Physical neural network
A physical neural network is a type of artificial neural network in which an electrically adjustable resistance material is used to emulate the function of a neural synapse. "Physical" neural network is used to emphasize the reliance on physical hardware used to emulate neurons as opposed to software-based approaches which simulate neural networks. More generally the term is applicable to other artificial neural networks in which a memristor or other electrically adjustable resistance material is used to emulate a neural synapse.
Types of physical neural networks
In the 1960s Bernard Widrow and Ted Hoff developed ADALINE (Adaptive Linear Neuron) which used electrochemical cells called memistors (memory resistors) to emulate synapses of an artificial neuron. The memistors were implemented as 3-terminal devices operating based on the reversible electroplating of copper such that the resistance between two of the terminals is controlled by the integral of the current applied via the third terminal. The ADALINE circuitry was briefly commercialized by the Memistor Corporation in the 1960s enabling some applications in pattern recognition. However, since the memistors were not fabricated using integrated circuit fabrication techniques the technology was not scalable and was eventually abandoned as solid state electronics became mature.
Physical Neural Network
Alex Nugent describes a physical neural network as one or more nonlinear neuron-like nodes used to sum signals and nanoconnections formed from nanoparticles, nanowires, or nanotubes which determine the signal strength input to the nodes. Alignment or self-assembly of the nanoconnections is determined by the history of the applied electric field performing a function analogous to neural synapses. Numerous applications for such physical neural networks are possible. For example, a temporal summation device  can be composed of one or more nanoconnections having an input and an output thereof, wherein an input signal provided to the input causes one or more of the nanoconnection to experience an increase in connection strength thereof over time. Another example of a physical neural network is taught by U.S. Patent No. 7,039,619 entitled "Utilized nanotechnology apparatus using a neural network, a solution and a connection gap," which issued to Alex Nugent by the U.S. Patent & Trademark Office on May 2, 2006.
A further application of physical neural network is shown in U.S. Patent No. 7,412,428 entitled "Application of hebbian and anti-hebbian learning to nanotechnology-based physical neural networks," which issued on August 12, 2008.
Nugent and Molter have shown that universal computing and general-purpose machine learning are possible from operations available through simple memristive circuits operating the AHaH plasticity rule. More recently, it has been argued that also complex networks of purely memristive circuits can serve as neural networks.  
Phase change neural network
In 2002, Stanford Ovshinsky described an analog neural computing medium in which phase change material has the ability to cumulatively respond to multiple input signals. An electrical alteration of the resistance of the phase change material is used to control the weighting of the input signals.
Memristive neural network
Greg Snider of HP Labs describes a system of cortical computing with memristive nanodevices. The memristors (memory resistors) are implemented by thin film materials in which the resistance is electrically tuned via the transport of ions or oxygen vacancies within the film. DARPA's SyNAPSE project has funded IBM Research and HP Labs, in collaboration with the Boston University Department of Cognitive and Neural Systems (CNS), to develop neuromorphic architectures which may be based on memristive systems .
- AI accelerator
- Brain simulation
- Neuromorphic engineering
- Optical neural network
- Quantum neural network
- Widrow, B.; Pierce, W. H.; Angell, J.B. (1961), "Birth, Life, and Death in Microelectronic Systems" (PDF), Technical Report No. 1552-2/1851-1
- Anderson, James; Rosenfeld, Edward (1998), Talking Nets: An Oral History of Neural Networks, MIT Press, ISBN 978-0-262-01167-9
- U.S. Patent 6,889,216
- U.S. Known Patents
- U.S. Patent No. 7,028,017
- Caravelli, F.; Traversa, F. L.; Di Ventra, M. (2017). "The complex dynamics of memristive circuits: analytical results and universal slow relaxation". Physical Review E. 95 (2): 022140. arXiv:1608.08651. Bibcode:2017PhRvE..95b2140C. doi:10.1103/PhysRevE.95.022140.
- Caravelli, Francesco (2019). "Asymptotic behavior of memristive circuits". Entropy. 21 (8): 789. arXiv:1712.07046. doi:10.3390/e21080789.
- U.S. Patent 6,999,953
- Snider, Greg (2008), "Cortical computing with memristive nanodevices", Sci-DAC Review, 10: 58–65
- Caravelli, Francesco; Carbajal, Juan Pablo (2018), "Memristors for the curious outsiders", Technologies, 6(4) (118), doi:10.3390/technologies6040118