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 propagating qubits (or propagating continuous variables), e.g. photons, to or from stationary qubits (or stationary continuous variables), e.g. atoms. 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 states of stationary systems, which allow for communication and quantum secret sharing tasks. Such long lived entanglement shared over a long distance requires transfer of entanglement from a long distance carrier, e.g. light, to long lived objects, e.g. atoms, 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.

Researchers from Europe have recently implemented an elementary quantum network consisting of two atom-cavity systems connected through a 60 m optical fibre link, and demonstrating both faithful quantum state transfer and entanglement generation between the stationary atoms of the two nodes. Entangling stationary qubits over large distances through optical links has by now also been realized in several other physical systems. These realizations include the entanglement of ions in spatially separated ion traps over 1 m, the entanglement of neutral atoms over separations up to 20 m, and the entanglement of electronic spins, nuclear spins and optical phonons, respectively, each located in distant solid state samples.

Heralded single-photon and 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 pair) 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). Efficient coherent upconversion of single photons has recently been demonstrated, which may relax the requirement for source specifications, as photons can be converted to the wanted wavelength.

Another promising outlook for long-distance quantum communication is to use satellite based platforms, which would allow the distribution of either single-photon qubits or of entanglement through space-born photon sources, hence allowing quantum communication on a global scale. Space-qualified sources for single photons and for entangled photons are now becoming available and first proof-of-concept tests are being prepared all over the world, including in Europe, Canada, Japan, Singapore and China. Heralded single-photon sources are also of relevance for ground based quantum communication, as the absence of multi-photon events allows for genuine security of quantum information through the no-cloning theorem, and for photonic quantum information processing, where they allow for the implementation of generalized (POVM) measurements.

Significant progress has been also made in solid-state based single-photon and entangled photon sources, including emission from semiconductor quantum dots and nitrogen vacancies in spin.

On-chip architectures for quantum computing and quantum simulation

The DiVincenzo criteria for quantum computing are currently approached from different directions. Also, in the current efforts to establish few-qubit quantum simulators, a set of less stringent requirements for quantum simulations has been suggested (by Cirac and Zoller).

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. Also, single-site addressing of atoms inside an optical lattice has now been realized. These technologies may 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.

Other on-chip implementations are also being pursued: in the context of photonic quantum simulations, integrated wave guides have recently enabled the first realization of efficient boson sampling, a task that can in principle not be efficiently computed on a classical computer. In the long run, the embedding of integrated waveguides in fiber networks may allow for securely delegated quantum computations. The main limitation for photonic waveguide technology to date is the overall optical loss that is acquired from photon generation, from optical coupling into and out of the wave guide and from (inefficient) photon detection. However, the latest advances in source- and detector technology in principle allow designing all relevant components (source, quantum gates, detectors) on one chip, thereby minimizing these losses. Optical microcavities and nano- and micro-optomechanical devices provide additional flexibility for photonic on-chip architectures in form of narrow linewidths, delay lines, optical nonlinearities or phononic quantum transducers.

Quantum processor architectures based on superconducting quantum circuits have recently been developed by researchers both in Europe and in the US. Today, these devices have already been used successfully to demonstrate, for example, logic gates like the three-qubit Toffoli-class OR phase gate, a combination of a quantum central processing unit (quCPU) and a quantum random-access memory (quRAM) which comprise two key elements of the quantum version of a classical von Neumann architecture. Also a three-qubit compiled version of Shor's algorithm to factor the number 15 and three-qubit quantum error correction has been implemented. Increasingly long coherence times (approaching the second regime) are now being reached through novel architectures. The next major challenge in the field is to improve the fidelity of the quantum logic operations and to interface the microwave domain to the optical domain, possibly through hybrid opto-electro-mechanical architectures.

Solid state quantum registers are now being implemented through nitrogen vacancy centers in diamond, as they allow for a simple optical interface and long-lived electronic spin qubit storage, whose lifetimes could even be improved additionally by dynamic decoupling techniques. Entanglement between pairs of NV centers have been created, both probabilistically over 3 meters distance using photons as a quantum channel and deterministically between neighboring centers via microwave fields acting on both NV centers operating even in a room temperature environment. With respect to quantum information processing a Grover algorithm between two spins has successfully been implemented. In order to bring these promising developments to the stage of real life technologies, further improvements are required with respect to fabrication techniques to place NV centers with high spatial control, to improvements on the collection efficiencies for light and to quantum gate operations and qubit storage for quantum registers.

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