and the cavity. DOI: 10.1103/PhysRevB.82.045306 PACS number͑s͒: 42.50.Pq, 85.35.Be Recent demonstrations of cavity quantum electrodynamics ͑CQED͒ with a single quantum dot ͑QD͒ coupled to a semiconductor microcavity show the great potential of this system for developing robust, scalable quantum information processing devices.1-4 However, unlike ultracold atoms, QDs constantly interact with their local environments and this interaction plays a significant role in CQED experiments with QDs. For example, several experiments have reported the observation of cavity emission even when the QD is far detuned ͑ϳ3-10 meV͒ from the cavity resonance, in contrast with atomic CQED experiments. This unexpected nonresonant QD-cavity coupling is observed both in photoluminescence, where the QD is excited by creating carriers above the band-gap of the GaAs surrounding the QD ͑Refs. 2, 5, and 6͒ and in the cavity luminescence under resonant excitation of the QD. 7,8 Recent theoretical investigations have attributed the off-resonant coupling to several different causes including pure dephasing, 9 phonon relaxation, 10 multiexciton complexes, 11 and charges surrounding the QD. 12In this paper, we experimentally study the process responsible for transferring photons between the QD and offresonant cavity mode, under resonant excitation of the QD or the cavity. We derive an analytical expression for the QD linewidth based on pure dephasing and coupling to the cavity, but find that experimentally obtained linewidths are larger than that predicted by the theory. We attribute this to an additional incoherent coupling mechanism between the QD and the cavity.When an off-resonant QD that is coupled to a cavity is coherently driven by a laser field, the QD is dressed by both the cavity and the laser field. In the absence of a driving laser, the dynamics of a coupled QD-cavity system is described by the Jaynes-Cummings HamiltonianHere, c and d are the cavity and the QD resonance frequency, respectively, is the lowering operator for the QD, a is the annihilation operator for the cavity photon and g is the coherent interaction strength between the QD and the cavity. The eigenfrequencies Ϯ of the coupled system are given bywhere 2 and 2␥ are the cavity energy decay rate and the QD spontaneous emission rate, respectively and ␦ is the QDcavity detuning d − c . When the coherent interaction strength g is greater than the decay rates and ␥, the system is in strong coupling regime, and the eigenstates of H JC are polaritons possessing the characteristics of both the cavity and the...