Development of quantum computers is an ambitious technological pursuit that can provide increased capability in code breaking, and its opposite, secure data and communications. One area of work is in development of solid-state quantum information technologies using the spin states of semiconductor quantum dots (QDs) and defects in solids. These systems have the advantage of a robust solid-state host, the ability to produce many quantum bits (qubits) on chip, coupling to photons, and the ability to incorporate qubits into sophisticated electronic, photonic, and mechanical devices.
Single photons are the central element in quantum technology applications. To generate photons, most systems currently use faint laser pulses as a photon source but laser pulses necessarily have a distribution in the number of photons in each pulse; not just one photon. If there is more than one photon in a pulse, the security in quantum communication is reduced and likewise the fidelity of the quantum computing operations is reduced.
Quantum dots represent a source of single photons and they can be incorporated into a solid-state optical cavity. The optical cavity has the dual purpose of coupling the QD to a single optical mode that can be efficiently emitted in a chosen direction, and of enhancing the transition rate of the QD, and thus, the ultimate repetition rate of single photons. A single, uncharged QD may be used for this purpose but uncertainty in the relaxation time (the time between excitement by the laser and the and the return to the ground state) presents a problem.
Navy researchers have addressed the above shortcoming. Instead of using a single uncharged QD, two singly-charged quantum dots that are separated by a thin tunnel barrier are utilized. This coupled quantum dot system is referred to as a quantum dot molecule (QDM). The energy levels of the quantum dot molecule are effectively a three-level system in an optical cavity. In a three-level system, a laser beam can trigger a Raman single photon, which is shifted from the laser beam. In an application, the two lowest energy states could also act as a solid state qubit – a stationary quantum memory.
- QDMs may be manufactured using self-assembled InAs quantum dots grown in a GaAs substrate
- The single photon emission is triggered through the Raman process in which the frequency shift is determined by the thickness of the tunnel barrier which can be sufficient to easily filter out the laser beam while still being nearly resonant with the QDs or cavity
- The single emitted photon frequency is tunable by tuning the laser frequency, in contrast to a two-level system in which the photon frequency is fixed by the QD frequency
- The photons can be more easily made phase coherent as compared to a two-level system: the laser beam is nearly degenerate with the QD/cavity and thus there is negligible timing jitter arising from uncertainty in the time required to relax to the emitting state
- US patent 9,619,754 available for license
- Potential for collaboration with Navy researchers