Regarding your PhD keep in mind a PhD is a learning experience. What you have to demonstrate are not results, but the hability to investigate. Demonstrating that something is wrong or explaining you did not get the expected results is a valid thesis. Just make extra enfasis in the state of the art and why your approach won't work. This will help some other experiment with similar properties to avoid losing 3 years due to poor experiment design.

Regarding your PhD keep in mind a PhD is a learning experience. What you have to demonstrate are not results, but the ability to investigate. Demonstrating that something is wrong or explaining you did not get the expected results is a valid thesis. Just make extra emphasis in the state of the art and why your approach won't work. This will help some other experiment with similar properties to avoid losing 3 years due to poor experiment design.