Quantum Computing & Simulation
The world is quantum!
Just often we don't realize it in normal life. But if quantum physics is used to elevate computing & simulation to the next level, a lot can happen. One envisages to solve difficult or even so far impossible calculations and optimizations. And one can utilize quantum simulators or quantum computers to analyze important quantum processes or difficult quantum systems for the good of humankind. To put it like 1965 Nobel Prize Winner Richard Feynman: "only quantum systems can simulate quantum physics". What he implied is to use one quantum system, the quantum simulator, to understand another quantum system of interest.
Quantum bits, quantum gates and quantum systems!
The fundamental unit of quantum computers and quantum simulators is the quantum bit or "Qubit", a thing that has at least two quantum states, which can be prepared, manipulated and read out. The manipulation of the quantum states has to be very precise and can be done e.g. by applying specific external fields in a quantum simulator (sometimes also called analog quantum computer) or by a sequence of typically different pulsed manipulation steps in a (digital) quantum computer. The latter are called quantum gates, and one usually needs two different type of gates, one acting on only one Qubit (single-qubit gate) and another one acting on two different qubits at the same time and conditionally (two-qubit gate). An assembly of many of such qubits is then called quantum system. Ions and atoms are not only excellent species for optical quantum clocks, but also excellent quantum systems for quantum computing and quantum simulation, providing accessible qubits with long life-time and high-fidelity addressability.
Lasers at the heart of quantum computers and quantum simulators
Qubits are very fragile. They get lost, are destroyed or disturbed easily in such a way that they can no longer serve for quantum computing or quantum simulation. Depending on the quantum system of choice, this can happen within milliseconds up to (only) many seconds. That's the time one has for one simulation or one computing sequence. Lasers are used to realize most of the quantum systems used for quantum computing and simulation. Lasers are utilized to prepare quantum states, to realize quantum gates, and to read out qubits. In addition, lasers are used to link different quantum systems (e.g. to connect quantum computers or quantum computing subunits), a major task of quantum communication. Tunable diode lasers, fiber amplifiers, optical frequency combs, and wavelength meters, ideally combined in laser rack systems as complete solutions, are the most important products.
The advantage of ions is that they can be selectively prepared, trapped in electrical and magnetic fields (Nobel Prize 1989 to Hans Dehmelt and Wolfgang Paul "for the development of the ion trap technique"), laser-cooled, well isolated from the environment to prevent decoherence, and laser manipulated. Ions played a crucial role in the development of the foundations for quantum computing, see Nobel Prize 2012 "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems" for Dave Wineland (jointly with Serge Haroche). Chip-scale ion traps can be realized and outline ways for scalability and manufacturability, especially if combined with photonic integrated circuits (PICs). Different laser systems are needed to convert atoms into ions by photo-ionization, to laser-cool the ions, to optically pump them into a specific quantum state, to realize one-qubit gates and two-qubit gates, and to read out the qubits. Typically, 6 to 10 different types of lasers with respect to power, wavelength, and linewidth are required to run an ion-based quantum computer. The lasers have to be actively frequency-stabilized, some of them to a linewidth on the order of one Hz. No lasers, no ion quantum computing and simulation!
Atoms are excellent quantum systems for both quantum computing and quantum simulation. Like ions, atoms can be selectively prepared, laser-cooled, optically pumped, manipulated via single-qubit and two-qubit quantum gates, and read out using advanced laser systems. Neutral atoms have also played a crucial role in "ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems" that were recognized with the Nobel Prize 2012 for Serge Haroche (jointly with Dave Wineland). For quantum computing and quantum simulation, atoms are trapped using laser light in form of optical tweezers (focused laser beams), optical lattices (counter-propagating laser beams forming optical standing waves), or combinations of both. Up to a dozen different types of lasers with respect to power, wavelength, and linewidth are required to run a neutral atom quantum computer. The lasers have to be actively frequency-stabilized, some of them to a linewidth on the order of one Hz.