If quantum computers are ever to be realized, they likely will be made of different types of parts that will need to share information with one another, just like the memory and logic circuits in today's computers do.
The goal to develop quantum computers—a long-awaited type of computer that could solve otherwise intractable problems, such as breaking complex encryption codes—has inspired scientists the world over to invent new devices that could become the brains and memory of these machines. Many of these tiny devices use particles of light, or photons, to carry the bits of information that a quantum computer will use.
But while each of these pieces of hardware can do some jobs well, none are likely to accomplish all of the functions necessary to build a quantum computer. This implies that several different types of quantum devices will need to work together for the computer or network to function. The trouble is that these tiny devices frequently create photons of such different character that they cannot transfer the quantum bits of information between one another. Transmuting two vastly different photons into two similar ones would be a first step toward permitting quantum information components to communicate with one another over large distances. We use parametric processes to make photons from dissimilar quantum systems compatible.
We considered the indistinguishability of photons produced by highly dissimilar sources: Photons produced from a single quantum dot (QD) and from parametric down-conversion (PDC) in a nonlinear crystal. A QD source is a deterministic source of single photons. On the other hand, a PDC process generates photon pairs, and the detection of one of the photons is used to herald the presence of the other. After spectral and temporal control of the PDC photons to optimize their overlap with the QD photons, we measure a two-photon coalescence and show partial indistinguishability of these photons.
Currently, we study properties of parametric up- (and down-) conversion for frequency translation of various states of light: from faint laser beams to entangled states. The goal here is to try not only to faithfully preserve the state, but in some cases to make a quantum state that is more useful, i.e. accomplish a few steps in one physical process. One such idea is a quantum eraser. A quantum eraser would use up-conversion to take a strongly-correlated bi-photon state and turn it into a frequency-translated, entangled state. To date, we have demonstrated the feasibility of up-converting for this and other purposes. Particularly, we developed an up-converter that does not add extra noise (background photons) in the process, or adds so few, that we cannot detect their presence no matter how hard we are trying. This is a unique up-converter, because most of the up-converted characterized to date suffer from quite a strong background, whereas converting quantum states less faithfully. Our result is due to the large detuning between the up-converting pump and a long-wavelength input.
A demonstration of indistinguishability of photons from dissimilar sources, ref. Coalescence of Single Photons Emitted by Disparate Single-Photon Sources: The Example of InAs Quantum Dots and Parametric Down-Conversion Sources, Sergey V. Polyakov, Andreas Muller, Edward B. Flagg, Alex Ling, Natalia Borjemscaia, Edward Van Keuren, Alan Migdall, and Glenn S. Solomon, Phys. Rev. Lett. 107, 157402 – Published 5 October 2011
A demonstration of background-free upconversion in a nonlinear waveguide, ref. Statistically background-free, phase-preserving parametric up-conversion with faint light, Y.-H. Cheng, Tim Thomay, Glenn S. Solomon, Alan L. Migdall, and Sergey V. Polyakov, Optics Express Vol. 23, Issue 14, pp. 18671-18678 (2015)