External quantum fluctuations select measurement contexts

Jonte R. Hance, Ming Ji, Tomonori Matsushita +1 more

Quantum paradoxes show that the outcomes of different quantum measurements cannot be described by a single measurement-independent reality. Any theoretical description of a quantum measurement implies the selection of a specific measurement context. Here, we investigate generalised quantum measurements, in order to identify the mechanism by which this specific context is selected. We show that external quantum fluctuations, represented by the initial state of the measurement apparatus, play an essential role in the selection of the context. This has the non-trivial consequence that, when considering measurements other than just idealised projection-valued measures, different outcomes of a single measurement setup can represent different measurement contexts. We further show this result underpins recent claims that contextuality can occur in scenarios without measurement incompatibility.

Z. Ficek, R. Tanaś

Holger Friedrich Hofmann

The quantum fluctuations of a physical property can be observed in the measurement statistics of any measurement that is at least partially sensitive to that physical property. Quantum theory indicates that the effective distribution of values taken by the physical property depends on the specific measurement context based on which these values are determined and weak values have been identified as the contextual values describing this dependence of quantum fluctuations on the measurement context. Here, the relation between classical statistics and quantum contextuality is explored by considering systems entangled with a quantum reference. The quantum fluctuations of the system can then be steered by precise projective measurements of the reference, resulting in different contextual values of the quantum fluctuations depending on the effective state preparation context determined by the measurement of the reference. The results show that mixed-state statistics are consistent with a wide range of potential contexts, indicating that the precise definition of a context requires maximal quantum coherence in both state preparation and measurement.

EDWARD B. NELSON

Ian C. M. Littler, Stefan Balle, Klaas Bergmann +2 more

We have demonstrated the detection of weak optical signals via the decay of an unstable state. A Q-switched laser is an example of an unstable state, the decay of which has to be triggered either by quantum fluctuations or by an injected external signal. Measurement of the initiation time of the laser intensity allows us to determine the strength of the external signal with a dynamic range of 7 orders of magnitude. It is possible to detect signal levels that correspond to less than one photon in a selected cavity mode.

EDWARD B. NELSON