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Edward Flagg
Department of Physics and Astronomy

Indistinguishable Photons

Photons emitted by quantum dots (QDs) have the potential to be extremely useful in many quantum information processing (QIP) applications, but currently that potential is limited by their significantly less-than-ideal indistinguishability. The most likely cause of reduced indistinguishability is spectral diffusion–fluctuations in the nearby charge distribution that cause shifts in the energy of the QD states. Addressing spectral diffusion is currently a critical issue in the field of QD-based photon sources. Results from correlation measurements have provided evidence that QDs can emit photons one at a time [Michler et al., 2000; Santori et al., 2001], making them strong candidates for the single-photon sources that are required for many QIP applications. Some QIP applications additionally require two-photon interference measurements. Examples include linear optical quantum computation [Knill et al., 2001], entanglement swapping [Zeilinger et al., 1997; Pan et al., 1998; Moehring et al., 2007], and quantum repeaters [Briegel et al., 1998; Zhao et al., 2003].

Schematic depiction of two-photon interference

The fidelity of two-photon interference in QIP depends sensitively on the indistinguishability, or degree of similarity, of the two photons [Hong et al., 1987; Kiraz et al., 2004; Flagg et al., 2010]. Indistinguishable photons must have the same wavelength, polarization, and temporal and spatial extent. In addition, the photons must be “transform-limited” or “Fourier-limited”, meaning that the only existing decoherence process is radiative decay. If there is any dephasing or other decoherence process, then the photons will not be transform-limited. For this reason, indistinguishability is also termed coherence. Spectral diffusion is a process by which the emission wavelength of a QD changes over a time scale longer than the emission lifetime [Portis, 1956; Robinson and Goldberg et al. 2000; Neuhauser et al., 2000; Beyler et al., 2013]. Two different QDs may have the same time-averaged emission wavelength, but their photons may have different instantaneous wavelengths because of spectral diffusion. Similarly, two photons from the same QD may still have different wavelengths because they were emitted at different times. Therefore, spectral diffusion will reduce photons’ coherence and the fidelity of two-photon interference.

The objective of this research is to identify, model, and establish effective strategies to address the primary environmental factors responsible for spectral diffusion in QDs, in order to significantly improve their photons’ level of coherence. The central hypothesis is that decoherence is primarily caused by spectral diffusion, which in turn is primarily caused by fluctuating impurity charge-states in the local environment of the QD, and that it can be significantly reduced by a combination of extrinsic doping (to counteract intrinsic doping), the application of electric fields from p-i-n structures, and leveraging resonant excitation of the QD itself. The rationale for this approach is that reducing or eliminating spectral diffusion in a variety of situations is expected to enable the design of QD-based photon sources with significantly higher fidelity performance in a variety of interference-based QIP protocols than is currently possible.

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This research is supported by the National Science Foundation grant number DMR-1452840.

Related publications

  1. Lander, G. R., Isaac, S. D., Chen, D., Demircan, S., Solomon, G. S., Flagg, E. B. "Auger recombination-induced neutralization and stretched exponential recharging in an InAs quantum dot." Quantum Dots and Nanostructures: Growth, Characterization, and Modeling XVI, 10929:109290F (2019). doi:10.1117/12.2506555
  2. Chen, D., Lander, G. R., Solomon, G. S. & Flagg, E. B. “Polarization-Dependent Interference of Coherent Scattering from Orthogonal Dipole Moments of a Resonantly Excited Quantum Dot.” Phys. Rev. Lett. 118, 37401 (2017). doi:10.1103/PhysRevLett.118.037401
  3. Chen, D., Lander, G. R., Krowpman, K. S., Solomon, G. S. & Flagg, E. B. “Characterization of the local charge environment of a single quantum dot via resonance fluorescence.” Phys. Rev. B 93, 115307 (2016). doi:10.1103/PhysRevB.93.115307


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