J. Miguel-Sanchez

Observation of entanglement between a quantum dot spin and a single photon

Entanglement has a central role in fundamental tests of quantum mechanics as well as in the burgeoning field of quantum information processing. Particularly in the context of quantum networks and communication, a main challenge is the efficient generation of entanglement between stationary (spin) and propagating (photon) quantum bits. Here we report the observation of quantum entanglement between a semiconductor quantum dot spin and the colour of a propagating optical photon. The demonstration of entanglement relies on the use of fast, single-photon detection, which allows us to project the photon into a superposition of red and blue frequency components. Our results extend the previous demonstrations of single-spin/single-photon entanglement in trapped ions, neutral atoms and nitrogen–vacancy centres to the domain of artificial atoms in semiconductor nanostructures that allow for on-chip integration of electronic and photonic elements. As a result of its fast optical transitions and favourable selection rules, the scheme we implement could in principle generate nearly deterministic entangled spin–photon pairs at a rate determined ultimately by the high spontaneous emission rate. Our observation constitutes a first step towards implementation of a quantum network with nodes consisting of semiconductor spin quantum bits.

Quantum teleportation from a propagating photon to a solid-state spin qubit

A quantum interface between a propagating photon used to transmit quantum information and a long-lived qubit used for storage is of central interest in quantum information science. A method for implementing such an interface between dissimilar qubits is quantum teleportation. Here we experimentally demonstrate transfer of quantum information carried by a photon to a semiconductor spin using quantum teleportation. In our experiment, a single photon in a superposition state is generated using resonant excitation of a neutral dot. To teleport this photonic qubit, we generate an entangled spin-photon state in a second dot located 5 m away and interfere the photons from the two dots in a Hong-Ou-Mandel set-up. Thanks to an unprecedented degree of photon-indistinguishability, a coincidence detection at the output of the interferometer heralds successful teleportation, which we verify by measuring the resulting spin state after prolonging its coherence time by optical spin-echo.

Cavity quantum electrodynamics with charge-controlled quantum dots coupled to a fiber Fabry–Perot cavity

We demonstrate non-perturbative coupling between a single self-assembled InGaAs quantum dot and an external fiber-mirror-based microcavity. Our results extend the previous realizations of tunable microcavities while ensuring spatial and spectral overlap between the cavity mode and the emitter by simultaneously allowing for deterministic charge control of the quantum dots. Using resonant spectroscopy, we show that the coupled quantum dot cavity system is at the onset of strong coupling, with a cooperativity parameter of C ≈ 2.0 ± 1.3. Our results constitute a milestone in the progress toward the realization of a high-efficiency solid-state spin–photon interface.

Electron mean free path from angle-dependent photoelectron spectroscopy of aerosol particles

We propose angle-resolved photoelectron spectroscopy of aerosol particles as an alternative way to determine the electron mean free path of low energy electrons in solid and liquid materials. The mean free path is obtained from fits of simulated photoemission images to experimental ones over a broad range of different aerosol particle sizes. The principal advantage of the aerosol approach is twofold. First, aerosol photoemission studies can be performed for many different materials, including liquids. Second, the size-dependent anisotropy of the photoelectrons can be exploited in addition to size-dependent changes in their kinetic energy. These finite size effects depend in different ways on the mean free path and thus provide more information on the mean free path than corresponding liquid jet, thin film, or bulk data. The present contribution is a proof of principle employing a simple model for the photoemission of electrons and preliminary experimental data for potassium chloride aerosol particles.

Observation of quantum jumps of a single quantum dot spin using submicrosecond single-shot optical readout.

Single-shot readout of individual qubits is typically the slowest process among the elementary single- and two-qubit operations required for quantum information processing. Here, we use resonance fluorescence from a single-electron charged quantum dot to read out the spin-qubit state in 800 nanoseconds with a fidelity exceeding 80%. Observation of the spin evolution on longer time scales reveals quantum jumps of the spin state: we use the experimentally determined waiting-time distribution to characterize the quantum jumps.

Resonant Spectroscopy on Charge Tunable Quantum Dots in Photonic Crystal Structures

We demonstrate resonant excitation of a semiconductor quantum dot embedded in a p-i-n diode structure in and out of a photonic crystal cavity. We investigate the modification of the spontaneous emission rate and the photon collection efficiency of quantum dots due to the presence of a photonic bandgap. The resonant excitation together with deterministic charging of the quantum dot embedded in a photonic crystal, can be used for single electron spin measurement.

Electrically tunable artificial gauge potential for polaritons

Neutral particles subject to artificial gauge potentials can behave as charged particles in magnetic fields. This fascinating premise has led to demonstrations of one-way waveguides, topologically protected edge states and Landau levels for photons. In ultracold neutral atoms, effective gauge fields have allowed the emulation of matter under strong magnetic fields leading to realization of Harper-Hofstadter and Haldane models. Here we show that application of perpendicular electric and magnetic fields effects a tunable artificial gauge potential for two-dimensional microcavity exciton polaritons. For verification, we perform interferometric measurements of the associated phase accumulated during coherent polariton transport. Since the gauge potential originates from the magnetoelectric Stark effect, it can be realized for photons strongly coupled to excitations in any polarizable medium. Together with strong polariton–polariton interactions and engineered polariton lattices, artificial gauge fields could play a key role in investigation of non-equilibrium dynamics of strongly correlated photons.

Current Spreading Efficiency and Fermi Level Pinning in GaInNAs–GaAs Quantum-Well Laser Diodes

The role of the current spreading efficiency, ?s, on the degradation of the figures of merit of stripe geometry GaInNAs-GaAs quantum well (QW) laser diodes (LDs) is studied as a function of the N content in the range from 0 to 3.3%. It is found that, in N-containing devices, Jth is strongly dependent on the current injection location along the stripe. This is attributed to a poor spreading of carriers along the length of the laser stripe when N is present in the diodes. If the current is injected using two parallel probes, the threshold current turns out to be nearly independent on the position of the current injection sites and carrier distribution along the laser stripe is similar in N-free and N-containing devices. A model is proposed to explain this phenomenon in which the low-populated portions of the QW are pumped optically by reabsorption of the photons emitted by the high-populated portions of the QW. Local heating in N-containing devices would cause a temperature gradient along the stripe that hinders this optical pumping of the lowly-injected portion of the cavity. The value of the lateral component of ?s, ?s lat, is evaluated by measuring the degree of above-threshold Fermi level pinning in the QW using two probes so any variation in ?s will arise from variations in ?s lat only. To do so, the partially amplified spontaneous emission from the laser diodes is measured above and below threshold, and the result is used to calculate ?s. It is found that ?s decreases by ~ 18% upon addition of N. This reduction can account for half of the observed reduction in the internal quantum efficiency, ?i, in N-containing LDs with respect to N-free devices. The rest of the degradation of ?i could be accounted for by another recombination mechanism such as non-radiative recombination at defects in the barriers. The physical mechanisms responsible for the degradation of ?s are discussed and various alternative models are proposed.

Influence of carrier localization on the performance of MBE grown GaInNAs/GaAs QW light emitting diodes and laser diodes

The influence of carrier localization on the opto-electronic properties of GaInNAs/GaAs quantum well (QW) light emitting diodes (LED) and laser diodes (LD) grown by molecular beam epitaxy is studied. The external quantum efficiency of the LEDs at low temperature is found to be strongly affected by emission from localized states, and its evolution with the injected current is modified compared to the typical one of a QW LED. The light-current characteristics of GaInNAs LDs are measured for different temperatures between 15 and 295 K, and an anomalous behaviour of the threshold current with temperature is obtained comparing to a reference InGaAs laser. In particular, a negative or infinite T0 is obtained at very low temperatures, followed by a region of very small T0. In addition, if the temperature is further increased, a change to a higher T0 is obtained at a temperature which is in the range of the typical delocalization temperatures in GaInNAs QWs. All these features are attributed to the influence of carrier localization. The temperature induced changes in the relative carrier population of the localized states and the band edge states change the lineshape of the gain spectrum and its peak value, and consequently the threshold current of GaInNAs QW lasers.

Quantum dot spin-photon entanglement and photon-to-spin teleportation

Entanglement plays a central role in fundamental tests of quantum mechanics as well as in the burgeoning field of quantum information processing. Particularly in the context of quantum networks and communication, a major challenge is the efficient generation of entanglement between stationary (spin) and flying (photon) qubits. Here we report the observation of quantum entanglement between a semiconductor quantum dot spin and the color of a propagating optical photon. As an extension experiment, we report the generation of a single-photon frequency qubit, interference of resonance fluorescence from two distant quantum dots and the teleportation from a flying photon to a quantum dot spin.