Efficiency Optimization of Coherent Down-conversion Process for Visible Light to the Telecomm Range

Hello, my name is Andrew Kindseth and I am a rising junior at Northwestern University. I am majoring in Physics and Integrated Sciences. This summer I am working with Joseph Heremans, a physicist and material’s scientist who is working on the Quantum link and solid state defects that can be used as quantum bits. My project involves making optically addressable solid state defects compatible with existing fiber optic infrastructure. I am also attempting to improve on transmission distances. I expect to accomplish these goals using a process called difference frequency generation.

Difference frequency generation (DFG) is a three wave mixing process which takes place in a nonlinear crystal with a high X(2) term.  This term refers to the polarization response of the crystal to an electric field. When two waves of frequency w1, w2 pass through the crystal the X(2) term results in the generation of electric waves of 2w1,2w2, w1+w2, and w1-w2. A process called phase-matching can then be used to increase the amount of mixed light received from the process. The technique applied uses a process called quasi-phase matching, which is performed by using a crystale that has opposite orientations of the crystal lattice with specified period. This is called periodically poling. Our crystal is periodically poled with period such that we will generate photons of frequency w1 -w2 when performing our experiment.

Figure 1: Schematic of usage of periodically poled crystal for amplification of mixed waves. For input waves v1,v2 by selecting the poling we can preferentially generate photons through one of these mixing processes.

By controlling the relative energy densities of each of the respective frequencies of light, the conversion efficiency can be made to rise. This efficiency maximum occurs when one of the light frequencies is much stronger in power then the other. It is the lower power light that is converted efficiently. In the case of single photon pulses on one or either of the input light frequencies the conversion efficiency can become significant, and values of 80% have been demonstrated in experiments.

When one or both of the sources is pulsed the photon emission from the crystal is time correlated with the presence of both frequencies of light in the crystal. If energy imbalance and pulsing are combined, using a single photon pulse on one of the input frequencies, the efficiency and time correlation results in single photon down-conversion.

I am working on creating a down-conversion setup here at Argonne for downconversion of light from 637nm to 1536.4nm. The input light to the crystal will then be 637nm light and 1088nm light, with the 1088nm light having a vastly stronger power. The focus of my project is overall efficiency of the process, as well as minimizing the stray photons that may be detected through optical and spatial filtering. For spatial filtering I am using a prism pinhole pair in order to spatially separate the two input wavelengths from the output wavelength of 1536.4nm. This will require painstaking alignment and spectrum analysis. For optical filtering I am using a free space bragg grating. The bragg grating will filter out the Raman and Stokes-Raman noise that is much closer in wavelength to my output wavelength than the input wavelengths.

Figure 2: Spectrometer readings demonstrating the optical filtering ability and necessity of a Bragg grating. This figure was taken from Sebastian Zaske, Andreas Lenhard, and Christoph Becher, “Efficient frequency downconversion at the single photon level from the red spectral range to the telecommunications C-band,” Opt. Express 19, 12825-12836 (2011)

The relevance of this project arises from the field of quantum information. One of the goals of the field of quantum information is the creation of quantum information networks, as well as transmission of quantum information over large distances. One of the prime candidates as a carrier of quantum information is the photon. Photons are coherent over arbitrarily long distances in free space, and have low transmission loses. However, existing infrastructure for transport of photons only maintain the desirable properties of photons for photons of particular energies. The telecomm band is the region for which fiber optic cable has low transmission loss. For classical computers this frequency range is not problematic because the digital information is transmitted at the desired frequency. However, transmission of quantum information has only be performed with certain quantum bits (qbits). The qbits which are most technologically mature emit photons in the visible range. This is problematic, the emission wavelength of the qbits cannot be changed, and the transmission losses of visible light are prohibitive for the creation of long-distance communication or networks.

One of the most used photon emitting defects is the Nitrogen Vacancy center (NV center). The NV center is a qbit that emits photons of wavelength 637nm. Being able to down-convert the light from an NV center would allow NV centers to be used, with all of their intrinsic benefits, without crippling the ability to form large networks or send information. Initially, I am using a laser to simulate photons being emitted from the NV center, and trying to improve upon the total efficiency realized. Once the efficiency has been optimized with the laser imitator, the laser imitator will be replaced with an NV center.

Connections between nodes in quantum information networks are not the same as for classical networks. One method for forming a connection is to establish entanglement between nodes. This has been done over short distances between two of the same type of qbit. Our overall goal is twofold: to establish entanglement between two different types of qbits, and to do so over a longer distance than has been possible before. I expect that successful and efficient down-conversion will enable this to be acomplished.