T1-echo sequence: Protecting the state of a qubit in the presence of coherent interaction

Abstract

We propose a sequence of pulses intended to preserve the state of a qubit in the presence of strong, coherent coupling to another quantum system. The sequence can be understood as a generalized SWAP sequence and works in formal analogy to the well-known spin echo. Since the resulting effective decoherence rate of the qubit state is strongly influenced by the additional system, this sequence might serve to protect its quantum state as well as negating the effects of the coherent coupling. A possible area of application is in large-scale quantum computing architectures, where spectral crowding of the resources might necessitate a method to mitigate residual couplings.

Observation of directly interacting coherent two-level systems in an amorphous material

“…First signatures of coherent TLS in phase qubits were found in spectroscopy data, where observed avoided level crossings manifest the defects’ two-level quantum character 3 4 . Often, these defects show longer coherence times than the qubit itself 5 , and thus might be useful as quantum memories 6 and resources for quantum algorithms 7 8 . Phase qubits were used in several attempts to identify the physical origin of those TLS, for example, by obtaining statistics on frequencies and coupling strengths 9 , estimating their density 4 , measuring the temperature dependence of their coherence times 5 or verifying theoretical models describing their origin 10 .…”