Is Quantum Computing for Real?

[Telemachus]

Can somebody explain the technical difficulties and what means that the program has to be runned in one step? I imagine it has to do with the collapse of the wave function, one need to has the superposition of entangled states to have the qbits running, is that right? and what about decoherence? that has to do with the life time of the wave function in the superposition state?

It is not clear to me neither this thing of the 'one step'. What I'm going to say is perhaps just stupid, but if you can make an algorithm that arrives at the solution on only one step, why do you need a quantum computer to run it? because of the volume of data in the input?

 

[.Scott]

Can somebody explain the technical difficulties and what means that the program has to be runned in one step? I imagine it has to do with the collapse of the wave function, one need to has the superposition of entangled states to have the qbits running, is that right? and what about decoherence? that has to do with the life time of the wave function in the superposition state?

It is not clear to me neither this thing of the 'one step'. What I'm going to say is perhaps just stupid, but if you can make an algorithm that arrives at the solution on only one step, why do you need a quantum computer to run it? because of the volume of data in the input?

I suppose it's "one step" in the sense that you only read the inputs once. But actually, you probably need to perform the quantum operations many times to get a significant result.
When the term "one step" was used in the previous posts, I think it refers to a superposition that is being processed - not just a "position".
In fact, even the simplest quantum algorithms require a setup and a read - two steps. More often it's dozens of steps to set up the entanglements and quantum state before the system of qubits is "measured". Then there are other steps before and after the quantum processing to get the data into and out of the form used to set up the quantum state.

 

[FactChecker]

I suppose it's "one step" in the sense that you only read the inputs once. But actually, you probably need to perform the quantum operations many times to get a significant result.
When the term "one step" was used in the previous posts, I think it refers to a superposition that is being processed - not just a "position".
In fact, even the simplest quantum algorithms require a setup and a read - two steps. More often it's dozens of steps to set up the entanglements and quantum state before the system of qubits is "measured". Then there are other steps before and after the quantum processing to get the data into and out of the form used to set up the quantum state.

When I used the term "one step", I mean that it is not iterating through 2¹⁰⁰ possible combinations. Even if the quantum process includes several phases that take hours, that is much different from the billions of years it would take to iterate through all the combinations. There is work going on to develop error correcting methods for quantum computers that may greatly reduce the need to repeat the process. Everything about quantum computers is in its infancy now.

 

[Quotidian]

Nevertheless, when we have a dice in hand before we throw it the possibility of each face falling upside is one to six. In the moment it falls upon the table and immobilize, to us it's clear one can no more speak of probabilities, as one of the faces was defined. Its obvious, there is nothing mysterious in it, as even Einstein and Niels Bohr concurred.

This post was made some time ago, but I hadn't re-visited the thread. However, I believe this comment is incorrect. In the case of dice, we know that even before they are thrown, the dice actually exist, it is a matter of simple probability which way they will land. It is precisely this which is in question with respect to sub-atomic particles (so-called). Before the measurement is taken, it is not as if they're in some place or other, but we don't know where they are until the measurement is taken. The point is, they're not anywhere before the measurement is taken. They are in what is described as a 'super-position', which is not a particular location, but which is described by the wave function. They're nowhere in particular, not in some place we don't know, but not anywhere. But when they are measured, there they are! It is very freaky and a major outstanding issue in philosophy of physics.

With respect to your claim that 'there is nothing mysterious in it', you would do well to recall Bohr's warning that 'those who have not been shocked by quantum mechanics have not understood it'.

Furthermore, Einstein and Bohr did not concur on the major points of interpretation of these findings. They had fundamental disagreements as to the meaning of 'uncertainty' and it is a testimony to their character and maturity that their friendship remained strong regardless.

See https://amzn.com/1400079969 , by David Lindley,
https://amzn.com/0393339882 by Manjit Kumar

 

[FactChecker]

This post was made some time ago, but I hadn't re-visited the thread. However, I believe this comment is incorrect. In the case of dice, we know that even before they are thrown, the dice actually exist, it is a matter of simple probability which way they will land. It is precisely this which is in question with respect to sub-atomic particles (so-called). Before the measurement is taken, it is not as if they're in some place or other, but we don't know where they are until the measurement is taken. The point is, they're not anywhere before the measurement is taken.

I like @Tollendal's use of an unthrown die to represent an entity in multiple states with a probability distribution. We can look at the unthrown die conceptually, not as it's current position, but rather as something with the potential of having state 1..6. The act of throwing the die is like observing the state of qubits and it results in a collapsed state of one number.

 

[FactChecker]

Can somebody explain the technical difficulties and what means that the program has to be runned in one step? I imagine it has to do with the collapse of the wave function, one need to has the superposition of entangled states to have the qbits running, is that right? and what about decoherence? that has to do with the life time of the wave function in the superposition state?

I know that I have been guilty of using the confusing term "one step" to describe one instance of collapsing the entangled states of many qubits into one state. I should have said one execution of that process (with several steps).

It is not clear to me neither this thing of the 'one step'. What I'm going to say is perhaps just stupid, but if you can make an algorithm that arrives at the solution on only one step, why do you need a quantum computer to run it? because of the volume of data in the input?

Not the volume of inputs, but rather the enormous number of possible combinations of a relatively small number of binary logicals. We can imagine a problem where a traditional computer algorithm takes inputs of 100 binary states and must test if that combination is the unique solution to a puzzle (e.g. the precise key to decode a message). That is not a lot of inputs but finding the solution might require iterating through the 2¹⁰⁰ > 10²⁹ possible combinations till a solution is found. It would require billions of years for the fastest traditional computer to do it. A quantum computer with 100 entangled qubits could conceivably represent all 2¹⁰⁰ possible states at once and collapse to the solution in one execution of the process. Even if that requires hours and several steps, it is still possible. Getting all that to work is going to be very difficult, but the potential is enormous.

 

[Quotidian]

I like @Tollendal's use of an unthrown die to represent an entity in multiple states with a probability distribution. We can look at the unthrown die conceptually, not as it's current position, but rather as something with the potential of having state 1..6. The act of throwing the die is like observing the state of qubits and it results in a collapsed state of one number.

The problem with the 'unthrown die' image is that it is too concrete. It is simply rationalising the problem so as to make sense out of the highly unintuitive reality of the situation. A proper analogy would be more like, a cloud of vapour that turns into a die when it has landed.

 

[FactChecker]

The problem with the 'unthrown die' image is that it is too concrete. It is simply rationalising the problem so as to make sense out of the highly unintuitive reality of the situation. A proper analogy would be more like, a cloud of vapour that turns into a die when it has landed.

Ha! That's right. I like that.