Quantum measurement back action is fundamentally unavoidable when manipulating electron spins. One of the main dephasing mechanisms for the localized carrier spins in semiconductors is the coupling to the fluctuating nuclear spin environment. Here we present an experimental observation on the effects of the quantum back action under pulsed optical measurements of spin ensemble and demonstrate that the nuclei-induced spin relaxation can be influenced. We show that the fast measurements freeze the spin dynamics and increase the effective spin relaxation time, the so-called quantum Zeno effect. Furthermore, we demonstrate that if the measurement rate is comparable with the spin precession frequency in the effective magnetic field, the spin relaxation rate increases and becomes faster than in the absence of the measurements, an effect known as the quantum antiZeno effect. A theory describing both regimes allows us to extract the system parameters and the strength of the quantum back action [1].

Modification of quantum system dynamics due to the interaction with a measurement apparatus can always be described microscopically, for example, based on the Schrödinger equation [2]. In many cases, however, the general concepts of strong or weak measurements [3] can be applied. The former, also known as von Neumann type of measurements, is widely discussed nowadays for measurement-based quantum computation [4], while weak measurements are often implemented experimentally to minimize the system perturbation.
Frequent measurements can lead to freezing of the quantum dynamics, known as quantum Zeno effect [5], which requires measurements with a repetition period TR shorter than the Zeno time [6], τZ , the time of non-Markovian relaxation. The less known and less universal is the quantum anti-Zeno effect which is the acceleration of the system relaxation due to the quantum back action [7–9]. It can occur when the repetition period is longer than the Zeno time [6]. In fact, this condition can be easily realized, but often the measurement involves additional heating and other perturbations, from which the quantum anti-Zeno effect can be challenging to separate.
The quantum Zeno effect is important for quantum information processing [10,11], especially with spin-based qubits, as it can be used to increase the electron spin relaxation time; the quantum anti-Zeno effect, by contrast, allows one to quickly erase spin polarization so that both effects should be taken into account when measuring spin qubits. However, the short Zeno time of free charge carriers strongly hinders reaching the regime of the quantum Zeno effect.