Quantum entanglement is a process by which microscopic objects like electrons or atoms lose their individuality to become better coordinated with each other. Entanglement is at the heart of quantum technologies that promise large advances in computing, communications and sensing, for example detecting gravitational waves.
Entangled states are famously fragile: in most cases even a tiny disturbance will undo the entanglement. For this reason, current quantum technologies take great pains to isolate the microscopic systems they work with, and typically operate at temperatures close to absolute zero. The ICFO team, in contrast, heated a collection of atoms to 450 Kelvin, millions of times hotter than most atoms used for quantum technology. Moreover, the individual atoms were anything but isolated; they collided with each other every few microseconds, and each collision set their electrons spinning in random directions.
The researchers used a laser to monitor the magnetization of this hot, chaotic gas. The magnetization is caused by the spinning electrons in the atoms, and provides a way to study the effect of the collisions and to detect entanglement. What the researchers observed was an enormous number of entangled atoms — about 100 times more than ever before observed. They also saw that the entanglement is non-local — it involves atoms that are not close to each other. Between any two entangled atoms there are thousands of other atoms, many of which are entangled with still other atoms, in a giant, hot and messy entangled state.
What they also saw, as Jia Kong, first author of the study, recalls, “is that if we stop the measurement, the entanglement remains for about 1 millisecond, which means that 1000 times per second a new batch of 15 trillion atoms is being entangled. And you must think that 1 ms is a very long time for the atoms, long enough for about fifty random collisions to occur. This clearly shows that the entanglement is not destroyed by these random events. This is maybe the most surprising result of the work.”
The observation of this hot and messy entangled state paves the way for ultra-sensitive magnetic field detection. For example, in magnetoencephalography (magnetic brain imaging), a new generation of sensors uses these same hot, high-density atomic gases to detect the magnetic fields produced by brain activity. The new results show that entanglement can improve the sensitivity of this technique, which has applications in fundamental brain science and neurosurgery.
As ICREA Prof. at ICFO Morgan Mitchell states, “this result is surprising, a real departure from what everyone expects of entanglement.” He adds “we hope that this kind of giant entangled state will lead to better sensor performance in applications ranging from brain imaging to self-driving cars to searches for dark matter.”
A Spin Singlet and QND
A spin singlet is one form of entanglement where the multiple particles’ spins–their intrinsic angular momentum–add up to 0, meaning the system has zero total angular momentum. In this study, the researchers applied quantum non-demolition (QND) measurement to extract the information of the spin of trillions of atoms. The technique passes laser photons with a specific energy through the gas of atoms. These photons with this precise energy do not excite the atoms but they themselves are affected by the encounter. The atoms’ spins act as magnets to rotate the polarization of the light. By measuring how much the photons’ polarization has changed after passing through the cloud, the researchers are able to determine the total spin of the gas of atoms.
The SERF regime
Current magnetometers operate in a regime that is called SERF, far away from the near absolute zero temperatures that researchers typically employ to study entangled atoms. In this regime, any atom experiences many random collisions with other neighboring atoms, making collisions the most important effect on the state of the atom. In addition, because they are in a hot medium rather than an ultracold one, the collisions rapidly randomize the spin of the electrons in any given atom. The experiment shows, surprisingly, that this kind of disturbance does not break the entangled states, it merely passes the entanglement from one atom to another.
Reference:”Measurement-induced, spatially-extended entanglement in a hot, strongly-interacting atomic system” by Jia Kong, Ricardo Jiménez-Martínez, Charikleia Troullinou, Vito Giovanni Lucivero, Géza Tóth and Morgan W. Mitchell, 15 May 2020, Nature Communications. DOI: 10.1038/s41467-020-15899-1
Change for a Paradigm? New Experiment Shows How Time May Emerge From Quantum Entanglement
March 4th, 2014
When cosmologists describe the formation of the Universe as occurring through the Big Bang – the logical question most people ask is “what happened before the Big Bang?” And the proper response to whatever explanation is given to that inquiry would logically be “oh, I see, well what happened before that?” This is much akin to asking what exists outside of the Universe – and it is an excellent line of inquiry because it forces our normally finite thought processes to ponder the nature of infinity. And perhaps most salient of all, it is an examination of the nature of causality and raises the question – What is time?
This is a question that has evaded a clear and definite answer within physics right up to our modern day theories. Time is absolutely fundamental to most of our conceptions of reality, and yet there is no consensus about what exactly time is. In the theory of Relativity, Einstein showed that time is relative – replacing the Newtonian framework of absolute time, wherein events in the Universe take place within an unchanging frame-of-reference, yet do not influence it in anyway. Interestingly, this still seems to be the conception of time harbored by most individuals (scientists included) – even after Einstein’s theory suggested that our experience of time is only relative to our frame of reference, such as how fast we are moving (different velocities will experience an objective difference in the passage of time, with inertial frames of reference that are traveling at greater velocities experiencing a slower progression of time relative to slower moving ones – so called time dilation factors). As Einstein put it – “put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT’S relativity.” Not so difficult to understand after all?
Note, since photons are moving at the velocity c (the speed of light, 300,000 km/s) – this would suggest that time is “standing still” for these particles – relative to an observer traveling at sub-c.
Yet, saying that time is relative does not explain what time is. In his original publication– On the Electrodynamics of Moving Bodies – wherein the principle of relativity is proposed, Einstein explains his definition of time in the following way:
The “time” of an event is that which is given simultaneously with the event by a stationary clock located at the place of the event, this clock being synchronous, and indeed synchronous for all time determinations, with a specified stationary clock.
According to this definition, which is the basis for the theory of General Relativity, and hence modern physics – time is the correspondence of events with a particular configuration of the hands on a clock located at the place of that event. While this definition certainly sufficed to describe the principles behind the relativity of time – there are many major assumptions within the definition. Because the logical question to ask is, what is a clock? What is a clock’s relation to the concept of time? The answer to these questions is – yet again – that what we call time is the relative correspondence of the geometric configurations of objects in space. To a certain extent this makes sense as our entire conception of time is based on the Earth’s movement through space: its orbital rotation and revolution around the Sun, begging the question “does time perhaps arise from spin?”
Regarding what Physics has to say on the nature of time, it is being shown more and more that time is perhaps an emergent phenomenon. That is, it does not underlie events, as something fundamental to physical processes – it emerges from them. Indeed, many posited solutions to quantum gravity do not involve time as a physical parameter within the calculations – such as the Wheeler-DeWitt equation, and more recently the geometric solution for particle interactions known as the Amplituhedron (see our articleA Jewel at the Heart of Quantum Physics). For a truly enlightening exploration of the concept of time and a new theory regarding its nature, see the work of Julian Barbour, and his book The End of Time. Since this plays very predominantly into the framework of quantum gravity, understanding the nature of time may be instrumental to the unification of Physics – a Unified Field Theory.
When General Relativity was first quantized (becoming a theory of quantum gravity) in the 1960’s by John Wheeler, the result predicted a static state of the Universe, that is – no change, i.e. timelessness. This particular solution to the quantization of General Relativity is known as the Wheeler-DeWitt equation. The result seemed to be paradoxical – because how can the Universe be static and unchanging – when our every experience is of change. Like the seeming axiom, ‘the only thing that stays the same is change’.
Recently, an experiment has been performed with entangled photons that suggests time may be related to the quantum correlation – entanglement – of a subsystem with itself. A subsystem is much like the particular inertial frames of reference within General Relativity (an inertial frame of reference is an observer-dependent area that is defined as interacting uniquely from adjacent areas or systems). The static entangled photons appears to change to an internal observer when one of the photons is used as a clock to measure the evolution of the other. This is done by depriving the internal observer of an external clock, and only allowing the evolution of the subsystem to be determined by correlation measurements (such as changes in polarity). As the measurements are made, the internal observer becomes nonlocally correlated with the subsystem of photons, and this quantum spacetime interconnectivity makes it appear to the entangled observer that the photons have changed.
The remarkable occurs when it is demonstrated that if the entangled photons are measured without becoming entangled with them – which is accomplished by measuring their global, or overall state instead of the correlation between them, as is done for the internal observer – it can be shown that no change has occurred. The experimenters utilize an interesting technique to accomplish measuring the state of the entangled photons without actually disturbing the quantum correlation between them. The experiment uses a form of quantum erasure, in which the measurement that normally destroys the quantum “superposition” of a wave-particle can be reversed through operations performed on an entangled counterpart.
Basic design of a quantum erasure experiment. The Down-Converter (a beta barium borate crystal) produces two quantum entangled photons, each directed into a separate channel. One of the photons goes directly to a detector, while polarizing filters “mark” the other photon as it passes through a double slit mask. Marking the photon in this way will inhibit formation of a wave interference pattern. If however, the other entangled photon (which does not pass through a double-slit but instead goes directly to a detector) is passed through a polarizing filter it “unmarks” the entangled counterpart – which will then produce a wave-interference pattern. In this way, it is thought that the operation on the first photon removes, or erases, the measurement performed on the second by the polarizing filters – allowing for “self-interference” of the photon passing through the double-slits, which will then result in a wave-interference pattern. Hence the appellation – quantum erasure.
Recall that one of the entangled photons was utilized as a clock to measure the evolution of the second, and this produces entanglement of the observer with that system. However, if those measurements are coherently erased, through a quantum erasure, then the observer can in a sense measure the global configuration of the subsystem without becoming entangled with it. From this vantage perspective, the so called super-observer can now determine if the global state of the photons evolves. And remarkably, the results suggest that the super-observer can show that there is no evolution of the subsystem, simply by not becoming correlated with the entangled photons.
The photons remain unchanged and static even while the entangled measurement of their quantum correlation gives the internal observer an apparent change – but it is an apparition of time! While this is all accomplished with a very clever experimental design – it does demonstrate how the appearance of change, or time, can emerge even within a static, timeless Universe. In this sense, there is time, but it is only the appearance of change within subsystems that are strongly correlated together – but less so with the overall system, being the Universe itself, which at that global scale, according to their result, experiences no net change, and within this model would be eternal.
In Haramein’s model (Quantum Gravity and the Holographic Mass) time is a function of the relationship of orbiting bodies relative to each other leaving a memory imprint on the structure of spacetime (encoded in Planck information pixels). In fact, he is able to show that the Planck pixels volume-to-surface ratio relationship generate the mass/energy of objects. However, the rotational relationships of all systems in an infinite boundary division Universe or multiverse would all cancel out so that each center of rotation would be the stillness that centers the rotation of all other relationships to infinity. Therefore, the global collective scale sees no change, while the local scale is obviously continuously changing. One could think of this as the center of your experience being the infinitely unchanging present emerging from the continuous changes of the past and the possible path of changes of the future.
Physics team entangles photons that never coexisted in time
May 28, 2013 by Bob Yirka
(Phys.org) —Researchers at the Hebrew University of Jerusalem have succeeded in causing entanglement swapping between photons that never coexisted in time. In their paper published in the journal Physical Review Letters, the team explains how their experiment proves true an entanglement phenomenon first described by researchers last year at the University of Erlangen-Nuremberg.
The idea seems not just counterintuitive, but impossible—that photons could be entangled that never existed at the same time—but that’s just what the team in Germany, led by Joachim von Zanthier, suggested. In this new effort, the team in Israel, led by Hagai Eisenberg, has proven it’s possible by actually doing it.
Entanglement is, of course, where the quantum states of two particles are linked—what happens to one happens to the other regardless of the distance between them. This new work shows that they can be linked via time as well.
To prove it, the researchers first used a laser to cause entanglement between a pair of photons, P1, P2. They then measured the polarization of P1, which was immediately followed by the entangling of another pair of photons, P3, P4. This was followed by measuring P2 and P3 simultaneously and causing them to become entangled with one another—a process known as projective measurement. Then, P4 was measured. Measuring P1 caused its demise of course—before P4 was born—but the measurement of P4 showed that it had become entangled with P1 nevertheless, if only for a very short period of time.
The researchers suggest that the outcome of their experiment shows that entanglement is not a truly physical property, at least not in a tangible sense. To say that two photons are entangled, they write, doesn’t mean they have to exist at the same time. It shows that quantum events don’t always have a parallel in the observable world.
Being able to entangle particles that don’t exist at the same time opens up the door to new encryption techniques for building ultra-secure networks—communications could occur between physical locations, for example, that never actually sent an encrypted key directly to one another. It could also perhaps lead to new developments by researchers hoping to create a true quantum computer.
The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the nonlocality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spacelike separation, but also to particles with timelike separation. In order to demonstrate these principles, we generated and fully characterized an entangled pair of photons that have never coexisted. Using entanglement swapping between two temporally separated photon pairs, we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed two-photon state demonstrates that entanglement can be shared between timelike separated quantum systems.
Weird! Quantum Entanglement Can Reach into the Past
Clara Moskowitz, LiveScience Senior Writer
Date: 30 April 2012 Time: 09:42 AM ET
Scientists have entangled particles in such a way that a future decision can affect the past states of the particles.
CREDIT: Jon Heras, Equinox Graphics Ltd.
Spooky quantum entanglement just got spookier.
Entanglement is a weird statewhere two particles remain intimately connected, even when separated over vast distances, like two die that must always show the same numbers when rolled. For the first time, scientists have entangled particles after they’ve been measured and may no longer even exist.
If that sounds baffling, even the researchers agree it’s a bit “radical,” in a paper reporting the experiment published online April 22 in the journal Nature Physics.
“Whether these two particles are entangled or separable has been decided after they have been measured,” write the researchers, led by Xiao-song Ma of the Institute for Quantum Optics and Quantum Information at the University of Vienna.
Essentially, the scientists showed that future actions may influence past events, at least when it comes to the messy, mind-bending world of quantum physics.
In the quantum world, things behave differently than they do in the real, macroscopic world we can see and touch around us. In fact, when quantum entanglement was first predicted by the theory of quantum mechanics, Albert Einstein expressed his distaste for the idea, calling it “spooky action at a distance.”
The researchers, taking entanglement a step further than ever before, started with two sets of light particles, called photons.
The basic setup goes like this:
Both pairs of photons are entangled, so that the two particles in the first set are entangled with each other, and the two particles in the second set are entangled with each other. Then, one photon from each pair is sent to a person named Victor. Of the two particles that are left behind, one goes to Bob, and the other goes to Alice.
But now, Victor has control over Alice and Bob’s particles. If he decides to entangle the two photons he has, then Alice and Bob’s photons, each entangled with one of Victor’s, also become entangled with each other. And Victor can choose to take this action at any time, even after Bob and Alice may have measured, changed or destroyed their photons.
“The fantastic new thing is that this decision to entangle two photons can be done at a much later time,” said research co-author Anton Zeilinger, also of the University of Vienna. “They may no longer exist.”
Such an experiment had first been predicted by physicist Asher Peres in 2000, but had not been realized until now.
“The way you entangle them is to send them onto a half-silvered mirror,” Zeilinger told LiveScience. “It reflects half of the photons, and transmits half. If you send two photons, one to the right and one to the left, then each of the two photons have forgotten where they come from. They lose their identities and become entangled.”
Zeilinger said the technique could one day be used to communicate between superfast quantum computers, which rely on entanglement to store information. Such a machine has not yet been created, but experiments like this are a step toward that goal, the researchers say.
“The idea is to create two particle pairs, send one to one computer, the other to another,” Zeilinger said.”Then if these two photons are entangled, the computers could use them to exchange information.”
Two Diamonds Linked by Strange Quantum Entanglement
Clara Moskowitz, LiveScience Senior Writer
Date: 01 December 2011 Time: 02:00 PM ET
The vibrational states of two spatially separated, millimeter-sized diamonds are entangled at room temperature by beaming laser light at them (green). The researchers verified this entanglement by studying the subsequent laser pulses beamed through the system.
Scientists have linked two diamonds in a mysterious process called entanglement that is normally only seen on the quantum scale.
Entanglement is so weird that Einstein dubbed it “spooky action at a distance.” It’s a strange effect where one object gets connected to another so that even if they are separated by large distances, an action performed on one will affect the other. Entanglement usually occurs with subatomic particles, and was predicted by the theory of quantum mechanics, which governs the realm of the very small.
But now physicists have succeeded in entangling two macroscopic diamonds, demonstrating that quantum mechanical effects are not limited to the microscopic scale.
“I think it’s an important step into a new regime of thinking about quantum phenomena,” physicist Ian Walmsley of England’s University of Oxford said.”That is, in this regime of the bigger world, room temperatures, ambient conditions. Although the phenomenon was expected to exist, actually being able to observe it in such a system we think is quite exciting.”
Another study recently used quantum entanglement to teleport bits of light from one place to another. And other researchers have succeeded in entangling macroscopic objects before, but they have generally been under special circumstances, prepared in special ways, and cooled to cryogenic temperatures. In the new achievement, the diamonds were large and not prepared in any special way, the researchers said.
“It’s big enough you can see it,” Walmsley told LiveScience of the diamonds.”They’re sitting on the table, out in plain view. The laboratory isn’t particularly cold or particularly hot, it’s just your everyday room.”
Walmsley, along with a team of physicists led by Oxford graduate student Ka Chung Lee, accomplished this feat by entangling the vibration of two diamond crystals. To do so, the researchers set up an apparatus to send a laser pulse at both diamonds simultaneously. Sometimes, the laser light changed color, to a lower frequency, after hitting the diamonds. That told the scientists it had lost a bit of energy.
Because energy must be conserved in closed systems (where there’s no input of outside energy), the researchers knew that the “lost” energy had been used in some way. In fact, the energy had been converted into vibrational motion for one of the diamonds (albeit motion that is too small to observe visually). However, the scientists had no way of knowing which diamond was vibrating.
Then, the researchers sent a second pulse of laser light through the now-vibrating system. This time, if the light emerged with a color of higher frequency, it meant it had gained the energy back by absorbing it from the diamond, stopping its vibration.
The scientists had set up two separate detectors to measure the laser light — one for each diamond.
If the two diamonds weren’t entangled, the researchers would expect each detector to register a changed laser beam about 50 percent of the time. It’s similar to tossing a coin, where random chance would lead to heads about half the time and tails the other half the time on average.
Instead, because the two diamonds were linked, they found that one detector measured the change every time, and the other detector never fired. The two diamonds, it seemed, were so connected they reacted as a single entity, rather than two individual objects.
The scientists report their results in the Dec. 2 issue of the journal Science.
“Recent advances in quantum control techniques have allowed entanglement to be observed for physical systems with increasing complexity and separation distance,” University of Michigan physicist Luming Duan, who was not involved in the study, wrote in an accompanying essay in the same issue of Science.”Lee et al. take an important step in this direction by demonstrating entanglement between oscillation patterns of atoms—phonon modes—of two diamond samples of millimeter size at room temperature, separated by a macroscopic distance of about 15 cm.”
In addition to furthering scientists’ understanding of entanglement, the research could help develop faster computers called photonic processors, relying on quantum effects, said Oxford physicist Michael Sprague, another team member on the project.
“The long-term goal is that if you can harness the power of quantum phenomena, you can potentially do things more efficiently than is currently possible,” Sprague said.