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arXiv:1103.1117 · Applied Physics Letters

A spintronic source of circularly polarized single photons

Pablo Aßhoff, Andreas Merz, H. Kalt, M. Hetterich

Published Mar 14, 2011• 31 citations
arXivHTMLPDF

Abstract

We present a spintronic single photon source which emits circularly polarized light, where the helicity is determined by an applied magnetic field. Photons are emitted from an InGaAs quantum dot inside an electrically operated spin light-emitting diode, which comprises the diluted magnetic semiconductor ZnMnSe. The circular polarization degree of the emitted light is high, reaching 83% at an applied magnetic field of 2 T and 96% at 6 T. Autocorrelation traces recorded in pulsed operation mode prove the emitted light to be antibunched. The two circular polarization states could be used for representing quantum states |0⟩ and |1⟩ in quantum cryptography implementations.

DOI
10.1063/1.3564893

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We present a spintronic single photon source which emits circularly polarized light, where the helicity is determined by an applied magnetic field. Photons are emitted from an InGaAs quantum dot inside an electrically operated spin light-emitting diode, which comprises the diluted magnetic semiconductor ZnMnSe. The circular polarization degree of the emitted light is high, reaching 83% at an applied magnetic field of 2 T and 96% at 6 T. Autocorrelation traces recorded in pulsed operation mode prove the emitted light to be antibunched. The two circular polarization states could be used for representing quantum states |0⟩ and |1⟩ in quantum cryptography implementations.
Electrically spin-injected ensembles of InGaAs/GaAs quantum dots emit circularly polarized single photons, with the emission helicity determined by the spin polarization of the injected electrons. A ferromagnetic CoFeB spin-injection layer with perpendicular magnetic anisotropy, combined with an MgO tunnel barrier, enables spin-polarized electron injection into the quantum-dot active region, resulting in circularly polarized photon emission at zero applied magnetic field. These devices realize direct spin-charge-photon conversion in an electrically driven, microelectronics-compatible quantum light source that operates without an external pump laser. Using an extended Hanbury Brown-Twiss interferometer with four detectors and time-tagged readout, we measure time-resolved photon-number statistics that resolve up to ∣2⟩|2\rangle∣2⟩ Fock states within a single excitation cycle. While single-photon (n=∣1⟩n=|1\ranglen=∣1⟩) emission tracks the electrical excitation pulse as expected, the two-photon (n=∣2⟩n=|2\ranglen=∣2⟩) statistics exhibit pronounced fluctuations at multiple temporal positions within the pulse. These fluctuations vanish upon demagnetization of the spin-injection layer, establishing a direct link between the multi-photon emission dynamics and the spin polarization of the injected carriers. The observations indicate that spin injection controls not only the helicity of the emitted photons but also higher-order temporal correlations of the emission. The behavior is phenomenologically captured by inserting both a variable excitation mechanism and a set of multi-emitter collapse operators within a stochastic wave function evolution. The platform thus opens a route to electrically driven, spin-controlled single and multi-photon Fock-state sources and provides a testbed for spin-conditioned multi-emitter dynamics in solid-state quantum light sources.
We show storage of the circular polarization of an optical field, transferring it to the spin-state of an individual electron confined in a single semiconductor quantum dot. The state is subsequently read out through the electronically-triggered emission of a single photon. The emitted photon shares the same polarization as the initial pulse but has a different energy, making the transfer of quantum information between different physical systems possible. With an applied magnetic field of 2 T, spin memory is preserved for at least 1000 times more than the exciton's radiative lifetime.
An in-plane spin-photon interface is essential for the integration of quantum dot spins with optical circuits. The optical dipole of a quantum dot lies in the plane and the spin is optically accessed via circularly polarized selection rules. Hence, a single waveguide, which can transport only one in-plane linear polarization component, cannot communicate the spin state between two points on a chip. To overcome this issue, we introduce a spin-photon interface based on two orthogonal waveguides, where the polarization emitted by a quantum dot is mapped to a path-encoded photon. We demonstrate operation by deducing the spin using the interference of in-plane photons. A second device directly maps right and left circular polarizations to antiparallel waveguides, surprising for a nonchiral structure but consistent with an off-center dot.
Abstract Single-photon sources based on semiconductor quantum dots find several applications in quantum information processing due to their high single-photon indistinguishability, on-demand generation, and low multiphoton emission. In this context, the generation of entangled photons represents a challenging task with a possible solution relying on the interference in probabilistic gates of identical photons emitted at different pulses from the same source. In this work, we implement this approach via a simple and compact design that generates entangled photon pairs in the polarization degree of freedom. We operate the proposed platform with single photons produced through two different pumping schemes, the resonant excited one and the longitudinal-acoustic phonon-assisted configuration. We then characterize the produced entangled two-photon states by developing a complete model taking into account relevant experimental parameters, such as the second-order correlation function, Hong–Ou–Mandel visibility, multiphoton emission and pump laser filtering. Our source shows long-term stability and high quality of the generated entangled states, thus constituting a reliable building block for optical quantum technologies.
We report the realization of the first electrically-driven single photon source. The device is based on a GaAs p-i-n diode containing self-assembled InAs quantum dots embedded within the i-region. Time resolved electroluminescence from the device yields information on the radiative lifetimes of the single quantum dots involved. Applying a continuous current a dip is measured in the second-order correlation function at zero time delay indicating that the luminescence is antibunched. Under pulsed excitation by subnanosecond voltage pulses, strong suppression of multi-photon pulses is achieved with respect to a laser of the same average intensity. The results suggest that conventional semiconductor technology can be used to mass-produce a single photon source for applications in quantum information technology.
Definite circular and linear polarizations of room-temperature single-photon sources, which can serve as polarization bases for quantum key distribution, are produced by doping planar-aligned liquid crystal hosts with single fluorescence emitters. Chiral 1-D photonic bandgap microcavities for a single handedness of circularly polarized light were prepared from both monomeric and oligomeric cholesteric liquid crystals. Fluorescent emitters, such as nanocrystal quantum dots, nitrogen vacancy color centers in nanodiamonds, and rare-earth ions in nanocrystals, were doped into these microcavity structures and used to produce circularly polarized fluorescence of definite handedness. Additionally, we observed circularly polarized resonances in the spectrum of nanocrystal quantum dot fluorescence at the edge of the cholesteric microcavity's photonic stopband. For this polarization we obtained a ~4.9 enhancement of intensity compared to the polarization of the opposite handedness that propagates without photonic bandgap microcavity effects. Such a resonance is indicative of coupling of quantum dot fluorescence to the cholesteric microcavity mode. We have also used planar-aligned nematic liquid crystal hosts to align DiI dye molecules doped into the host, thereby providing a single-photon source of linear polarization of definite direction. Antibunching is demonstrated for fluorescence of nanocrystal quantum dots, nitrogen vacancy color centers, and dye molecules in these liquid crystal structures.