# Quantum decoherence is the strong-coupling limit of classical noise

Quantum effects have fascinated physicists for over 100 years, but their study is experimentally challenging for one major reason. Quantum coherence is fragile and breaks once the quantum system under study is “observed”. Interestingly, the “observation” of a quantum system does not require human interaction. Instead, it can be performed by the system's environment, e.g. by the omnipresent electrons, photons, nuclei or atoms. This effect, known as decoherence, has been suggested as an explanation for the measurement problem, which lies at the heart of many quantum paradoxa and most prominently Schrödinger's cat.

In practice, the study of quantum systems is complicated by another challenge. Quantum coherence is extremely sensitive to external noise, such as magnetic or electric fields. This fact is turned into an advantage in atomic accelerometers and magnetometers, where the highly sensitive quantum systems provide excellent sensors. However, it is a serious problem for quantum computers and atomic clocks, the performance of which can be easily spoiled by the stray field of a nearby subway or radio station which breaks their coherence.

These two challenges – exposure to the observation by the environment and to classical noise – have long been regarded as entirely different effects, the former as a fundamental one and the latter merely as an experimental complication. In a recent study [1] we show that they are fundamentally linked and can be regarded as limiting cases of strong and weak coupling to the environment, respectively. We experimentally prove this view by studying an NV center in diamond. It has long been known that this system loses its coherence when it is “observed” by the nuclear spins of the surrounding carbon atoms in the diamond lattice [2]. However, the loss of coherence can equally be explained by a purely classical mechanism: nuclei have a magnetic moment and can thus be considered as a source of classical spin noise, a permanently fluctuating magnetic field.

We demonstrated that this spin noise is indeed indistinguishable from classical magnetic field noise (such as a radio station) if the coupling of the nuclei to the NV center is weak. Moreover, we managed to vary the strength of the coupling and thus to continuously tune the system into the regime of either strong coupling and quantum decoherence or weak coupling and classical spin noise.

[1] Friedemann Reinhard et.al., Phys.Rev.Lett. 108, 200402 (2012)

[2] Lilian I. Childress et.al., Science 314, 281 (2006)