Electronic Enhancement of the Exciton Coherence Time in Charged Quantum Dots

Abstract

Minimizing decoherence due to coupling of a quantum system to its fluctuating environment is at the forefront of quantum information and photonics research. Nature sets the ultimate limit, however, given by the strength of the system’s coupling to the electromagnetic field. Here, we establish the ability to electronically control this coupling and enhance the optical coherence time of the charged exciton transition in quantum dots embedded in a photonic waveguide. By manipulating the electronic wavefunctions through an applied lateral electric field, we increase the coherence time from ~ 1.4 ns to ~ 2.7 ns. Numerical calculations reveal that longer coherence arises from the separation of charge carriers by up to ~ 6 nm, which leads to a 30% weaker transition dipole moment. The ability to electronically control the coherence time opens new avenues for quantum communication and novel coupling schemes between distant qubits.

Quadrature demodulation of a quantum dot optical response to faint light fields

“…The modulation depth is determined by comparing the power in the sidebands and the signal-LO beatnote. Maximum modulation occurs at zero detuning, and the half-width at half-maximum of a Lorentzian fit function (solid lines) yields the transform-limited charged exciton homogeneous linewidth that has been previously characterized [23]. The upper and lower sideband lineshapes are symmetric, i.e.…”

“…Phase shifts approaching π might be achieved by increasing the density and number of QD layers in the waveguide, but at the expense of a reduction in the efficiency [5]. Larger phase shifts at lower optical power may be possible by optimizing the optical mode overlap with the QDs and by tailoring the electronic wavefunctions using external magnetic and electric fields [23,34]. …”

“…The lenses and sample are hermetically sealed with 15 Torr of ultra-high purity helium exchange gas in a stainless steel insert, which is fixed at 4.2 K inside a liquid helium bath magneto-cryostat. The QD photoluminescence is centered at 1045 nm with an inhomogeneous linewidth of ~50 nm due to spatial variation in the QD size and shape [23]. …”

“…This measurement indicates how much power is transferred from the carrier to the sidebands, but information on the exact mechanism responsible for this is lost by averaging over the individual quadratures. The power in the sidebands can be measured as the pump-probe detuning is varied, providing details of the charged exciton dephasing and lifetime [23]. …”

Implementations of more general solid-state (SWAP) 1 / m and controlled-(swap) 1 / m gates

“…T 2 ∼μs is the electron spin coherence time, which is limited by the spin-relaxation time T 1 ∼ms. The optical dephasing has a slight impact on the photon-QD emitter because the optical coherence time [58] (several hundred picoseconds [59]) is ten times longer than the cavity photon lifetime τ (tens of picoseconds [60]). The hole spin coherence time (T 100 h 2 > ns) is three orders of magnitude longer than the cavity photon lifetime [59][60][61], causing the spin dephasing to be safely neglected.…”