News of MRS Members/Materials Researchers
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Nature Materials, the researchers prepared a multiple-quantum-well structure consisting of 100 CdTe layers 58 Å thick separated by 19-Å MnTe barriers. The Mn2+ impurity ions, present at ~0.5% doping levels, were not introduced into the CdTe intentionally, but diffused from the barrier layers. Photoluminescence (PL) as well as PL excitation and Raman scattering spectra were acquired by using a CW Ti:sapphire laser. An additional modelocked Ti:sapphire laser emitting subpicosecond pulses was used to obtain the time-domain reflectivity data and generate the exciton states. The resonant Raman spectra show spin flip (SF) transitions of electrons bound to donors and to the Mn ions. The Mn spectrum exhibits a series of peaks at multitudes of the fundamental paramagnetic resonance. The researchers were able to create entangled states by selectively irradiating the quantum-well structures with light pulses of central energy at 1.677 eV. The signature of entanglement was the observation of overtones of the SF transition in the coherent time-domain spectra. Specifically, their data reveal the first and second SF harmonics implying the existence of entangled states that involve three donor impurities. Their results also show indirect evidence of two Mn ion entanglement. Because exciton generation can be tuned by varying the wavelength of the exciting light, this system can in principle be used to generate and precisely control multiple sets of entangled states for an arbitrarily large number of impurities. GREG KHITROV
GaAs Quantum Dots Exhibit Triggered Single Photon Emission Materials capable of emitting Fourier transform limited single photons are needed for quantum computing and quantum cryptography schemes. Semiconductor quantum dots are attractive single photon emitters but usually oper-
Figure. Microphotoluminescence spectrum measured in a 1-µm-diameter microdisk. The arrow points to the quantum-dot line selected for the photon correlation measurements. Inset: Image of a microdisk obtained by scanning electron microscopy.
ate far from the Fourier transform limit. As reported in the April 7 issue of Applied Physics Letters, a team of researchers from CNRS, Alcatel R&I, and CEA in France has recently observed single-photon emission from quantum dots formed at interface fluctuations of GaAs/GaAlAs quantum wells (see figure). The researchers indicate that the quantum dots have potential as a source of Fourier transform limited single photons. The group used molecular-beam epitaxy to grow the sample, which included a single 3-nm GaAs quantum well surrounded by 50-nm Ga0.67Al0.33As barriers. Quantum dots resulted from fluctuations in the thickness of the narrow well. Electron-beam lithography and chemical etching were used to cut microdisks from the sample, isolating several quantum dots. After measuring the emission spectrum of the isolated dots at 10 K and selecting a spectral line corresponding to an exciton transition, the team carried out photon correlation experiments using a Ti:sapphire laser delivering 1.5 ps pulse
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