In a quantum network, information is stored, processed, and transmitted from one device to another in the form of quantum states. The quantum nature of the network gives it certain advantages over classical networks, such as greater security.
One promising method for implementing a quantum network involves using both atoms and photons for their unique advantages. While atoms are useful as nodes (in the form of quantum memories and processors) due to their long storage times, photons are useful as links (on optical fibers) because they're better at carrying quantum information over large distances.
However, using both atoms and photons requires that quantum states be converted between single atoms and single photons. This in turn requires a high degree of control over the emission and absorption processes in which single atoms act as senders and receivers of single photons. Because it's difficult to achieve complete overlap between the atomic and photonic modes, photon-to-atom state transfer usually suffers from low fidelities of below 10%. This means that more than 90% of the time the state transfer is unsuccessful.
In a new paper published in Nature Communications, a team of researchers led by Jürgen Eschner, Professor at Saarland University in Saarbrucken, Germany, has experimentally demonstrated photon-to-atom quantum state transfer with a fidelity of more than 95%. This drastic improvement marks an important step toward realizing future large-scale quantum networks. The researchers' protocol consists of transferring the polarization state of a laser photon onto the ground state of a trapped calcium ion. To do this, the researchers prepared the calcium ion in a quantum superposition state, in which it simultaneously occupies two atomic levels. When the ion absorbs a photon emitted by a laser at an 854-nm wavelength, the photon's polarization state gets mapped onto the ion. Upon absorbing the photon, the ion returns to its ground state and emits a single photon at a 393-nm wavelength. Detection of this 393-nm photon signifies a successful photon-to-atom quantum state transfer.
he researchers showed that this method achieves very high fidelities of 95-97% using a variety of atomic states and both linear and circular polarizations. The method also has a relatively high efficiency of 0.438%. The researchers explain that the large fidelity improvement is due in large part to the last step involving the detection of the 393-nm photon.