Realism is an idea that a state of the object is determined before its observation. Although this idea is widely believed to hold in our lives, it has been shown that "realism" is broken in a microscopic world. The microscopic world is governed by the principles of quantum mechanics that allows a state to have a superposition. For a superposed state, the measurable values are not determined until they are observed. Since the macroscopic world is composed of microscopic objects, quantum mechanics could be valid even in this macroscopic world. Or there could be so limitation to the application of the quantum mechanics to the macroscopic world. Currently, there is still no consensus about which is likely to be true.
Here we aim to address this. A superconducting flux qubit is a circuit composed of several Josephson junctions and a superconducting loop. Due to the non-linearity of the Josephson junctions, this device provides a two-level system with clockwise and anti-clockwise current states. These two states contain 1012 electrons flowing around the circuit per a second. We test whether realism is broken with this macroscopic device or not.
In our normal world, it is accepted that we can measure macroscopic objects without disturbing them (non-invasiveness). If realism is true, any non-invasively measured results on this device should satisfy the Leggett-Garg inequality. However, if quantum mechanics is true, we could observe a violation of the inequality using non-invasive measurements.
We show that the Leggett-Garg inequality is mathematically equivalent to an experimental test to check if the non-invasive measurements affect the state of the device. Interestingly, even if the measurement is designed to be non-invasive, observation by the measurement induces a projection according to quantum mechanics. This can change the state of the device.
In our test, we perform two experiments. First we check the non-invasiveness of the measurements by evaluating the classical disturbance that our measurement device on the flux qubit has. Second, we compare the measured state of the flux qubit with a not measured state of the flux qubit. From these experiments, we have shown that there is a large difference in these measured results, which cannot be explained by the classical disturbance [Fig.1(a)]. This is a manifestation of quantum superposition. Our results conclusively demonstrate that "realism" is broken even for a macroscopic device such as a superconducting qubit [Fig. 1(b)] .
This work was partly supported by JSPS KAKENHI JP15H05870 and JP15H05867.
Fig. 1. (a) Differences by observation. (b) Quantum superposition of current state.