Sony has successfully made a 7-layer holographic disc by reading and writing data using a "Micro-Reflector method", where multi-layer recording is carried out using holographic recording.  They were also able to calculate the error rates of the reproduced signals with practical results using signal conditioning similar to that on DVD, unlike an earlier 4-layer experiment that only announced the eye patterns of the reproduced signals.  This new 7-layer version was announced at the ISOM '07, an international conference on optical technology, which took place October 21st to 25th this year in Singapore.

When the Micro-Reflector recording method takes place, lights are shed on both sides of a holographic medium and adjusted such that interference fringes form in a recording layer from the interaction of the lights.  Each interference fringe stores one data bit.  Data is read by shedding light on one side of the medium.

While this technology records data in multiple layers on a holographic disc, it has a significant advantage over other common multilayer recording technology that requires the disc to be produced with multiple recording layers.  However, as this technology stores data on multiple layers on a holographic disc by focusing light at different depths to effectively form layers, the company defines the technology as multilayer recording technology and not holographic recording technology

So far, this experiment has been carried out on an 8cm disc with a memory density that would calculate at 1.2GB per layer in terms of a 12cm disc.  Sony is still working to improve the memory density per layer and also increase the number of recording layers.  In its experiments so far, they managed to achieve up to a memory density of 3GB per layer in terms of a 12cm disc with favourable reproduced signals as well as a 10-layer disc where each layer stored the equivalent of 1.2GB calculated in terms of a 12cm disc, also with favourable reproduced signals.  Sony aims to achieve a capacity of 500GB involving 20 x 25GB layers on a 12cm disc by about 2010.

Further more in-depth technical info can be found in this Tech-On Source.