For a highly secured environment, like banking service or capital market service, can we use cloud computing with 100% data security? Are any of the banks using it? 

Yes, you can utilize the cloud, and yes banks are using the cloud now. There are three generally agreed upon delivery models for the cloud: public, private, and hybrid. With a private cloud, you can get the benefits of cloud such as elasticity, rapid deployment, and autonomic behavior, while still retaining complete control over the components (including data) that make up your environment. In this way, you have a more efficient way to deliver your services, but you do not cede control over your components to another party.

Hi, 
I think that's not, even if it seems logical and increased security, but when data are in the internet they aren't you anymore and you can't protect them
That's what a manager told me and i think it's a bit true  

The data is protected by the IT staffers. It doesn't matter if they are your employees or of a cloud provider's.

I'd rather have security experts who are Harvard/ MIT/ Stanford graduates with experience in Google, Microsoft and IBM and who are paid more than $100 000 yearly  protect my data  than those I could hire with much much less experience and skill.

The only thing you should pay attention to is the SLA. Try to contact multiple cloud vendors find the most suitable one and try to negotiate a SLA that best suits your needs.

However, if the data is critical and you're bound by compliance regulations, the best solution here is a hybrid/private cloud. Banking nowadays still relies heavily on mainframes  but the hybrid/private cloud does really give the same level of hightened security and comes at a greatly cheaper price.