Resources

The physical theory that describes the behaviour of particles at tiny length scales (roughly at or below the size of individual molecules) or at extremely low temperatures (often a fraction of a degree above absolute zero temperature).

The theoretical and experimental study of how the principles of quantum physics can be used to collect, process and transmit information in ways that are not possible using conventional technology.

An artificial device or system that harnesses the principles of quantum physics for practically useful applications. Quantum technology is closely related to quantum information science. Quantum technology is often divided in three overlapping subcategories: quantum computing, quantum sensing and quantum communications. Atomic clocks (within quantum sensing) are the only quantum technology that have already proven to be practically useful, but many other types of quantum technology are advancing quickly and may soon prove useful as well.

The smallest and simplest possible quantum system. A qubit is equivalent to the classical digital bit in traditional, classical computing. A classical bit can be in one of two states. One state usually represents a “0” for the purpose of computation, and the other state represents a “1”. By contrast, a qubit (short for “quantum bit”) can be in either of two physical states or in a superposition (see entry below) of both. The development and manipulation of qubits is fundamental to advancing quantum computing and other quantum technologies. An actual real-world qubit can take on several different physical forms, known as qubit modalities (see entry below). Qubits are often subdivided into physical qubits and logical qubits (see entries below).

A quantum system can exist in a superposition of different physical states (e.g. the concept that a qubit is in a combination of two distinct states). This roughly means that the system is in all of those states at the same time, although not necessarily to equal degrees. 

When two or more particles interact and become correlated so that the state of one particle affects the state of the others no matter how far apart they are. Roughly speaking, two or more quantum systems are entangled if they have jointly entered together into a single combined superposition. Entangled quantum systems can experience very complicated statistical correlations that can only be created using quantum physics. The complexity of an entangled system grows very quickly as more particles are entangled together. For example, a quantum computer contains many qubits entangled together; the complexity of an entangled state is what makes quantum computers so powerful. Entangled systems tend to be very physically delicate; entanglement is easily “broken”. Maintaining robust entanglement across many qubits is ultimately one of the main engineering challenges facing many quantum technologies. 

A model of computation that harnesses the properties of quantum physics including entanglement and superposition. Quantum computers operate on fundamentally different principles from ordinary (“classical”) computers, and they are considered to be much more powerful for some types of calculations. The fundamental building blocks of classical computers are bits (0’s and 1’s), but the fundamental building blocks of quantum computers are qubits.

Controllable systems which reproduce the behaviour of another quantum system and are easier to construct compared to a full quantum computer. While quantum computers once realised, will be able to tackle complex problems, fault-tolerant machines may not be available until far into the future, so in the short term, quantum simulators are being developed. 

A physical, real-world quantum system that can be in one of two physical states, or in a superposition of both. In order to be useful, a physical qubit must be able to maintain its physical state for a significant amount of time, and it must be easily manipulated by an operator (for example, the operator might switch one qubit between its 0 and 1 state, or might perform elementary logic operations on a pair of qubits). In practice, physical qubits are significantly affected by external noise that causes the qubit to lose its state, reducing the accuracy of a quantum computer’s calculations. For example, environmental noise might cause a qubit to switch between its 0 and 1 state without the operator’s knowledge or intention. More complicated types of errors with no classical analogue are also possible. Today’s physical qubits come in many different hardware architectures or modalities. 

The hardware design architecture of a physical qubit. These are the different physical systems or methods used to fabricate, store or manipulate qubits. Commonly used qubit modalities include superconducting qubits, trapped ions, neutral atoms, photonic qubits, silicon-spin qubits, and topological qubits. These different qubit modalities have very different physical behaviours and underlying technological considerations, and each modality has its own advantages and disadvantages compared to the others.

External environmental effects that interfere with a quantum computer and cause errors that reduce the accuracy of the computation. 

There are several different usages of the term “logical qubit”, but all of them relate to the idea of a qubit that is resistant to the effects of external noise and maintains its logical state (0, 1, or a superposition of both) for a long time. Some people reserve the term “logical qubit” to refer to an idealised theoretical qubit that never experiences any errors; others use the term more broadly to refer to a qubit that experiences errors rarely enough that it can participate in a long and complex calculation without experiencing any errors. The technique of quantum error correction can be used to turn several physical qubits into one much more stable logical qubit

A method of implementing a quantum computation that makes it more robust against external noise, thereby improving the accuracy of the final output. Quantum computers today are characterised by high error rates and noise that significantly impact their accuracy and reliability. Implementing quantum error correction is a major engineering challenge that is still in the early stage of development. The full potential of quantum computers (including building practical and scalable quantum computers) may not be realised until QEC is effectively implemented to mitigate the noise and errors arising from the inherent fragility of quantum systems. QEC works by combining many physical qubits into a much smaller number of logical qubits that experience lower error rates. For example, a quantum computer with 200 physical qubits might be designed to group its 200 physical qubits into 10 groups of 20 physical qubits each. Each group of 20 physical qubits would form one logical qubit with a lower error rate than any of its constituent physical qubits, so the computer would effectively contain 10 logical qubits. QEC greatly decreases the effective error rate of the qubits in a calculation, at the expense of greatly decreasing the number of qubits available (in this case, from 200 to just 10). 

A quantum computer with roughly 50-1000 qubits that can perform certain calculations too difficult for any conventional supercomputer. A NISQ computer does not utilise quantum error correction, so external noise causes its accuracy to decrease quickly over a calculation’s runtime. Therefore, the maximum runtime that a NISQ computer can sustain over a single calculation is inherently limited.

A still-hypothetical quantum computer that implements quantum error correction that extends its accuracy and runtime far beyond what is possible without quantum error correction. Many (but not all) known quantum algorithms can only run on a fault-tolerant quantum computer. 

A computational algorithm (that is, a detailed list of step-by-step mathematical operations that solve some computational problem) that can only be run on a quantum computer. There are many known quantum algorithms, although they tend to cluster into a few broad categories: modelling physical systems, performing numerical optimisation, solving systems of linear equations, or factoring large numbers. 

One of the most important quantum algorithms, developed by mathematician Peter Shor in 1994 to efficiently factorise large numbers. A large-scale and stable quantum computer would be able to run Shor’s algorithm to break much of the cryptography used to protect today’s internet traffic, with significant consequences for cybersecurity. Shor’s algorithm can only be run on a fault-tolerant quantum computer (see entry above), which is not yet known to exist.

Mathematical cryptographic algorithms designed for protecting information against interception or interference. These algorithms can be implemented on a conventional computer and do not require a quantum computer. Unlike many cryptographic algorithms used today, PQC algorithms are not believed to be vulnerable to Shor’s algorithm or to any other attack from a quantum computer. PQC and quantum key distribution (QKD) are the two main proposed countermeasures to future-proof quantum computer attacks against cryptography. 

The transmission of information encoded within qubits over a distance. These qubits usually take the physical form of photons (individual packets of light). The term quantum communications is closely related to (and sometimes used synonymously with) the term quantum networking. There are two main categories of application for quantum communications: (1) security against eavesdropping through a protocol such as quantum key distribution , and (2) networking together other quantum devices, such as quantum computers or quantum sensors.   

A communication protocol that transmits a stream of individual photons instead of classical bits. Two physically distant parties can use QKD to encrypt information transmitted between them. When a QKD system is implemented correctly, any eavesdropper who attempts to intercept the transmission inevitably leaves a signature of their interception. Therefore, a well-implemented QKD system provides security against the interception of an encrypted message. Unlike today’s conventional cryptography, QKD is not vulnerable to an attack from a quantum computer of any size. QKD and PQC are the two main proposed countermeasures to future quantum computer attacks against cryptography. 

the use of quantum physics to take measurements of physical phenomena such as time, gravity, acceleration, rotation, electric or magnetic fields. In some cases, quantum sensors have higher accuracy than is possible for conventional sensors, sometimes approaching the highest sensitivities that are allowed by the laws of physics. In other cases, quantum sensors can offer practical advantages over conventional sensors, such as higher stability or reductions in cost, size, weight or power consumption. Many practical applications of quantum sensors have been proposed, in areas as varied as biomedicine, underground prospecting, environmental monitoring, navigation in areas where GPS is unavailable and robust communications. The term quantum sensing is often used to include quantum timing via highly sensitive atomic clocks, although some experts consider atomic clocks to be a separate category of quantum technology. 

A technology with both civilian (i.e. commercial or industrial) and military applications. Recently, this term has sometimes been applied in the context of potential weaponisation and exploitation of technology and applications by malicious actors. Examples of dual-use technology include certain types of biotechnology, artificial intelligence, quantum technologies, drones, advanced materials and manufacturing, space technologies and nuclear power technologies. The dual-use nature of these technologies presents specific challenges and considerations, especially concerning regulation, export control and ethical implications.