Quantum computing

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Short description: Study of a model of computation
IBM Q System One (2019), the first circuit-based commercial quantum computer

Quantum computing is a type of computation that harnesses the collective properties of quantum states, such as superposition, interference, and entanglement, to perform calculations. The devices that perform quantum computations are known as quantum computers.[1]:I-5 They are believed to be able to solve certain computational problems, such as integer factorization (which underlies RSA encryption), substantially faster than classical computers. The study of quantum computing is a subfield of quantum information science. Expansion is expected in the next few years as the field shifts toward real-world use in pharmaceutical, data security and other applications.[2]

Quantum computing began in 1980 when physicist Paul Benioff proposed a quantum mechanical model of the Turing machine.[3] Richard Feynman and Yuri Manin later suggested that a quantum computer had the potential to simulate things a classical computer could not feasibly do.[4][5] In 1994, Peter Shor developed a quantum algorithm for factoring integers with the potential to decrypt RSA-encrypted communications.[6] Despite ongoing experimental progress since the late 1990s, most researchers believe that "fault-tolerant quantum computing [is] still a rather distant dream."[7] In recent years, investment in quantum computing research has increased in the public and private sectors.[8][9] On 23 October 2019, Google AI, in partnership with the U.S. National Aeronautics and Space Administration (NASA), claimed to have performed a quantum computation that was infeasible on any classical computer,[10][11] but whether this claim was or is still valid is a topic of active research.[12][13]

There are several types of quantum computers (also known as quantum computing systems), including the quantum circuit model, quantum Turing machine, adiabatic quantum computer, one-way quantum computer, and various quantum cellular automata. The most widely used model is the quantum circuit, based on the quantum bit, or "qubit", which is somewhat analogous to the bit in classical computation. A qubit can be in a 1 or 0 quantum state, or in a superposition of the 1 and 0 states. When it is measured, however, it is always 0 or 1; the probability of either outcome depends on the qubit's quantum state immediately prior to measurement.

Efforts towards building a physical quantum computer focus on technologies such as transmons, ion traps and topological quantum computers, which aim to create high-quality qubits.[1]:2–13 These qubits may be designed differently, depending on the full quantum computer's computing model, whether quantum logic gates, quantum annealing, or adiabatic quantum computation. There are currently a number of significant obstacles to constructing useful quantum computers. It is particularly difficult to maintain qubits' quantum states, as they suffer from quantum decoherence and state fidelity. Quantum computers therefore require error correction.[14][15]

Any computational problem that can be solved by a classical computer can also be solved by a quantum computer.[16] Conversely, any problem that can be solved by a quantum computer can also be solved by a classical computer, at least in principle given enough time. In other words, quantum computers obey the Church–Turing thesis. This means that while quantum computers provide no additional advantages over classical computers in terms of computability, quantum algorithms for certain problems have significantly lower time complexities than corresponding known classical algorithms. Notably, quantum computers are believed to be able to quickly solve certain problems that no classical computer could solve in any feasible amount of time—a feat known as "quantum supremacy." The study of the computational complexity of problems with respect to quantum computers is known as quantum complexity theory.

Quantum circuit

The Bloch sphere is a representation of a qubit, the fundamental building block of quantum computers.


Main pages: Quantum circuit, Quantum logic gate, and Qubit

The prevailing model of quantum computation describes the computation in terms of a network of quantum logic gates.[17] This model can be thought of as an abstract linear-algebraic generalization of a classical circuit. Since this circuit model obeys quantum mechanics, a quantum computer capable of efficiently running these circuits is believed to be physically realizable.

A memory consisting of [math]\displaystyle{ n }[/math] bits of information has [math]\displaystyle{ 2^n }[/math] possible states. A vector representing all memory states thus has [math]\displaystyle{ 2^n }[/math] entries (one for each state). This vector is viewed as a probability vector and represents the fact that the memory is to be found in a particular state.

In the classical view, one entry would have a value of 1 (i.e. a 100% probability of being in this state) and all other entries would be zero. In quantum mechanics, probability vectors can be generalized to density operators. The quantum state vector formalism is usually introduced first because it is conceptually simpler, and because it can be used instead of the density matrix formalism for pure states, where the whole quantum system is known.

We begin by considering a simple memory consisting of only one bit. This memory may be found in one of two states: the zero state or the one state. We may represent the state of this memory using Dirac notation so that [math]\displaystyle{ |0\rangle := \begin{pmatrix} 1 \\ 0 \end{pmatrix};\quad |1\rangle := \begin{pmatrix} 0 \\ 1 \end{pmatrix} }[/math] A quantum memory may then be found in any quantum superposition [math]\displaystyle{ |\psi\rangle }[/math] of the two classical states [math]\displaystyle{ |0\rangle }[/math] and [math]\displaystyle{ |1\rangle }[/math]: [math]\displaystyle{ |\psi\rangle := \alpha\,|0\rangle + \beta\,|1\rangle = \begin{pmatrix} \alpha \\ \beta \end{pmatrix};\quad |\alpha|^2 + |\beta|^2 = 1. }[/math] In general, the coefficients [math]\displaystyle{ \alpha }[/math] and [math]\displaystyle{ \beta }[/math] are complex numbers. In this scenario, one qubit of information is said to be encoded into the quantum memory. The state [math]\displaystyle{ |\psi\rangle }[/math] is not itself a probability vector but can be connected with a probability vector via a measurement operation. If the quantum memory is measured to determine whether the state is [math]\displaystyle{ |0\rangle }[/math] or [math]\displaystyle{ |1\rangle }[/math] (this is known as a computational basis measurement), the zero state would be observed with probability [math]\displaystyle{ |\alpha|^2 }[/math] and the one state with probability [math]\displaystyle{ |\beta|^2 }[/math]. The numbers [math]\displaystyle{ \alpha }[/math] and [math]\displaystyle{ \beta }[/math] are called quantum amplitudes.

The state of this one-qubit quantum memory can be manipulated by applying quantum logic gates, analogous to how classical memory can be manipulated with classical logic gates. One important gate for both classical and quantum computation is the NOT gate, which can be represented by a matrix [math]\displaystyle{ X := \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}. }[/math] Mathematically, the application of such a logic gate to a quantum state vector is modelled with matrix multiplication. Thus [math]\displaystyle{ X|0\rangle = |1\rangle }[/math] and [math]\displaystyle{ X|1\rangle = |0\rangle }[/math].

The mathematics of single qubit gates can be extended to operate on multi-qubit quantum memories in two important ways. One way is simply to select a qubit and apply that gate to the target qubit whilst leaving the remainder of the memory unaffected. Another way is to apply the gate to its target only if another part of the memory is in a desired state. These two choices can be illustrated using another example. The possible states of a two-qubit quantum memory are [math]\displaystyle{ |00\rangle := \begin{pmatrix} 1 \\ 0 \\ 0 \\ 0 \end{pmatrix};\quad |01\rangle := \begin{pmatrix} 0 \\ 1 \\ 0 \\ 0 \end{pmatrix};\quad |10\rangle := \begin{pmatrix} 0 \\ 0 \\ 1 \\ 0 \end{pmatrix};\quad |11\rangle := \begin{pmatrix} 0 \\ 0 \\ 0 \\ 1 \end{pmatrix}. }[/math] The CNOT gate can then be represented using the following matrix: [math]\displaystyle{ \operatorname{CNOT} := \begin{pmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 1 \\ 0 & 0 & 1 & 0 \end{pmatrix}. }[/math] As a mathematical consequence of this definition, [math]\displaystyle{ \operatorname{CNOT}|00\rangle = |00\rangle }[/math], [math]\displaystyle{ \operatorname{CNOT}|01\rangle = |01\rangle }[/math], [math]\displaystyle{ \operatorname{CNOT}|10\rangle = |11\rangle }[/math], and [math]\displaystyle{ \operatorname{CNOT}|11\rangle = |10\rangle }[/math]. In other words, the CNOT applies a NOT gate ([math]\displaystyle{ X }[/math] from before) to the second qubit if and only if the first qubit is in the state [math]\displaystyle{ |1\rangle }[/math]. If the first qubit is [math]\displaystyle{ |0\rangle }[/math], nothing is done to either qubit.

In summary, a quantum computation can be described as a network of quantum logic gates and measurements. However, any measurement can be deferred to the end of quantum computation, though this deferment may come at a computational cost, so most quantum circuits depict a network consisting only of quantum logic gates and no measurements.

Any quantum computation (which is, in the above formalism, any unitary matrix over [math]\displaystyle{ n }[/math] qubits) can be represented as a network of quantum logic gates from a fairly small family of gates. A choice of gate family that enables this construction is known as a universal gate set, since a computer that can run such circuits is a universal quantum computer. One common such set includes all single-qubit gates as well as the CNOT gate from above. This means any quantum computation can be performed by executing a sequence of single-qubit gates together with CNOT gates. Though this gate set is infinite, it can be replaced with a finite gate set by appealing to the Solovay-Kitaev theorem.

Quantum algorithms

Main page: Engineering:Quantum algorithm

Progress in finding quantum algorithms typically focuses on this quantum circuit model, though exceptions like the quantum adiabatic algorithm exist. Quantum algorithms can be roughly categorized by the type of speedup achieved over corresponding classical algorithms.[18]

Quantum algorithms that offer more than a polynomial speedup over the best known classical algorithm include Shor's algorithm for factoring and the related quantum algorithms for computing discrete logarithms, solving Pell's equation, and more generally solving the hidden subgroup problem for abelian finite groups.[18] These algorithms depend on the primitive of the quantum Fourier transform. No mathematical proof has been found that shows that an equally fast classical algorithm cannot be discovered, although this is considered unlikely.[19] Certain oracle problems like Simon's problem and the Bernstein–Vazirani problem do give provable speedups, though this is in the quantum query model, which is a restricted model where lower bounds are much easier to prove and doesn't necessarily translate to speedups for practical problems.

Other problems, including the simulation of quantum physical processes from chemistry and solid-state physics, the approximation of certain Jones polynomials, and the quantum algorithm for linear systems of equations have quantum algorithms appearing to give super-polynomial speedups and are BQP-complete. Because these problems are BQP-complete, an equally fast classical algorithm for them would imply that no quantum algorithm gives a super-polynomial speedup, which is believed to be unlikely.[20]

Some quantum algorithms, like Grover's algorithm and amplitude amplification, give polynomial speedups over corresponding classical algorithms.[18] Though these algorithms give comparably modest quadratic speedup, they are widely applicable and thus give speedups for a wide range of problems.[21] Many examples of provable quantum speedups for query problems are related to Grover's algorithm, including Brassard, Høyer, and Tapp's algorithm for finding collisions in two-to-one functions,[22] which uses Grover's algorithm, and Farhi, Goldstone, and Gutmann's algorithm for evaluating NAND trees,[23] which is a variant of the search problem.

Potential applications


Main pages: Engineering:Quantum cryptography and Post-quantum cryptography

A notable application of quantum computation is for attacks on cryptographic systems that are currently in use. Integer factorization, which underpins the security of public key cryptographic systems, is believed to be computationally infeasible with an ordinary computer for large integers if they are the product of few prime numbers (e.g., products of two 300-digit primes).[24] By comparison, a quantum computer could efficiently solve this problem using Shor's algorithm to find its factors. This ability would allow a quantum computer to break many of the cryptographic systems in use today, in the sense that there would be a polynomial time (in the number of digits of the integer) algorithm for solving the problem. In particular, most of the popular public key ciphers are based on the difficulty of factoring integers or the discrete logarithm problem, both of which can be solved by Shor's algorithm. In particular, the RSA, Diffie–Hellman, and elliptic curve Diffie–Hellman algorithms could be broken. These are used to protect secure Web pages, encrypted email, and many other types of data. Breaking these would have significant ramifications for electronic privacy and security.

Identifying cryptographic systems that may be secure against quantum algorithms is an actively researched topic under the field of post-quantum cryptography.[25][26] Some public-key algorithms are based on problems other than the integer factorization and discrete logarithm problems to which Shor's algorithm applies, like the McEliece cryptosystem based on a problem in coding theory.[25][27] Lattice-based cryptosystems are also not known to be broken by quantum computers, and finding a polynomial time algorithm for solving the dihedral hidden subgroup problem, which would break many lattice based cryptosystems, is a well-studied open problem.[28] It has been proven that applying Grover's algorithm to break a symmetric (secret key) algorithm by brute force requires time equal to roughly 2n/2 invocations of the underlying cryptographic algorithm, compared with roughly 2n in the classical case,[29] meaning that symmetric key lengths are effectively halved: AES-256 would have the same security against an attack using Grover's algorithm that AES-128 has against classical brute-force search (see Key size).

Quantum cryptography could potentially fulfill some of the functions of public key cryptography. Quantum-based cryptographic systems could, therefore, be more secure than traditional systems against quantum hacking.[30]

Search problems

The most well-known example of a problem admitting a polynomial quantum speedup is unstructured search, finding a marked item out of a list of [math]\displaystyle{ n }[/math] items in a database. This can be solved by Grover's algorithm using [math]\displaystyle{ O(\sqrt{n}) }[/math] queries to the database, quadratically fewer than the [math]\displaystyle{ \Omega(n) }[/math] queries required for classical algorithms. In this case, the advantage is not only provable but also optimal: it has been shown that Grover's algorithm gives the maximal possible probability of finding the desired element for any number of oracle lookups.

Problems that can be addressed with Grover's algorithm have the following properties:

  1. There is no searchable structure in the collection of possible answers,
  2. The number of possible answers to check is the same as the number of inputs to the algorithm, and
  3. There exists a boolean function that evaluates each input and determines whether it is the correct answer

For problems with all these properties, the running time of Grover's algorithm on a quantum computer scales as the square root of the number of inputs (or elements in the database), as opposed to the linear scaling of classical algorithms. A general class of problems to which Grover's algorithm can be applied[31] is Boolean satisfiability problem, where the database through which the algorithm iterates is that of all possible answers. An example and (possible) application of this is a password cracker that attempts to guess a password. Symmetric ciphers such as Triple DES and AES are particularly vulnerable to this kind of attack. This application of quantum computing is a major interest of government agencies.[32]

Simulation of quantum systems

Main page: Physics:Quantum simulator

Since chemistry and nanotechnology rely on understanding quantum systems, and such systems are impossible to simulate in an efficient manner classically, many believe quantum simulation will be one of the most important applications of quantum computing.[33] Quantum simulation could also be used to simulate the behavior of atoms and particles at unusual conditions such as the reactions inside a collider.[34] Quantum simulations might be used to predict future paths of particles and protons under superposition in the double-slit experiment. About 2% of the annual global energy output is used for nitrogen fixation to produce ammonia for the Haber process in the agricultural fertilizer industry while naturally occurring organisms also produce ammonia. Quantum simulations might be used to understand this process increasing production.[35]

Quantum annealing and adiabatic optimization

Quantum annealing or Adiabatic quantum computation relies on the adiabatic theorem to undertake calculations. A system is placed in the ground state for a simple Hamiltonian, which is slowly evolved to a more complicated Hamiltonian whose ground state represents the solution to the problem in question. The adiabatic theorem states that if the evolution is slow enough the system will stay in its ground state at all times through the process.

Machine learning

Main page: Engineering:Quantum machine learning

Since quantum computers can produce outputs that classical computers cannot produce efficiently, and since quantum computation is fundamentally linear algebraic, some express hope in developing quantum algorithms that can speed up machine learning tasks.[36][37] For example, the quantum algorithm for linear systems of equations, or "HHL Algorithm", named after its discoverers Harrow, Hassidim, and Lloyd, is believed to provide speedup over classical counterparts.[38][37] Some research groups have recently explored the use of quantum annealing hardware for training Boltzmann machines and deep neural networks.[39][40][41]

Computational biology

Main page: Engineering:Quantum machine learning

In the field of computational biology, computing has played a big role in solving many biological problems. One of the well-known examples would be in computational genomics and how computing has drastically reduced the time to sequence a human genome. Given how computational biology is using generic data modeling and storage, its applications to computational biology are expected to arise as well.[42]

Computer-aided drug design and generative chemistry

Main page: Engineering:Quantum machine learning

Deep generative chemistry models emerge as powerful tools to expedite drug discovery. However, the immense size and complexity of the structural space of all possible drug-like molecules pose significant obstacles, which could be overcome in the future by quantum computers. Quantum computers are naturally good for solving complex quantum many-body problems [43] and thus may be instrumental in applications involving quantum chemistry. Therefore, one can expect that quantum-enhanced generative models[44] including quantum GANs[45] may eventually be developed into ultimate generative chemistry algorithms. Hybrid architectures combining quantum computers with deep classical networks, such as Quantum Variational Autoencoders, can already be trained on commercially available annealers and used to generate novel drug-like molecular structures.[46]

Quantum supremacy

Main page: Quantum supremacy

John Preskill has introduced the term quantum supremacy to refer to the hypothetical speedup advantage that a quantum computer would have over a classical computer in a certain field.[47] Google announced in 2017 that it expected to achieve quantum supremacy by the end of the year though that did not happen. IBM said in 2018 that the best classical computers will be beaten on some practical task within about five years and views the quantum supremacy test only as a potential future benchmark.[48] Although skeptics like Gil Kalai doubt that quantum supremacy will ever be achieved,[49][50] in October 2019, a Sycamore processor created in conjunction with Google AI Quantum was reported to have achieved quantum supremacy,[51] with calculations more than 3,000,000 times as fast as those of Summit, generally considered the world's fastest computer.[52] In December 2020, a group at USTC implemented a type of Boson sampling on 76 photons with a photonic quantum computer Jiuzhang to demonstrate quantum supremacy.[53][54][55] The authors claim that a classical contemporary supercomputer would require a computational time of 600 million years to generate the number of samples their quantum processor can generate in 20 seconds.[56] Bill Unruh doubted the practicality of quantum computers in a paper published back in 1994.[57] Paul Davies argued that a 400-qubit computer would even come into conflict with the cosmological information bound implied by the holographic principle.[58]


There are a number of technical challenges in building a large-scale quantum computer.[59] Physicist David DiVincenzo has listed these requirements for a practical quantum computer:[60]

  • Physically scalable to increase the number of qubits
  • Qubits that can be initialized to arbitrary values
  • Quantum gates that are faster than decoherence time
  • Universal gate set
  • Qubits that can be read easily

Sourcing parts for quantum computers is also very difficult. Many quantum computers, like those constructed by Google and IBM, need Helium-3, a nuclear research byproduct, and special superconducting cables made only by the Japanese company Coax Co.[61]

The control of multi-qubit systems requires the generation and coordination of a large number of electrical signals with tight and deterministic timing resolution. This has led to the development of quantum controllers which enable interfacing with the qubits. Scaling these systems to support a growing number of qubits is an additional challenge.

Quantum decoherence

Main page: Physics:Quantum decoherence

One of the greatest challenges involved with constructing quantum computers is controlling or removing quantum decoherence. This usually means isolating the system from its environment as interactions with the external world cause the system to decohere. However, other sources of decoherence also exist. Examples include the quantum gates, and the lattice vibrations and background thermonuclear spin of the physical system used to implement the qubits. Decoherence is irreversible, as it is effectively non-unitary, and is usually something that should be highly controlled, if not avoided. Decoherence times for candidate systems in particular, the transverse relaxation time T2 (for NMR and MRI technology, also called the dephasing time), typically range between nanoseconds and seconds at low temperature.[62] Currently, some quantum computers require their qubits to be cooled to 20 millikelvins in order to prevent significant decoherence.[63] A 2020 study argues that ionizing radiation such as cosmic rays can nevertheless cause certain systems to decohere within milliseconds.[64]

As a result, time-consuming tasks may render some quantum algorithms inoperable, as maintaining the state of qubits for a long enough duration will eventually corrupt the superpositions.[65]

These issues are more difficult for optical approaches as the timescales are orders of magnitude shorter and an often-cited approach to overcoming them is optical pulse shaping. Error rates are typically proportional to the ratio of operating time to decoherence time, hence any operation must be completed much more quickly than the decoherence time.

As described in the Quantum threshold theorem, if the error rate is small enough, it is thought to be possible to use quantum error correction to suppress errors and decoherence. This allows the total calculation time to be longer than the decoherence time if the error correction scheme can correct errors faster than decoherence introduces them. An often cited figure for the required error rate in each gate for fault-tolerant computation is 10−3, assuming the noise is depolarizing.

Meeting this scalability condition is possible for a wide range of systems. However, the use of error correction brings with it the cost of a greatly increased number of required qubits. The number required to factor integers using Shor's algorithm is still polynomial, and thought to be between L and L2, where L is the number of digits in the number to be factored; error correction algorithms would inflate this figure by an additional factor of L. For a 1000-bit number, this implies a need for about 104 bits without error correction.[66] With error correction, the figure would rise to about 107 bits. Computation time is about L2 or about 107 steps and at 1 MHz, about 10 seconds.

A very different approach to the stability-decoherence problem is to create a topological quantum computer with anyons, quasi-particles used as threads and relying on braid theory to form stable logic gates.[67][68]

Physicist Mikhail Dyakonov has expressed skepticism of quantum computing as follows:

"So the number of continuous parameters describing the state of such a useful quantum computer at any given moment must be... about 10300... Could we ever learn to control the more than 10300 continuously variable parameters defining the quantum state of such a system? My answer is simple. No, never."[69][70]


Quantum computing models

There are a number of quantum computing models, distinguished by the basic elements in which the computation is decomposed. The four main models of practical importance are:

The quantum Turing machine is theoretically important but the physical implementation of this model is not feasible. All four models of computation have been shown to be equivalent; each can simulate the other with no more than polynomial overhead.

Physical realizations

For physically implementing a quantum computer, many different candidates are being pursued, among them (distinguished by the physical system used to realize the qubits):

The large number of candidates demonstrates that quantum computing, despite rapid progress, is still in its infancy.

Relation to computability and complexity theory

Computability theory

Any computational problem solvable by a classical computer is also solvable by a quantum computer.[16] Intuitively, this is because it is believed that all physical phenomena, including the operation of classical computers, can be described using quantum mechanics, which underlies the operation of quantum computers.

Conversely, any problem solvable by a quantum computer is also solvable by a classical computer; or more formally, any quantum computer can be simulated by a Turing machine. In other words, quantum computers provide no additional power over classical computers in terms of computability. This means that quantum computers cannot solve undecidable problems like the halting problem and the existence of quantum computers does not disprove the Church–Turing thesis.[96]

As of yet, quantum computers do not satisfy the strong Church thesis. While hypothetical machines have been realized, a universal quantum computer has yet to be physically constructed. The strong version of Church's thesis requires a physical computer, and therefore there is no quantum computer that yet satisfies the strong Church thesis.

Quantum complexity theory

Main page: Quantum complexity theory

While quantum computers cannot solve any problems that classical computers cannot already solve, it is suspected that they can solve certain problems faster than classical computers. For instance, it is known that quantum computers can efficiently factor integers, while this is not believed to be the case for classical computers.

The class of problems that can be efficiently solved by a quantum computer with bounded error is called BQP, for "bounded error, quantum, polynomial time". More formally, BQP is the class of problems that can be solved by a polynomial-time quantum Turing machine with an error probability of at most 1/3. As a class of probabilistic problems, BQP is the quantum counterpart to BPP ("bounded error, probabilistic, polynomial time"), the class of problems that can be solved by polynomial-time probabilistic Turing machines with bounded error.[97] It is known that BPP[math]\displaystyle{ \subseteq }[/math]BQP and is widely suspected that BQP[math]\displaystyle{ \subsetneq }[/math]BPP, which intuitively would mean that quantum computers are more powerful than classical computers in terms of time complexity.[98]

The suspected relationship of BQP to several classical complexity classes.[20]

The exact relationship of BQP to P, NP, and PSPACE is not known. However, it is known that P[math]\displaystyle{ \subseteq }[/math]BQP[math]\displaystyle{ \subseteq }[/math]PSPACE; that is, all problems that can be efficiently solved by a deterministic classical computer can also be efficiently solved by a quantum computer, and all problems that can be efficiently solved by a quantum computer can also be solved by a deterministic classical computer with polynomial space resources. It is further suspected that BQP is a strict superset of P, meaning there are problems that are efficiently solvable by quantum computers that are not efficiently solvable by deterministic classical computers. For instance, integer factorization and the discrete logarithm problem are known to be in BQP and are suspected to be outside of P. On the relationship of BQP to NP, little is known beyond the fact that some NP problems that are believed not to be in P are also in BQP (integer factorization and the discrete logarithm problem are both in NP, for example). It is suspected that NP[math]\displaystyle{ \nsubseteq }[/math]BQP; that is, it is believed that there are efficiently checkable problems that are not efficiently solvable by a quantum computer. As a direct consequence of this belief, it is also suspected that BQP is disjoint from the class of NP-complete problems (if an NP-complete problem were in BQP, then it would follow from NP-hardness that all problems in NP are in BQP).[99]

The relationship of BQP to the basic classical complexity classes can be summarized as follows:

[math]\displaystyle{ \mathsf{P \subseteq BPP \subseteq BQP \subseteq PP \subseteq PSPACE} }[/math]

It is also known that BQP is contained in the complexity class #P (or more precisely in the associated class of decision problems P#P),[99] which is a subclass of PSPACE.

It has been speculated that further advances in physics could lead to even faster computers. For instance, it has been shown that a non-local hidden variable quantum computer based on Bohmian Mechanics could implement a search of an [math]\displaystyle{ N }[/math]-item database in at most [math]\displaystyle{ O(\sqrt[3]{N}) }[/math] steps, a slight speedup over Grover's algorithm, which runs in [math]\displaystyle{ O(\sqrt{N}) }[/math] steps. Note, however, that neither search method would allow quantum computers to solve NP-complete problems in polynomial time.[100] Theories of quantum gravity, such as M-theory and loop quantum gravity, may allow even faster computers to be built. However, defining computation in these theories is an open problem due to the problem of time; that is, within these physical theories there is currently no obvious way to describe what it means for an observer to submit input to a computer at one point in time and then receive output at a later point in time.[101][102]

See also


  1. 1.0 1.1 The National Academies of Sciences, Engineering, and Medicine (2019). Quantum Computing : Progress and Prospects (2018). Washington, DC: National Academies Press. p. I-5. doi:10.17226/25196. ISBN 978-0-309-47969-1. OCLC 1081001288. 
  2. "Scopus for Corporate Research & Development". https://www.elsevier.com/solutions/scopus/who-uses-scopus/research-and-development#trends. 
  3. Benioff, Paul (1980). "The computer as a physical system: A microscopic quantum mechanical Hamiltonian model of computers as represented by Turing machines". Journal of Statistical Physics 22 (5): 563–591. doi:10.1007/bf01011339. Bibcode1980JSP....22..563B. 
  4. Feynman, Richard (June 1982). "Simulating Physics with Computers". International Journal of Theoretical Physics 21 (6/7): 467–488. doi:10.1007/BF02650179. Bibcode1982IJTP...21..467F. https://people.eecs.berkeley.edu/~christos/classics/Feynman.pdf. Retrieved 28 February 2019. 
  5. Manin, Yu. I. (1980) (in ru). Vychislimoe i nevychislimoe. Sov.Radio. pp. 13–15. http://publ.lib.ru/ARCHIVES/M/MANIN_Yuriy_Ivanovich/Manin_Yu.I._Vychislimoe_i_nevychislimoe.(1980).%5bdjv-fax%5d.zip. Retrieved 4 March 2013. 
  6. Mermin, David (28 March 2006). "Breaking RSA Encryption with a Quantum Computer: Shor's Factoring Algorithm". Physics 481-681 Lecture Notes (Cornell University). http://people.ccmr.cornell.edu/~mermin/qcomp/chap3.pdf. 
  7. Preskill, John (2018). "Quantum Computing in the NISQ era and beyond". Quantum 2: 79. doi:10.22331/q-2018-08-06-79. 
  8. Gibney, Elizabeth (2 October 2019). "Quantum gold rush: the private funding pouring into quantum start-ups". Nature 574 (7776): 22–24. doi:10.1038/d41586-019-02935-4. PMID 31578480. Bibcode2019Natur.574...22G. 
  9. Rodrigo, Chris Mills (12 February 2020). "Trump budget proposal boosts funding for artificial intelligence, quantum computing". The Hill. https://thehill.com/policy/technology/482402-trump-budget-proposal-boosts-funding-for-artificial-intelligence-quantum. 
  10. Gibney, Elizabeth (2019-10-23). "Hello quantum world! Google publishes landmark quantum supremacy claim" (in en). Nature 574 (7779): 461–462. doi:10.1038/d41586-019-03213-z. https://www.nature.com/articles/d41586-019-03213-z. 
  11. Aaronson, Scott (2019-10-30). "Opinion | Why Google’s Quantum Supremacy Milestone Matters" (in en-US). The New York Times. ISSN 0362-4331. https://www.nytimes.com/2019/10/30/opinion/google-quantum-computer-sycamore.html. 
  12. "On 'Quantum Supremacy'" (in en-US). 22 October 2019. https://www.ibm.com/blogs/research/2019/10/on-quantum-supremacy/. 
  13. Pan, Feng; Zhang, Pan (2021-03-04). "Simulating the Sycamore quantum supremacy circuits". arXiv:2103.03074 [physics, physics:quant-ph]. http://arxiv.org/abs/2103.03074. 
  14. Franklin, Diana; Chong, Frederic T. (2004). "Challenges in Reliable Quantum Computing". Nano, Quantum and Molecular Computing. pp. 247–266. doi:10.1007/1-4020-8068-9_8. ISBN 1-4020-8067-0. 
  15. Pakkin, Scott; Coles, Patrick (10 June 2019). "The Problem with Quantum Computers". Scientific American. https://blogs.scientificamerican.com/observations/the-problem-with-quantum-computers/. 
  16. 16.0 16.1 Nielsen, p. 29
  17. Nielsen, Michael A.; Chuang, Isaac L. (2010). Quantum Computation and Quantum Information: 10th Anniversary Edition. Cambridge: Cambridge University Press. doi:10.1017/cbo9780511976667. ISBN 9780511976667. http://cds.cern.ch/record/465953. 
  18. 18.0 18.1 18.2 Quantum Algorithm Zoo – Stephen Jordan's Homepage
  19. Schiller, Jon (19 June 2009). Quantum Computers. ISBN 9781439243497. https://books.google.com/books?id=l217ma2sWkoC&pg=PA11. 
  20. 20.0 20.1 Nielsen, p. 42
  21. Nielsen, p. 7
  22. Brassard, Gilles; Høyer, Peter; Tapp, Alain (2016), Kao, Ming-Yang, ed. (in en), Quantum Algorithm for the Collision Problem, New York, NY: Springer, pp. 1662–1664, doi:10.1007/978-1-4939-2864-4_304, ISBN 978-1-4939-2864-4, https://doi.org/10.1007/978-1-4939-2864-4_304, retrieved 6 December 2020 
  23. Farhi, Edward; Goldstone, Jeffrey; Gutmann, Sam (23 December 2008). "A Quantum Algorithm for the Hamiltonian NAND Tree" (in EN). Theory of Computing 4 (1): 169–190. doi:10.4086/toc.2008.v004a008. ISSN 1557-2862. http://www.theoryofcomputing.org/articles/v004a008. 
  24. Lenstra, Arjen K. (2000). "Integer Factoring". Designs, Codes and Cryptography 19 (2/3): 101–128. doi:10.1023/A:1008397921377. http://sage.math.washington.edu/edu/124/misc/arjen_lenstra_factoring.pdf. 
  25. 25.0 25.1 Bernstein, Daniel J. (2009). "Introduction to post-quantum cryptography". Post-Quantum Cryptography. 549. 1–14. doi:10.1007/978-3-540-88702-7_1. ISBN 978-3-540-88701-0. 
  26. See also pqcrypto.org, a bibliography maintained by Daniel J. Bernstein and Tanja Lange on cryptography not known to be broken by quantum computing.
  27. McEliece, R. J. (January 1978). "A Public-Key Cryptosystem Based On Algebraic Coding Theory". DSNPR 44: 114–116. Bibcode1978DSNPR..44..114M. http://ipnpr.jpl.nasa.gov/progress_report2/42-44/44N.PDF. 
  28. Kobayashi, H.; Gall, F.L. (2006). "Dihedral Hidden Subgroup Problem: A Survey". Information and Media Technologies 1 (1): 178–185. doi:10.2197/ipsjdc.1.470. 
  29. Bennett, Charles H.; Bernstein, Ethan; Brassard, Gilles; Vazirani, Umesh (October 1997). "Strengths and Weaknesses of Quantum Computing". SIAM Journal on Computing 26 (5): 1510–1523. doi:10.1137/s0097539796300933. Bibcode1997quant.ph..1001B. 
  30. Katwala, Amit (5 March 2020). "Quantum computers will change the world (if they work)". Wired UK. https://www.wired.co.uk/article/quantum-computing-explained. 
  31. Ambainis, Ambainis (June 2004). "Quantum search algorithms". ACM SIGACT News 35 (2): 22–35. doi:10.1145/992287.992296. Bibcode2005quant.ph..4012A. 
  32. Rich, Steven; Gellman, Barton (1 February 2014). "NSA seeks to build quantum computer that could crack most types of encryption". The Washington Post. https://www.washingtonpost.com/world/national-security/nsa-seeks-to-build-quantum-computer-that-could-crack-most-types-of-encryption/2014/01/02/8fff297e-7195-11e3-8def-a33011492df2_story.html. 
  33. Norton, Quinn (15 February 2007). "The Father of Quantum Computing". Wired. http://archive.wired.com/science/discoveries/news/2007/02/72734. 
  34. Ambainis, Andris (Spring 2014). "What Can We Do with a Quantum Computer?". Institute for Advanced Study. http://www.ias.edu/ias-letter/ambainis-quantum-computing. 
  35. "Lunch & Learn: Quantum Computing". Sibos TV. 21 November 2018. https://www.youtube.com/watch?v=7susESgnDv8. 
  36. Biamonte, Jacob; Wittek, Peter; Pancotti, Nicola; Rebentrost, Patrick; Wiebe, Nathan; Lloyd, Seth (September 2017). "Quantum machine learning" (in en). Nature 549 (7671): 195–202. doi:10.1038/nature23474. ISSN 0028-0836. PMID 28905917. Bibcode2017Natur.549..195B. http://www.nature.com/articles/nature23474. 
  37. 37.0 37.1 Preskill, John (6 August 2018). "Quantum Computing in the NISQ era and beyond" (in en-GB). Quantum 2: 79. doi:10.22331/q-2018-08-06-79. https://quantum-journal.org/papers/q-2018-08-06-79/. 
  38. Harrow, Aram; Hassidim, Avinatan; Lloyd, Seth (2009). "Quantum algorithm for solving linear systems of equations". Physical Review Letters 103 (15): 150502. doi:10.1103/PhysRevLett.103.150502. PMID 19905613. Bibcode2009PhRvL.103o0502H. 
  39. Benedetti, Marcello; Realpe-Gómez, John; Biswas, Rupak; Perdomo-Ortiz, Alejandro (9 August 2016). "Estimation of effective temperatures in quantum annealers for sampling applications: A case study with possible applications in deep learning". Physical Review A 94 (2): 022308. doi:10.1103/PhysRevA.94.022308. Bibcode2016PhRvA..94b2308B. 
  40. Ajagekar, Akshay; You, Fengqi (5 December 2020). "Quantum computing assisted deep learning for fault detection and diagnosis in industrial process systems" (in en). Computers & Chemical Engineering 143: 107119. doi:10.1016/j.compchemeng.2020.107119. ISSN 0098-1354. http://www.sciencedirect.com/science/article/pii/S0098135420308322. 
  41. Ajagekar, Akshay; You, Fengqi (2021-12-01). "Quantum computing based hybrid deep learning for fault diagnosis in electrical power systems" (in en). Applied Energy 303: 117628. doi:10.1016/j.apenergy.2021.117628. ISSN 0306-2619. https://www.sciencedirect.com/science/article/pii/S030626192100996X. 
  42. Outeiral, Carlos; Strahm, Martin; Morris, Garrett; Benjamin, Simon; Deane, Charlotte; Shi, Jiye (2021). "The prospects of quantum computing in computational molecular biology". WIREs Computational Molecular Science 11. doi:10.1002/wcms.1481. 
  43. Lloyd, S. (23 August 1996). "Universal Quantum Simulators". Science 273 (5278): 1073–1078. doi:10.1126/science.273.5278.1073. 
  44. Gao, Xun; Anschuetz, Eric R.; Wang, Sheng-Tao; Cirac, J. Ignacio; Lukin, Mikhail D. (20 January 2021). "Enhancing Generative Models via Quantum Correlations". arXiv:2101.08354 [cond-mat, physics:quant-ph, stat]. https://arxiv.org/abs/2101.08354. 
  45. Li, Junde; Topaloglu, Rasit; Ghosh, Swaroop (9 January 2021). "Quantum Generative Models for Small Molecule Drug Discovery". arXiv:2101.03438 [quant-ph]. https://arxiv.org/abs/2101.03438. 
  46. Gircha, A. I.; Boev, A. S.; Avchaciov, K.; Fedichev, P. O.; Fedorov, A. K. (26 August 2021). "Training a discrete variational autoencoder for generative chemistry and drug design on a quantum annealer". arXiv:2108.11644 [quant-ph, q-bio]. https://arxiv.org/abs/2108.11644. 
  47. Boixo, Sergio; Isakov, Sergei V.; Smelyanskiy, Vadim N.; Babbush, Ryan; Ding, Nan; Jiang, Zhang; Bremner, Michael J.; Martinis, John M. et al. (2018). "Characterizing Quantum Supremacy in Near-Term Devices". Nature Physics 14 (6): 595–600. doi:10.1038/s41567-018-0124-x. Bibcode2018NatPh..14..595B. 
  48. Savage, Neil (5 July 2017). "Quantum Computers Compete for "Supremacy"". Scientific American. https://www.scientificamerican.com/article/quantum-computers-compete-for-supremacy/. 
  49. "Quantum Supremacy and Complexity". 23 April 2016. https://rjlipton.wordpress.com/2016/04/22/quantum-supremacy-and-complexity/. 
  50. Kalai, Gil. "The Quantum Computer Puzzle". AMS. https://www.ams.org/journals/notices/201605/rnoti-p508.pdf. 
  51. Arute, Frank; Arya, Kunal; Babbush, Ryan; Bacon, Dave; Bardin, Joseph C.; Barends, Rami; Biswas, Rupak; Boixo, Sergio et al. (23 October 2019). "Quantum supremacy using a programmable superconducting processor". Nature 574 (7779): 505–510. doi:10.1038/s41586-019-1666-5. PMID 31645734. Bibcode2019Natur.574..505A. 
  52. "Google researchers have reportedly achieved 'quantum supremacy'". https://www.technologyreview.com/f/614416/google-researchers-have-reportedly-achieved-quantum-supremacy/. 
  53. Ball, Philip (2020-12-03). "Physicists in China challenge Google's 'quantum advantage'" (in en). Nature 588 (7838): 380. doi:10.1038/d41586-020-03434-7. PMID 33273711. Bibcode2020Natur.588..380B. 
  54. Garisto, Daniel. "Light-based Quantum Computer Exceeds Fastest Classical Supercomputers" (in en). https://www.scientificamerican.com/article/light-based-quantum-computer-exceeds-fastest-classical-supercomputers/. 
  55. Conover, Emily (2020-12-03). "The new light-based quantum computer Jiuzhang has achieved quantum supremacy" (in en-US). https://www.sciencenews.org/article/new-light-based-quantum-computer-jiuzhang-supremacy. 
  56. Zhong, Han-Sen; Wang, Hui; Deng, Yu-Hao; Chen, Ming-Cheng; Peng, Li-Chao; Luo, Yi-Han; Qin, Jian; Wu, Dian et al. (2020-12-03). "Quantum computational advantage using photons" (in en). Science 370 (6523): 1460–1463. doi:10.1126/science.abe8770. ISSN 0036-8075. PMID 33273064. Bibcode2020Sci...370.1460Z. https://science.sciencemag.org/content/early/2020/12/02/science.abe8770. 
  57. Unruh, Bill (1995). "Maintaining coherence in Quantum Computers". Physical Review A 51 (2): 992–997. doi:10.1103/PhysRevA.51.992. PMID 9911677. Bibcode1995PhRvA..51..992U. 
  58. Davies, Paul. "The implications of a holographic universe for quantum information science and the nature of physical law". Macquarie University. https://arxiv.org/ftp/quant-ph/papers/0703/0703041.pdf. 
  59. Dyakonov, Mikhail (15 November 2018). "The Case Against Quantum Computing". IEEE Spectrum. https://spectrum.ieee.org/computing/hardware/the-case-against-quantum-computing. 
  60. DiVincenzo, David P. (13 April 2000). "The Physical Implementation of Quantum Computation". Fortschritte der Physik 48 (9–11): 771–783. doi:10.1002/1521-3978(200009)48:9/11<771::AID-PROP771>3.0.CO;2-E. Bibcode2000ForPh..48..771D. 
  61. Giles, Martin (17 January 2019). "We'd have more quantum computers if it weren't so hard to find the damn cables". MIT Technology Review. https://www.technologyreview.com/s/612760/quantum-computers-component-shortage/. 
  62. DiVincenzo, David P. (1995). "Quantum Computation". Science 270 (5234): 255–261. doi:10.1126/science.270.5234.255. Bibcode1995Sci...270..255D.  (Subscription content?)
  63. Jones, Nicola (19 June 2013). "Computing: The quantum company". Nature 498 (7454): 286–288. doi:10.1038/498286a. PMID 23783610. Bibcode2013Natur.498..286J. 
  64. Vepsäläinen, Antti P.; Karamlou, Amir H.; Orrell, John L.; Dogra, Akshunna S.; Loer, Ben et al. (August 2020). "Impact of ionizing radiation on superconducting qubit coherence" (in en). Nature 584 (7822): 551–556. doi:10.1038/s41586-020-2619-8. ISSN 1476-4687. PMID 32848227. Bibcode2020Natur.584..551V. https://www.nature.com/articles/s41586-020-2619-8. 
  65. Amy, Matthew; Matteo, Olivia; Gheorghiu, Vlad; Mosca, Michele; Parent, Alex; Schanck, John (30 November 2016). "Estimating the cost of generic quantum pre-image attacks on SHA-2 and SHA-3". arXiv:1603.09383 [quant-ph].
  66. Dyakonov, M. I. (14 October 2006). "Is Fault-Tolerant Quantum Computation Really Possible?". Future Trends in Microelectronics. Up the Nano Creek: 4–18. Bibcode2006quant.ph.10117D. 
  67. "Topological quantum computation". Bulletin of the American Mathematical Society 40 (1): 31–38. 2003. doi:10.1090/S0273-0979-02-00964-3. 
  68. Monroe, Don (1 October 2008). "Anyons: The breakthrough quantum computing needs?". New Scientist. https://www.newscientist.com/channel/fundamentals/mg20026761.700-anyons-the-breakthrough-quantum-computing-needs.html. 
  69. Dyakonov, Mikhail. "The Case Against Quantum Computing". https://spectrum.ieee.org/computing/hardware/the-case-against-quantum-computing. 
  70. Dyakonov, Mikhail (24 March 2020). Will We Ever Have a Quantum Computer?. Springer. ISBN 9783030420185. https://www.springer.com/gp/book/9783030420185. Retrieved 22 May 2020. 
  71. Das, A.; Chakrabarti, B. K. (2008). "Quantum Annealing and Analog Quantum Computation". Rev. Mod. Phys. 80 (3): 1061–1081. doi:10.1103/RevModPhys.80.1061. Bibcode2008RvMP...80.1061D. 
  72. Nayak, Chetan; Simon, Steven; Stern, Ady; Das Sarma, Sankar (2008). "Nonabelian Anyons and Quantum Computation". Reviews of Modern Physics 80 (3): 1083–1159. doi:10.1103/RevModPhys.80.1083. Bibcode2008RvMP...80.1083N. 
  73. Clarke, John; Wilhelm, Frank K. (18 June 2008). "Superconducting quantum bits". Nature 453 (7198): 1031–1042. doi:10.1038/nature07128. PMID 18563154. Bibcode2008Natur.453.1031C. https://www.semanticscholar.org/paper/7ee1053ce63f33a62f2ea555547c514ce5f21054. 
  74. Kaminsky, William M.; Lloyd, Seth; Orlando, Terry P. (12 March 2004). Scalable Superconducting Architecture for Adiabatic Quantum Computation. Bibcode2004quant.ph..3090K. 
  75. Khazali, Mohammadsadegh; Mølmer, Klaus (11 June 2020). "Fast Multiqubit Gates by Adiabatic Evolution in Interacting Excited-State Manifolds of Rydberg Atoms and Superconducting Circuits". Physical Review X 10 (2): 021054. doi:10.1103/PhysRevX.10.021054. Bibcode2020PhRvX..10b1054K. 
  76. Henriet, Loic; Beguin, Lucas; Signoles, Adrien; Lahaye, Thierry; Browaeys, Antoine; Reymond, Georges-Olivier; Jurczak, Christophe (22 June 2020). "Quantum computing with neutral atoms". Quantum 4: 327. doi:10.22331/q-2020-09-21-327. 
  77. Imamog¯lu, A.; Awschalom, D. D.; Burkard, G.; DiVincenzo, D. P.; Loss, D.; Sherwin, M.; Small, A. (15 November 1999). "Quantum Information Processing Using Quantum Dot Spins and Cavity QED". Physical Review Letters 83 (20): 4204–4207. doi:10.1103/PhysRevLett.83.4204. Bibcode1999PhRvL..83.4204I. 
  78. Fedichkin, L.; Yanchenko, M.; Valiev, K. A. (June 2000). "Novel coherent quantum bit using spatial quantization levels in semiconductor quantum dot". Quantum Computers and Computing 1: 58. Bibcode2000quant.ph..6097F. 
  79. Ivády, Viktor; Davidsson, Joel; Delegan, Nazar; Falk, Abram L.; Klimov, Paul V.; Whiteley, Samuel J.; Hruszkewycz, Stephan O.; Holt, Martin V. et al. (6 December 2019). "Stabilization of point-defect spin qubits by quantum wells". Nature Communications 10 (1): 5607. doi:10.1038/s41467-019-13495-6. PMID 31811137. Bibcode2019NatCo..10.5607I. 
  80. "Scientists Discover New Way to Get Quantum Computing to Work at Room Temperature". interestingengineering.com. 24 April 2020. https://interestingengineering.com/scientists-discover-new-way-to-get-quantum-computing-to-work-at-room-temperature. 
  81. Bertoni, A.; Bordone, P.; Brunetti, R.; Jacoboni, C.; Reggiani, S. (19 June 2000). "Quantum Logic Gates based on Coherent Electron Transport in Quantum Wires". Physical Review Letters 84 (25): 5912–5915. doi:10.1103/PhysRevLett.84.5912. PMID 10991086. Bibcode2000PhRvL..84.5912B. 
  82. Ionicioiu, Radu; Amaratunga, Gehan; Udrea, Florin (20 January 2001). "Quantum Computation with Ballistic Electrons". International Journal of Modern Physics B 15 (2): 125–133. doi:10.1142/S0217979201003521. Bibcode2001IJMPB..15..125I. 
  83. Ramamoorthy, A; Bird, J P; Reno, J L (11 July 2007). "Using split-gate structures to explore the implementation of a coupled-electron-waveguide qubit scheme". Journal of Physics: Condensed Matter 19 (27): 276205. doi:10.1088/0953-8984/19/27/276205. Bibcode2007JPCM...19A6205R. 
  84. Leuenberger, Michael N.; Loss, Daniel (April 2001). "Quantum computing in molecular magnets". Nature 410 (6830): 789–793. doi:10.1038/35071024. PMID 11298441. Bibcode2001Natur.410..789L. 
  85. Harneit, Wolfgang (27 February 2002). "Fullerene-based electron-spin quantum computer". Physical Review A 65 (3): 032322. doi:10.1103/PhysRevA.65.032322. Bibcode2002PhRvA..65c2322H. https://www.researchgate.net/publication/257976907. 
  86. Igeta, K.; Yamamoto, Y. (1988). "Quantum mechanical computers with single atom and photon fields". International Quantum Electronics Conference. https://www.osapublishing.org/abstract.cfm?uri=IQEC-1988-TuI4. 
  87. Chuang, I.L.; Yamamoto, Y. (1995). "Simple quantum computer". Physical Review A 52 (5): 3489–3496. doi:10.1103/PhysRevA.52.3489. PMID 9912648. Bibcode1995PhRvA..52.3489C. 
  88. Knill, G. J.; Laflamme, R.; Milburn, G. J. (2001). "A scheme for efficient quantum computation with linear optics". Nature 409 (6816): 46–52. doi:10.1038/35051009. PMID 11343107. Bibcode2001Natur.409...46K. https://www.semanticscholar.org/paper/054b680165a7325569ca6e63028ca9cee7f3ac9a. 
  89. Nizovtsev, A. P. (August 2005). "A quantum computer based on NV centers in diamond: Optically detected nutations of single electron and nuclear spins". Optics and Spectroscopy 99 (2): 248–260. doi:10.1134/1.2034610. Bibcode2005OptSp..99..233N. https://www.semanticscholar.org/paper/a7598ca24265e5537f14dc61b7c3a1d5b5953162. 
  90. Dutt, M. V. G.; Childress, L.; Jiang, L.; Togan, E.; Maze, J.; Jelezko, F.; Zibrov, A. S.; Hemmer, P. R. et al. (1 June 2007). "Quantum Register Based on Individual Electronic and Nuclear Spin Qubits in Diamond". Science 316 (5829): 1312–1316. doi:10.1126/science.1139831. PMID 17540898. Bibcode2007Sci...316.....D. 
  91. Neumann, P. et al. (6 June 2008). "Multipartite Entanglement Among Single Spins in Diamond". Science 320 (5881): 1326–1329. doi:10.1126/science.1157233. PMID 18535240. Bibcode2008Sci...320.1326N. 
  92. Anderlini, Marco; Lee, Patricia J.; Brown, Benjamin L.; Sebby-Strabley, Jennifer; Phillips, William D.; Porto, J. V. (July 2007). "Controlled exchange interaction between pairs of neutral atoms in an optical lattice". Nature 448 (7152): 452–456. doi:10.1038/nature06011. PMID 17653187. Bibcode2007Natur.448..452A. 
  93. Ohlsson, N.; Mohan, R. K.; Kröll, S. (1 January 2002). "Quantum computer hardware based on rare-earth-ion-doped inorganic crystals". Opt. Commun. 201 (1–3): 71–77. doi:10.1016/S0030-4018(01)01666-2. Bibcode2002OptCo.201...71O. 
  94. Longdell, J. J.; Sellars, M. J.; Manson, N. B. (23 September 2004). "Demonstration of conditional quantum phase shift between ions in a solid". Phys. Rev. Lett. 93 (13): 130503. doi:10.1103/PhysRevLett.93.130503. PMID 15524694. Bibcode2004PhRvL..93m0503L. 
  95. Náfrádi, Bálint; Choucair, Mohammad; Dinse, Klaus-Peter; Forró, László (18 July 2016). "Room temperature manipulation of long lifetime spins in metallic-like carbon nanospheres". Nature Communications 7 (1): 12232. doi:10.1038/ncomms12232. PMID 27426851. Bibcode2016NatCo...712232N. 
  96. Nielsen, p. 126
  97. Nielsen, p. 41
  98. Nielsen, p. 201
  99. 99.0 99.1 Bernstein, Ethan; Vazirani, Umesh (1997). "Quantum Complexity Theory". SIAM Journal on Computing 26 (5): 1411–1473. doi:10.1137/S0097539796300921. http://www.cs.berkeley.edu/~vazirani/bv.ps. 
  100. Aaronson, Scott. "Quantum Computing and Hidden Variables". http://www.scottaaronson.com/papers/qchvpra.pdf. 
  101. Aaronson, Scott (2005). "NP-complete Problems and Physical Reality". ACM SIGACT News 2005. Bibcode2005quant.ph..2072A.  See section 7 "Quantum Gravity": "[…] to anyone who wants a test or benchmark for a favorite quantum gravity theory,[author's footnote: That is, one without all the bother of making numerical predictions and comparing them to observation] let me humbly propose the following: can you define Quantum Gravity Polynomial-Time? […] until we can say what it means for a 'user' to specify an 'input' and ‘later' receive an 'output'—there is no such thing as computation, not even theoretically." (emphasis in original)
  102. "D-Wave Systems sells its first Quantum Computing System to Lockheed Martin Corporation". D-Wave. 25 May 2011. http://www.dwavesys.com/en/pressreleases.html#lm_2011. 

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