Chinese Researchers Achieve Quantum Entanglement Record

https://www.scientificamerican.com/article/chinese-researchers-achieve-stunning-quantum-entanglement-record/

by 0xbxd • 8 years ago

305 140 8 years ago

Even neater is that they are using quantum entanglement in radar to detect stealth planes: https://www.popsci.com/china-quantum-radar-detects-stealth-p...

misterprime4• 8 years ago

From the article: they are planning to do this, and hopeful that it will work. Not actually using it yet.

seanmcdirmid2• 8 years ago

Also, if they got it to work they would surely keep it secret.

barbegal4• 8 years ago

I understand the core concept of quantum radar but does it actually work in practice as a radar? i.e. you can resolve targets with a small range and angular resolution.

mike006322• 8 years ago

Theoretically you would combine the new tech with the old tech. Quantum state would be another measurement to make on the incoming wave on top of measurements like frequency and amplitude.

madeuptempacct3• 8 years ago

Yea, you have to explain this. It made no sense when I first read it, and it still makes no sense.

Here is how radar works: You shine a flashlight somewhere. If there is an object within range, some of the light is reflected back at you and you know there is an object. This is literally what radar does.

So, what part of this, and how, does this "quantum" magic affect?

nabla94• 8 years ago

It works like normal radar, except that they send one entangled photon towards the plane and keep the other.

When the reflection returns, they can use the entanglement to sort out signal from noise much better and radar is safe from jamming and spoofing.

mike006322• 8 years ago

Think of the quantum state as another measurement to make on the incoming signals. The receiver already measures many characteristics of incoming waves, like frequency and amplitude, and now it has yet another measurement that can be helpful. It's helpful specifically because they kept a reference wave with the same quantum state. With more data you can get higher fidelity which means you can perhaps see further or see the same distance with less power.

angrygoat• 8 years ago

Here's a pre-print of the USTC team's paper:

18-qubit entanglement with photon's three degrees of freedom

https://arxiv.org/abs/1801.04043

foxes3• 8 years ago

I visited USTC several years ago, (2014) and in this time they have basically doubled the number of qubits they can entangle, so this is a great step forward. At around ~100 qubits quantum computing becomes very useful, so maybe we won't have to wait too long.

ssdd• 8 years ago

That's great. Now next thing that needs to be solved is to make this tech available for general public use just like computers are these days. Problems that are waiting to be solved by it are very large and of huge impact.

reaperducer• 8 years ago

Problems that are waiting to be solved by it are very large and of huge impact

I know virtually nothing about quantum computing, so can you give some examples? Whenever I've asked anyone who seemed to know anything about the field, all they can come up with is weather forecasting and simulating nuclear explosions. Not exactly "general use," as you put it.

In the back of my mind, I know that quantum computing is a big deal, and we're in ENIAC days with it. But I don't have a good understanding of where it goes or why.

jblow2• 8 years ago

We all have these things in our pockets that are massive supercomputers by the standards of my college days. And we all use them just to rant at each other about political things that we in reality are totally misinformed about ... and otherwise just waste our lives. I am not sure why quantum computing is supposed to change this.

Even software that is supposed to be useful is so terribly slow. Computers are between 2 and 4 orders of magnitude faster than programmers today experientially believe they are, because today’s culture of programming has rotted so thoroughly. Do you really need a quantum computer when 3 orders of magnitude are just sitting there on the table waiting to be picked up?

Cobord2• 8 years ago

Because of the expense it will have to start off as QCaaS. You send the really hard part of your problem to whichever person actually has the hardware and they'll return the answer. If you are concerned with them not knowing your data, there are workarounds to that as well. So available to general public in a weaker sense.

bscphil2• 8 years ago

What's the size needed for cracking large-primes based encryption? If not at 100 qubits, what are some useful calculations that we can do at that size?

deepGem2• 8 years ago

So the number of raw qubits are in the order of a million for large prime factorization or to make Shor's algorithm a reality. This is because the physical qubits are highly unstable and prone to errors. The error correction methodologies will result in a fault-tolerant logical qubit. Depending on the error correction algorithm, the ratio of logical: data/physical qubit is 1:1000 at the minimum. You need 4000 logical qubits at least for large prime factorization give or take 4 million raw qubits.

dzhiurgis2• 8 years ago

A lot of things regarding chemistry and biology - finding catalysts for nitrate fertiliser production (uses 5% of worlds power), perhaps some catalysts in cement production (my idea), invention of new medicines, either due to better cell simulations or (my idea) faster protein folding.

rurban2• 8 years ago

2048 would break current crypto, >20 would help solving most unsolvable NP hard problems.

ArtWomb• 8 years ago

Congrats! US House science committee also set to pass national initiative in quantum tech with on order of $1B funding:

National Quantum Initiative - Action Plan

https://www.lightourfuture.org/getattachment/85484dca-465a-4...

ahelwer3• 8 years ago

Crash course in entanglement:

The state of each qbit is represented by a state vector of two complex numbers [a, b] where |a|^2 + |b|^2 = 1. There are two special qbit values called the classical basis: [1, 0] which is the classical bit 0, and [0, 1] which is the classical bit 1. If a qbit is not in one of the two classical states, we say it is in superposition. When a qbit is in superposition, we can measure it[0] and it will collapse probabilistically to 0 or 1; for a qbit [a, b], the probability that it collapses to 0 is |a|^2 and the probability that it collapses to 1 is |b|^2.

Things get more interesting when we have multiple qbits. If we have two qbits [a, b] and [c, d], we define their product state as their tensor product [ac, ad, bc, bd]. For example, if we have two qbits both in state [1/sqrt(2), 1/sqrt(2)], their product state would be [1/2, 1/2, 1/2, 1/2]. We use the product state to calculate the action of a quantum logic gate that operates on multiple qbits - for a gate which operates on two qbits, we can always represent its action as a 4x4 matrix.

Usually we can move back and forth between the product state representation and writing out the individual qbits states. However, in certain scenarios something very special happens: we cannot factor the product state back into the individual state representation! Consider the product state [1/sqrt(2), 0, 0, 1/sqrt(2)]. If you try to write this as a tensor product of two states [a, b] and [c, d], you cannot! It cannot be factored; the qbits have no individual value, and we say they are entangled.

Well, what does this mean? It means when you measure one qbit, even if the qbits are very far apart, you instantly know the value of the other qbit. So if I entangled two qbits in the state [1/sqrt(2), 0, 0, 1/sqrt(2)], give you one, then we go to opposite ends of the universe, if I measure my qbit and see a 0 I'll know your qbit instantly also collapsed to 0 (or collapsed to 1 if I measured 1). This phenomenon has been experimentally-verified to occur faster than light. It is instantaneous, as far as we can tell. So, local realism is wrong! Spooky action at a distance is real!

There is an important caveat: while the qbits seem to coordinate in some faster-than-light way, you cannot use this to communicate in a faster-than-light way. All we have is a shared random number generator. I can't send some chosen bit from my reference frame to yours. This is called the no-communication theorem.

If you found this interesting, I have a full video on quantum computing for computer scientists here: https://youtu.be/F_Riqjdh2oM

[0] IDGAF about your chosen quantum mechanics interpretation, don't @ me

bdamm3• 8 years ago

The part I don't understand is the "spooky action at a distance." Isn't this just the same as if the qbits were already in whatever their final state was as soon as they were entangled? Or in other words, is there any experimental basis for determining that the "cat" stayed alive at all? It seems a lot less magical when we just determine that in fact the value is the same no matter when or where you measure it, although it may not be predictable. And since the "entangled" qbit always had the same value, no matter when you measured it, then there is no "spooky action at a distance". The action occurred when d=0, and what you actually have is two copies of the same ROM.

ahelwer3• 8 years ago

The idea that the qbits choose which value they'll collapse to at time of entanglement is called local hidden variable theory, which John Bell disproved in 1964: https://en.wikipedia.org/wiki/Local_hidden_variable_theory

In more practical terms, the theory also falls apart when you start doing more complicated things with entangled qbits than just measuring them, like quantum teleportation or error correction.

stouset3• 8 years ago

The explanation you're supposing for the "spooky action at a distance" is called a hidden variable theory, and as others have said, this explanation has been disproven. You might be wondering how, and I'll tell you the scenario (adapted from The Elegant Universe) that helped me understand.

Imagine you and I each have a device: a small box with one openable window on the top, one on the front, and one on the back. When one of these windows is opened, it shows either white or black. Further, I tell you that these boxes are entangled: if we both open the same window, the same color will be displayed on both boxes. This only works for the first window opened and only if we both open the same window: if a second window is opened or if we open two different windows, the colors shown will no longer be correlated in any way.

I actually have many of these pairs of boxes, each pair numbered uniquely to identify its matching twin, and we run the experiment for an arbitrarily large number of them until you're satisfied that my claim holds true. You see nothing special about these boxes and argue that they're simply preprogrammed from the factory to display a set of colors the first time a window is opened (for example, BWB or WWB). I counter that entanglement is special and the boxes are not preprogrammed from the factory. In fact, they decide at random which color to show right at the moment one of the boxes' windows is opened!

My claim seems crazy and at first glance possibly even unprovable! But it turns out with a bit of cleverness we can test this.

We take an arbitrarily large number of boxes and begin independently opening their windows at random (without consulting or coordinating with each other when deciding which window to open), recording box numbers, window choices, and the resulting colors. When finished, we compare notes. Sure enough, when we both open the same window on entangled boxes, the colors match as expected. Upon a closer look at the data, we discover something surprising. Ignoring which window we opened, each pair of entangled boxes showed the same color exactly 50% of the time. How is this surprising? A little bit of probabilistic analysis conclusively demonstrates that such a result is fundamentally incompatible with any hidden variable theory.

Assume for the sake of argument that the boxes are preprogrammed. The only possible programming for each box is some form of XXX or XXY. Consider: if both boxes are programmed WWW (or BBB) and we both open a window at random, it will show the same color 100% of the time. If both are programmed WWB (or BWB, or BWW, or any equivalent variant), 2/3 of the time there's a 2/3 chance of displaying the same color when choosing windows at random and 1/3rd of the time there's a 1/3 chance. Combined, whenever the colors are in an XXY configuration, there's a 5/9 chance of showing the same color when windows are chosen at random. Note that in both scenarios the odds of seeing the same color from randomly-chosen windows are greater than 50%! Assuming the preprogrammed colors are chosen at random, some form of XXX will be chosen 2/8 of the time and XXY will be chosen 6/8 of the time, we see that the total odds of seeing the same color if the boxes were preprogrammed should be 2/8 * 100% + 6/8 * 5/9 = 2/3. In theory, our data should have shown a 66% agreement in color, but our experiment consistently shows colors matching 50% of the time.

A hidden variable theory—any hidden variable theory—runs into this probabilistic hurdle.

divan• 8 years ago

I think this video deserves much more attention by HN readers, it's truly great introduction into quantum computing.

3pt14159• 8 years ago

Since speed warps spacetime, does the relative speed of the qubits change this? Can they go out of sync if one is moved much faster than the other for a prolonged period of time? If not, could this property be used for a universal clock?

Florin_Andrei• 8 years ago

"Much faster than the other" relative to what?

There is no absolute motion or speed. There is only relative motion or speed.

xpuente• 8 years ago

uhmmm ...

https://www.quantamagazine.org/gil-kalais-argument-against-q...

Cobord2• 8 years ago

Take that more as a challenge and a caution to avoid overhype.

nabla94• 8 years ago

Question:

Why superfluid 4He is not quantum-entangled? What differentiates macroscopic many-body quantum states from many-body quantum entanglement?

barbegal4• 8 years ago

Superfluid 4He is quantum-entangled and non-local effects are observable. https://www.sciencedaily.com/releases/2017/03/170321110344.h...

nabla94• 8 years ago

So why it's not taken into account when quantum-entanglement records are broken?

You can have flasks of superfluid 4He with thousands of moles of entangled atoms clearly visible.

tzakrajs3• 8 years ago

“It’s as though you took six bits in your computer, but each bit tripled in how much information it could hold,” Schreppler said

"Each bit is tripled in how much information it could hold"? Normally bits have two states, but Shreppler is claiming these qubits can have six states? What am I missing?

albertgoeswoof• 8 years ago

I thought qubits had an infinite range of states?

tzakrajs3• 8 years ago

Oh yeah, thats right.. Isn't it based on orientation in a spherical sense? I guess you could have an infinite number of those orientations.

barbegal4• 8 years ago

It's a confusing quote. They have achieved 18 qubits from 6 photons. But 18 qubits can store more information than 18 regular bits however when the 6 photon system is measured the quantum state collapses to 18 bits of data.

How do you get 18 qubits from 6 photons? You use multiple degrees of freedom. Whereas a bit in traditional memory has a single degree of freedom (electric charge), photons have many degrees of freedom such as polarity, direction of travel and angular momentum.

21• 8 years ago

Even in traditional memory, you can use a single degree of freedom to store multiple bits, by quantizing the charge. Like in TLC flash memory, where you can store 3 bit per cell.

This is because in traditional memory, a single "cell" stores many elementary charges (electrons).

jxy• 8 years ago

There are 18 qubits stored in 6 photons, where each photon effectively have 3 qubits from its 3 different degrees of freedom.

Reading the arxiv document prepared by Microsoft Word is so painful. Can't Microsoft learn something from TeX?

xtrapolate• 8 years ago

Is entanglement an actual physical phenomenon? What exactly needs to happen in-order for a photon to "become" entangled?

dvh• 8 years ago

Physical? As in everyday Newtonian physical? It doesn't work on quantum levels, just like Bohr's model of atom doesn't work during big bang because gravity we were successfully ignoring suddenly is on the same scale or bigger then electrostatic force. We will never be able to intuitively (physically) grasp quantum world because our intuition was trained on trees, rocks and cats.

mundo4• 8 years ago

Not sure what "actual physical phenomenon" means, but probably no. If you mean can you look at a photon and observe something about it that tells you it's entangled to some other photon, no. If you mean do we know what physical process underlies entanglement, also no. If you mean does entanglement really happen as opposed to being theoretical, yes. The link from the first paragraph of the article may help: https://www.scientificamerican.com/article/chinese-researche...

kelvinsilva• 8 years ago

https://www.reddit.com/r/QRL/

Chinese Researchers Achieve Quantum Entanglement Record

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