Research

Quantum entanglement is one of the most intriguing properties of quantum mechanics, and is commonly visualized as a correlation of two or more quantum particles that form a single quantum object. But entanglement does not necessarily involve two quantum particles. Possibly even more fascinating, a single photon can entangle two modes of the vacuum. Here we show that this single-particle entanglement can also be generated by coherently controlling the quantum dynamics of nuclei. First, the nuclei are excited with a synchrotron radiation pulse. Then, by applying a sequence of weak magnetic fields to the nuclei, they are forced to emit a single photon in the x-ray regime such that it entangles field modes at energies about 10000 times higher than usual. This could pave the way for entanglement research in nuclear physics, based on experiments exploiting nuclear complexity with synchrotron radiation sources and free-electron lasers in an as-yet unexplored energy regime.

Decoherence is one of the key limiting factors for many applications, most prominently in quantum information science. A possible solution is provided by decoherence-free subspaces (DFS) which is a subspace of the complete Hilbert space with purely unitary time evolution. We have considered two dipole-dipole interacting four-level atoms with a singlet ground and triplet excited state as a candidate for a DFS. Both parallel and orthogonal dipole-dipole coupling are considered. An analysis of the system dynamics reveals that in the limit of vanishing interatomic distance, the state space of our system contains a four-dimensional DFS. The excited states in the DFS are the generalization of the well-known sub-radiant Dicke state found in two two-level systems. Their lifetime and thus the operation time of the DFS depends on the interatomic distance, and becomes infinite in the limit of vanishing interatomic distance. Using a single cw laser field, the DFS can be populated probabilistically without requiring, e.g., field gradients on the wavelength distance scale as employed by previously proposed schemes. After preparing the system in the DFS, arbitrary single-qubit operations can be executed between two excited states within the DFS either using a static magnetic field or a rf field.

Quantum entanglement is known to be the key resource in many applications of quantum information and quantum computing. Photons are considered as a candidate system, but it still remains a challenge to generate entanglement in macroscopic light rather than on the few photon level. Recently, a laser consisting of a single-atom interacting with a single cavity mode was experimentally demonstrated in Kimble's group.
Based on this result, we have shown that a single atom that interacts with two quantized modes of a doubly resonant cavity via two lasing transitions can lead to macroscopic entangled light. Macroscopic entanglement can be achieved over a wide range of control parameters and initial states of the cavity field.