They did so in the course of developing a “Universal quantum bus”-a novel system to allow photonic connections between disparate components of a quantum computer, each of which can operate at a very different and narrow range of photon frequency.
Quantum circuits may have to transfer information via photons among quantum dots, ensembles of atoms, trapped ions, or other materials systems.
A signal produced by one component, such as a quantum dot, may have to be transferred to a trapped ion which is sensitive only to photons at a much higher frequency than the original dot signal.
In passing through the crystal, the energies of both the input and the pump are joined, producing a single output photon of a higher frequency and therefore higher energy.
One persistent difficulty with the technique is that the pump beam can contain so much power that when it hits the crystal it generates a large amount of “Noise” in the form of unwanted photons that can swamp the delicate quantum states.
The project team used a pump beam of continuous, high-power light at a standard telecom wavelength of 1550 nanometers, and merged it with input photons at a near-infrared wavelength of 920 nm.
Because the high-energy, up-converted output photons register as larger peaks in the detector than most low energy dark counts, it is possible to adjust the detector system so that it filters out all signals that fall below a certain energy threshold.