# Is quantum information really the future? Will we ever get a working quantum computer?

Some people think there may be fundamental limits that stop you from building a quantum computer (large scale coherence)

This may be able to be tested soon in optomechanical systems.

What are the applications if we even have one? So far they are very limited (and often contrived)

Its not really the future — its the present.

Fundamental limitations are no more fundamental then other fields.

Application to studying quantum gravity.

If we had a real pressing need could we have a quantum computer (analogous to the manhattan project to build the bomb) Ie. if we had infinite resources would we have a quantum computer. Are the problems technological or more fundamental?

Are current approaches to QI the best way? Eg. does fault-tolerance and error correction have too much overhead to be practically useful on a large scale. Do we need a new approach? Possibly depends on the application.

QI (and especially entanglement) is playing an important role in other fields (e.g. condensed matter, quantum gravity).

The history of humanity can be classified by scientific discoveries or advancements that where revolutionary for mankind. For instance,  the invention of the steam engine lead to the industrial revolution as well as the invention of the computer and later the internet has lead to what can be called an information revolution. One can think that having a working a quantum computer can have a similar impact, however, this seems to be more as a nice topic for science fiction than reality.  There are claims nowadays that the so-called d-wave computer is a working quantum computer, however is a classical computer in which quantum systems can be simulated.  In other words its a quantum simulator. Although recently the concept of information has penetrated in almost every field on physics, its to extreme to say that quantum information will be the future. It is true that several physical theories and descriptions can be rewritten  and understood in terms of  an information-theoretic language. Although this paradigm offers new insights in a broad number of fields, it is not fundamental. Thus, quantum information will be part of the future for sure, but it will not be the future.

The meaning of the future is not clear. The quantum information can help physicists to simulated condensed matter systems or particle physics system. In this sense, it can change the future.

In the context of AdS/CFT, it is not clear how to use CFT data to construct local operators. New tools from quantum information, for example, quantum error correction, bring a fresh way to study the old problem.

We already have working quantum computers. Now Problems are how it can be scaled up. There are many skeptics that quantum computers break down at a certain scale. Then it is a good question to ask at which scale it must be break down, like de-coherence sits in or quantum correction theory breaks down.

Suppose we can run a large-scale quantum computer, does that mean we have new physics, which cannot be observed otherwise? If quantum computer is living on the boundary of some gravity theory, and that quantum computer we have very strong control, does that mean we can create any kind of entanglement? Is there any consequence in the gravity side?

If you have a condensed matter system, people build effective field theory to describe the system. Is it a kind quantum computer? There are differences between a quantum emulator and a universal quantum computer. People have not built a theory to describe how to in general simulate any quantum system we want. There are some studies for certain theories but they are not general.

Unless there is a theoretical restriction for QM, there is no reason to believe that we cannot get quantum computers.
Moore law makes quantum computing inevitable. We are going to get there eventually, because of going to smaller sizess and quantum effects becoming important .

There are 5 different levels of quantum computers. We might be able to make some of them.

Limitations (like propagation of signals) on building quantum computers are causing delay.

techniques from QI has helped us in real physics and condensed matter, for example in Ads-CFT, entanglement entropy. QI has helped us in providing new (operational) ways to do stuff in other fields of physics.

QI has already produced quantum cryptography. Quantum simulations in 10 years will be provided.

Quantum information useful

Less certain about fully quantum computer (scaling problems)

Scattered direction of research

When will we get a fully quantum computer – can do something we can’t currently do with an analog one

What is time scale?

Practitioners more optimistic – might happen?

Criterion: can we scale to larger – not just a couple q-bits

Optical lattice as a technique

Semi-optimistic

We think it should be possible to build a quantum computer, in principle and in practice. The only reason it might not be possible is if something breaks down on some scale that we haven’t reached yet. It was suggested that Moore’s law makes quantum computing inevitable; as we are able to build more and more precise electronics, we will naturally arrive at quantum computers. However, Moore’s law is not really a law, merely an observation, and once we enter the quantum regime things might be different.

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