A unified theory of spin decoherence and relaxation in molecular spin qubits

The loss of information about the relative phase between two quantum states, known as decoherence, is one of the main obstacles to the use of molecular spin qubits for quantum information processing applications. Many experimental studies have attempted to prolong the coherence time of molecular qubits, but these efforts have been hampered by the uncertainty about the actual mechanism underlying spin decoherence. Indeed, whilst a qualitative understanding of the role of spin-spin and spin-phonon interactions taking place at low and high temperatures, respectively, is available, the fine details of these processes and how to mitigate them remain unclear. In this contribution, I will demonstrate that open quantum systems theory, combined with first-principles methods[1], is able to quantitatively reproduce the entire corpus of evidence available for molecular qubits. For the first time, both spin-spin and spin-phonon contributions to both T₁ and T₂ will be reported and fully elucidated across the entire temperature range, from a few K to room temperature. Several molecular qubits will be used as examples to settle several topical debates. For instance, I will discuss the relative role of electronic and nuclear spins in determining T₂ at low temperature and establish the real limits achievable through deuteration[2]. Moreover, the role of excited states, low and high-energy phonons in determining relaxation and decoherence rates will be fully explained [3]. Finally, a critical discussion of the limits for T₂ achievable in molecular qubits and comparisons with other quantum platforms [4] will be drawn.