Cryostats are cooling devices that generate very low and constant temperatures. Like a thermos flask, the design of the container ensures good thermal insulation. For cooling a cooling liquid is used. For very low temperatures, this is helium, which becomes liquid at - 269 °C. With the help of pumps and by using certain isotopes of helium, the temperature in the cryostat can eventually be lowered to a few millikelvin above absolute zero (- 273.15 °C), i.e. to temperatures colder than in outer space.
Decoherence describes the process by which quantum mechanical superposition is lost when quantum objects interact with their environment. For example, if the quantum object is an atom, it can scatter light or collide with air molecules, and in this way interact with its environment. Since objects in nature constantly interact with their environment, decoherence is the reason why we cannot observe phenomena such as superposition in everyday life. But even in laboratories, where researchers try to isolate quantum objects in the best possible way, influences from the environment cannot be avoided 100%, which is why decoherence also occurs here after a short time.
If two quantum particles, e.g. two electrons, are entangled with each other, then these two particles cannot be considered as separate particles anymore. They behave like a closed unit, even if they are (infinitely) far apart from each other. The entanglement leads to the fact that an action at one of the two particles, can instantaneously influence the state of the other particle. Instantaneously means that no information transfer takes place. So one must not imagine that information is transferred through space and time from one particle to the other. There is no classical interaction between the entangled particles and yet an action on one particle affects its entangled partner. Entanglement is not limited to two parties, also multiple particles can be entangled.
In classical computing, the so-called logic gates, or gates for short, are physical components on a computer chip that process a binary input signal into a binary output signal. Usually, today's gates consist of transistors and the binary signals are voltage signals. On a classical processor there are very many of these gates. An example of a logic gate is the AND gate which outputs the binary value 1 if all input signals are 1, and outputs 0 if at least one input signal is 0. During the so-called compilation, a programmer's code is translated into machine language, i.e., into a sequence of instructions that are passed on to the processor. The processor executes these instructions with the help of the gates. These are so to say the last instance with which the original algorithm is implemented on the hardware.
NISQ is the acronym for "Noisy Intermediate-Scale Quantum". A NISQ computer is a quantum computer that has only a limited number of qubits (intermediate scale). In addition, the accuracy of the individual computing operations (= gates) is limited, which is why errors occur. The gates are therefore called "noisy". NISQ computers available today have between 50 and a few hundred qubits. The error rates of the gates are in the range of 1%.
Quanta are indivisible, thus elementary packages of which our world is composed. On the one hand, there are the building blocks of matter, such as electrons. On the other hand there are the elementary energy packages, the light quanta, also called light particles or photons.
In physics, it has been found that certain physical quantities, such as the energy of an electron in an atom or the electric charge, cannot be varied continuously, but can only take on discrete values that are multiples of an elementary quantity, i.e., multiple of a quantum. The quantity is said to be quantized.
A quantum circuit is a sequence of quantum gates that describes a quantum computation on data stored in qubits, analogous to the logical circuit in classical computer science, which describes a computation on data stored in bits as a sequence of logical gates.
Not all quantum computers are the same. There are various physical systems that can be used to realize quantum computing hardware, from atoms to light particles and electron spins to superconducting circuits. The physical system on which the hardware is based is often referred to as a hardware platform or modality.
In order to guarantee reliable computing results in future (more powerful) quantum computers, error correction methods are required. These are implemented using logical qubits, which means that error correction requires a very large number of physical qubits. In order to correct errors, they must first be detected. However, error detection is not trivial, as the measurements should provide information about the errors that may have occurred, but should not interfere with the encoded logical information. Error correction is necessary because the superposition states of the qubits used in quantum computing are extremely sensitive (see also decoherence), and quantum computation is therefore much more error-prone than computation on classical computers.
Like classical gates, quantum gates process information and are, so to say, the last instance with which an algorithm is implemented on the hardware. An important example of a quantum gate is the Hadamard gate which transforms the binary values 0 and 1 into a superposition of 0 and 1, respectively. However, unlike classical gates, quantum gates are not physical components placed on the processor. While a classical processor consists of gates, the quantum processor consists of qubits (qubits can be realized, for example, with single atoms; the processor is then a regular arrangement of individual atoms). The quantum gates are physical operations which influence deterministically the state of the qubits on the processor (for example, a time-limited laser pulse used to exite an atom).
Quantum bits are the elementary information carriers in a quantum computer. The quantum bit, or qubit, is therefore the quantum-physical analogue of the classical bit, the elementary information carrier in classical information technology. The bit is the smallest storage and computing unit of conventional computers. These work with the binary system, that is information is encoded in sequences of zeros and ones. In a classical computer, for example, "power on" means a 1 and "power off" means a 0. If we consider a quantum system which classically can be in exactly two different states (e.g. excited and non-excited state of an electron in the atom), then we can use this system to realize a qubit. A qubit is the superposition of exactly two complementary states. In contrast to the classical bit, where one has either the state zero or one, the states zero and one are realized simultaneously in a qubit.
The term ‘physical qubit’ refers to the physical system with which the qubit is realized. This can be, for example, a single atom, a single superconducting circuit or a single light particle (photon). These different physical systems that make up the qubits, and thus the hardware of the quantum computer, are also referred to as the quantum computing platforms.
When several physical qubits are combined to form a memory and computing unit, this is referred to as a logical qubit. More precisely, the information of the logical qubit is encoded in a joint entangled state of the physical qubits. The information is therefore not stored locally, but delocally, and is therefore protected from errors that occur locally on one of the physical qubits. The state of the logical qubit can be restored after the occurrence of such a local error. The concept of the logical qubit is required for quantum error correction.
The scalability of a quantum computer refers to the ability to increase the number of qubits on the processor to a large number. Thus, a hardware platform is scalable if the physical and technical requirements for connecting many qubits are given. For many practical applications, future quantum processors will need to scale to thousands or millions of qubits, which is an enormous challenge.
A quantum object can be in complementary states at the same time, that is in states which are mutually exclusive according to classical understanding. This is called "superposition" of states. For example, an electron can be in different places at the same time. But you could also put it differently and say that the electron simply does not have the property of being in a particular place. While in classical physics an object is always in a concrete state and has concrete properties, quantum physics allows these superpositions. Importantly, this superposition describes a physical property of the quantum object and has nothing to do with ignorance of the observer.