Abstract
We demonstrate that the quantum dot-confined dark exciton forms a long-lived integer spin solid-state qubit that can be deterministically on-demand initiated in a pure state by one optical pulse. Moreover, we show that this qubit can be fully controlled using short optical pulses, which are several orders of magnitude shorter than the life and coherence times of the qubit. Our demonstrations do not require an externally applied magnetic field, and they establish that the quantum dot-confined dark exciton forms an excellent solid-state matter qubit with some advantages over the half-integer spin qubits, such as the confined electron and hole, separately. Since quantum dots are semiconductor nanostructures that allow integration of electronic and photonic components, the dark exciton may have important implications for implementations of quantum technologies consisting of semiconductor qubits.
- Received 10 April 2014
DOI:https://doi.org/10.1103/PhysRevX.5.011009
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Published by the American Physical Society
Popular Summary
Coherently controlling and exploiting matter quantum systems is essential for realizing future technologies based on processing quantum information. Optical approaches are preferred since they are state selective, require no contacts, and are ultrafast. Semiconductors play an important role among the matter venues of choice because they dovetail with contemporary leading technologies. In semiconductors, optical excitations—in which electrons from the valence band hop to the conduction band after the absorption of a photon—form electron-hole pairs with antiparallel spins, which are called bright excitons. In semiconductor quantum dots, single bright excitons have an advantage over single half-integer carrier spins since the coherent state of their two levels can be written, read, and manipulated using single ultrafast optical pulses. However, their typically short radiative lifetimes (around 1 ns) limit their usefulness. Here, we circumvent the optical inactivity of an electron-hole pair with parallel spins, a dark exciton, whose lifetime is orders of magnitude longer (about ). We experimentally demonstrate on-demand initiation and control of this dark exciton using single optical pulses.
We use an InGaAs quantum dot deposited on a substrate, with the system held at 4 K. We demonstrate deterministic generation of the dark exciton using a single optical pulse and find that short optical pulses, lasting on the order of picoseconds, can control its spin. Our methodology does not require a magnetic field, and it shows that the dark exciton can act as a matter qubit—a coherent two-level system that can be written and controlled optically. Our all-optical, time-resolved, polarization-sensitive measurements set a lower limit on the coherence time of the dark exciton of approximately 100 ns.
By verifying that short optical pulses can deterministically generate and control the spin state of the long-lived and coherent dark exciton, our work paves the way for future studies of entangled dark exciton spins and photons, providing an important building block for quantum information processing.