The control of quantum systems in the solid state has appealing prospects, not least for quantum information processing. But, compared with isolated atomic systems, a solid-state quantum bit, or qubit, is strongly coupled to its environment and so tends to lose coherence very quickly. Now, in a paper published on Science Express, Jason Petta and colleagues1 show how coherent manipulation of a qubit based on two coupled electron spins in semiconductor quantum dots can result in an extended coherence time.
The two electrons of the qubit reside in a double-well potential, defined by a double quantum dot. Inside this trap, the electrons can be both in one single well, or separated, with one electron in each well. Depending on the state, the electron spins prefer different arrangements. When both electrons are in one well, only the singlet state (with a total spin 0) is accessible; when there is one electron in each well, the singlet and the triplet state (with total spin 1) can be populated. If magnetic fields are introduced to split off one of these states, the two spins can be made to constitute an effective two-level system — a qubit.
Switching on and off the applied voltage controls the amount of exchange interaction between the two electrons. Petta and colleagues used this tool to drive Rabi oscillations between the two levels, which in turn enabled the implementation of a fast SWAP gate, taking only fractions of a nanosecond. However, hyperfine interactions with the environment — which in this case consists of roughly one million spin-bearing GaAs nuclei — dephase coherences between the electron spin states within a few nanoseconds. Coherent control offers a solution: by inducing so-called spin echoes, Petta et al. were able to refocus substantial parts of the dephasing, to obtain coherence lifetimes of the order of a microsecond. For quantum-information applications, this translates directly into useable operation times.
