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Register a new userCoherent Population Trapping of Electron Spins in a Semiconductor
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In high-purity n-type GaAs under strong magnetic field, we are able to isolate a lambda system composed of two Zeeman states of neutral-donor bound electrons and the lowest Zeeman state of bound excitons. When the two-photon detuning of this system is zero, we observe a pronounced dip in the excited-state photoluminescence indicating the creation of the coherent population-trapped state. Our data are consistent with a steady-state three-level density-matrix model. The observation of coherent population trapping in GaAs indicates that this and similar semiconductor systems could be used for various EIT-type experiments.
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We report high resolution coherent population trapping on a single hole spin in a semiconductor quantum dot. The absorption dip signifying the formation of a dark state exhibits an atomic physics-like dip width of just 10 MHz. We observe fluctuations in the absolute frequency of the absorption dip, evidence of very slow spin dephasing. We identify this process as charge noise by, first, demonstrating that the hole spin g-factor in this configuration (in-plane magnetic field) is strongly dependent on the vertical electric field, and second, by characterizing the charge noise through its effects on the optical transition frequency. An important conclusion is that charge noise is an important hole spin dephasing process.
Optical spin rotations and cycling transitions for measurement are normally incompatible in quantum dots, presenting a fundamental problem for quantum information applications. Here we show that for a hole spin this problem can be addressed using a trion with one hole in an excited orbital, where strong spin-orbit interaction tilts the spin. Then, a particular trion triplet forms a double $Lambda$ system, even in a Faraday magnetic field, which we use to demonstrate fast hole spin initialization and coherent population trapping. The lowest trion transitions still strongly preserve spin, thus combining fast optical spin control with cycling transitions for spin readout.
Hyperfine interactions with a nuclear spin environment fundamentally limit the coherence properties of confined electron spins in the solid-state. Here, we show that a quantum interference effect in optical absorption from two electronic spin states of a solid-state emitter can be used to prepare the surrounding environment of nuclear spins in well-defined states, thereby suppressing electronic spin dephasing. The evolution of the coupled electron-nuclei system into a coherent population trapping state by optical excitation induced nuclear spin diffusion can be described in terms of Levy flights, in close analogy with sub-recoil laser cooling of atoms. The large difference in electronic and nuclear time scales simultaneously allow for a measurement of the magnetic field produced by nuclear spins, making it possible to turn the lasers that cause the anomalous spin diffusion process off when the strength of the resonance fluorescence reveals that the nuclear spins are in the desired narrow state.
Because of their long coherence times and potential for scalability, semiconductor quantum-dot spin qubits hold great promise for quantum information processing. However, maintaining high connectivity between quantum-dot spin qubits, which favor linear arrays with nearest neighbor coupling, presents a challenge for large-scale quantum computing. In this work, we present evidence for long-distance spin-chain-mediated superexchange coupling between electron spin qubits in semiconductor quantum dots. We weakly couple two electron spins to the ends of a two-site spin chain. Depending on the spin state of the chain, we observe oscillations between the distant end spins. We resolve the dynamics of both the end spins and the chain itself, and our measurements agree with simulations. Superexchange is a promising technique to create long-distance coupling between quantum-dot spin qubits.
Coherent population trapping (CPT) refers to the steady-state trapping of population in a coherent superposition of two ground states which are coupled by coherent optical fields to an intermediate state in a three-level atomic system. Recently, CPT has been observed in an ensemble of donor bound spins in GaAs and in single nitrogen vacancy centers in diamond by using a fluorescence technique. Here we report the demonstration of CPT of an electron spin in a single quantum dot (QD) charged with one electron.
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