Typically noise is unwanted in experiments, and the challenge is minimising it. In the experiment of generating and detecting a „squeezed“ vacuum, the noise is the signal that reveals the existence of quantum entanglement. Even though the number of atoms in both gases (marked in red and blue) fluctuates extremely, their difference (marked in black) is very small. In order to obtain a correct analysis, a few experiments (on the left) are not sufficient. The noise has to be analysed in long series of measurements (on the right).
Journal Nature - Atomic homodyne detection of continuous-variable entangled twin-atom states
The breakthrough in the quantum state discovered and created by Prof. Oberthaler and his team lies in the quantum entanglement of continuous variables. This means that in principle, individual measurements of the two gases randomly produce many different values. After measuring one gas, however, all the other measurements on the second – entangled – gas can be precisely predicted. To create and detect a “squeezed” quantum vacuum with its unique characteristics in the laboratory, the researchers worked with a Bose Einstein condensate. This condensate is an extreme aggregate state of a system of indistinguishable particles, most of which are in the same quantum mechanical state. The condensate used was comprised of Rubidium atoms cooled to an ultracold temperature of 0.000 000 1 Kelvin above absolute zero.
“The setup of the experiment had to be extraordinarily stable since we took measurements continuously for many days in a row to gather enough data to verify the generation of a quantum entanglement”, explains Prof. Oberthaler. For this purpose, the researchers had to guarantee the stability of magnetic fields that is 10,000 times smaller than of the magnetic field of the earth. They also needed to detect a gas consisting of 500 atoms with an error tolerance of less than eight atoms since the particle number fluctuations served as the signal for a successful generation of an entanglement. Prof. Oberthaler: “Normally you don’t want noise in experiments, but in our investigations careful examination of the noise actually proved the presence of the quantum entanglement.” The challenge for the Heidelberg team was suppressing the technical noise enough to allow the quantum noise to dominate.
Prof. Oberthaler and his colleagues hope not only that their research results lead to an application for precise atomic interferometry, but also see their findings as an important step in the investigation of fundamental questions of quantum mechanical entanglement of massive particles.
Historically, the completeness of quantum theory has been questioned using the concept of bipartite continuous-variable entanglement1. The non-classical correlations (entanglement) between the two subsystems imply that the observables of one subsystem are determined by the measurement choice on the other, regardless of the distance between the subsystems. Nowadays, continuous-variable entanglement is regarded as an essential resource, allowing for quantum enhanced measurement resolution, the realization of quantum teleportation and quantum memories or the demonstration of the Einstein–Podolsky–Rosen paradox. These applications rely on techniques to manipulate and detect coherences of quantum fields, the quadratures. Whereas in optics coherent homodyne detection of quadratures is a standard technique, for massive particles a corresponding method was missing. Here we report the realization of an atomic analogue to homodyne detection for the measurement of matter-wave quadratures. The application of this technique to a quantum state produced by spin-changing collisions in a Bose–Einstein condensate reveals continuous-variable entanglement, as well as the twin-atom character of the state. Our results provide a rare example of continuous-variable entanglement of massive particles. The direct detection of atomic quadratures has applications not only in experimental quantum atom optics, but also for the measurement of fields in many-body systems of massive particle
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