Jul 16, 2016 02:33 AM
The birth of quantum holography: Making holograms of single light particles!
https://www.sciencedaily.com/releases/20...133218.htm
RELEASE: Until quite recently, creating a hologram of a single photon was believed to be impossible due to fundamental laws of physics. However, scientists at the Faculty of Physics, University of Warsaw, have successfully applied concepts of classical holography to the world of quantum phenomena. A new measurement technique has enabled them to register the first ever hologram of a single light particle, thereby shedding new light on the foundations of quantum mechanics.
Scientists at the Faculty of Physics, University of Warsaw, have created the first ever hologram of a single light particle. The spectacular experiment, reported in the journal Nature Photonics, was conducted by Dr. Radoslaw Chrapkiewicz and Michal Jachura under the supervision of Dr. Wojciech Wasilewski and Prof. Konrad Banaszek. Their successful registering of the hologram of a single photon heralds a new era in holography: quantum holography, which promises to offer a whole new perspective on quantum phenomena.
"We performed a relatively simple experiment to measure and view something incredibly difficult to observe: the shape of wavefronts of a single photon," says Dr. Chrapkiewicz.
In standard photography, individual points of an image register light intensity only. In classical holography, the interference phenomenon also registers the phase of the light waves (it is the phase which carries information about the depth of the image). When a hologram is created, a well-described, undisturbed light wave (reference wave) is superimposed with another wave of the same wavelength but reflected from a three-dimensional object (the peaks and troughs of the two waves are shifted to varying degrees at different points of the image). This results in interference and the phase differences between the two waves create a complex pattern of lines. Such a hologram is then illuminated with a beam of reference light to recreate the spatial structure of wavefronts of the light reflected from the object, and as such its 3D shape.
One might think that a similar mechanism would be observed when the number of photons creating the two waves were reduced to a minimum, that is to a single reference photon and a single photon reflected by the object. And yet you'd be wrong! The phase of individual photons continues to fluctuate, which makes classical interference with other photons impossible. Since the Warsaw physicists were facing a seemingly impossible task, they attempted to tackle the issue differently: rather than using classical interference of electromagnetic waves, they tried to register quantum interference in which the wave functions of photons interact.
Wave function is a fundamental concept in quantum mechanics and the core of its most important equation: the Schrödinger equation. In the hands of a skilled physicist, the function could be compared to putty in the hands of a sculptor: when expertly shaped, it can be used to 'mould' a model of a quantum particle system. Physicists are always trying to learn about the wave function of a particle in a given system, since the square of its modulus represents the distribution of the probability of finding the particle in a particular state, which is highly useful.
"All this may sound rather complicated, but in practice our experiment is simple at its core: instead of looking at changing light intensity, we look at the changing probability of registering pairs of photons after the quantum interference," explains doctoral student Jachura.
Why pairs of photons? A year ago, Chrapkiewicz and Jachura used an innovative camera built at the University of Warsaw to film the behaviour of pairs of distinguishable and non-distinguishable photons entering a beam splitter. When the photons are distinguishable, their behaviour at the beam splitter is random: one or both photons can be transmitted or reflected. Non-distinguishable photons exhibit quantum interference, which alters their behaviour: they join into pairs and are always transmitted or reflected together. This is known as two-photon interference or the Hong-Ou-Mandel effect.
"Following this experiment, we were inspired to ask whether two-photon quantum interference could be used similarly to classical interference in holography in order to use known-state photons to gain further information about unknown-state photons. Our analysis led us to a surprising conclusion: it turned out that when two photons exhibit quantum interference, the course of this interference depends on the shape of their wavefronts," says Dr. Chrapkiewicz.
Quantum interference can be observed by registering pairs of photons. The experiment needs to be repeated several times, always with two photons with identical properties. To meet these conditions, each experiment started with a pair of photons with flat wavefronts and perpendicular polarisations; this means that the electrical field of each photon vibrated in a single plane only, and these planes were perpendicular for the two photons. The different polarisation made it possible to separate the photons in a crystal and make one of them 'unknown' by curving their wavefronts using a cylindrical lens. Once the photons were reflected by mirrors, they were directed towards the beam splitter (a calcite crystal). The splitter didn't change the direction of vertically-polarised photons, but it did diverge diplace horizontally-polarised photons. In order to make each direction equally probable and to make sure the crystal acted as a beam splitter, the planes of photon polarisation were bent by 45 degrees before the photons entered the splitter. The photons were registered using the state-of-the-art camera designed for the previous experiments. By repeating the measurements several times, the researchers obtained an interference image corresponding to the hologram of the unknown photon viewed from a single point in space. The image was used to fully reconstruct the amplitude and phase of the wave function of the unknown photon.
The experiment conducted by the Warsaw physicists is a major step towards improving our understanding of the fundamental principles of quantum mechanics. Until now, there has not been a simple experimental method of gaining information about the phase of a photon's wave function. Although quantum mechanics has many applications, and it has been verified many times with a great degree of accuracy over the last century, we are still unable to explain what wave functions actually are: are they simply a handy mathematical tool, or are they something real?
"Our experiment is one of the first allowing us to directly observe one of the fundamental parameters of photon's wave function -- its phase -- bringing us a step closer to understanding what the wave function really is," explains Jachura.
The Warsaw physicists used quantum holography to reconstruct wave function of an individual photon. Researchers hope that in the future they will be able to use a similar method to recreate wave functions of more complex quantum objects, such as certain atoms. Will quantum holography find applications beyond the lab to a similar extent as classical holography, which is routinely used in security (holograms are difficult to counterfeit), entertainment, transport (in scanners measuring the dimensions of cargo), microscopic imaging and optical data storing and processing technologies?
"It's difficult to answer this question today. All of us -- I mean physicists -- must first get our heads around this new tool. It's likely that real applications of quantum holography won't appear for a few decades yet, but if there's one thing we can be sure of it's that they will be surprising," summarises Prof. Banaszek.
The Noise at the Bottom of the Universe
http://nautil.us/issue/38/noise/the-nois...e-universe
EXCERPT: To a physicist, perfect quiet is the ultimate noise. Silence your cellphone, still your thoughts, and muffle every kind of vibration, and you would still be left with quantum noise. It represents an indeterminacy deep within nature, bursts of static and inexplicable motions that cannot be gotten rid of, or made sense of. It seems devoid of meaning.
Considering how pervasive this noise is, you might presume that physicists would have a good explanation for it. But it remains one of the great unsolved problems in science. Quantum theory is silent not just on where the noise comes from, but on how exactly it enters the world. The theory’s defining equation, the Schrödinger equation, is completely deterministic. There is no noise in it at all. To explain why we observe quantum particles to be noisy, we need some additional principle.
For physicists in the Niels Bohr tradition, the act of observation itself is decisive. The Schrödinger equation defines a menu of possibilities for what a particle could do, but only when measured does the particle actually do anything, choosing at random from the menu. Identical particles will make different choices, causing the outcomes of fundamental processes to vary in an uncontrollable way. On Bohr’s view, quantum noise cannot be explained further. It is what physicist John Wheeler called “an elementary act of creation,” with no antecedents. Genesis was not a singular event in the distant past, but an ongoing process that we bring about. We create the world by observing it.
To skeptics such as Einstein, that view is both wonderfully romantic and completely incoherent. Who are “we”? What is “observing”? Physicists and philosophers have spent the better part of a century seeking a less hand-wavy explanation, taking one of two general directions. Maybe quantum noise, like the noise we encounter in daily life, has a meaning that escapes us. It may seem indeterministic, but could be produced by deterministic processes that, for whatever reason, we can’t see. It might, for example, be a consequence of living in one of countless parallel universes and not being able to tell which is ours. The noise, in essence, tells us where we live. All those little upticks and swerves in particle behavior are the quirks that differentiate our universe from others, and they are “noise” inasmuch as our location is pure happenstance, like being put in hotel room 314 rather than 159.
The GRW theory supposes that noise sporadically strikes particles and causes them to materialize in one of the locations open to them. This can’t happen very often, or else particle behavior would deviate from the Schrödinger equation all the time. Once every 100 million years is enough, because when the blow does come, its effect is greatly amplified by quantum entanglement—the spooky interconnection of particles. A hit on one particle is felt by all those it is entangled with.
This multiplier effect would neatly explain why we observe quantum behavior on the particle level but not in everyday life—why a particle can have no determinate properties but a macroscopic object always does. Although a person or a planet might exist fleetingly in an indeterminate state, just like an isolated particle, it presents a much bigger target and one of its particles will quickly get struck by noise. The noise will fix the location of that particle and all its entangled partners. An object composed of 1023 mutually entangled particles will be pinged every 10 nanoseconds or so.
The noise acts on spatial position, but also affects other properties indirectly. Schrödinger’s famous both-dead-and-alive cat is quickly forced to be either alive or dead, because those two conditions correspond to different spatial arrangements of particles in the cat’s body, and the GRW mechanism will select among them.
The GRW mechanism also demystifies observation. To observe is simply to correlate a particle with some large piece of apparatus, so that the particle’s properties become accessible to you. In so doing you expose the particle to the noise that strikes the apparatus. You may not create reality directly, as Bohr thought, but do make it possible for the all-pervading noise to act on objects it would otherwise pass over.
The GRW theory, like quantum mechanics itself, is a mathematical formalism and does not prescribe what the world is made of—what philosophers call the ontology. Its creators originally envisioned a particle ontology: an atomistic world. But the theory also works if the universe consists instead of smeared-out matter or force fields. What distinguishes GRW from other interpretations of quantum mechanics is that the noise is untriggered. The hits just happen spontaneously. So, they do not strictly require the existence of anything else. The theory thus opens up a wholly new possibility: that the universe is made purely out of noise....
https://www.sciencedaily.com/releases/20...133218.htm
RELEASE: Until quite recently, creating a hologram of a single photon was believed to be impossible due to fundamental laws of physics. However, scientists at the Faculty of Physics, University of Warsaw, have successfully applied concepts of classical holography to the world of quantum phenomena. A new measurement technique has enabled them to register the first ever hologram of a single light particle, thereby shedding new light on the foundations of quantum mechanics.
Scientists at the Faculty of Physics, University of Warsaw, have created the first ever hologram of a single light particle. The spectacular experiment, reported in the journal Nature Photonics, was conducted by Dr. Radoslaw Chrapkiewicz and Michal Jachura under the supervision of Dr. Wojciech Wasilewski and Prof. Konrad Banaszek. Their successful registering of the hologram of a single photon heralds a new era in holography: quantum holography, which promises to offer a whole new perspective on quantum phenomena.
"We performed a relatively simple experiment to measure and view something incredibly difficult to observe: the shape of wavefronts of a single photon," says Dr. Chrapkiewicz.
In standard photography, individual points of an image register light intensity only. In classical holography, the interference phenomenon also registers the phase of the light waves (it is the phase which carries information about the depth of the image). When a hologram is created, a well-described, undisturbed light wave (reference wave) is superimposed with another wave of the same wavelength but reflected from a three-dimensional object (the peaks and troughs of the two waves are shifted to varying degrees at different points of the image). This results in interference and the phase differences between the two waves create a complex pattern of lines. Such a hologram is then illuminated with a beam of reference light to recreate the spatial structure of wavefronts of the light reflected from the object, and as such its 3D shape.
One might think that a similar mechanism would be observed when the number of photons creating the two waves were reduced to a minimum, that is to a single reference photon and a single photon reflected by the object. And yet you'd be wrong! The phase of individual photons continues to fluctuate, which makes classical interference with other photons impossible. Since the Warsaw physicists were facing a seemingly impossible task, they attempted to tackle the issue differently: rather than using classical interference of electromagnetic waves, they tried to register quantum interference in which the wave functions of photons interact.
Wave function is a fundamental concept in quantum mechanics and the core of its most important equation: the Schrödinger equation. In the hands of a skilled physicist, the function could be compared to putty in the hands of a sculptor: when expertly shaped, it can be used to 'mould' a model of a quantum particle system. Physicists are always trying to learn about the wave function of a particle in a given system, since the square of its modulus represents the distribution of the probability of finding the particle in a particular state, which is highly useful.
"All this may sound rather complicated, but in practice our experiment is simple at its core: instead of looking at changing light intensity, we look at the changing probability of registering pairs of photons after the quantum interference," explains doctoral student Jachura.
Why pairs of photons? A year ago, Chrapkiewicz and Jachura used an innovative camera built at the University of Warsaw to film the behaviour of pairs of distinguishable and non-distinguishable photons entering a beam splitter. When the photons are distinguishable, their behaviour at the beam splitter is random: one or both photons can be transmitted or reflected. Non-distinguishable photons exhibit quantum interference, which alters their behaviour: they join into pairs and are always transmitted or reflected together. This is known as two-photon interference or the Hong-Ou-Mandel effect.
"Following this experiment, we were inspired to ask whether two-photon quantum interference could be used similarly to classical interference in holography in order to use known-state photons to gain further information about unknown-state photons. Our analysis led us to a surprising conclusion: it turned out that when two photons exhibit quantum interference, the course of this interference depends on the shape of their wavefronts," says Dr. Chrapkiewicz.
Quantum interference can be observed by registering pairs of photons. The experiment needs to be repeated several times, always with two photons with identical properties. To meet these conditions, each experiment started with a pair of photons with flat wavefronts and perpendicular polarisations; this means that the electrical field of each photon vibrated in a single plane only, and these planes were perpendicular for the two photons. The different polarisation made it possible to separate the photons in a crystal and make one of them 'unknown' by curving their wavefronts using a cylindrical lens. Once the photons were reflected by mirrors, they were directed towards the beam splitter (a calcite crystal). The splitter didn't change the direction of vertically-polarised photons, but it did diverge diplace horizontally-polarised photons. In order to make each direction equally probable and to make sure the crystal acted as a beam splitter, the planes of photon polarisation were bent by 45 degrees before the photons entered the splitter. The photons were registered using the state-of-the-art camera designed for the previous experiments. By repeating the measurements several times, the researchers obtained an interference image corresponding to the hologram of the unknown photon viewed from a single point in space. The image was used to fully reconstruct the amplitude and phase of the wave function of the unknown photon.
The experiment conducted by the Warsaw physicists is a major step towards improving our understanding of the fundamental principles of quantum mechanics. Until now, there has not been a simple experimental method of gaining information about the phase of a photon's wave function. Although quantum mechanics has many applications, and it has been verified many times with a great degree of accuracy over the last century, we are still unable to explain what wave functions actually are: are they simply a handy mathematical tool, or are they something real?
"Our experiment is one of the first allowing us to directly observe one of the fundamental parameters of photon's wave function -- its phase -- bringing us a step closer to understanding what the wave function really is," explains Jachura.
The Warsaw physicists used quantum holography to reconstruct wave function of an individual photon. Researchers hope that in the future they will be able to use a similar method to recreate wave functions of more complex quantum objects, such as certain atoms. Will quantum holography find applications beyond the lab to a similar extent as classical holography, which is routinely used in security (holograms are difficult to counterfeit), entertainment, transport (in scanners measuring the dimensions of cargo), microscopic imaging and optical data storing and processing technologies?
"It's difficult to answer this question today. All of us -- I mean physicists -- must first get our heads around this new tool. It's likely that real applications of quantum holography won't appear for a few decades yet, but if there's one thing we can be sure of it's that they will be surprising," summarises Prof. Banaszek.
The Noise at the Bottom of the Universe
http://nautil.us/issue/38/noise/the-nois...e-universe
EXCERPT: To a physicist, perfect quiet is the ultimate noise. Silence your cellphone, still your thoughts, and muffle every kind of vibration, and you would still be left with quantum noise. It represents an indeterminacy deep within nature, bursts of static and inexplicable motions that cannot be gotten rid of, or made sense of. It seems devoid of meaning.
Considering how pervasive this noise is, you might presume that physicists would have a good explanation for it. But it remains one of the great unsolved problems in science. Quantum theory is silent not just on where the noise comes from, but on how exactly it enters the world. The theory’s defining equation, the Schrödinger equation, is completely deterministic. There is no noise in it at all. To explain why we observe quantum particles to be noisy, we need some additional principle.
For physicists in the Niels Bohr tradition, the act of observation itself is decisive. The Schrödinger equation defines a menu of possibilities for what a particle could do, but only when measured does the particle actually do anything, choosing at random from the menu. Identical particles will make different choices, causing the outcomes of fundamental processes to vary in an uncontrollable way. On Bohr’s view, quantum noise cannot be explained further. It is what physicist John Wheeler called “an elementary act of creation,” with no antecedents. Genesis was not a singular event in the distant past, but an ongoing process that we bring about. We create the world by observing it.
To skeptics such as Einstein, that view is both wonderfully romantic and completely incoherent. Who are “we”? What is “observing”? Physicists and philosophers have spent the better part of a century seeking a less hand-wavy explanation, taking one of two general directions. Maybe quantum noise, like the noise we encounter in daily life, has a meaning that escapes us. It may seem indeterministic, but could be produced by deterministic processes that, for whatever reason, we can’t see. It might, for example, be a consequence of living in one of countless parallel universes and not being able to tell which is ours. The noise, in essence, tells us where we live. All those little upticks and swerves in particle behavior are the quirks that differentiate our universe from others, and they are “noise” inasmuch as our location is pure happenstance, like being put in hotel room 314 rather than 159.
The GRW theory supposes that noise sporadically strikes particles and causes them to materialize in one of the locations open to them. This can’t happen very often, or else particle behavior would deviate from the Schrödinger equation all the time. Once every 100 million years is enough, because when the blow does come, its effect is greatly amplified by quantum entanglement—the spooky interconnection of particles. A hit on one particle is felt by all those it is entangled with.
This multiplier effect would neatly explain why we observe quantum behavior on the particle level but not in everyday life—why a particle can have no determinate properties but a macroscopic object always does. Although a person or a planet might exist fleetingly in an indeterminate state, just like an isolated particle, it presents a much bigger target and one of its particles will quickly get struck by noise. The noise will fix the location of that particle and all its entangled partners. An object composed of 1023 mutually entangled particles will be pinged every 10 nanoseconds or so.
The noise acts on spatial position, but also affects other properties indirectly. Schrödinger’s famous both-dead-and-alive cat is quickly forced to be either alive or dead, because those two conditions correspond to different spatial arrangements of particles in the cat’s body, and the GRW mechanism will select among them.
The GRW mechanism also demystifies observation. To observe is simply to correlate a particle with some large piece of apparatus, so that the particle’s properties become accessible to you. In so doing you expose the particle to the noise that strikes the apparatus. You may not create reality directly, as Bohr thought, but do make it possible for the all-pervading noise to act on objects it would otherwise pass over.
The GRW theory, like quantum mechanics itself, is a mathematical formalism and does not prescribe what the world is made of—what philosophers call the ontology. Its creators originally envisioned a particle ontology: an atomistic world. But the theory also works if the universe consists instead of smeared-out matter or force fields. What distinguishes GRW from other interpretations of quantum mechanics is that the noise is untriggered. The hits just happen spontaneously. So, they do not strictly require the existence of anything else. The theory thus opens up a wholly new possibility: that the universe is made purely out of noise....