On the one hand, natural atoms (such as neutral atoms and ions) have long coherence times, and could be stored in large arrays, providing ideal “quantum memories”. On the other hand, artificial atoms (such as superconducting circuits or semiconductor quantum dots) have the advantage of custom-designed features and could be used as “quantum processing units”. Natural and artificial atoms can be coupled with each other and can also be interfaced with photons for long-distance communications. Hybrid devices made of natural/artificial atoms and photons may provide the next-generation design for quantum computers.
The experimental realization of Quantum Computation (QC) has been a challenge for more than a decade. While a fully operational quantum computer that could factorize thousand-digit numbers is still a distant goal, with the new technologies for the coherent manipulation of atoms, photons, and electrons, nowadays applications like quantum cryptography and quantum communication are already commercially available. Since potential QC implementations come in many shapes and sizes, it is difficult to quantify the overall progress in the field of QC. In order to assess the current state of the art in QC, a comparison between the various approaches is needed. However, because these approaches are very different (in terms of the underlying physical processes, experimental techniques, and how well the physical system is understood), we should be careful not to compare apples with oranges. We would rather like to compare apples with apples, or in our case, atoms with atoms. Therefore, in this paper we consider natural and artificial atoms for implementing QC.
In both natural and artificial atoms, almost all the basic requirements for realizing QC have been demonstrated (i.e., (i) a scalable system with well-characterized qubits; (ii) initialization of the qubits; (iii) reasonably long decoherence times; (iv) a universal set of quantum gates; (v) measurement of the qubits). The online supplementary material provides a brief snapshot of the current experimental status for several types of qubits.
The current challenges are to attain increased controllability (and minimize decoherence) and scale the existing systems to tens and hundreds of qubits and many-gate operations. At this stage, new milestones, such as the creation of many-particle entangled states, the implementation of small quantum algorithms, and other applications (i.e., quantum simulation), and the realization of quantum communication by interfacing the qubits with photons, are being targeted.
“Quantum supercomputers” for factorizing large numbers are still a distant goal. The first-generation of practical quantum computers may be either specialized devices for scientific applications like quantum simulations, or integrated in complex quantum networks. As the very positive results summarized above point out, the first-generation quantum computers may be available in the near future. Furthermore, they may come as hybrids consisting of natural atoms, artificial atoms, and photons.