Friction and Measurements that Preserve rather than Destroy Quantum Entanglement

Entangled states constitute a key resource in fundamental quantum physics, quantum cryptography and quantum computation. Unfortunately, these states are fragile and easily degrade by unwanted interactions with a dissipative environment. Thus, it has generally been assumed that the creation of such states requires the system to have minimal dissipation and decoherence. However, some recent theoretical studies have shown that dissipative interactions combined with appropriately chosen drives can be employed to preserve coherence, and in particular to stabilize entanglement autonomously. Following these ideas, we have built experimentally an autonomous feedback scheme to stabilize an entangled Bell state of a quantum register of two superconducting qubits for an arbitrary time [1]. We further show that the fidelity of the Bell state can be increased by continuously monitoring one dissipative channel involved in the stabilization. Finally, we contrast our autonomous stabilization with a more conventional measurement-based feedback scheme involving an external controller, and find that our autonomous approach provides greater robustness to errors. The general approach we are following may be applied in the future to build a logical qubit holding quantum information in a fault-tolerant way and with minimal hardware [2].

[1] Shankar et al., Nature 504, 419 (2013)
[2] Mirrahimi et al., New J. Phys. 16, 045014 (2014)