Wallraff group demonstrate exchange interactions, superradiant states and subradiant states for two qubits placed two centimeters apart in an open one-dimensional space.
|Figure 1: In a transmission line photons are forced to travel along the line connecting the two qubits, mediating interactions between the two. By tuning the qubit emission wavelength we tune the effective distance between two qubits.|
The Wallraff group at ETH Zürich have demonstrated for the first time that strong interactions between two distant superconducting qubits in an open environment exist. When the two qubits are separated by multiples of λr/2, we observe the formation of super- and subradiant states. The subradiant state was found to have a lifetime more than 100 times as long as the lifetime of the superradiant state. For perspective, in initial experiments showing similar effects in ions, the subradiant state lived about 1.03 times as long as the superradiant state. Finding a two-qubit state in an open environment with a lifetime two orders of magnitude longer than a single qubit suggests that these systems could potentially be used as a quantum memory.
Furthermore, when the qubits are separated by odd multiples of λr/4, an exchange interaction between the qubits emerges which is mediated by the vacuum fluctuations of the 1D continuum at all frequencies. To our knowledge, our work presents the first observation of an exchange-type interaction mediated by photons over larger distances, i.e. the first instance of quantum systems interacting coherently at a distance, in a free space. We find good agreement of our experimental findings with the theory (Lalumiere et. al., Phys Rev A 88, 043806, 2013).
|Figure 2: Interaction effects are revealed when measuring resonance fluorescence. An exchange splitting is seen at qubit separations of 3λr/4, while super- and subradiant states with drastically different lifetimes form for qubits separated by λr.|