Quantum physics first: Researchers observe single photons in two-slit interferometer experiment

June 2, 2011, University of Toronto

Quantum mechanics is famous for saying that a tree falling in a forest when there's no one there doesn't make a sound. Quantum mechanics also says that if anyone is listening, it interferes with and changes the tree. And so the famous paradox: how can we know reality if we cannot measure it without distorting it?

An international team of researchers, led by University of Toronto physicist Aephraim Steinberg of the Centre for and Quantum Control, have found a way to do just that by applying a modern to the historic two-slit interferometer experiment in which a shone through two slits results in an interference pattern on a screen behind.

That famous experiment, and the 1927 Neils Bohr and debates, seemed to establish that you could not watch a particle go through one of two slits without destroying the interference effect: you had to choose which phenomenon to look for.

"Quantum measurement has been the philosophical elephant in the room of for the past century," says Steinberg, who is lead author of Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer, to be published in Science on June 2. "However, in the past 10 to 15 years, technology has reached the point where detailed experiments on individual really can be done, with potential applications such as and computation."

With this new experiment, the researchers have succeeded for the first time in experimentally reconstructing full trajectories which provide a description of how move through the two slits and form an . Their technique builds on a new theory of weak measurement that was developed by Yakir Aharonov's group at Tel Aviv University. Howard Wiseman of Griffith University proposed that it might be possible to measure the direction a photon (particle of light) was moving, conditioned upon where the photon is found. By combining information about the photon's direction at many different points, one could construct its entire flow pattern ie. the trajectories it takes to a screen.

"In our experiment, a new single-photon source developed at the National Institute for Standards and Technology in Colorado was used to send photons one by one into an interferometer constructed at Toronto. We then used a quartz calcite, which has an effect on light that depends on the direction the light is propagating, to measure the direction as a function of position. Our measured trajectories are consistent, as Wiseman had predicted, with the realistic but unconventional interpretation of quantum mechanics of such influential thinkers as David Bohm and Louis de Broglie," said Steinberg.

The original double-slit experiment played a central role in the early development of quantum mechanics, leading directly to Bohr's formulation of the principle of complementarity. Complementarity states that observing particle-like or wave-like behaviour in the double-slit experiment depends on the type of measurement made: the system cannot behave as both a particle and wave simultaneously. Steinberg's recent experiment suggests this doesn't have to be the case: the system can behave as both.

"By applying a modern measurement technique to the historic double-slit experiment, we were able to observe the average particle trajectories undergoing wave-like interference, which is the first observation of its kind. This result should contribute to the ongoing debate over the various interpretations of quantum theory," said Steinberg. "It shows that long-neglected questions about the different types of measurement possible in quantum mechanics can finally be addressed in the lab, and weak measurements such as the sort we use in this work may prove crucial in studying all sorts of new phenomena.

"But mostly, we are all just thrilled to be able to see, in some sense, what a photon does as it goes through an interferometer, something all of our textbooks and professors had always told us was impossible."

Explore further: Observing a Photon no Longer a Seek-and-Destroy Mission

More information: Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer, Science 3 June 2011: Vol. 332 no. 6034 pp. 1170-1173 DOI: 10.1126/science.1202218

A consequence of the quantum mechanical uncertainty principle is that one may not discuss the path or “trajectory” that a quantum particle takes, because any measurement of position irrevocably disturbs the momentum, and vice versa. Using weak measurements, however, it is possible to operationally define a set of trajectories for an ensemble of quantum particles. We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes. The results provide an observationally grounded description of the propagation of subensembles of quantum particles in a two-slit interferometer.

Related Stories

Observing a Photon no Longer a Seek-and-Destroy Mission

June 2, 2004

A team of University of Queensland, Australia physicists has devised a sophisticated measurement system for single particles of light, or photons, enabling them to investigate fascinating behaviour in the quantum world. ...

Physicists resolve a paradox of quantum theory

January 14, 2009

University of Toronto quantum physicists Jeff Lundeen and Aephraim Steinberg have shown that Hardy's paradox, a proposal that has confounded physicists for over a decade, can be confirmed and ultimately resolved, a task which ...

Recommended for you

Trembling aspen leaves could save future Mars rovers

March 18, 2019

Researchers at the University of Warwick have been inspired by the unique movement of trembling aspen leaves, to devise an energy harvesting mechanism that could power weather sensors in hostile environments and could even ...

Quantum sensing method measures minuscule magnetic fields

March 15, 2019

A new way of measuring atomic-scale magnetic fields with great precision, not only up and down but sideways as well, has been developed by researchers at MIT. The new tool could be useful in applications as diverse as mapping ...

Researchers report new light-activated micro pump

March 11, 2019

Even the smallest mechanical pumps have limitations, from the complex microfabrication techniques required to make them to the fact that there are limits on how small they can be. Researchers have announced a potential solution—a ...


Adjust slider to filter visible comments by rank

Display comments: newest first

5 / 5 (6) Jun 02, 2011
".... thrilled to be able to see what a photon does as it goes through an interferometer.."

Well, just what does it do?

5 / 5 (2) Jun 02, 2011
They seem to be saying that the experiment shows that a photon exhibits both particle-like and wave-like behavior at the same time. They claim that this result somehow contradicts Bohr's Complementary Principle.
2 / 5 (8) Jun 02, 2011
ummm let's see the evidence and explanation of what it is doing, this article was a bunch of hot air!
2.4 / 5 (5) Jun 02, 2011
"we were able to observe the average particle trajectories undergoing wave-like interference,"

could this be from the fabric of space, the wave patern. would make more sence to me
2.2 / 5 (13) Jun 02, 2011
God, this debate of quantum uncertainty has always bugged me, the original experiment is easily explainable and no big mystery but thanks to the idiotic need to understand something that is technologically decades or possibly centuries ahead of us we have formulated a crude understanding of the situation that only proceeds to cloud our judgement even more.

It's time to move on to more important things people, wait for gravimetric sensors that are sensitive enough to detect your quantum particles otherwise your measurements will ALWAYS affect the experiment in an observable way thus fouling the scientific process and giving you incomprehensible results.
4 / 5 (6) Jun 02, 2011

Things may not be as simple as you suspect.

Variations on the double-slit experiment send one of an entangled photon pair through the double-slit apparatus. In these experiments, by making measurements on the entangled photon, the interference pattern can be collapsed without making any measurements of the photon that goes through the double-slit itself.

Basically measuring the *other* photon, and by inference *knowing* something about the double-slit photon, *still* collapses the interference pattern. This isn't a case of the measurement affecting the photon going through the double-slit (at least not without invoking some of the seemingly "incomprehensible" aspects of QM).

While much of quantum mechanics can seem "incomprehensible", in reality it is indeed possible to understand. Perhaps it's best to call it "unintuitive but comprehensible".
4.5 / 5 (8) Jun 02, 2011
Well I still like the experiment that was done with a wave tank.
He placed a vibration source under the tank and watched the surface oscillate. He then turned down the vibration until the surface vibration appeared null. Then he dropped liquid (the same as that in the tank) via dropper into the tank. The drop would hover above the surface due to the force of the falling drop compressing air in front of it, which pushed down on the liquid just before impact. The liquid would push back with equal force and cause the drop to hover. Part of this "repulsive" force was due to the interaction with the liquid in the tank, which was oscillating below the visible threshhold.

Anyway when barriers were placed in the tank, and the drop nudged, it behaved the same as the double slit experiment. The key here is that the drop was a particle, however the medium (liquid in the tank) acted as a wave. The mathematics of fluid dynamics described everything perfectly, which also matched that of Q.M.
4.5 / 5 (11) Jun 02, 2011
So from this, I can't help but think that SPACE itself provides the wave like properties to matter, as does the liquid in the above mentioned tank experiment. Curled up energy, ie MATTER, gives the particle like behaviour. The two interact, and presto you can explain the behaviour, and feel good about the problem of both WAVE and PARTICLE behaviour. I believe all of our understanding problems in Q.M. stem from not knowing enough about SPACE itself, and forgetting that IT IS SOMETHING and PART of the SYSTEM being observed, and not merely the stage on which the play is acted out on.
5 / 5 (4) Jun 02, 2011
The experiment did say they were able to track light as a single photon through it's whole path while still producing interference patterns over time.

The big deal is: Light stayed as a single and whole particle the whole time.

I just wish they showed us a few examples of the path.
1 / 5 (5) Jun 02, 2011
Measuring the dark matter/energy associated with the photon on an ultra-microscopic level should also yield accurate results. An omniverse scanner would be an ideal term to use for such a device.
1.8 / 5 (5) Jun 02, 2011
@ Ober:

5 / 5 (1) Jun 02, 2011
Well I still like the experiment that was done with a wave tank.

Do you have a link or reference for that? I'm interested in hearing more.
5 / 5 (2) Jun 02, 2011
One thing that can keep me up at night is whether or not we'll ever be able to distinguish space and vacuum.

In other words: Is there such a fabric as spacetime without matter/energy in it? :x
2 / 5 (4) Jun 02, 2011
In other words: Is there such a fabric as spacetime without matter/energy in it? :x
Yes there is a hypothesis/theory for a void or bubble or something like that. There was an article or somebody talking about it recently. If I find it I will let you know. There are lots of parallel dimensions, so empty space would be difficult to believe.
3.4 / 5 (5) Jun 02, 2011
So this looks like the death knell of the Copenhagen interpretation, and (finally) an end to the Many Worlds Interpretation (which has made for some great sci-fi, but equally terrible theoretical physics).

Theres a graphic representation of the electron trajectory pathways in the double slit experiment at the Bohm-De Broglie wiki page, which offers some clarity on the wave distribution pathways of particles according to pilot wave theory:


Briefly put, in pilot wave theory the wavefunction of the particle travels through both slits, interfering with itself between the slits and the screen. The particle is attracted to the constructive interference regions, and repelled by the destructive interference regions, which provides a deterministic mechanism for a probabilistic distribution pattern on the screen.
4 / 5 (4) Jun 02, 2011
Here's the abstract page, with the experimentally-derived single-photon trajectories chart:

2 / 5 (4) Jun 02, 2011
... The key here is that the drop was a particle, however the medium (liquid in the tank) acted as a wave. The mathematics of fluid dynamics described everything perfectly, which also matched that of Q.M.

So, you're saying your a good, old-fashioned 'aether-ist'?
2 / 5 (4) Jun 02, 2011
... The key here is that the drop was a particle, however the medium (liquid in the tank) acted as a wave. The mathematics of fluid dynamics described everything perfectly, which also matched that of Q.M.

So, you're saying your a good, old-fashioned 'aether-ist'?

So, you're saying you're illiterate?
Jun 03, 2011
This comment has been removed by a moderator.
2 / 5 (2) Jun 03, 2011
I have every reason to trust that wave dynamics are the key here. The paradox is real. It is the significance of the consensus that leaves open the abstraction of what we are dealing with here. Spend less time doubting and more time wondering, and then all you have to do is inquire. After all, it is the imagination we are contesting when it comes to righting all the wrong perceptions.
5 / 5 (3) Jun 03, 2011
These experiment nicely complement with recent of Couder, Fort - they've recreated wave-particle duality on macroscopic oil droplets - bouncing on liquid surface and so coupled with waves they create.
They observes interference for them (corpuscle goes one way, but waves goes both), tunneling effect, orbit quantization ...
3.7 / 5 (3) Jun 03, 2011

Except that De BroglieBohm theory (pilot-wave theory) contains the same "many worlds" of dynamically separate branches as the Everett interpretation (because it is based on same global wave function as Everett many-worlds interpretation).

Or like David Deutsch said:

"Pilot-wave theories are parallel-universe theories in a state of chronic denial"

So this experiment can be looked at as a proof of many-worlds interpretation! Hurrah!
4 / 5 (2) Jun 03, 2011
Just oppositely, MickeyMike - that we don't need out of physics story tales, but Lagrangian mechanics is already enough (like QFT) - where is a place for splitting of universe in Euler-Lagrange equations?
Look at Couder, Ford oil droplet interference paper above or about tunneling or orbit quantization already for classical field theory:
2.5 / 5 (2) Jun 03, 2011
... The key here is that the drop was a particle, however the medium (liquid in the tank) acted as a wave. The mathematics of fluid dynamics described everything perfectly, which also matched that of Q.M.

So, you're saying your a good, old-fashioned 'aether-ist'?

So, you're saying you're illiterate?

No, not really .. What im saying is that you are stupid in your analogy.
3 / 5 (2) Jun 03, 2011
So Deutsch is saying that the only viable contender to the MWI, is also a MWI. That's awfully convenient, coming from the guy who wrote the book on MWIs. I'd like to know what evidence, if any, he would accept as falsification of his favorite pet theory (which, frankly, offers little more than sheer audacity).

Contrarily, Antony Valentini makes a compelling argument that the PWT is not a MWI. Here's a quote from his paper on this topic:

from a de Broglie-Bohm viewpoint, alternative trajectories are merely mathematical and not ontological. Thus, from the perspective of pilot-wave theory itself, many worlds are an illusion. It is further argued that, even leaving pilot-wave theory aside, the theory of many worlds is rooted in the intrinsically unlikely assumption that quantum measurements should be modeled on classical measurements, and is therefore unlikely to be true.

3 / 5 (4) Jun 04, 2011
photon is a packet of energy which by moving into quantum vacuum creates waves in the vacuum itself.....
3 / 5 (1) Jun 04, 2011
... The key here is that the drop was a particle, however the medium (liquid in the tank) acted as a wave. The mathematics of fluid dynamics described everything perfectly, which also matched that of Q.M.

So, you're saying your a good, old-fashioned 'aether-ist'?

So, you're saying you're illiterate?

Here, for the brainless:
""Science and engineering

Aether (classical element), a concept, historically, used in science (as a medium) and in philosophy (as a substance)"

2.7 / 5 (3) Jun 04, 2011
Well, the situation with quantum mechanics is, it can only compute the shape of diffusive flabelliform patterns during double slit experiment. But the double slit experiment still brings somewhat deeper information - the location of individual particles, which appear like spots at the target. We should realize, it's not the same, like the probability function: the observable reality is substantially richer and we have no theory for explaining it, predicting the less - despite this phenomena is real. The probabilistic character of quantum mechanics doesn't mean, the particles appear like the probability function - they're still appear like the particles with distinct paths.
3.5 / 5 (4) Jun 05, 2011
elementary particles motion is affecting quantum vacuum and vice versa....quantum vacuum waves are affecting particles
not rated yet Jun 06, 2011
Forgive me for not being a quantum physicist, but the abstract does state that the path was reconstructed from measurements. Isn't that different than recording concurrently? And this analogy may not be apt; but from a series of photos of one horse in the Grand National, one might reconstruct the race. But that's different than riding another horse in the race and immediately experiencing the first horse's behavior. (Crowded me at the last fence, the bugger!)
not rated yet Jun 06, 2011
"Is "quantum vacuum" continuous or discrete?
The is where the marriage ends.
Where everyone files for divorce.
The GR-ers and the QM-ers are single again."

This "quantum vacuum" is both continuous and discrete, until this paradox is understood as real the answer will elude understanding.
5 / 5 (1) Jun 06, 2011
Here's the abstract page, with the experimentally-derived single-photon trajectories chart:


Excellent! The image in that PDF speaks volumes as well. Look at how each photon changes direction along its path, being attracted to certain paths. It's like there are static attractors (and/or repellents) in the space between the plane of the slits and the screen. Interesting also that the photons are not traveling in straight lines.
1.8 / 5 (5) Jun 07, 2011
"Mirrored Quantum Reverberations" may explain this!

Rip theory predicts that the Probability Waves calculated by Quantum Physics are the mathematical expressions that calculate the culmination of source waves, their "reflections", and their "reverberations", through both Common Space and True Space. Rip Theory predicts that all rips are connected by a minimum of one continuous dimension of True Space, this "connectedness" facilitates that the edge movements of a source rip are "mirrored" by all other rips in the universe. The subsequent vibrations/movements due to this "mirroring" are referred to as "mirrored reverberations". These "mirrored reverberations" then travel to all other rips, including the source rip, through both True Space and Common Space. (The vibrations/movements "travelling" through True Space would be somewhat "instantaneous", the vibrations travelling through Common Space would be limited to the speed of light. Gravity Tubes/True Space Tunnels are facilitated by the one continuous dimension of True Space that exists between all rips.) Rip Theory Predicts that as these multitudes of waves crisscross back and forth through both Common and True Space, and proposes that the culmination of their amplitudes at each point in space are represented by the Probability Waves of Quantum Physics.
When a Source-rip vibrates/moves, that movement/information/energy is transferred through true space to all the U-rips in the universe "instantaneously" and through Common Space at or below the speed of light. (The term U-rip refers to all the "other" rips in the universe.) The U-rips add this information to theirs and these new culminations of movement/information/energy emanate outward from each U-rip through both Common Space and True Space. All the U-rips and the source rip then get this information and add it to theirs, then emanate this new culmination through both True Space and Common Space to all other rips. (The "senders" now see the Source-rip as a U-rip.) As these "waves" of movement/information/energy travel to other rips, they are themselves mirrored by U-rips (these new culminations are "mirrored reverberations" of the source waves). The different magnitude source waves and reverberated waves that travel outward from each individual rip though Common Space will sooner or later cross paths. At the location where they cross paths there will be a culmination.
The "mirrored waves" that reverberate through True Space, lengthening and shortening, constricting and dilating the Gravity Tubes between the rips, thus the influencing the rips at that location. Sooner, or later, the waves emanating from all the U-rips through both Common Space and True Space reach back to the Source-rip. At that point The Source-rip adds this movement/information/energy to it's present movement/information/energy and the process starts all over. At this time there are a multitude of subsequent "mirrored" waves still reverberating back and forth through both True Space and Common Space. These waves reverberate back and forth between all rips. Each time a wave or movement is received by a rip, it adds that movement/information/energy to it's present culmination, then this new culmination is emanated outward through both True Space and Common Space to all other rips. The culminations of these Mirrored Quantum Reverberations produce multitudes of peaks and troughs of differing amplitudes between and around particles. Rip Theory also predicts that the ever-changing cross-sectional areas of the Gravity Tubes between rips are the source of the Gravity Fluctuations that occur at small scales. The ever-changing length, cross-sectional area, and location of the Gravity Tubes between rips cause the gravity at any given location to change moment from moment.
© Copyright 2011 Thomas A. Sullivan
not rated yet Jun 13, 2011
Above comment reported for abuse, it is at least 3x as long as the 1k character limit.
1 / 5 (2) Jun 17, 2011
A good experiment would show a distinction between the photon model and some alternative. No matter how you do the double slit experiment, it will demonstrate the paradox of the photon model. I did a simple decisive experiment with singly emitted gamma-rays, a beam splitter, and two detectors measuring coincidences. Photons should measure no coincidences beyond chance. I, UnDr Unquantum, break chance many times, many ways. There are no photons, its classical light. So what makes the particle-like detection event? It loads up to a threshold and 'pops.'

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.