Understand Decoherence in Quantum Computers: How Heat Affects Qubits

In summary, the conversation discusses the role of reversible and irreversible logic gates in quantum computers. It is explained that irreversible gates generate heat and cause decoherence in the qubits, which can affect the behavior of a quantum circuit. The concept of a 45 degree rotation in the state space is also mentioned and its relation to a square root not gate is discussed. Overall, the conversation highlights the importance of understanding the effects of reversible and irreversible gates in efficient quantum algorithms.
  • #1
michael879
698
7
ok I just read something that doesn't make much sense to me. It said that quantum computers can only use reversible logic gates because irreversible gates generate heat/lost information. It makes sense to me that everything in physics is reversible and that NAND gates produce heat because there is a loss of information but what I don't get is how the heat generated by a NAND gate in a quantum computer would cause the computer to stop working.

This article said that the heat would cause decoherence in the qubits which makes sense since the heat gives information about their state. What I don't get is how that would effect a quantum circuit. The qubits are still randomly 1 or 0 with whatever probability you made them so wouldn't whatever algorithm your running still come out right? would using irreversible gates in a quantum computer than can factor 15 really cause it to not give 5 and 3 as an answer?
 
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  • #2
If I have the state

|0>

and do a 45° rotation in the state space, I now have (a multiple of)

|0>+|1>

If I do another 45° rotation, I now have

|1>

And when I measure the result, I'm guaranteed to get |1>.

-----------------------------

If I have the state

|0>

and do a 45° rotation in the state space, I now have (a multiple of)

|0>+|1>

If decoherence happens, I now have

50% chance of |0>
50% chance of |1>

If I do another 45° rotation, I now have

50% chance of |0> + |1>
50% chance of -|0> + |1>

And when I measure the result, I have a 50% chance of getting a |1>, and a 50% chance of getting a |0>.


An efficient quantum algorithm relies on doing similar sorts of things, and as we see, decoherence will change the behavior of your program.
 
  • #3
is a 45 degree rotation the same as a square root not gate? If its not I still get what you mean cause the same thing would happen with those. Thanks a lot for the help, I just started seriously reading about quantum computers the other day.
 
  • #4
and wait if you do 2 45 degree rotations on a |0> your guaranteed to get a |1>? That sounds kinda weird. The description of a sqrt-not gate I read was to send a photon at an electron with half of the energy required to make it change states (a not gate would have twice the energy). If you do this twice to an electron in the ground state it will always be excited in the end?
 
  • #5
By a 45° rotation, I mean the operation that sends

a |0> + b |1>

to

[(a - b) |0> + (a + b) |1>] / sqrt(2)


Appling this transformation twice will leave you with

b |0> + a |1>

which is, indeed, a not gate.


If what you described is a physical realization of that transformation, then yes, that's what I mean.
 

What is decoherence?

Decoherence is the process by which a quantum system loses its coherence and becomes entangled with its environment. This causes the quantum state of the system to become uncertain and indeterminate, making it difficult to extract useful information from the system.

How does heat affect qubits in quantum computers?

Heat can cause qubits, the basic units of information in quantum computers, to lose their coherence and become entangled with their environment. This can lead to errors in quantum computations and make it difficult to extract meaningful results.

Why is understanding decoherence important in quantum computing?

Understanding decoherence is crucial in quantum computing because it is one of the biggest obstacles to developing reliable and scalable quantum computers. By understanding how heat affects qubits and causes decoherence, researchers can work towards mitigating its effects and improving the performance of quantum computers.

What are some strategies for mitigating decoherence in quantum computers?

Some strategies for mitigating decoherence in quantum computers include using error correction codes, implementing quantum error correction algorithms, and developing more stable qubits that are less susceptible to environmental influences such as heat.

What are some potential applications of understanding decoherence in quantum computers?

Understanding decoherence in quantum computers can lead to the development of more reliable and powerful quantum computers, which could have a wide range of applications in fields such as cryptography, optimization, and simulation of quantum systems. It could also pave the way for advancements in quantum communication and quantum sensing technologies.

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