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Comment Re:I Think this article might be a bit misleading. (Score 1) 189

Sorry, I'm hopeless at explaining hence why I could never teach.

There aren't two identical "messages" in quantum teleportation. That would violate the no-cloning theorem. Instead there's one message that originally exists at point A and later exists at point B instead.

But - and this is the bit where it involves QM weirdness - there is no way to "read" a complete quantum state and record it classically. Think of it as a two bit word in a computer where every time you read one bit the reading circuit randomly disturbs the other bit.

QM teleportation lets us move that word from one place to another without changing either bit (but note that the original word (letter) is destroyed in the process.)

Comment Re:I Think this article might be a bit misleading. (Score 1) 189

I don't know what else you saw but basically the entanglement leaves the target (teleportation) end in a superposition of four states, only one of which is the one you want the others are complementary states.

The sender makes a measurement at their end to determine which one of the four states is the correct one and then transmits it to the receiver. The receiver can then isolate the correct state from the others that would otherwise cancel out all knowledge of the original state.

Comment Re:Can Anyone Explain This To Me? (Score 2) 189

Oh well. I tried to write a comment with a diagram but hit submit instead of preview :-(

Consider four directions on a plane. x axis (we'll call that |+>), y axis (we'll call that |->) y=-x (we'll call that |0>) and y=x (we'll call that |1>)

Modulo some constant factors, I hope it's obvious that you can build up some of those vectors from others:

|1> = |+> + |->
|-> = |0> + |1>

These are the directions of a plane polarized photon.

We setup some photons that are polarized in the |1> direction and then pass them through a polarization filter.

If the filter points along the |1> direction then all of them pass. If the filter passes along the |0> direction then none of them pass.

Now we put the filter along the |-> direction. What happens.

|1> = s|+> + s|-> (s is 1/sqrt(2) - which can be deduced from standard trig - the lines must be the same length)

When we measure along the |-> direction the s|-> part will pass the filter but the s|+> part wont.

But an individual photon can't get dimmer therefore it must either pass or not. Half the photons do pass and half don't (and it's random whether any one photon gets through the detector)

The ones that do get through are now in state |-> which is also |0>+|1> (again with factors of sqrt 2)

If we now measure along the |1> direction again we now lose half the photons again (due to that |0> component)

Quantum teleportation involves taking a photon in state a|0> + b|1> (for unknown values of a and b) and taking very careful measurements that don't destroy a and b but instead transfer them to another photon without us actually knowing what they are.

Comment Re:Coincidence circuit again, i.e. filtering (Score 3, Informative) 189

You got the first paragraph right. But then got sidetracked by tennis balls.

There are *two* complementary quantum states that you can measure. Measuring one destroys all knowledge of the other.

There is no classical system that behaves like this, therefore any analogy that doesn't invoke some magic artificial property of a classical object won't represent what happens in QM.

In your example you need tennis balls that randomly change colour when you measure their spin and can magically reverse spin when you look what colour they are.

Comment Re:Confused (Score 1) 189

Quantum teleportation requires the use of a classical channel. The entangled particles can be exchanged in advance (provided they can be stored without breaking the entanglement which is difficult in practice but trivial in theory)

The classical data can only be transferred at the time the teleportation is done - hence that limits the speed of the teleportation to the speed of light.

Comment Re:I Think this article might be a bit misleading. (Score 5, Informative) 189

It's effectively** equivalent to having two identical letters containing a random message

No. you're describing entanglement.

Teleportation is subtly different.

Teleportation consists of transferring the quantum state of one particle to another particle via the use of entangled particles (and a classical channel)

The beauty of this is that the entangled state can be set up in advance. You then give me a particle that you might or might not know something about its quantum state (but importantly, I do not know what you know about it so cannot measure that quantum state in advance). I can transfer the state of that particle to another particle that Bob has via some entangled particles we exchanged earlier *plus* some standard classical information that goes over classical channels (it's this classical information that limits the teleportation to the speed of light)

The particle that Bob ends up with is in an identical state the the one you gave me (and which I still have).

N.B. This is quantum teleportation, not quantum cloning which is not possible. The act of getting the quantum state to Bob affects my particle in a way that means I cannot also extract any information from it about the original state of your particle.

Comment Re:Coincidence circuit again, i.e. filtering (Score 2) 189

I think you misunderstand.

The experiments that are discarded are where the two end points don't measure the same quantum variable.

For photons, for example, you can measure whether linear polarization is up-down/left-right or diagonal-left-up diagonal-right-down/diagonal-right-up diagonal-left down.

If both ends measure the up-down/left-right state then one will get up-down, one left-right. If both measure the diagonal polarization then again they will get complementary results. But if one measures up-down and the other measures diagonal then we cannot tell anything useful any more than trying to compare two sweets where one person says what shape it is and the other says what flavour it is so those results get discarded.

There is additional statistical analysis - due to the fact that these experiments are done on single photons and sometimes detectors fire when there is no photon and sometimes they don't fire when there is so we cannot expect 100% correlation - but that's nothing to do with discarding some of the results.

Comment Re:Nobody knows yet (Score 1) 165

Basically, they have promised to veto any Article 50 agreement that doesn't continue to allow free travel (with ID) for their citizens to the UK, as is currently the case. Any Article 50 agreement requires a unanamous vote in favour - all 27 remaining countries

This isn't quite correct. An article 50 agreement requires, iirc at least 50% of member states representing at least 66% of the EU population.

However, I think disconnecting access to the free market from freedom of travel does require unanimity. That's an independent rule of the EU IIUC unrelated to article 50.

What isn't obvious at this point is whether the UK can negotiate some (acceptable) restrictions in trade in return for some (acceptable) restrictions in movement.

It may well end up with WTO rules due to the 2 year negotiating period expiring. Britain will then, of course, lose it's passporting rights to the financial markets. Whether the finance industry will have relocated in time is debatable - it probably depends on how early on failure of negotiations becomes apparent.

Comment Re:points of interest (Score 1) 532

How does the energy efficiency of this drive compare to a normal rocket?

Utter crap. But what it would give (assuming it works) is freedom from having to carry reaction mass.

But if it works, then it's going to open a whole new world in physics that hasn't been considered up until now - and it's likely to be obsolete within a few years once a theory that explains it comes to light.

(I'm betting on measurement error but hoping for something much more exciting)

Could this allow interstellar travel, by humans, within a normal human lifespan?

In theory yes - provided you can carry enough energy in a small enough mass. But interstellar travel precludes the use of solar energy so it's most likely to be useful in manoeuvring spacecraft around the inner solar system.

What kind of reletavistic effects happen at high speed? I would assume thrust would drop as you approach C.

Comment Re:I hate Apple, but no (Score 1) 564

However, you can't claim you're owed past money when Apple wasn't hiding anything.

I don't know about EU law but in the UK this is, indeed the case.

Tax avoidance schemes have to be registered with HMRC. If they are deemed to be invalid then you have to pay the tax that was due.

If you decide to fight it through the courts and the courts find in HMRCs favour then your tax liability is a multiple (greater than or equal to 1) of the tax that you tried to avoid. (plus any legal fees)

There was a very recent case along exactly these lines:

And another that looks like it will go HMRCs way.

Comment Re:Math is hard (Score 1) 145

Was MB ever 1024K? (except for memory)

I didn't ever see/use a 8 inch floppy.

Early 5 1/4 floppy was 40 tracks, 18 sectors/track - 360K. That was 1024 bytes/K

Early 3 1/2 floppy was 80 tracks - 720K.

Double sided - 1.44M (we'd already started confusing multipliers)

At some point the remaining factor of 1024 got dropped - probably when we stopped thinking about heads, tracks, sectors/track. Prior to that the first 1024 was baked into the disk geometry. I don't remember enough detail of the early hard disks to recall whether a 30MB disk was 30000000 bytes or 30720000 bytes.

Comment Re:Well, (Score 2) 643

Be confident in yourself and be confident in your future.

Just try not to be arrogant or entitled.

All of the above are hard to do all the time (unless you're so insecure you're incapable of being arrogant or so arrogant that you're incapable of harbouring doubts about yourself) so forgive others who get it wrong some of the time - and hope that they forgive you.

And if you want to get laid, find something cheap to do that you enjoy doing and that has people of the right sex there that you can talk to and don't hurry things.

I would guess that volunteer groups tend to have a reasonably equal gender balance and if you're doing something that you believe in then you're likely to meet people you can connect with.

Most of my time is already committed (not volunteer work) but if it weren't I'd probably start somewhere like here:

Probably something similar in your area of the world.

Comment Re:Good thing you have a choice (Score 3, Insightful) 537

How the *fuck* did we function during the 1960s, 1970s, 1980s and 1990s without mobile phones

We used to plan.

There would be an agreed meeting point if we got separated (or common sense - go back to the place where we last saw each other)

People would arrive on time. If people hadn't turned up, five minute after the agreed time we'd be off and the latecomers were on their own.

Nowadays, people text five minutes before the agreed time to say they're going to be an hour late[1]. People also assume the most optimistic times for a journey instead of a realistic time.

Late entry into theatres and concerts has, IME, become much more common. 20 years ago there might one one or two couples who were let in in the first break - and you felt sorry for them because obviously there'd been an accident or something else completely unexpected that had delayed them excessively. Now it's dozens of people - often so many that it's not actually possible to seat them all in the few minutes before the second piece starts.

[1] This is the one that really pisses me off. It's taken me an hour to get to our agreed meeting point. I've arrived a good ten minutes early out of courtesy, and then I'm kept waiting around for another hour.

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