In physics and computer science, quantum information is information that is held in the state of a quantum system. Quantum information is the basic entity of study in quantum information theory, and can be manipulated using engineering techniques known as quantum information processing. Much like classical information can be processed with digital computers, transmitted from place to place, manipulated with algorithms, and analyzed with the mathematics of computer science, so also analogous concepts apply to quantum information. While the fundamental unit of classical information is the bit, in quantum information it is the qubit.
Quantum information differs strongly from classical information, epitomized by the bit, in many striking and unfamiliar ways. Among these are the following:
- A unit of quantum information is the qubit. Unlike classical digital states (which are discrete), a qubit is continuous-valued, describable by a direction on the Bloch sphere. Despite being continuously valued in this way, a qubit is the smallest possible unit of quantum information, as despite the qubit state being continuously-valued, it is impossible to measure the value precisely.
- A qubit cannot be (wholly) converted into classical bits; that is, it cannot be "read". This is the no-teleportation theorem.
- Despite the awkwardly-named no-teleportation theorem, qubits can be moved from one physical particle to another, by means of quantum teleportation. That is, qubits can be transported, independently of the underlying physical particle.
- An arbitrary qubit can neither be copied, nor destroyed. This is the content of the no cloning theorem and the no-deleting theorem.
- Although a single qubit can be transported from place to place (e.g. via quantum teleportation), it cannot be delivered to multiple recipients; this is the no-broadcast theorem, and is essentially implied by the no-cloning theorem.
- Qubits can be changed, by applying linear transformations or quantum gates to them, to alter their state. While classical gates correspond to the familiar operations of Boolean logic, quantum gates are physical unitary operators that in the case of qubits correspond to rotations of the Bloch sphere.
- Due to the volatility of quantum systems and the impossibility of copying states, the storing of quantum information is much more difficult than storing classical information. Nevertheless, with the use of quantum error correction quantum information can still be reliably stored in principle. The existence of quantum error correcting codes has also led to the possibility of fault tolerant quantum computation.
- Classical bits can be encoded into and subsequently retrieved from configurations of qubits, through the use of quantum gates. By itself, a single qubit can convey no more than one bit of accessible classical information about its preparation. This is Holevo's theorem. However, in superdense coding a sender, by acting on one of two entangled qubits, can convey two bits of accessible information about their joint state to a receiver.
- Quantum information can be moved about, in a quantum channel, analogous to the concept of a classical communications channel. Quantum messages have a finite size, measured in qubits; quantum channels have a finite channel capacity, measured in qubits per second.
- Quantum information, and changes in quantum information, can be quantitatively measured by using an analogue of Shannon entropy, called the von Neumann entropy. Given a statistical ensemble of quantum mechanical systems with the density matrix , it is given by Many of the same entropy measures in classical information theory can also be generalized to the quantum case, such as Holevo entropy and the conditional quantum entropy.
- In some cases quantum algorithms can be used to perform computations faster than in any known classical algorithm. The most famous example of this is Shor's algorithm that can factor numbers in polynomial time, compared to the best classical algorithms that take sub-exponential time. As factorization is an important part of the safety of RSA encryption, Shor's algorithm sparked the new field of post-quantum cryptography that tries to find encryption schemes that remain safe even when quantum computers are in play. Other examples of algorithms that demonstrate quantum supremacy include Grover's search algorithm, where the quantum algorithm gives a quadratic speed-up over the best possible classical algorithm. The complexity class of problems efficiently solvable by a quantum computer is known as BQP.
- Quantum key distribution (QKD) allows unconditionally secure transmission of classical information, unlike classical encryption, which can always be broken in principle, if not in practice. Do note that certain subtle points regarding the safety of QKD are still hotly debated.
The study of all of the above topics and differences comprises quantum information theory.
Relation to quantum mechanics
Quantum mechanics studies how microscopic physical systems change dynamically in nature. In the field of quantum information theory, the quantum systems studied are abstracted away from any real world counterpart. A qubit might for instance physically be a photon in a linear optical quantum computer, an ion in a trapped ion quantum computer, or it might be a large collection of atoms as in a superconducting quantum computer. Regardless of the physical implementation, the limits and features of qubits implied by quantum information theory hold as all these systems are all mathematically described by the same apparatus of density matrices over the complex numbers. Another important difference with quantum mechanics is that, while quantum mechanics often studies infinite-dimensional systems such as a harmonic oscillator, quantum information theory concerns itself primarily with finite-dimensional systems.
Many journals publish research in quantum information science, although only a few are dedicated to this area. Among these are
- International Journal of Quantum Information
- Quantum Information & Computation
- Quantum Information Processing
- npj Quantum Information
- Categorical quantum mechanics
- Einstein's thought experiments
- Information theory
- Interpretations of quantum mechanics
- POVM (positive operator valued measure)
- Quantum clock
- Quantum cognition
- Quantum computing
- Quantum foundations
- Quantum information science
- Quantum statistical mechanics
- Typical subspace
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- Quantiki - A wiki portal for quantum information with introductory tutorials.
- Charles H. Bennett and Peter W. Shor, "Quantum Information Theory," IEEE Transactions on Information Theory, Vol 44, pp 2724–2742, Oct 1998
- Gregg Jaeger's book on Quantum Information(published by Springer, New York, 2007, ISBN 0-387-35725-4)
- Lectures at the Institut Henri Poincaré (slides and videos)
- International Journal of Quantum Information World Scientific
- Quantum Information Processing Springer
- Michael A. Nielsen, Isaac L. Chuang, "Quantum Computation and Quantum Information"
- Wilde, Mark M. (2017), Quantum Information Theory, Cambridge University Press, arXiv:1106.1445, Bibcode:2011arXiv1106.1445W, doi:10.1017/9781316809976.001
- John Preskill, Course Information for Physics 219/Computer Science 219 Quantum Computation, Caltech 
- Charles H. Bennett, Peter W. Shor, "Quantum Information Theory"
- Vlatko Vedral, "Introduction to Quantum Information Science"
- Masahito Hayashi, "Quantum Information: An Introduction"
- Masahito Hayashi, "Quantum Information Theory: Mathematical Foundation"
- Christian Weedbrook, Stefano Pirandola, Raul Garcia-Patron, Nicolas J. Cerf, Timothy C. Ralph, Jeffrey H. Shapiro, Seth Lloyd "Gaussian Quantum Information", arXiv:1110.3234
- Samuel L. Braunstein, Peter van Loock "Quantum information with continuous variables", arXiv:quant-ph/0410100
- Quantum information can be negative
- Centres and groups doing research on quantum information include:
- Quantum Information Theory at ETH Zurich
- Quantum Information Perimeter Institute for Theoretical Physics
- Institute for Quantum Computing - The Institute for Quantum Computing, based in Waterloo, ON Canada, is a research institute working in conjunction with the University of Waterloo and Perimeter Institute on the subject of Quantum Information.
- Centre for Quantum Technologies - CQT is a research centre at the National University of Singapore bringing together quantum physicists and computer scientists
- Center for Quantum Computation - The CQC, part of Cambridge University, is a group of researchers studying quantum information, and is a useful portal for those interested in this field.
- Quantum Information Group The quantum information research group at the University of Nottingham.
- Research Group on Mathematics and Quantum Information Madrid
- Institute for Quantum Information and Matter Caltech
- Quantum Information Theory Imperial College
- Quantum Information University College London
- Quantum Information Technology Toshiba Research
- USC Center for Quantum Information Science & Technology
- Center for Quantum Information and Control Theoretical and experimental groups from University of New Mexico and University of Arizona with adjunct faculty associates from the Sandia National Labs and the Los Alamos National Labs. Another site to view their research activities (more on the UNM site) is at http://cquic.unm.edu/.
- Group of Quantum Information Theory Kyungnam University in Korea
- The Joint Center for Quantum Information and Computer Science
- The Joint Quantum Institute
- Quantum Science & Technologies Group at Louisiana State University