Lab sets a new record for creating heralded photons

May 20, 2013 by Phillip F. Schewe
Light produced in the process of spontaneous parametric down conversion. Credit: NIST

(Phys.org) —Entanglement, by general consensus of physicists, is the weirdest part of quantum science. To say that two particles, A and B, are entangled means that they are actually two parts of an inseparable quantum thing. An important consequence of this inherent kinship is that measuring a property of A (say, the particle's polarization) is necessarily to know the corresponding property of B, even if you're not there with a detector to observe B and even if (as explained below) the existence of that property had no prior fixed value until the moment particle A was detected.

To create such entanglement it is generally necessary to generate two at a time and to generate them so that they are born with this connected property. The most basic step in measuring such a system is to measure and detect both particles and to do so efficiently. So it had better be the case that if one detector registers a particle, the other detector should collect and register the other particle. Because we know that if we see one particle, the other must exist, we say that the detection of one particle "heralds" the existence of the other, just as medieval heralds, with their banners and bugles, signified the arrival of a king. Although in this case, because with these particles born in twos, one photon is no more regal than the other, so we can equally well say that one photon heralds the other and vice versa. But as in the case of a king, in real life even though the herald announces the king he may be waylaid and never appear.

An experiment conducted at the Joint Quantum Institute establishes a new record for heralding for a pair of entangled photons (particles of light). The JQI work is published in the May 15 issue of the journal Optics Letters. What happens is this: about 84% of the time the researchers observe photon A they also observe photon B just where it should be, and vice versa.

The JQI detection scheme will be useful for a number of reasons: it should help experiments to tighten remaining loopholes over the fundamental sway of quantum reality; it shows that sources of single heralded photons can achieve a certain level of reliability; and that might be a critical ingredient in producing a source of random numbers in a way that guarantees that any nefarious attempts to "load the dice" are impossible.

Indeterminacy

The JQI experiment demonstrates a photon source which could allow one to get to the heart of counter-intuitive nature of quantum reality by looking at indeterminacy. In common experience a coin facing up has a definite value: it is a head or a tail. Even if you don't look at the coin you trust that it must be a head or tail. In quantum experience the situation is more unsettling: material properties of things do not exist until they are measured. Until you "look" (measure the particular property) at the coin, as it were, it has no fixed face up.

What this indeterminacy means is that until it is observed an object has no definite value for that property. So the property in question, whether it is position, velocity, charge, polarization, or some other attribute, cannot even be said to exist. Instead the object is said to be in a superposition of states and its physical attributes can potentially take on a variety of values. When describing the existence of this particle, we can do no more than specify a set of probabilities that the object's properties have certain values. At the moment measurement occurs the object undergoes a "collapse of probability." The probability estimates in play just before measurement become superfluous. The property being measured—-the polarization of a photon, say—-has assumed a definite value, horizontal or vertical in this case.

Einstein's reservations

Describing reality in terms of indeterminacy and probability bothered Albert Einstein. Surely, he said, a particle's property exists before it is measured and a theory more complete than quantum mechanics would include the existence of those properties before they were measured. Those properties before measurement must be contained in some variables hidden from the standard quantum mechanical representation. The search for those "hidden variables" pertaining to the existence of things occupied a lot of Einstein's time in the latter part of his life, and has been a topic of concern with physicists ever since.

In the 1960s John Bell proposed a number of experiments designed to test the validity of things like entanglement and indeterminacy. So far all such tests have supported the validity of quantum indeterminacy and have discouraged the idea of any hidden variables. But for some skeptics, loopholes remain, and they argue that the reality of entanglement has not yet been adequately demonstrated. One reason for this is the difficulty in measuring properties of two or more (supposedly entangled) objects with sufficient efficiency. The relatively poor measurement efficiency, resulting in the failure to detect one or the other of the pair of entangled photons, allowed skeptics to assert that the measured sample of pairs did not constitute a good enough representation of the overall set of objects to be able to say something definitive about entanglement.

JQI experiment

The experiment effort in Alan Migdall's JQI lab specifically targets the efficiency of the heralding process. To start, the researchers send a beam of ultraviolet photons into a special crystal where, at a rate of about one per billion, a UV photon is turned into a pair of entangled photons. This process is called spontaneous parametric down-conversion (PDC). The laws of physics dictate that the momentum and energy of the incoming photon (from the pump beam) should be split between the daughter photons (one is called the "signal" and the other the "idler"). In this picture omega is the frequency of the respective photon and is proportional to its energy.

The daughters might, for instance, be a green photon plus a near-infrared photon, or two red photons, or any other combination of colors so long as the sum of the energies of the photons adds up the energy of the pump photon.

Each of the two photons makes its way through a lens and into a fiber so narrow that only a single mode can propagate. That is, if we think of the light not as a particle (photon) but as a bundle of electric and magnetic fields, the lateral profile of the ray will have a simple Gaussian shape. This kind of fiber, aligned to exacting standards, ensures that photons of a very specific energy and direction will be channeled into a photodetector where its presence and time of arrival can be determined.

Photon or vacuum?

"In effect the observation of photon A brings photon B into existence," says Alan Migdall, "at least if these are true entangled photons." This entanglement between the existence of a photon and no photon (or vacuum) is not what is usually considered to be entanglement but it is nonetheless.

The aim of this JQI experiment is not itself to test the Bell criteria for entanglement (as it turns out the polarizations of photons A and B are known be forehand), but rather to optimize the process of heralding—-the ability to say that if A is here then B is there. For some theories a heralding efficiency must at least 82% if loopholes are to be closed.

New heralding record

The JQI have now exceeded this yardstick. They typically observe about 50,000 signal photons (photon A) per second in their detector. And when this happens about 84% of the time a photon is seen in detector B. And simultaneously, when the roles of the two detectors are reversed a comparable percentage is registered. This is the highest symmetric heralding efficiency for a single-mode fiber yet seen in any experiment.

Migdall says that because of the random nature of observing a photon with an appropriately prepared state, the measurement of a heralded can be turned into a number that is truly random and guaranteed to be free of tampering. Such random numbers can, in turn, be used in various schemes to encrypt messages that can never be cracked.

Explore further: Researchers find qubits based on trapped ions offer a promising scalable platform for quantum computing

More information: "Demonstrating highly symmetric single-mode, single-photon heralding efficiency in spontaneous parametric downconversion," Marcelo Da Cunha Pereira, Francisco E Becerra, Boris L Glebov, Jingyun Fan, Sae Woo Nam, and Alan Migdall, Optics Letters, May 15, 2013.

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vacuum-mechanics
1 / 5 (15) May 20, 2013
Entanglement, by general consensus of physicists, is the weirdest part of quantum science. To say that two particles, A and B, are entangled means that they are actually two parts of an inseparable quantum thing….

Yes, but this something like saying that God (nature) is a magician; maybe this physical view could hint that actually it is human ignorance…
http://www.vacuum...19〈=en
gwrede
1.7 / 5 (6) May 20, 2013
This entanglement between the existence of a photon and no photon (or vacuum) is not what is usually considered to be entanglement but it is nonetheless.
"But it is, nonetheless?" So we can entangle a photon with no photon? I really wish I had an older brother at Cern or somewhere, who would answer all my questions properly.

The hard part is having to read about research in physics, where there is no guarantee that the experimenter has a clue, and that is written by some editor who definitely is clueless -- and then trying to build one's understanding of the world based on that. I'd read a good book on this any day, but I haven't found any that are both accessible to the interested layman and yet are written by somebody who really knows what he's writing. The nearest thing to this has been watching through the Feynman lectures on Youtube. Now there really was a guy who had a clue. (Yes, he said _nobody_ understands this crap, but that's not really what I am talking about here.)
axemaster
1 / 5 (1) May 20, 2013
I'm always annoyed whenever I read or hear someone saying that quantum physics and entanglement are "weird" or "don't make sense". I can't imagine a physics construction that wouldn't have these properties. In fact, I think the universe we have is incredibly, in fact bizarrely close to being fully classical.

The absence of useful nonlocal information transfer is by far the weirdest thing to me, and I think it says a lot about the way space is constructed, and how time effects propagate through it. My personal view is that nonlocal effects are completely dominant behind the scenes, but observing a system and "collapsing the state" (another horrifying misconception propagated among the legions of physics students) generates a return signal specific to the observer, while simultaneously reorienting the observed object such that it can't be re-observed in the exact same way (thus creating the so-called time's arrow).
axemaster
1 / 5 (1) May 20, 2013
@gwrede

The thing you're not understanding is that "entangled" really just means "logically related". So when we say that two electrons are entangled, we just mean that there is some logical relation between their properties. For example: two electrons are entangled such that their spin axes are pointing in opposite directions. We detect one of them and its spin axis is pointing down. Because they are entangled, we know that the other electron must be pointing up. That's really all there is to it.

Entangling a photon with no photon simply means that we are making the logical statement that because we saw a photon, another photon can't exist.

There's a ton of fuss about entanglement violating the speed of light or whatever, but that's because people choose to assume that time can't run backward, that nonlocal effects don't exist, and so on. Unfortunately physicists are always trained in classical physics first, and quantum second, so it's a difficult shell to break out of.
Telekinetic
1 / 5 (5) May 20, 2013
"In fact, I think the universe we have is incredibly, in fact bizarrely close to being fully classical."-axemaster

If you're as bold as your statement, then it'll be easy for you to accept the precepts of David Deutsch, who believes that what happens on this microcosmic scale will also happen on the macrocosmic scale. After all, he asks, are we not made of the same particles? Be prepared to detect your entangled twin in infinite variations of outcomes in an infinite array of universes.

ValeriaT
1.8 / 5 (5) May 20, 2013
There's a ton of fuss about entanglement violating the speed of light or whatever, but that's because people choose to assume that time can't run backward, that nonlocal effects don't exist, and so on.
This is equivalent from perspective of violation "the speed of light or whatever". The special relativity doesn't allow the time run backward in the same way, like it doesn't allow variable speed of light.
axemaster
3 / 5 (2) May 21, 2013
There's a ton of fuss about entanglement violating the speed of light or whatever, but that's because people choose to assume that time can't run backward, that nonlocal effects don't exist, and so on.
This is equivalent from perspective of violation "the speed of light or whatever". The special relativity doesn't allow the time run backward in the same way, like it doesn't allow variable speed of light.

I should probably rephrase "time can't run backward" to "effects can't propagate backwards through time". And this doesn't violate anything, it's integral to the mathematics of quantum theory.

@Telekinetic
I don't agree with the many worlds interpretation of quantum mechanics for the following reasons:
- It isn't necessary for quantum mechanics to work
- There's absolutely no evidence to support it
- It doesn't even do what it's supposed to do - it merely shifts the question of state collapse to the observer, i.e. why do we perceive one reality if many others exist
MaiioBihzon
2.3 / 5 (12) May 21, 2013
Axemaster, do you reject string theory? True, it's fairly unproven at this stage, but many theoretical physicists feel that, even if string theory as currently conceived is not correct, then there still must be something operating similar to it which incorporates both quantum mechanics and relativity. String theory supports the inflationary view of our cosmos, and predicts more than one spacetime bubble, of which ours is but one, as well as collisions between said spacetimes. WMAP found evidence in the CMBR of such a collision, which was recently confirmed by Planck:

http://www.newsci...vil.html

More controversially, we also get predictions of time-traveling Higgs singlets:

http://www.smh.co...4qq.html

String theory: other dimensions, other universes, time-travel.
eduard_burian_3
1 / 5 (2) May 21, 2013
axemaster: I don't agree with the many worlds interpretation of quantum mechanics for the following reasons:
- It isn't necessary for quantum mechanics to work
- There's absolutely no evidence to support it
- It doesn't even do what it's supposed to do - it merely shifts the question of state collapse to the observer, i.e. why do we perceive one reality if many others e


axemaster given you are rather young, and would have grandchild in timeframe of 40 years, and she or he will one day optimize her/his new avatar on a school 128k-qubit QC, by directly applying the quantum parallelism, and you would say just what you wrote there, guess how bad she/he will look upon you?

visual
1 / 5 (1) May 21, 2013
why do we perceive one reality if many others exist

why are you yourself and you are not me?
why are you the you of now and not the you of 24 hours ago, or the you of 10 years in the future?
equally pointless questions...

I'm with you though - i dont like many worlds either. but at least i can see its appeal, it definitely makes more sense to me than giving dices to god or giving the poorly defined act of observation some magical meaning.
axemaster
not rated yet May 21, 2013
axemaster given you are rather young, and would have grandchild in timeframe of 40 years, and she or he will one day optimize her/his new avatar on a school 128k-qubit QC, by directly applying the quantum parallelism, and you would say just what you wrote there, guess how bad she/he will look upon you?

As I just said, quantum mechanics does not require multiple universes to work. The many-worlds interpretation is just that - a philosophical interpretation of the science. It has absolutely no bearing on the actual workings or mathematics of quantum mechanics, and never has.

@visual
But the thing is, many worlds does give observation a magical meaning. Rather than say that a particle "collapses" into one state at random, it says that both states of the particle still exist, but our perception of the particle collapses at random. So the problem isn't solved at all - it's the same problem, only they move it from the system to the observer.
axemaster
not rated yet May 21, 2013
@MaiioBihzon
String theory supports the inflationary view of our cosmos, and predicts more than one spacetime bubble, of which ours is but one, as well as collisions between said spacetimes.

I don't reject string theory, though I kind of doubt it's correct. In any case, the multiple spacetime bubbles from inflation are an entirely separate issue from the many-worlds interpretation in quantum mechanics. It's quite possible for multiple spacetime bubble universes to exist, regardless of the veracity of the many-worlds interpretation.
MaiioBihzon
2.3 / 5 (12) May 22, 2013
"I don't reject string theory, though I kind of doubt it's correct."

Well, that's the thing. There is more than one string theory. But they all turn out to be aspects of M-theory. While no one seems to be exactly sure how it all works, most theoretical physicists agree that some version or other of the concept should do the trick.

"I don't agree with the many worlds interpretation of quantum mechanics for the following reasons:
- It isn't necessary for quantum mechanics to work
- There's absolutely no evidence to support it"

Double-slit experiments? Wave-particle duality? Entanglement?

"So the problem isn't solved at all - it's the same problem, only they move it from the system to the observer."

Doesn't decoherence dispense with dependence upon the observer, which is kind of the point of many-worlds? Instead of making the observer independent of and outside the system, the observer is part of the system. The importance of the system is paramount in many-worlds.
axemaster
not rated yet May 22, 2013
Double-slit experiments? Wave-particle duality? Entanglement?

As I already said, there are no effects in quantum mechanics that require the many-worlds interpretation to work. Many-worlds is a PHILOSOPHICAL VIEW - it contributes nothing to the mathematics and makes no testable predictions of any kind. If you're interested in the subject, I suggest this wikipedia page:

http://en.wikiped...echanics

Doesn't decoherence dispense with dependence upon the observer, which is kind of the point of many-worlds? Instead of making the observer independent of and outside the system, the observer is part of the system.

Not really. Many worlds just says that when a state collapses, the "unchosen" outcomes end up in other universes (rather than simply vanishing). The observer also splits apart. The point I'm making is that the randomness has not been dispensed with - it is moved from the system to the observer, since the observer "chooses".
MaiioBihzon
2.5 / 5 (13) May 22, 2013
"In any case, the multiple spacetime bubbles from inflation are an entirely separate issue from the many-worlds interpretation in quantum mechanics."

Because the equations used in physics to describe our world are unitary, the multiiverse continues to branch at every event with a random outcome, producing a plenitude of parallel universes. In addition to everything else, this argues for the many-worlds interpretation.

Minds much more immersed in this topic than mine show far better than I ever could that our universe could be described by a single wave function, you CAN be in two places at once, and there are therefore a great many worlds. As a result, the majority of physicists feel the many-worlds interpretation to be the correct one.

The dissenting minority must content with the case put forward by no less than Max Tegmark and John Wheeler. If you can disprove them, you will have won me over as another dissenter:

http://arxiv.org/...77v1.pdf
MaiioBihzon
2.5 / 5 (13) May 22, 2013
"[T]he observer 'chooses'" versus "The observer also splits apart." In the first case, many-worlds is not operating and the observer determines outcomes. In the second, many-worlds operates, and it is the system ~ or relationship ~ which produces outcomes. Pick one.

Again, I refer you to the link (above) to "100 Years of the Quantum" and the reasoning and evidence provided by Tegmark and Wheeler. If you can refute that, you have made your case against the many-worlds interpretation.

[In my previous post, please read "content" as "contend."]
LarryD
not rated yet May 22, 2013
Back in the 1960's I was discussing Logic with an Indian friend (Joshi) who had just majored in Physics/QM. Our discussion usually went on for hours but his basic argument was that Logic did not and could not function at the QM level while I argued that Logic could be applied to any and every level. However, what was clear to us both was that we couldn't get passed 'okay if we have a em wave what is it made of?' and so on. In a way I was leaning towards SS theory (although I din't know it). But SS today still bothers me because I would have to ask 'What is the string's composition and what are the Branes composed of?' Where does it end? Does there have be to and end?
But then this is perhaps a 'classical' line of thought and perhaps the 'many world'/'multiverse' is the answer...or is it/they just an excuse to stop asking questions....
antialias_physorg
3 / 5 (2) May 22, 2013
I don't agree with the many worlds interpretation of quantum mechanics for the following reasons:
- It isn't necessary for quantum mechanics to work
- There's absolutely no evidence to support it

...and it pretty much postulates that every moment an infinity of energy/universes are created - and the next instant an infinity times an infinity such universe ..etc. .

Which sort spoils the whole thing for me, since these universes originate from each other (i.e. are connected) but must also be perfectly detached from each other (otherwise we'd feel the exponential accumulations of 'parallel' mass/energy at some point in our universe)

It's a weird thought experiment and makes for some good (and not so good) SciFi. But I can't really get it to grok.
Telekinetic
1 / 5 (4) May 22, 2013
David Deutsch doesn't write science fiction, in fact, the quantum computing research industry depends on his work for practical applications. These theories aren't "flights of fancy" concocted for our entertainment, but carefully considered outgrowths of prior work. We can't "feel" other worlds, we need new instrumentation when are senses fall short. Through what channels does entanglement manifest? What regions does a teleported particle travel through before arriving at a place of detection? The MWI is not an outrageous postulation, I think it just scratches the surface.
MaiioBihzon
2.2 / 5 (13) May 23, 2013
"..and it pretty much postulates that every moment an infinity of energy/universes are created - and the next instant an infinity times an infinity such universe ..etc. . "

Antialias, you make good points. To the one raised above, that is one reason I introduced cosmological inflation with its multitude of bubble universes. A quantum fluctuation (of low, but nonzero, probability) produces our universe. Eventually, our universe will decay back into the vacuum which produced it, in obedience to conservation. But not before spawning other inflationary spacetimes. Guth did call this "the ultimate free lunch."

The birth of each inflationary region is the very thing that separates it from the others. But interaction is not necessarily be forbidden. Gravity may operate across the bulk space lying between spacetimes, for instance. And collisions are also possible (but not probable, due to the higher-dimensionality of the bulk space). Inflation provides a working multiverse model.
antialias_physorg
5 / 5 (1) May 23, 2013
A quantum fluctuation (of low, but nonzero, probability) produces our universe.

There's always the issue what the quantum fluctuation was in (a universe is much more than just matter/energy coming into being. It's spacetime coming into being, also).
And, of course, there's the problem that we tend to think in causal/time-linear way which may or may not hold for such events as a 'beginning of a universe' - and if it does then one just has moved the goalpost for the problem to that larger multiverse in which our universe then came to be

The beginning of a universe/multiverse is just one of these things where we currently have to admit: we don't know. (though that doesn't mean we can't come up a lot of interesting - and even possibly correct - theories). And it's a particularly hard problem since there is only one known instance of the event - and that one doesn't seem to be observable "from the other side"

Why did the symmetry break? And is "why" even the correct question?
visual
1 / 5 (3) May 23, 2013
axemaster:
but our perception of the particle collapses at random.

Um, no. Strictly speaking, there is no randomness. All states exist, all states are perceived. The "why am I me" "randomness" is something way to subjective, philosophical even, and is not a real issue for an objective physical model.
The same would be observed if we could duplicate matter... if we made a copy of you, both instances may be wondering "why am i this instance and not the other one" but that makes no more sense than the questions from my previous post.

Or, why am I me and not Queen Elizabeth? Randomness!
visual
1 / 5 (3) May 23, 2013
That said, I'll repeat that I also don't like MW. It just is better than CHI in that it is deterministic and logical, no dice-rolls. But if after collapse/decoherence the different "worlds" were completely separated and unobservable, they are indeed redundant entities in the model that should be dealt away with by Occam's razor. And we get back to CHI's dice rolls...

But I think the different "worlds" can still interact, even after a "collapse". This is the key difference from CHI.

Also, I do agree the "worlds" terminology is a bit poor. We can just stick to calling it a superimposed state... it is a purely semantical, interpretation point. The key again is that instead of CHI's "collapse" we get superimposed observer state.

And eventually it will be testable.
MaiioBihzon
2.3 / 5 (12) May 23, 2013
"There's always the issue what the quantum fluctuation was in"

A fluctuation in what? Cosmologists refer to the inflating field, which Linde describes as a foam giving rise to bubbles of false vacuum. Fluctuations in it tend to end naturally, just as quantum fluctuations in the vacuum do. But some fluctuations continue to expand, birthing universes.

Omega appears to be very close to one. The negative energy of gravity seems to perfectly balance the positive energy of matter. This is precisely what is to be expected of a quantum fluctuation.
LarryD
not rated yet May 23, 2013
The article quotes 'To start, the researchers send a beam of ultraviolet photons into a special crystal where, at a rate of about one per billion, a UV photon is turned into a pair of entangled photons. This process is called spontaneous parametric down-conversion (PDC).'
'...one in a billion...' seems very low to me so that I wonder why it should happen at all?
I assume that as a relatively small experiment the pump beam source was a 'lamp' of some kind and not, say, protons from a reactor...could someone explain which is most likely to produce the articles 'one in billion'? (Me being a layman...thanks)

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