QUROPE Quantum Information Processing and Communication in Europe

Quantum Interfaces

Quantum interfaces between quantum information carriers (quantum states of light) and quantum information storage and processors (atoms, ions, solid state) are required as essential parts of a full-scale quantum information system. Such interfaces should thus be developed for connecting quantum computers in small networks, or more generally for quantum communication purposes. Let us first contrast the quantum technology required here to its classical counterpart. In classical optical communication, information is transferred encoded in pulses of light, which are possibly amplified, and then detected by photo detectors, transformed into electrical current pulses, amplified by electronics, and sent to computers, phones, etc. This transformation of light into electrical signals forms a classical light-matter interface. But in quantum information processing, classical amplification or detection of light is inadequate, because it destroys the quantum state by adding extra noise to it. Hence a quantum interface has to be developed, in order to transfer the quantum state of light qubits (or light continuous variables) to or from atomic qubits (or atomic continuous variables). Quantum interfaces usually involve storage elements (quantum memories), and processing elements (deterministic or conditional quantum gates). They often involve also long-distance quantum teleportation of long lived atomic states, which allow for communication and quantum secret sharing tasks. Such long lived entanglement shared over a long distance requires transfer of entanglement from light (the long distance carrier) to atoms (the long lived objects), realized by the quantum interface. Many different quantum technologies can be used to implement the interfaces, e.g. atomic ensembles, cavity QED, solid state devices, etc.

Heralded entangled photon-pair sources

Point to point earth based quantum communication is limited in distance by the losses of optical fibers. For long distance quantum communication (>500km) protocols with quantum repeaters are needed. Such schemes require, among other things, high quality sources of pairs of entangled photon, either on demand or heralded. Today’s sources are probabilistic, based on spontaneous parametric down conversion. Future sources should keep or improve on the optical quality of the existing ones (compatible with single-mode optical fibers, Fourier-transform limited, and coherence length of several centimetres), provide larger rates and yields (probability of a photon pairs) while reducing the probability of multi-pairs. The exact type of entanglement is not essential, but should involve two photons, one in each of two quantum channels (i.e. the entanglement obtained by bunching two single photons on a beam splitter is not appropriate). At least one of the photons should be at the telecom wavelength around 1.55 microns. Depending on the protocol, the second photon can be around the same wavelength or at a shorter one, below one micron (but one should bear in mind that future progress in quantum communication protocols may affect the required specifications).

Chip traps for quantum computing

The DiVincenzo criteria for quantum computing are currently approached from different directions. To date, ion traps offer the possibility to precisely manipulate and read out single qubits and to perform entangling gate operations, while the size of the system is currently limited to a few qubits. In contrast, with neutral atoms large ensembles of entangled qubits have been created while the manipulation of single atoms and their detection present a major challenge. Both these approaches – bottom up for ions and top down for atoms – need to be further developed to take quantum computation the next scale. Chip technology for trapping ions or neutral atoms will play a major role in this development. For neutral atoms, chip traps offer precise positioning that enables controlled interactions and detection of single atom states. The first on-chip implementation of a high-finesse fibre resonator has very recently been demonstrated, offering at the same time a tool for manipulation, entanglement and detection of ions. In addition, this should be used in the future to establish an interface between stationary (atoms) and flying qubits (photons). For ions, the chip traps serve to increase the number of qubits that can be handled. The segmentation of trap electrodes in microscopic traps allows for a multitude of miniature ion traps on one chip. Future developments have to meet two major challenges: finding a trap technology that features small heating rates and long coherence times, and a trap design that allows for transport of the ions (along with their contained quantum information) between all miniature traps on the chip. An integration of optical cavities as demonstrated for neutral atoms would be desirable, too.