Researchers from the Quantum Dynamics Division headed by Professor Gerhard Rempe from the Max Planck Institute of Quantum Optics have at present accomplished a significant advancement in the development of quantum memories for forming global quantum networks.
They exhibited the lasting storage of a photonic qubit into a single atom confined inside an optical resonator. The stored quantum bit’s coherence time is more than 100 ms and hence correlates with the demand for forming a global quantum network where qubits are teleported directly between the end nodes. “The coherence times that we achieve represent an improvement by two orders of magnitude compared to the current state-of-the-art,” stated Professor Rempe.
Light is an optimal carrier for the transfer of quantum information encoded into single photons. However, this transfer is unreliable and ineffective over longer distances owing to losses. Direct teleportation between a network’s end nodes can be adopted to arrest the loss of valuable quantum bits. The initial step involves developing a remote entanglement between the nodes. Next, an appropriate measurement from the sender side initiates the “spooky action at a distance”, that is, the spontaneous transfer of the qubit to the receiver-side node. Yet, the quantum bit may be rotated upon reaching the receiver and so must be returned. Therefore, the required information must be classically conveyed from the sender to the receiver. This requires a specific amount of time in which the qubit should be conserved by the receiver. Taking into account two network nodes at the farthest positions on earth, this relates to a time span of 66 ms.
In the year 2011, Professor Rempe and his colleagues exhibited an acknowledged method for the storage of a photonic quantum bit on a single atom. The atom is positioned at the centre of an optical cavity created by two high-precision mirrors and held in position by standing light waves. A single photon carrying the quantum bit in a coherent superposition of two polarization states begins to actively interact with the single atom when it is directed into the resonator. Eventually, the atom absorbs the photon and the quantum bit is conveyed to a coherent superposition of the two atomic states. The task here is to preserve the atomic superposition for as longer a time as feasible. In earlier experiments, the storage time was restricted to only a few hundred microseconds.
The major problem for storing quantum bits is the phenomenon of dephasing, characteristic of a quantum bit is the relative phase of the wave functions of the atomic states that are coherently superimposed. Unfortunately, in real-world experiments, this phase relation is lost over time mostly due to interaction with fluctuating ambient magnetic fields.
Stefan Langenfeld, a doctoral candidate at the experiment.
In the present study, the researchers have taken innovative steps to overcome the effect of these fluctuations. When the information is conveyed from the photon to the atom, an atomic state’s population is coherently transferred to a different state. This is performed by adopting a pair of laser beams to initiate a Raman transition. In this innovative configuration, the stored qubit is nearly 500 times less sensitive to fluctuations in the magnetic field.
Prior to the recovery of the stored photonic quantum bit, the researchers reversed the Raman transition. For a 10 ms storage time, the overlap of the stored photon over the recovered photon is nearly 90%, indicating that just transferring the atomic qubit to a less sensitive state increases the coherence time by a factor of 10. Another factor of 10 was achieved by including a “spin echo” to the experimental series. In this instance, the population of the two atomic states adopted for storing the quantum bit is retrieved in the middle of the storage time.
The new technique allows us to preserve the quantum nature of the stored bit for more than 100 milliseconds, although an envisioned global quantum network which allows for secure and reliable transport of quantum information still demands a lot of research, the long-lived storage of quantum bits is one of the key technologies and we believe that the current improvements will bring us a significant step closer to its realization.
Matthias Körber, a doctoral candidate at the experiment.