Nano-Electro-Optical Gates to control single colloidal quantum dot blinking and local charge noise: towards coherent control of the bright states – NEOGate

Proposal
Given recent advances in resonant laser spectroscopy, it becomes evermore tantalizing to envision coherent manipulation of the excited-state populations of single II-VI colloidal quantum dots (CQDs). Such experiments would establish these economical, easy-to-tune, highly fluorescent nano-objects as middle-term alternatives to epitaxial QDs – which are expensive, complex to grow and wavelength-restricted – in state-of-the art photonics applications.

Before such ambitious experiments become feasible with II-VI CQDs, several fundamental issues remain to be resolved, which we propose to do in the NEOGate project.

Quick position of the problem
Coherent manipulation of single CQDs has so far been precluded by the propensity of these nanoparticles to exhibit so-called blinking behavior, as well as by dephasing induced by charge-noise. Blinking is generally believed to be the consequence of ionization of the CDQ core and has been held responsible for spectral fluctuations observed in non-resonant/quasi-resonant emission spectra. We propose to address the causes of blinking and spectral fluctuations, which currently act as roadblocks for coherent-control experiments.

Blinking suppression via CQD core neutralization
We will couple an electric force microscope (EFM) tip, to a confocal microscope so that the tip can serve as a nano-electrode to control the charge state of single CQDs embedded in a semiconductor matrix. We want to achieve local control of the core charge state of a single CQD with a local electric field (E-field) by controlling the tunneling times of electrons and holes. The advantage of an EFM tip as compared to the commonly-used patterned electrodes is that the tip will allow us to address single nano-objects independently. We expect to suppress blinking with a bandwidth of up to several tens of kHz, which is more than one order of magnitude faster than currently-existing schemes and corresponds to the fastest timescales over which blinking is known to take place. The underlying dynamic feedback principle should be applicable to many other types of blinking nano-emitters, which highlights the universality of our concept.

Local charge noise control (4 K)
Spectral fluctuations are attributed to local charge noise in the CQDs immediate environment, which we aim to control by combining resonant excitation, weak non-resonant interaction and external E-field. To this end, we will implement a resonance-fluorescence confocal microscope based on polarization rejection of scattered laser light. This apparatus will allow the first study of the resonance-fluorescence signal as a function of the local population of quantum traps. The non-resonant optical excitation will serve as a local optical gate to control the local E-field in our scheme; by controlling the local charge noise in this manner, we expect to reduce spectral fluctuations and hence to narrow the emission linewidth of single CQCDs.

Coherent/incoherent parts of the resonance fluorescence signal (4 K)
Finally, we intend to prove that single CQDs coupled to a quiet electrical surrounding can be used as two-level systems for quantum information, starting by probing the high power (incoherent) part of the resonance fluorescence signal. Measuring Rabi oscillations will show that strong light-matter interaction is possible in these nano-objects and it will demonstrate a one-qubit rotation in a single CQD for the first time, which is important for proposals using QD excitons for quantum computing. The corresponding spectral measurement, the hallmark of resonance fluorescence, is the observation of the Mollow triplet. On the other hand, at sufficiently small Rabi frequencies, the character of the scattered photons changes from incoherent to coherent, leading to a source of single-photons with sub-natural linewidth, potentially tunable over the entire visible range, a necessary step towards hybrid quantum memories coupling single CQDs to single atoms.