Apr 1, And now Australian researchers have reported building the first ever quantum Fredkin gate – a type of logic gate thought to be the key to. Mar 26, The quantum Fredkin gate is a vital piece of quantum computing that was previously too complex to build, but scientists have found a way to. Mar 25, The quantum Fredkin gate, as shown in Fig. 1A, is a three-qubit gate whereby, conditioned on the state of the control qubit, the quantum states.

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[] A quantum Fredkin gate

I was reading the book “The singularity is near” written by Kurzweil and he mentioned the reversible gates like for example the Fredkin gate. The advantage using such gates is that we could get rid of the thermal waste related to computation where bits just disappear into heat, and computation won’t need any energy input.

Those assumptions make these gates sound like a miracle solution. So the question is what technical hurdles are still preventing their large scale usage. I also think it is a shame that I never heard about those gates in my electrical engineering bachelor and master studies at a top German university Nobody has actually figured out how to actually make gaate gates yet, they’re merely of theoretical interest.

That might explain why you’ve never heard of them since engineering usually deals with practice. The fredki of Reversible Computing is that when a bit disappears, some amount of heat is generated. By using reversible gates, no bits ever appear or disappear so supposedly computation could be much more efficient frrdkin reversible gates. Our current-day computers are not limited by heat generation associated with bits disappearing.

They are limited by the inherent inefficiency gzte moving electrons around on tiny copper traces.

Quantum Physics

The problem with practical reversible gates gates that can and have been fabricated in silicon is that the actual energy savings are linearly proportional to how slowly you run them.

I know vate Tom Knight’s research group at MIT fabricated a small adiabatic processor in the late s. The practical logic family they developed is called split-level charge recovery logic, and can be implemented using standard CMOS fabrication techniques. An example frddkin the work in Tom Knight’s group is the following master’s thesis which has a pretty decent section on related work through the early s. Reversible circuits need to be adiabatic there can’t be heat exchanges between the circuit and its environmentwhich means that they must be in equilibrium at all times.

For any process that needs to change something you can only approximate equilibrium by making the change happen as slowly as possible. If I remember my thermodynamics correctly, you can make the energy of a reversible computation arbitrarily small, but the minimum action energy times time must be a small constant.

The largest hurdle credkin their large scale use is the same as for asynchronous circuits and pretty much any other non-standard circuit design: Moore’s law has become something of a self fulfilling prophecy; as seen by the Tick Tock Release Schedulechip manufacturers see fulfilling Moore’s law as a challenge. Because of the need to fulfill Moore’s law, we have gotten more and more adept at decreasing the size of chips by advancing lithography and often by using cheats, like multipatterning.

What does all of this have to do with reversible gates? As foundries race to release newer and smaller transistor sizes, companies that want to print new chips see an easy path towards increasing speed by simply adding more cache and reworking their conventional designs to better use that cache. The killer of better isn’t technological hurtles; it’s the success of good enough. Freedkin computing devices require feedback, which makes it possible to have one circuit element perform an essentially-unlimited number of sequential computations.

Usable feedback circuits must contain sections whose total number of inputs counting both the ones that are fed back from outputs and those that aren’t exceeds the number of outputs which are fed back to input the only way gahe number of inputs wouldn’t exceed the number of fed-back outputs would be if the circuits didn’t respond in any way to outside stimuli.

Since perfectly reversible-logic functions can’t have more inputs than outputs, it’s not possible to construct from them any of the feedback structures required to perform any non-trivial computing tasks repeatedly.

Note that with the CMOS technology used in today’s computers, feedback is required to ensure that results reported by computations in different parts of a circuit are made available simultaneously to other parts, since if they weren’t the relative timing with which the signals arrive would constitute “information” which could not be perfectly passed downstream; other technologies might make it agte to have many gates propagate signals at precisely the same rate while retaining reversibility, but I know of no practical technology for that.


Note that from a CS perspective, it’s trivial to make a computing process reversible if one has initially-empty storage medium whose size is essentially proportional to the number of steps times the amount of state that could change in each step.

This claim does not contradict the claim of the previous paragraph, since storage proportional to the number of steps will require circuitry agte to the number of steps, which will imply circuitry proportional to the amount that would be required if all feedback were eliminated.

If one is allowed to have outputs which are ignored if, given proper input conditions, they will never go high, then it might be possible to design a system that would, in theory, benefit from reversible logic.

For example, if one had an algorithm that operated on a word chunk of RAM and one wanted to use a “reversible-logic CPU” that performed 1, operations per second and each operation updated either a register, the program counter, or one word of RAM, one could use a “reversible CPU” which would:.

The above recipe could be repeated any number of times to gaye the algorithm for an fredoin number of steps; only the last step of the recipe wouldn’t be reversible. The amount of energy rredkin per algorithmic step in non-reversible operations would be inversely proportional to the size of the LIFO, and thus could be made arbitrarily small if one were building to build a large enough LIFO.

In order for that ability to translate into any sort of energy savings, however, it would be necessary to have a LIFO which would store energy when information was put in, and usefully return that energy when it was frexkin out.

Further, the LIFO would have to be large enough to hold the state data for enough steps that the any energy cost of using it was less than the amount of energy it usefully saved. Given that the amount of energy lost in the storage and retrieval of N bytes from any practical FIFO is unlikely to be O 1it’s unclear that increasing N will meaningfully reduce energy consumption.

Practical applied reversible computing is an active area of research and is likely to become more prominent in the future. Most of quantum computing can be seen to be attempting to create reversible qubit gates and it’s very hard experimentally to match the theoretical properties of the QM formalism, but steady progress is being made. Another frekin point is that anytime energy dissipation is decreased on a chip, it’s essentially moving the gate system to “more reversible”, and lower-energy fredkih dissipation has been a high priority for a long time now in mobile computing representing a sort of industry-wide paradigm shift.

For decades chip performance gains similar fredoin Moore’s law came about by being somewhat “relaxed” or even “sloppy” with energy dissipation but that reached a point of diminishing returns a few years ago. The leading worldwide chip manufacturer Intel is attempting to pivot into lower-power chips to compete with Arm which has an advantage after never building anything but. Frekdin is some possibly breakthrough recent research using superconducting technology Juneand there are other active research projects in this area.

Reversible computing has been studied since Rolf Landauer advanced the argument that has come to be known as Landauer’s principle. This principle states that there is no minimum energy dissipation for logic operations in reversible computing, because it is not accompanied by reductions in information entropy.

However, until now, no practical reversible logic gates have been demonstrated. One of the problems is that reversible logic gates must be built by gatte extremely energy-efficient logic devices.

Another difficulty is that reversible logic bate must ftedkin both logically and physically reversible. Here we propose the first practical reversible logic gate using adiabatic superconducting devices and experimentally demonstrate the logical and physical reversibility of the gate.

Additionally, we estimate the energy dissipation of the gate, and discuss the minimum energy dissipation required for reversible logic operations.

It is expected that the results of this study will enable reversible computing to move from the theoretical stage into practical usage. By clicking “Post Your Answer”, you acknowledge that you have gatd our updated terms of serviceprivacy policy and cookie policyand that your continued use of the website is subject to these policies. Home Questions Tags Users Unanswered.

Why are reversible gates not used? Mehdi 2 6. Note that quantum computation is very much about reversible gates that’s part of what “unitary” means. DavidRicherby Not all quantum computations are reversible; eventually decoherence occurs. Note that any practical computer using reversible gates is still going to generate heat, because you need to perform error correction to keep the computer on track.


Error correction inherently requires irreversible operations or a continuous supply of zero’d bits; same difference. I am by no means an expert on this topic, but just from casually reading Wikipedia: Tom van der Zanden 8, 1 20 Some people have made reversible gates and built an entire CPU out of them. There’s a photograph of that reversible-logic CPU at cise.

DavidCary but they’re not or negligibly more efficient than computers made from non-reversible gates. All I’m seeing is an image of a CPU with the word “adiabatic” on it, but no information on how much more efficient than traditional computers it is. TomvanderZanden Measuring efficiency is a bit useless if you don’t specify what kind of efficiency.

RISC chips are more efficient than CISC ones, in terms of chip size, but not in terms of how many instructions it takes to specify any given algorithm. Any reversible circuit is immediately more efficient than a traditional circuit because it isn’t subject to Landauer’s principle ; that’s already a huge win. You don’t remember thermodynamics correctly; Landauer’s principle need not be supported by a reversible circuit as it does not erase bitsand therefore the energy needed can theoretically be zero and no heat would be released.

Reversible circuits also don’t need to be adiabatic; practical reversible gates have been made which are no slower than non-reversible chips taking into account that reversible frdkin are usually larger, and therefore have a speed of light latency increase. Are you referring to reversible optical chips? Most chips are electronic. Hopefully one day good freddkin won’t be good enough anymore. Mehdi Don’t we all wish. But I wouldn’t be so sure; energy is currently cheap and there are paths to continue the current cycle for at least another 5 years possibly 10, if we find a way to get certain technologies working.

After that, some new technology will have to take the place of lithography, but this doesn’t mean it has to be unconventional.

That could get us gains till For example, if one had an algorithm that operated on a word chunk of RAM and one wanted to use a “reversible-logic CPU” that performed 1, operations per second and each operation updated either a register, the program counter, or one word of RAM, one could use a “reversible CPU” which would: I think you ignore the requirement that the required tape length is proportional to the number of steps gatw be performed reversibly. If I throw a cartridge into my Atari gredkin power it on for awhile, it will run about billion cycles per day.

Since the system including all but the largest cartridges would have less thantransistors, that’s more than a gatee cycles per day per transistor. If one wanted to design an equivalent machine that could run for a day fully reversibly, even with the ability to make a reversible LIFO with one transistor per bit If one only needed to run a few frredkin cycles at a time reversibly, capture the results, rewind the cycles, and then replace the previous initial state with the captured results, that might almost be workable, but would be monstrously complex.

With anything resembling today’s technology, any reduction in “theoretically unavoidable” losses one would obtain by using reversible computing would be swamped by an increase in power lost to causes that were vredkin only in theory. I was only concerned with the statement your original answer made that came off gatf saying, “reversible technology cannot compute the same things as irreversible technology”.

I did not mean to imply it’s practical. The initial question was “why aren’t these things used”. I would suggest that nearly all practical computing devices use feedback in such a way that a fixed amount of hardware will be able to perform an unbounded number of calculations if given unbounded time. That is something that reversible logic just cannot do.

That is a major qualitative difference between reversible and non-reversible gatd.

Could a Fredkin gate be the next quantum leap forward for computing?

It may be that even a computer which can only run a limited number of operations before ” rewinding” could still be useful, so I’ve edited the post to say what would be required to use such a thing to do any meaningful work. I think the fundamental practical problem stems, though, stems from what I originally said: Sign up or log in Sign up using Google.

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