A hybrid approach to couple different quantum systems is considered as a promising way to realize quantum computation, because such an approach has a potential advantage to combine the best properties of each system. One of the attractive hybrid approaches is to use a superconducting flux qubit and a spin ensemble in diamond [1-3]. The former system plays a role of a processor while the latter system is useful for a memory. In this hybrid system, an unknown long-lived state was observed [1, 2]. Although this long-lived state has a potential application for a memory, there was no theoretical explanation why the long-lived system was observed, which made it difficult to use this state for quantum computation.
We have found the mechanism how the state was so-long-lived, and have shown that this is an evidence of a dark state in the diamond . A dark state has a destructive interference so that the signal from this state should be significantly suppressed. However, we theoretically show that, in this hybrid system, magnetic fields and strain variations decrease the effect of the destructive interference, which leads to a detectable signal from the dark state. We experimentally show that the lifetime of the dark state was around 150 ns , which is much longer than the previously observed lifetime of 20 ns in the diamond .
Since we understand the origin of this long-lived state, our results open a new way to realize a quantum memory for the storage of information by using this dark state. If such a quantum memory is realized, we can reduce the resource and cost for quantum computation to provide us with ultra-fast calculation. This work was supported by FIRST and NICT.