Ask a physicist anything. (3)

[canehdianhotstuff]

Precisely, ions stay in solution, water evaps.

 

[catzrfluffy]

A man is riding a subway. He meets a one-armed man, who pulls
out a gun and shoots him.

 

[Wiccan_Child]

A man is riding a subway. He meets a one-armed man, who pulls
out a gun and shoots him.

What needs explaining?

 

[Cabal]

I love google.

Spoiler: Ah cheated :D

Several men were shipwrecked together. They agreed to survive by eating each other a piece at a time. Each of them in turn gives up an arm, but before they get to the last one, they're rescued. They all demand that the last man live up to his end of the deal. Instead, he kills a bum and sends the bum's arm to the others in a box to
demonstrate that he fulfilled the bargain. Later, one of them sees him on the subway, holding onto the overhead rings with the arm he supposedly cut off, realizes he cheated, and kills him.

 

[catzrfluffy]

well done!

 

[AV1611VET]

A man is riding a subway. He meets a one-armed man, who pulls out a gun and shoots him.

The man who was shot is Dr. Richard Kimble (David Janssen).

 

[Wiccan_Child]

The man who was shot is Dr. Richard Kimble (David Janssen).

 

[GrowingSmaller]

I heard that QM is used in modern computers, though transistors or their modern equivalent. What aspect of QM is this, and why is it necessary to know the QM?

 

[Wiccan_Child]

I heard that QM is used in modern computers, though transistors or their modern equivalent. What aspect of QM is this, and why is it necessary to know the QM?

I've heard of transistors will soon reach a hard limit on how small they can get, because quantum mechanical effects will interfere with their operation. For example, they'll be so small that charge will be able to tunnel from one to the other enough to short-circuit the device (which isn't good when you have literally trillions doing this at once!).

I've also heard of quantum computers, which use 'qubits', which are particles that are either absent ('0'), present, ('1'), or a mixture of the two ('0/1'). Thus you can have a trinary computing language without having to delve into the world of analogue circuitry. It's meant to make for much more powerful computers, since it exponentially increases the power of existing technology.

Or so I hear.
I've not heard of quantum mechanics actually being used in modern equipment though.

 

[Cabal]

I've heard of transistors will soon reach a hard limit on how small they can get, because quantum mechanical effects will interfere with their operation. For example, they'll be so small that charge will be able to tunnel from one to the other enough to short-circuit the device (which isn't good when you have literally trillions doing this at once!).

I've also heard of quantum computers, which use 'qubits', which are particles that are either absent ('0'), present, ('1'), or a mixture of the two ('0/1'). Thus you can have a trinary computing language without having to delve into the world of analogue circuitry. It's meant to make for much more powerful computers, since it exponentially increases the power of existing technology.

Or so I hear.
I've not heard of quantum mechanics actually being used in modern equipment though.

Semiconductor physics is essentially quantum mechanical. There was one key attempt to explain it classically (Drude model) with some success, but ultimately it was treating conduction electrons as an ideal gas - and that doesn't chime with Fermi-Dirac statistics, Pauli exclusion etc. Put those into the mix and you get a quite different model to Drude's.

I would go into more detail, but semiconductor physics is one of my weaknesses and I don't have any literature with me.

As for quantum computing, there's no real limit on the superposition you can use - I've seen some fairly freaky theoretical talks where they discuss possible uses of qutrits (3 basis states plus all their possible superpositions), and generalised qunits (n basis states etc).

Keraaaaaazy stuff.

 

[GrowingSmaller]

I have found this and this.

Moreover, electrons acting like a wave can sometimes burrow right through a barrier. Understanding this odd behavior of electrons was necessary as scientists tried to control how current flowed through the first transistors.

And certainly it was a thorough understanding of these new laws which helped Bardeen, Brattain, and Shockley invent the transistor.

You'll hardly ever find anyone discuss transistors in quantum terms, because the quantum effects give rise to classical-looking physics which is much easier to talk about. But all of the classical-looking physics is really based on quantum mechanics, and without an understanding of QM it is extremely unlikely that anyone would have invented the Field Effect Transistor.

 

[Wiccan_Child]

Semiconductor physics is essentially quantum mechanical. There was one key attempt to explain it classically (Drude model) with some success, but ultimately it was treating conduction electrons as an ideal gas - and that doesn't chime with Fermi-Dirac statistics, Pauli exclusion etc. Put those into the mix and you get a quite different model to Drude's.

I would go into more detail, but semiconductor physics is one of my weaknesses and I don't have any literature with me.

As for quantum computing, there's no real limit on the superposition you can use - I've seen some fairly freaky theoretical talks where they discuss possible uses of qutrits (3 basis states plus all their possible superpositions), and generalised qunits (n basis states etc).

Keraaaaaazy stuff.

Well I suppose semiconductors maaaaay need a quantum explanation. I didn't think of that actually. Though I suppose it's a bit like quantum thermodynamics; a classical approach with a few quantum things thrown in (spin states, etc).

 

[Wiccan_Child]

I have found this and this.

Kewl

 

[Cabal]

Well I suppose semiconductors maaaaay need a quantum explanation. I didn't think of that actually. Though I suppose it's a bit like quantum thermodynamics; a classical approach with a few quantum things thrown in (spin states, etc).

Well, I guess the simplest models are "semiclassical" in that sense - but the impression I get is the more and more complicated ones have more and more quantum aspects. Seems to be how most semiclassical theories develop after a while.

Kewl

Darn, tunnelling. KNEW there was some obvious quantum weirdery that I'd forgotten.

 

[Maxwell511]

Well I suppose semiconductors maaaaay need a quantum explanation. I didn't think of that actually. Though I suppose it's a bit like quantum thermodynamics; a classical approach with a few quantum things thrown in (spin states, etc).

There is no "maaaay" about it. It is impossible to explain how semiconductors work without quantum physics. If you pick up any undergraduate book on Solid-State Physics the introductory chapters are all about quantum physics and explaining details of it that are necessary to understand semi-conductors. They don't use analogies to classical physics because there are none.

You cannot classically derive the behaviour of semiconductors. You need quantum ideas such as disallowed energy states, the exclusion principle and tunnelling.

 

[gipsy]

I heard that QM is used in modern computers, though transistors or their modern equivalent. What aspect of QM is this, and why is it necessary to know the QM?

There's also another very interesting field where QM is used in computers, or at least it's already technically possible: Quantum cryptography.

The big advantages against any other cryptography techniques are physical security (not depending on algorithms and processing power) and reliable detection of a possible listening third party.

Wikipedia gives a good overview about quantum cryptography ...

And of course also interesting: we (Austrians) were the first

 

[Cabal]

There's also another very interesting field where QM is used in computers, or at least it's already technically possible: Quantum cryptography.

The big advantages against any other cryptography techniques are physical security (not depending on algorithms and processing power) and reliable detection of a possible listening third party.

Wikipedia gives a good overview about quantum cryptography ...

And of course also interesting: we (Austrians) were the first

Were IQOQI involved with that? They usually are. Those guys are made of win.

 

[gipsy]

Were IQOQI involved with that? They usually are. Those guys are made of win.

I'm not sure of this, but as Zeilinger is a member (even director) of IQOQI it would be very feasible that they were involved

 

[Wiccan_Child]

There is no "maaaay" about it. It is impossible to explain how semiconductors work without quantum physics. If you pick up any undergraduate book on Solid-State Physics the introductory chapters are all about quantum physics and explaining details of it that are necessary to understand semi-conductors. They don't use analogies to classical physics because there are none.

You cannot classically derive the behaviour of semiconductors. You need quantum ideas such as disallowed energy states, the exclusion principle and tunnelling.

My 'maaaay' was facetious. I was acknowledging, in a humorous way, that Cabal had thought of something I hadn't.
That said, I maintain what I said: semiconductor physics is a semi-classical approach, like modern thermodynamics. It's a classical approach to a quantum problem (instead of classical particles, you have spin states, energy bands, etc).

 

[Wiccan_Child]

There's also another very interesting field where QM is used in computers, or at least it's already technically possible: Quantum cryptography.

The big advantages against any other cryptography techniques are physical security (not depending on algorithms and processing power) and reliable detection of a possible listening third party.

Wikipedia gives a good overview about quantum cryptography ...

And of course also interesting: we (Austrians) were the first

Is there any practical use to quantum cryptography? I heard that, once the next prime number is discovered, we'll have uncrackable public keys, or something like that.

My knowledge of cryptography is limited to a book on it that I've read, and Darren Brown's Digital Fortress, so I'm probably wrong on all counts