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The extraordinary promise of quantum technology—depend on quantum “entanglement,” in which the physical states of two or more objects such as atoms, photons or ions become so inextricably connected that the state of one particle can instantly influence the state of the other—no matter how far apart they are. Entanglement links the strange states of tiny quantum mechanical objects. For example, a top can spin either clockwise or counterclockwise, but an atom can spin both ways at once—at least until it is measured and that two-way state collapses one way or the other. Two atoms can be entangled so that each is in an uncertain two-way state, but their spins are definitely correlated, say, in opposite directions. So if physicists measure the first atom and find it spinning clockwise, they know instantly the other one must be spinning counterclockwise, no matter how far away it is.
The concept of entanglement can also be applied to photons, leading to applications in secure quantum communication, quantum computation and high-precision sensor technology.
Entangled photon pairs are important for the realization of quantum communication
The use of entangled photons for quantum cryptography was proposed by Arthur Ekert in 19916. Ekert’s protocol, known as E91, includes a photon pair entangled in polarization with paths split so one photon is received by Alice and the other by Bob. To ensure a secure
connection is present, a Bell’s theorem is performed to check for eavesdroppers. This protocol was first implemented in by Kwiat et al. in 19995 . This experiment showed the viability of Ekert’s protocol as well as an exploration of potential eavesdropping strategies.
Entanglement would be key to a fully quantum internet that would let quantum computers of the future communicate with one another and be immune to hacking. If hackers messed with communication, they would spoil the entanglement, revealing their presence. Various companies already sell systems that send messages in quantum states of light that are largely unhackable. But to use such links, the information must still be decoded at each network node, which is potentially vulnerable. In a quantum internet, any node could be entangled with any other, so messages between them couldn’t be decoded at intermediate nodes.
But developers must first stretch entanglement over greater distances. Previously, researchers had demonstrated entanglement of two bits of matter over 1.3 kilometers of optical fiber. In a new study, scientists have successfully transmitted entangled photons between a satellite and Earth at a distance of over 1,200 kilometres (750 miles). This smashes the previous record for entanglement distribution, which only reached up to 100 kilometres. “We have demonstrated the distribution of two entangled photons from a satellite to two ground stations that are 1,203 kilometres apart,” says Juan Yin, lead author and physicist at the Science and Technology University of China in Shanghai, in the research paper. Not only were the particles entangled in space (scientists have never done this before), but they retained their bizarre connection even after they’d been separated by a distance 10 times the previous record for what’s called “quantum teleportation.” “Long-distance entanglement distribution is essential for the testing of quantum physics and quantum networks.”
Quantum computing advances with control of entanglement
The power of quantum computers is derived from entanglement of quantum bit, or qubit, that can assume two values (0 and 1) at the same time. When two or more qubits are linked in a special “entangled” state, this property extends out and the power of qubits grows exponentially. Ten fully entangled qubits would be able to store as much information as 1,024 classical bits; 33 qubits could store one gigabyte; and 300 fully entangled qubits would store as many classical bits as there are atoms in the universe.
A quantum computer has capacity for show exponential speedups for searching and manipulating big data, performing data cryptography, analyzing protein folding to design better drugs, simulating the early Universe, and providing much more accurate weather forecasting, among many other things
In order to meet various requirements for various applications, an important technological development is to increase the number of available entangled qubits. In October 2019, researchers from Google claimed to have achieved a milestone known as quantum supremacy. They had created the first quantum computer that could perform a calculation that is impossible for a standard computer. “Quantum supremacy,” a term coined by John Preskill, a physicist at the California Institute of Technology in Pasadena, to denote a quantum computer that can do something beyond the ken of a classical computer, such as simulate molecular structures in chemistry and materials science, from code-breaking and cyber security to big data analysis and machine learning.
In particular, the researchers have shown that they can continue to create a quantum behavior known as the entangled state—entangling more than one million different physical systems, a world record that was only limited in their investigation by data storage space. “We have demonstrated the deterministic generation and verification of a fully inseparable dual-rail continuous variable (CV) cluster state consisting of more than one million qumodes of light, by employing a time-domain multiplexing scheme. Compared to the previous work, the cluster state generator does not degrade during operation, owing to the continuous feedback of the optical system. We can in principle further increase the number of qumodes, but we stopped at around one million qumodes because of the data size for verification. Time domain multiplexing is one of the key technologies of CV information processing
“There is a problem of the lifetime of qubits for quantum information processing. We have solved the problem, and we can continue to do quantum information processing for any time period we want,” explained Akira Furusawa, of the Department of Applied Physics, School of Engineering at the University of Tokyo and lead researcher on the study. “The most difficult aspect of this achievement was continuous phase locking between squeezed light beams, but we have solved the problem.” The report of their investigation appeared in the journal APL Photonics.
Quantum Entanglement technology
Entanglement takes place when a part of particles interacts physically. For instance, a laser beam fired through a certain type of crystal can cause individual light particles to be split into pairs of entangled photons.
Entanglement would be key to a fully quantum internet that would let quantum computers of the future communicate with one another and be immune to hacking. But developers must first stretch entanglement over greater distances to form quantum networks and Quantum internet.
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.
Researchers demonstrated the entanglement between 16 million atoms in a crystal crossed by a single photon in 2017
Quantum theory is unequivocal: it predicts that a vast number of atoms can be entangled and intertwined by a very strong quantum relationship even in a macroscopic structure. Until now, however, experimental evidence has been mostly lacking, although recent advances have shown the entanglement of 2,900 atoms.
Scientists at the University of Geneva (UNIGE), Switzerland, recently reengineered their data processing, demonstrating that 16 million atoms were entangled in a one-centimetre crystal. Florian Fröwis, a researcher in the applied physics group in UNIGE’s science faculty explained “it’s impossible to directly observe the process of entanglement between several million atoms since the mass of data you need to collect and analyse is so huge.” As a result, Fröwis and his colleagues chose a more indirect route, they examined the characteristics of light re-emitted by the crystal, as well as analysing its statistical properties and the probabilities, following two major avenues: that the light is re-emitted in a single direction rather than radiating uniformly from the crystal; and that it is made up of a single photon. In this way, the researchers succeeded in showing the entanglement of 16 million atoms when previous observations had a ceiling of a few thousand.
In a parallel work, scientists at University of Calgary, Canada, demonstrated entanglement between many large groups of atoms. “We haven’t altered the laws of physics,” points out Mikael Afzelius, a member of Professor Nicolas Gisin’s applied physics group. “What has changed is how we handle the flow of data.”
Entanglement in heated environment of 15 trillion atoms reported in June 2020 to better sensor performance
Quantum entanglement is a delicate phenomenon that only survives in ultracold, ultra-low-noise environments. But in June 2020, physicists at the ICFO in Barcelona, Spain used a technique called a quantum non-demolition measurement to show that at least 1.52 × 10¹³ out of the 5.32 × 10¹³ rubidium atoms in their 450 K sample were, in fact, entangled.
The ICFO team, 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 team, led by Morgan Mitchell and Jia Kong, also showed that this entanglement was non-local, meaning that it involved atoms that were not close to each other. As well as challenging assumptions about what entanglement looks like, the finding could be important for sensing technologies such as vapour-phase spin-exchange-relaxation-free (SERF) magnetometers that are based on hot, dense clouds of atoms.
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.
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.”
Quantum Entanglement at room temperature demonstrated in 2015
Entanglement is also one of nature’s most elusive phenomena. Realization of entanglement at the macroscopic scale, among huge numbers of particles is highly challenging as entanglement requires that particles start out in a highly ordered state, whereas the matter at atomic stage is highly disordered. “The macroscopic world that we are used to seems very tidy, but it is completely disordered at the atomic scale. The laws of thermodynamics generally prevent us from observing quantum phenomena in macroscopic objects,” said Paul Klimov, a graduate student in the Institute for Molecular Engineering and lead author of new research on quantum entanglement.
Previously, scientists have overcome the thermodynamic barrier and achieved macroscopic entanglement in solids and liquids by going to ultra-low temperatures (-270 degrees Celsius) and applying huge magnetic fields (1,000 times larger than that of a typical refrigerator magnet) or using chemical reactions. In Science Advances, Klimov and other researchers in David Awschalom’s group at the Institute for Molecular Engineering have demonstrated that macroscopic entanglement can be generated at room temperature and in a small magnetic field.
The researchers used infrared laser light to order (preferentially align) the magnetic states of thousands of electrons and nuclei and then used electromagnetic pulses, similar to those used for conventional magnetic resonance imaging (MRI), to entangle them. This procedure caused pairs of electrons and nuclei in a macroscopic 40 micrometer-cubed volume (the volume of a red blood cell) of the semiconductor SiC to become entangled.
“We know that the spin states of atomic nuclei associated with semiconductor defects have excellent quantum properties at room temperature,” said Awschalom, Liew Family Professor in Molecular Engineering and a senior scientist at Argonne National Laboratory. “They are coherent, long-lived and controllable with photonics and electronics. Given these quantum ‘pieces,’ creating entangled quantum states seemed like an attainable goal.” In addition to being of fundamental physical interest, “the ability to produce robust entangled states in an electronic-grade semiconductor at ambient conditions has important implications on future quantum devices,” Awschalom said.
In the short-term, the techniques used here in combination with sophisticated devices enabled by advanced SiC device-fabrication protocols could enable quantum sensors that use entanglement as a resource for beating the sensitivity limit of traditional (non-quantum) sensors. Given that the entanglement works at ambient conditions and the fact that SiC is bio-friendly, one particularly exciting application is biological sensing inside a living organism. “We are excited about entanglement-enhanced magnetic resonance imaging probes, which could have important biomedical applications,” said Abram Falk of IBM’s Thomas J. Watson Research Center and a co-author of the research findings.
In the long term, it might even be possible to go from entangled states on the same SiC chip to entangled states across distant SiC chips. Such efforts could be facilitated by physical phenomena that allow macroscopic quantum states, as opposed to single quantum states (in single atoms), to interact very strongly with one another, which is important for producing entanglement with a high success rate. Such long-distance entangled states have been proposed for synchronizing global positioning satellites and for communicating information in a manner that is fundamentally secured from eavesdroppers by the laws of physics.
Improving Quantum sensors using entanglement
Quantum sensors are measuring device that takes advantage of quantum correlations, such as states in a quantum superposition or entanglement, for better sensitivity and resolution than can be obtained by classical systems. Usually, a sensor’s precision is limited by something called the standard quantum limit. For example, smartphone GPS systems are usually accurate within a 16-foot radius. Quantum metrology uses entangled particles to break past the standard quantum limit and take ultrasensitive measurements.
Quantum sensor’s advantages survive entanglement breakdown, reported in 2015
But entanglement is very fragile, and the difficulty of preserving it is a major obstacle to developing practical quantum information systems. Members of the Optical and Quantum Communications Group at MIT’s Research Laboratory of Electronics have demonstrated that entanglement can also improve the performance of optical sensors, even when it doesn’t survive light’s interaction with the environment.
In the MIT researchers’ system, two beams of light are entangled, and one of them is stored locally—racing through an optical fiber—while the other is projected into the environment. When light from the projected beam—the “probe”—is reflected back, it carries information about the objects it has encountered. But this light is also corrupted by the environmental influences that engineers call “noise.” Recombining it with the locally stored beam helps suppress the noise, recovering the information.
The local beam is useful for noise suppression because its phase is correlated with that of the probe. If you think of light as a wave, with regular crests and troughs, two beams are in phase if their crests and troughs coincide. If the crests of one are aligned with the troughs of the other, their phases are anti-correlated. Quantum mechanics does not allow you to precisely measure the phase of each individual photon, instead, quantum mechanics interprets phase statistically.
When a probe beam interacts with the environment, the noise it accumulates also increases the uncertainty of the ensuing phase measurements. But that’s as true of classical beams as it is of entangled beams. Because entangled beams start out with stronger correlations, even when noise causes them to fall back within classical limits, they still fare better than classical beams do under the same circumstances. In experiments that compared optical systems that used entangled light and classical light, the researchers found that the entangled-light systems increased the signal-to-noise ratio—a measure of how much information can be recaptured from the reflected probe—by 20 percent. That accorded very well with their theoretical predictions.
But the theory also predicts that improvements in the quality of the optical equipment used in the experiment could double or perhaps even quadruple the signal-to-noise ratio. Since detection error declines exponentially with the signal-to-noise ratio that could translate to a million-fold increase in sensitivity. “This is a breakthrough,” says Stefano Pirandola, an associate professor of computer science at the University of York in England. “One of the main technical challenges was the experimental realization of a practical receiver for quantum illumination. Shapiro and Wong experimentally implemented a quantum receiver, which is not optimal but is still able to prove the quantum illumination advantage. In particular, they were able to overcome the major problem associated with the loss in the optical storage of the idler beam.”
“This research can potentially lead to the development of a quantum LIDAR which is able to spot almost-invisible objects in a very noisy background,” he adds. “The working mechanism of quantum illumination could in fact be exploited at short-distances as well, for instance to develop non-invasive techniques of quantum sensing with potential applications in biomedicine.”
In quantum entanglement first, scientists link distant large objects reported in Sep 2020
Scientists entangled two large quantum objects, both at different locations from each other, in a quantum mechanics first. The feat is a step towards practical application of a rather counterintuitive phenomenon and was accomplished by a team from the Niels Bohr Institute at the University of Copenhagen.
The researchers, led by Professor Eugene Polzik, used light particles photons to create an entanglement between a mechanical oscillator (“a vibrating dielectric membrane”) and a cloud of atoms, with each acting like a tiny magnet or “spin”. They picked these particular objects because atoms can be made to process quantum information while the membrane can store that information.
“With this new technique, we are on route to pushing the boundaries of the possibilities of entanglement,” stated professor Polzik. “The bigger the objects, the further apart they are, the more disparate they are, the more interesting entanglement becomes from both fundamental and applied perspectives. With the new result, entanglement between very different objects has become possible.” By entangling the systems, the scientists made them move in correlation with each other. If one object went left, so did the other.
The achievement can pave the way to new sensing technologies. One example would be getting rid of noisy fluctuations currently affecting the Laser Interferometer Gravitational-wave Observatory (LIGO), which detects gravity waves. If the researchers were able to take information from one system and apply it in another, they could get more precise readings.
While the new technology is promising, research into creating useable devices based on quantum mechanics is very challenging, as explained by Ph.D. student Christoffer Østfeldt: “Imagine the different ways of realizing quantum states as a kind of zoo of different realities or situations with very different qualities and potentials,” he shared. If one was to try to make a device using quantum states that would all have different functions, “it will be necessary to invent a language they are all able to speak. The quantum states need to be able to communicate, for us to use the full potential of the device. That’s what this entanglement between two elements in the zoo has shown we are now capable of,” Østfeldt added.
Russian Physicists restore the entanglement of “untangled” quantum light reported in 2015
The entangled states are very fragile; they break easily during transmission due to noise or optical losses. Scientists from the Russian Quantum Center (Moscow), lead by Prof. Alexander Lvovsky, have developed a method to restore quantum entanglement between pulses of light in two optical channels, which was almost completely destroyed after passing through a 20x optical losses. This corresponds to the level of loss in 65 kilometers of ordinary fiber optic cable.The results are published in prestigious scientific journal Nature Photonics. This research significantly broadens the possibilities of quantum communication and quantum cryptography.
In their experiment, a nonlinear crystal of periodically poled potassium titanyl phosphate was used as the source of entangled photons in the experiment. Picosecond pulses of light generated by a titanium-sapphire laser were “fired” at the crystal. As a result, entangled photon pairs were produced in the crystal and directed into two different optical channels. In one of them, the light was subjected to 20-fold attenuation using darkened glass, causing the level of entanglement to fall almost to zero. Then a special amplification procedure was applied, restoring the quantum properties of light in the channel to levels close to those that occurred before the loss.
Their procedure consisted of mixing the light pulse in the attenuated channel with a single “auxiliary” photon in a beam splitter (a partially transparent mirror). A single photon detector is placed at one of the outputs of the beam splitter. If the detector “clicks”, this means that the photon has entered into the beam splitter and left. It would seem that the state of the second pulse to enter the beam splitter (a part of the entangled state) should not change. But, because of paradoxical properties of quantum interference, the state changes in towards the “strengthening” of its quantum properties. This phenomenon, discovered by Lvovsky and colleagues in 2002, has been named quantum catalysis, because the “auxiliary” photon, like a chemical catalyst, is not itself involved in the reaction, but changes the state of light in the other channel.
“At that time it seemed no more than a curious phenomenon, of which there are plenty in quantum physics. Now it turns out to be of great practical use – it allows one to restore entanglement of quantum light,” says Lvovsky. According to him, this work is a step towards building a quantum repeater – a device capable of reducing the loss of quantum information over fiber optic connections. In the future, this will create a global system of transmission of quantum information, and will overcome constraints on quantum cryptography.
“Of course there is a price for the restoration of entanglement – out of one million weakly entangled pairs of photons there is one highly entangled one. But in this regard the level of correlation is restored to the original, and while the data transmission rate is somewhat reduced, we can get a stable connection at a much greater distance,” said study co-author Alexander Ulanov.
References and Resources also include:
https://www.sciencedaily.com/releases/2017/10/171013091023.htm
https://www.delta.tudelft.nl/article/first-version-quantum-internet-making
https://bigthink.com/surprising-science/quantum-entanglement?rebelltitem=4#rebelltitem4
