Atac Imamoglu

Optically active quantum dots in monolayer WSe2.

Semiconductor quantum dots have emerged as promising candidates for the implementation of quantum information processing, because they allow for a quantum interface between stationary spin qubits and propagating single photons. In the meantime, transition-metal dichalcogenide monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large bandgap with degenerate valleys and non-zero Berry curvature. Here, we report the observation of zero-dimensional anharmonic quantum emitters, which we refer to as quantum dots, in monolayer tungsten diselenide, with an energy that is 20-100 meV lower than that of two-dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host transition-metal dichalcogenide. The large ∼1 meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement that is most probably induced by impurities or defects. The possibility of achieving electrical control in van der Waals heterostructures and to exploit the spin-valley degree of freedom renders transition-metal-dichalcogenide quantum dots interesting for quantum information processing.

Valley Zeeman effect in elementary optical excitations of monolayer WSe2

Charge carriers in transition metal dichalcogenides have an extra degree of freedom known as valley pseudospin, which is associated with the shape of the energy bands. Experiments show that this pseudospin can be manipulated using magnetic fields.

Photon Antibunching in the Photoluminescence Spectra of a Single Carbon Nanotube

We report the first observation of photon antibunching in the photoluminescence from single carbon nanotubes. The emergence of a fast luminescence decay component under strong optical excitation indicates that Auger processes are partially responsible for inhibiting two-photon generation. Additionally, the presence of exciton localization at low temperatures ensures that nanotubes emit photons predominantly one by one. The fact that multiphoton emission probability can be smaller than 5% suggests that carbon nanotubes could be used as a source of single photons for applications in quantum cryptography.

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.

Strongly correlated photons on a chip

Researchers observe a continuous change in photon correlations from strong antibunching to bunching by tuning either the probe laser or the cavity mode frequency. These results, which demonstrate unprecedented strong single-photon nonlinearities in quantum dot cavity system, are explained by the photon blockade and tunnelling in the anharmonic Jaynes–Cummings model.

Cavity QED Based on Collective Magnetic Dipole Coupling : Spin Ensembles as Hybrid Two-Level Systems

We analyze the magnetic dipole coupling of an ensemble of spins to a superconducting microwave stripline structure, incorporating a Josephson junction based transmon qubit. We show that this system is described by an embedded Jaynes-Cummings model: in the strong coupling regime, collective spin-wave excitations of the ensemble of spins pick up the nonlinearity of the cavity mode, such that the two lowest eigenstates of the coupled spin wave-microwave cavity-Josephson junction system define a hybrid two-level system. The proposal described here enables new avenues for nonlinear optics using optical photons coupled to spin ensembles via Raman transitions. The possibility of strong coupling cavity QED with magnetic dipole transitions also opens up the possibility of extending quantum information processing protocols to spins in silicon or graphene, without the need for single-spin confinement.

Explanation of Photon Correlations in the Far-Off-Resonance Optical Emission from a Quantum-Dot-Cavity System

Martin Winger, Thomas Volz, Guillaume Tarel, Stefano Portolan, Antonio Badolato, Kevin J. Hennessy, Evelyn L. Hu, Alexios Beveratos, Jonathan Finley, Vincenzo Savona, and Ataç Imamoğlu Institute of Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne EPFL, CH-1015 Lausanne, Switzerland Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA California NanoSystems Institute, University of California, Santa Barbara, California 93106, USA CNRS Laboratoire Photonique et Nanostructures, Route de Nozay, F-91460 Marcoussis, France Walter Schottky Institut, Am Coulombwall 3, D-85748 Garching, Germany (Dated: November 23, 2009)

Observation of Faraday rotation from a single confined spin

The ability to read out the state of a single confined spin lies at the heart of solid-state quantum-information processing1. Although spin measurements using Faraday rotation of light polarization have been implemented in semiconductor spin ensembles2,3,4, single-spin read-out has only been achieved using transport measurements5,6. Here, we demonstrate an all-optical dispersive measurement of the time-averaged spin state of a single electron in a quantum dot. We obtain information on the spin state through conditional Faraday rotation of a spectrally detuned laser, induced by the polarization- and spin-selective trion (charged quantum dot) transitions. To assess the sensitivity of the technique, we use an independent resonant laser for spin-state preparation7. We infer that there are ∼10 spin-flip Raman scattering events (that is, back-action) within our measurement timescale. Straightforward improvements such as incorporating solid-immersion lenses8,9 and higher efficiency detectors should allow for back-action-evading spin measurements, without the need for a cavity.

Nuclear spin physics in quantum dots : an optical investigation

The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic,coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantum-mechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.

Knight-Field-Enabled Nuclear Spin Polarization in Single Quantum Dots

We demonstrate dynamical nuclear-spin polarization in the absence of an external magnetic field by resonant circularly polarized optical excitation of a single electron or hole charged quantum dot. Optical pumping of the electron spin induces an effective inhomogeneous magnetic (Knight) field that determines the direction along which nuclear spins could polarize and enables nuclear-spin cooling by suppressing depolarization induced by nuclear dipole-dipole interactions. Our experiments constitute a first step towards a quantum measurement of the Overhauser field.