Month: June 2017

QCJC: Yin 2017!

Wow! Hot off the presses from Science, we have a new world-record for quantum entanglement distance!

Front cover of the June 16th Issue of Science! Copyright AAAS

And I’m especially excited to be covering this article, since I have previously written about the Micius satellite here for the Yale Scientific, and I was fortunate enough to have worked in the same lab in Shanghai where they designed and built this amazing satellite!

As far as I can tell, this is the first time that Micius is being written about in a scientific journal, and it’s especially thrilling to me to see it on the cover of Science. But why wouldn’t it be? After all, we’ve only broken the previous record for entangled particles by 1000 kilometers, and oh, the sources were orbiting Earth in space and the photons were still able to be detected!

Most of the news sources that are covering the story are focusing on the implications of Quantum Key Distribution and the Bell Test, so I’ll go ahead and highlight a few that I thought did a very good job: LiveScience has a good article from Edd Gent with quotes from Dr. Pan, Nature’s Davide Castelvecchi does a very good job in placing Micius in the context of other quantum satellites, and Washington Post’s Sarah Keplan does an excellent job explaining the science of quantum entanglement in easy-to-understand terms.

But what about the satellite itself? How does it manage to transmit these entangled photons over such an incredible distance without getting out of sync? In my last blog post, I did some hand-waving regarding a function known as “fast feed-forward”, saying that this system “provides necessary support.” Now, with the publication of the journal article, we can understand how exactly that happens.

First, the basics. The Quantum Experiments at Space Scale (QUESS) Satellite, Micius, was launched last August, to begin a very bold experiment in setting up a Quantum Key Distribution (QKD) scheme. It planned on entangling two photons from the satellite, and then send those pairs of entangled photons down to different land-based locations within China, in Qinghai, Xinjiang, and Yunnan. These land-based stations are separated by a distance of around 1100-1200 kilometers, far exceeding any past records of quantum teleportation. These quantum effects were verified by a Bell Inequality Test, a standard manner of measuring if something is quantum or classical. The entangled photons received from Micius had a four standard deviation confirmation for it’s quantum, not classical, nature.

The primary motivation for such an experiment is that the distance between the satellite and the Earth, while generally being between 700-800 kilometers for a sun-synchronous orbit, only really travel through 16 kilometers of atmosphere. For the rest of that distance, those photons are traveling through near-vacuum, meaning that there is little environmental disturbances that could bump into the photon. This is the key problem with ground-based fiber optic cables; even with the best fiber optics, there is still a significant loss from bumping around the wire.

But of course, space is not very forgiving either. The biggest problem is the fact that you are distributing from a satellite, and hitting a tiny 1.2 meter telescope on the ground while whizzing around the Earth at 1100 miles per hour. The probability of this happening by chance is minuscule – “the equivalence of a quarter into a slot 100,000 meters away” as I mentioned last year. Furthermore, the implementation of this satellite is different from other systems used in Canada and Europe, in that Micius generates the photons on the satellite, rather than receive entangled photons from the ground.

This paper explains how the group was able to solve both of these problems, first by pulsing laser light through a crystal to generate the entangled photons, and then using laser signals that are sent with the primary quantum signal to implement the mysterious fast feed-forward system.

The production of entangled photon pairs happens on the satellite as a very weakly powered laser pumps light through a KTiOPO4 crystal on board. To give a sense of the scale here, the laser only uses 30 milliWatts, which is only 0.03% of the power needed for a normal 100 Watt light bulb. And yet it still produces almost 6 million entangled photons per second! But to qualify that, the 100 Watt light bulb would be producing somewhere around 6 Quintilian (18 zeros) photons per second. This might be a very weak beam of light, but the more important aspect is the quality of the entanglement, not the quantity of photons. As the process continues, photons are produced in the classic Bell entangled state of

|Ψ> =1/2 (|01> + |10>),

the necessary condition for photon entanglement. Now, to send them out to Earth!

To understand how the laser light is synchronized, imagine that you are jogging with your friend around a lake. But instead of jogging side by side, you want to jog exactly opposite the lake from your friend. If you have a very large lake, even if you think you are jogging at exactly the right speed to match your friend, you may find that the two of you start to drift further and further apart.

You and your friend are running around a lake, and you want to be exactly opposite each other as you run.

To make sure that the two of you are in the right place, you might want to send a signal to the other person, something to show your friend where your true location is. Let’s say that the easiest way to do this is by laser light – you have a green laser that you are pointing at your friend, and as long as your friend stays aligned with it, she would be directly across the lake from you. But what if your green laser starts slipping away from the true location? Then you would be back to square one!

Therefore, instead of using only one laser for alignment, suppose that we have two, one for you and one for your friend. As you jog, both of you keep your own lasers pointed directly across the pond, while trying to stay exactly on the laser of the other person. This two-fold alignment helps keeps both people in the right place at the right time.

With two lasers aligned on each other, the runners are able to have more certainty in keeping in pace with each other.

The same idea is exactly in play with the Micius satellite, but at a more complex level. Along with the primary 810nm signal, an additional signal of green light at 532nm is sent from the satellite, while a signal of red light at 671nm is being sent from the ground. These two additional signals feed directly into very sensitive motors near the mirrors of the satellites, allowing minuscule changes to be made, keeping everything properly aligned. There is additional complexity, since the motion of the satellite itself creates a drift in arrival time for the photons. This additional error needs to be accounted for by considering the relative motion between the satellite to each ground station, and quickly changing the polarization angle on Earth.

The thing that blew me away the most was this sentence: “Compared with the previous method of entanglement distribution by direct transmission of the same two-photon source – using the best performance … the effective link efficiency of our satellite-based approach … is 12 orders of magnitude higher.” 12 orders of magnitude! That is 1 TRILLION times better than any current fiber optics! And even at a theoretically perfect fiber optics, which have not been received yet, this first satellite experiment would be better by four to eight orders of magnitude, or between 10 thousand and 100 million times better. Wow.

Most of the remainder of the paper is in providing the method for determining Bell test violation. As a quick refresher from my previous post, the Bell inequality is a conclusion drawn from regular statistics. All classical physics is within the Bell inequality, but quantum physics violates the Bell inequality. Therefore, getting a result that violates the Bell inequality shows that the system is quantum, not classical. With Micius, the entangled photons were shown to violate the Bell inequality by four standard deviations, meaning that the probability of such a result happening by chance is around 1 in 16,000.

Honestly, this research is so exciting and fresh, and I absolutely can’t wait to read more about it. In the Nature article linked above, Dr. Pan says that their team has already performed QKD experiments, but are not yet ready for publication. But when it does publish, it will definitely change the world.

Reference: Yin, J. et al. Satellite-based Entanglement Distribution over 1200 Kilometers Science 356 1140-1144 (2017)

Disclaimer: Don’t take my blog post as unshakable truths! It’s very enriching to read the original paper itself, with it’s many beautiful diagrams. See link to Science here for the original publication. If you are a university student, you may need to use your VPN to get access.

QCJC: Aspect 1981

Digging further and further back now, for a very early paper on Bell’s Theorem. Honestly, I think the best reason I can come up with for reading this paper is that it just happens to be in the proper queue… so if you could suggest some better papers for me to read, please do! Everyone is free to comment here, or email me directly :)

Moving on to the paper! This paper is an experimental demonstration of Bell’s Inequality, the same issue that was raised by EPR (and was mentioned in the last QCJC). This inequality, and subsequent theorem, makes local hidden variable theory exclusive from quantum mechanics. Whenever Bell’s inequality holds, a local hidden variable theory is possible. However, quantum mechanics proposes a value that violates Bell’s inequality, and therefore, violates local hidden variable theory. When we speak of Bell states, we often speak of entangled particles that obey quantum mechanical laws and NOT local hidden variable theory.

A few more words about local hidden variable theories: Originally, in the EPR paper, the topic was dealing with just hidden variable theories in general. Hidden variable theories are just the idea our formulation of quantum mechanics is incomplete, and are actually reliant on some underlying “hidden” variables that we just don’t know about yet. This theory was proposed by Einstein, Polesky, and Rosen (yes got the name right this time!), and seemed to rely on common sense. After all, how would a pair of entangled particles transmit information faster than the speed of light? But, in time, this “spooky action at a distance” has shown to be in fact a real, verifiable fact through experiment.

This paper is one such demonstration of particles that violate Bell’s theorem, thus showing to be quantum mechanical and not local hidden variable theory dependent. But before we go further, we should probably state how Bell’s inequality is intended to be measured. (Aside: I thought I was quite familiar with the ideas of Bell’s Theorem when I started writing. But it turns out that I was more comprehensive of the philosophical implications, and wasn’t as thorough with the physics…)

Bell’s theorem by itself is simply derived from standard probabilities. If we take the original inequality, we have that

N(A, not B) + N(B, not C) >= N(A, not C)

We can prove this simply as the following:

N(A, not B, C) + N(not A, B, not C) >=0

N(A, not B, C) + N(not A, B, not C) + N(A, not B, not C) + N(A, B, not C) >= N(A, not B, not C) + N(A, B, not C)

N(A, not B) + N(B, not C) >= N(A, not C)

QED! Very simple, very direct. Supposing that A, B, and C are each independent variables, there shouldn’t be any variables that violate these rules. We have just shown it to be true simply by counting and through sets.

Now, we take A, B, and C, to be hidden variables corresponding to the spin of a photon, for instance. We might be interested in the spin of the photon along three different axes, such as a 0 degree axis, a 45 degree axis, and a 90 degree axis. We know that the spin can either be up or down in each of these directions, when measured. Furthermore, we know that a pair of antisymmetric photons is being produced. That is, if one of the photons has spin up in 0 degrees, the other photon must have spin down in 0 degrees, because at an earlier point, they had total spin 0.

Since we have two different antisymmetric particles, we can randomly test each individual particle with randomly chosen axis. Afterwards, we compile the information for each pair, and compute the inequality. Was it true that the number of pairs with spin up in 0 degrees, spin down in 45 degrees plus the number of pairs with spin up in 45 degrees and spin down in 90 degrees greater than the number of pairs with spin up in 0 degrees and spin down in 90 degrees? Or was this inequality violated, as quantum mechanics would predict?

After 650ish words, we have finally gotten to this paper! The paper provides a new way for experimentally checking Bell’s theorem. The authors first discuss the earlier tests of Bell’s theorem, which use positronium anihilation – when an electron meets a positron and annihilates each other. Then, they discuss a better way of conducting the tests using low-energy photons produced by atomic radiative cascades. They claim that the photons produced in this method is better for testing Bell’s theorem. I’m not entirely sure why this is the case, but it seems to have to do with detector efficiencies and/or efficient polarizers. The authors claim that it is able to not require “strong supplementary assumptions” that would otherwise apply.

The experimenters use the atomic radiative cascade of calcium, which yields two photons that have polarization correlations. They demonstrate how they set up their cascade, through irraddiating an atomic beam of calcium with a single-mode krypton ion laser, and a clockwise single-mode Rhodamine dye laser. The reason for choosing these two lasers is so that they have parallel polarizations, and have wavelengths that are corresponding to the different states of the Calcium, allowing “selective excitation”. By controlling these factors, the experimenters are able to very finely control the photon source, producing more data than previous experiments had done.

Next, the paper discusses the optical elements used in this experiment. They discuss the filters used to prevent photon reflections, as well as the two different polarizers that were constructed to perform the measurements. They have lots of specific information regarding how the piles of plates inclined near the Brewster’s angle (for polarization) would perform the polarization. They also provide data regarding the transmittances for each of the polarizers.

Then, the paper discusses the electronics that allow for coincidence counting to occur. It remarks on the TAC and Multichannel analysers that provide a time-delay spectrum, allowing for the monitoring of coincidences. This, of course, is crucial to the computation of Bell’s inequality and whether it is violated.

In the end, the group discovered a violation of Bell’s inequality (and a confirmation of quantum mechanics) by over 13 standard deviations, for both near and far (6.5 meter separation). This is great for quantum mechanics!

I think this blog post spent a bit too much time on the theory of Bell’s inequality, which is a shame given how interesting the experimental part is. But hopefully I will have a chance to explore other papers on Bell’s inequality and discover more there!

Continue reading “QCJC: Aspect 1981”