Sony boosts XQD memory card speeds with the S Series-- read and write speeds reach 168MB/s, up from the 125MB/s of the first XQD generation and the maximum limit of Compact Flash cards (167MB/s).
Available in 64GB and 32GB sizes, the XQD S Series card takes advantage of the PCI-E Gen interfaces and Thunderbolt connections on PCs.
The XQD format is a SanDisk, Sony and Nikon co-development, and has the backing of the Compact Flash Association. So far only Sony sells QXD cards, and the only commercially available camera with a QXD slot is the Nikon D4.