Inequalities [Honours project 06/07, Lim Jiaqin]
Bell inequalities are a standard tool to check for the presence of
entanglement and to study the non-local properties of quantum mechanics. The
simplest Bell inequalities are typically tested on pairs of maximally entangled
spin-1/2 particles. Under some conditions, such as in the presence of noise, it
can be advantageous to use higher dimensional Hilbert spaces. One way of
creating entanglement in higher dimensional spaces is to use the higher order
photon production in PDC. In this project we use 4 photons produced by a pulsed
pump to check several classes of Bell inequalities.
Narrow bandwidth PDC sources
[Honours project 06/07, Arpan Roy]
The most commonly used sources of entangled photons are based on the process of parametric down conversion (PDC). This is a non-linear process in which a pump photon has a small probability of splitting into two photons of lower energy which are highly correlated in their properties. However, while photons are ideal carriers of quantum information they are not very usefull for storage; for that, one would use atoms or other microscopic quantum systems. Unfortunately the spectral properties of photons produced in PDC too broad to interact efficiently with atomic levels. This project looks at creating narrower bandwidth PDC sources.
Fast polarization-independent switching [Honours project 06/07, Ng Tien Tjuen]
Entanglement is the main resource in quantum information protocols. One of the cleanest and easier to manipulate maximally entangled states is polarization entanglement between pairs of photons produced in PDC. For some applications one would like to be able to direct one of the members of the pair between several alternative measurements without affecting its polarization state. This project tries to implement polarization-independent switching in a time scale of a few nanoseconds.
Daylight entanglement distribution [Honours/Masters project 06/07, Caleb Ho]
Pairs of entangled photons are the basis for a number of quantum information
protocols (quantum cryptography, teleportation, etc). All these depend on
single photon counting and are therefore typically conducted in reasonably dark
laser laboratories. To be really useful though, some of these protocols really
are meant to be taken outside. What happens then, is that unless the photons
are fiber guided, the experiments are performed at night when the background
light level is more manageable. This project looks at the feasibility of
adapting a working quantum cryptography system for daylight operation.
Compact optical autocorrelator [Honours project 06, Syed Abdullah Aljunid]
Measuring the time duration of an ultrashort (100 fs) light pulse is not an easy task since there exist no electronics that can directly measure that time span. For short intense pulses of light, the standard solution is to use some information about the shape of the pulse together with a non-linear process to perform a physical autocorrelation measure. Frequency doubling in non-linear crystals is a common choice for the non-linear process. This has the disadvantage of relatively stringent phase matching conditions for the process, making it expensive and unnecesarily complex. An alternative is to use non-linearity in the absorption process, for example by using a semiconductor material where the energy of a single photon of the light to be measured is not enough to bridge the bandgap. In this project we use different color LEDs used as photodetectors to measure the length of a pulse at 780nm produced by a mode-locled Ti:Sapphire laser.