Single photon detectors

Superconducting nano-wire single photon detectors (SNSPD) are a good candidate to be used for photon detection. The nano-scale size of these detectors make them highly sensitive to absorbing a single photon. SNSPDs can be designed to work at different wavelengths. SNSPDs are highly efficient, with reported efficiencies of more than 97% [156].

Ideally, the detectors should be on chip. Currently, the superconducting nano- wires used for these detectors can’t tolerate the level of magnetic fields required to get the long coherence times of the rare-earth quantum memories. This means that unless superconducting nano-wires capable of remaining superconducting in magnetic fields larger than tens of Gauss become available, it will be necessary to have the detectors off chip to separate them from the rare-earth devices.

3.6

Summary

In this chapter, an alternative method of building a linear optics quantum processor chip has been presented. This passive waveguide architecture has the advantage that it is sample independent. Also, by tapering waveguide regions, isolated devices can be built on chip.

This method can bring non-classical single photon sources and quantum mem- ories, required for a quantum repeater, on to the same platform. In this integrated circuit, the bandwidth of the photon generator and the quantum memories are au- tomatically matched because they are both on the same chip.

The long storage times achievable using rare-earth ion doped quantum memories eliminates the need for high switching speeds and reduces the clock speed of the pro- cessor to a manageable speed. These long storage times give the quantum repeater node more robustness, ensuring that entanglement can be established in neighboring nodes and the entanglement swapping can be achieved between nodes.

The proposed architecture also incorporates passive elements like beam splitters and Mach-Zehnder interferometers onto the same integrated circuit. Using these

passive elements and electrical control, the light field can be manipulated in different parts of the beam path without interacting with the active ions.

Superconducting nano-wire single photon detectors (SNSPD) can be used to detect the output photons in this integrated chip. To-date, a SNSPD compatible with the magnetic field requirements of rare-earth ion doped quantum memories has not been built. So until this is achieved, the SNSPDs will have to be kept off chip.

In the next chapters, the proposed waveguide structure is investigated. First, the waveguide fabrication is presented and then the experimental analysis of the architecture is reported.

Chapter 4

Material Complexities and

Fabrication of Rare-Earth Planar

Waveguides

4.1

Introduction

The aim of this chapter is to introduce two rare-earth ion doped thin film approaches, as shown in Figure 4.1, and present some of the fabrication and material issues involved in each approach.

(a) (b)

Laserowavelength:o580onmo

Laserowavelength:o606onm

Euo3+_{othinofilmoexcitedoionso}

Pro3+_{osubstrateoexcitedoions}

Figure 4.1: Two rare-earth ion doped thin film architectures, (a) For the active rare- earth ion waveguide, a thin film of Eu3+:Y2O3was deposited on a sapphire substrate. The Eu3+ _{active ions were then probed by a laser at its excitation wavelength of} 580 nm, (b) For the passive rare-earth ion waveguide, a glass thin film of TeO2 was deposited on a Pr3+:Y2SiO5 substrate. The Pr3+ active ions in the substrate were then evanescently probed by a laser at its excitation wavelength of 606 nm.

The first method, which is primarily a PhD project conducted in the Laser Physics Center at the ANU by Paulraj et al.[23], was growing rare-earth ion doped

thin films on suitable substrates. These films were fabricated by depositing thin layers of Eu3+_:Y

2O3 (nthinf ilm = 1.95) on α-Al2O3 (nsubstrate = 1.76) substrates

using the Pulse Laser Deposition (PLD) method. In these thin films, the preser- vation of the optical coherence properties of the rare-earth depends on the growth mechanism and the lattice mismatch between the thin film and the substrate. Af- ter the deposition, these films were characterized by X-ray diffraction and optical spectroscopy.

The second approach, which is the focus of this thesis, was depositing glass thin films of TeO2 (nthinf ilm = 2.05) on rare-earth ion doped single crystalline sub-

strates of Pr3+:Y2SiO5 (nsubstrate = 1.806). This method has many advantages to

the previous approach, essentially avoiding issues caused by stressing the crystalline structure. Glass is a soft material compared to rare-earth crystals, so it can relax on the substrate and lattice matching is not essential. Even in the cold liquid he- lium temperatures needed for the experiments at hand, glass is still mobile. The characterization of these thin films will be presented in Chapter 6.

The deposition of suitable electrodes to produce the electric fields required to probe the substrate ions and Stark shift their frequencies is also presented. The material deposited to fabricate the electrodes on the glass thin films was gold. There were a few issues with the adhesion of these gold electrodes to the thin film surface, which had to be investigated and resolved.

4.2

Thin films with rare-earth doped crystal as