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The sentience quotient concept was introduced by Robert A. Freitas Jr. in the late 1970s. It defines sentience as the relationship between the information processing rate (bit/s) of each individual processing unit (neuron), the weight/size of a single unit and the total number of processing units (expressed as mass).
This is a non-standard usage of the word "sentience" which in standard usage relates to an individual organism's capacity to perceive the world subjectively (The word "sentience" is derived from the Latin "sentire" meaning "to feel" and is closely related to the word "sentiment." Intelligence or cognitive capacity is better denoted by the word "sapience" and not "sentience.")
The potential and total processing capacity of a brain, based on the amount of neurons and the processing rate and mass of a single one, combined with its design (myelin coating and specialized areas and so on) and programming, lays the foundations of the brain level of the individual. Not just in humans, but in all organisms, even artificial ones such as computers (although their "brain" is not based on neurons).
SQ's of various entities 
According to this equation, humans have an SQ of +13. A human neuron has an average mass of about 10−10 kg and one neuron can process 1000-3000 bit/s, giving us an SQ rating of +13. All other animals with a nervous system (or all "neuronal sentience") from insects to mammals, cluster within several points of the human value. Plants cluster around an SQ of −2. Carnivorous plants have an SQ of +1, while the Cray-1 had an SQ of +9. IBM Watson, which achieves 80 TFLOPS (using 64-bit words) and consists of 90 IBM Power 750 weighing approximately 100 kilograms (220 lb) each, has an SQ in the range of +11—+12.
The theoretical superconducting Josephson junction electronic gates could weigh 10−12 kg and process 1011 bits/s, giving an "electronic sentience" made of these components an SQ of +23.
SQ spectrum 
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SQ is not limited to sentient beings so far encountered by humans, but extends to all possible sentiences, defining an expected range.
The lowest SQ possible would have just one neuron with the mass of the whole universe (1052 kg) and require a time equal to the age of the universe (1018 seconds) to process just one bit, giving a minimum SQ of -70. It has been argued that under multiverse theory, an infinitely low SQ is theoretically possible, though Freitas is not known to have commented on this possibility himself.
The fundamental upper limit to brain efficiency is imposed by the laws of quantum mechanics: all information, to be acted upon, must be represented physically and be carried by matter-energy "markers." According to the Uncertainty Principle in quantum mechanics, the lower limit for the accuracy with which energy can be measured—the minimum measurable energy level for a marker carrying one bit–is given by Planck's constant h divided by T, the duration of the measurement. If one energy level is used to represent one bit, then the maximum bit rate of a brain is equal to the total energy available E ( = mc02) for representing information, divided by the minimum measurable energy per bit (h/T) divided by the minimum time required for readout (T): mc02/h = 1050 (bit/s)/kg. Hence the maximum possible SQ is +50.
Implications for interspecies communication 
According to Freitas, an alien civilization having their consciousness running on non-biological hardware (such as quantum-mechanical circuits) could have an SQ of 23+, 10 orders of magnitude more than the human SQ. Freitas states that such a gap in SQ "may affect our ability, and the desirability, of communicating with extraterrestrial beings...It may be that there is a minimum SQ "communication gap," an intellectual distance beyond which no two entities can meaningfully converse."  For example, an alien civilization may form a Matrioshka brain or a black hole and communicate using neutrinos or gamma-ray bursts at bandwidths that exceed our receiving capabilities.
At present, human scientists are attempting to communicate outside our species to primates and cetaceans, and in a limited way to a few other vertebrates. This is inordinately difficult, and yet it represents a gap of at most a few SQ points. The farthest we can reach in our "communication" with vegetation is when we plant, water, or fertilize it, but it is evident that messages transmitted across an SQ gap of 10 points or more cannot be very meaningful. What, then, could an SQ +50 Superbeing possibly have to say to us?
— Robert A. Freitas Jr
See also 
- Dr. Freitas, Robert A. Jr., Xenopsychology, Analog Science Fiction/Science Fact, Vol. 104, April 1984, pp 41-53
- Tony Pearson, IBM Watson - How to build your own "Watson Jr." in your basement, Inside System Storage
- IBM, 8233-E8B (IBM Power 750 Express)
Moravec, Hans. "When will computer hardware match the human brain?" Journal of Evolution and Technology 1998, Vol. 1. Last accessed 11 April 2008.
References to SQ 
- Hypnotic experiments with 'time sense', Dr. Bernard Aaronson, Bureau of Research in Neuropsychology & Psychiatry, described by Freitas in "Timeless Minds," Omni 6 (February 1984):38.
- Sentience Quotient is referenced in the Artificial Intelligence issue of Daedalus, Proceedings of the American Academy of Arts and Sciences, [Probably Volume 117, Winter 1988.]
- "Sentience Quotient" shows up in Star Trek Voyager episode "Eye of the Needle", broadcast Monday, February 20, 1995, in the review by Tim Wright.
- "Sentience Quotient" appears in the Transhuman Terminology Page by Anders Sandberg, in the Lextropicon collected by Max More, and in the Nanotechnology Glossary by 7th Wave Inc.
- "Sentience Quotient" as defined by Freitas is mentioned in: Linda MacDonald Glenn, "Biotechnology at the Margins of Personhood: An Evolving Legal Paradigm," Journal of Evolution and Technology, Vol. 13 (2002).