First step towards photonic quantum network

Illustration of a photon gun. A quantum dot (the yellow symbol) emits one photon (red wave packet) at a time. The quantum dot is embedded in a photonic crystal structure, which is obtained by etching holes (black circles) in a semiconductor.

Advanced photonic nanostructures are well on their way to revolutionizing quantum technology for quantum networks based on light. Researchers from the Niels Bohr Institute have now developed the first building blocks needed to construct complex quantum photonic circuits for quantum networks. This rapid development in quantum networks is highlighted in an article in the journal Nature.

Quantum technology based on light (photons) is called quantum photonics, while electronics is based on electrons. Photons (light particles) and electrons behave differently at the quantum level. A quantum entity is the smallest unit in the microscopic world. For example, photons are the fundamental constituent of light and electrons of electric current. Electrons are so-called fermions and can easily be isolated to conduct current one electron at a time. In contrast photons are bosons, which prefer to bunch together. But since information for quantum communication based on photonics is encoded in a single photon, it is necessary to emit and send them one at a time.

Increased information capacity
Information based on photons has great advantages; photons interact only very weakly with the environment - unlike electrons, so photons do not lose much energy along the way and can therefore be sent over long distances. Photons are therefore very well suited for carrying and distributing information and a quantum network based on photons will be able to encode much more information than is possible with current computer technology and the information could not be intercepted in route.

Many research groups around the world are working intensively in this research field, which is developing rapidly and in fact the first commercial quantum photonics products are starting to be manufactured.


Directional emission of photons. The figure shows the calculations of the photon emission in the new directional single-photon source. If the spin of the quantum dot's electron points up, the photon will be emitted in the one direction.

Control of the photons
A prerequisite for quantum networks is the ability to create a stream of single photons on demand and the researchers at the Niels Bohr Institute succeeded in doing exactly that.

"We have developed a photonic chip, which acts as a photon gun. The photonic chip consists of an extremely small crystal that is 10 microns wide and is 160 nanometers thick. Embedded in the middle of the chip is a light source, which is a so-called quantum dot. Illuminating the quantum dot with laser light excites an electron, which can then jump from one orbit to another and thereby emit a single photon at a time. Photons are usually emitted in all directions, but the photonic chip is designed so that all the photons are sent out through a photonic waveguide," explains Peter Lodahl, professor and head of the Quantum Photonics research group at the Niels Bohr Institute, University of Copenhagen.

In a long, laborious process, the research group further developed and tested the photonic chip until it achieved extreme efficiency and Peter Lodahl explains that it was particularly surprising that they could get the photon emission to occur in a way that was not previously thought possible. Normally, the photons are transmitted in both directions in the photonic waveguide, but in their custom-made photonic chip they could break this symmetry and get the quantum dot to differentiate between emitting a photon right or left, that means emit directional photons. This means full control over the photons and the researchers are beginning to explore how to construct complete quantum network systems based on the new discovery.

"The photons can be sent over long distances via optical fibers, where they whiz through the fibers with very little loss. You could potentially build a network where the photons connect small quantum systems, which are then linked together into a quantum network - a quantum internet," explains Peter Lodahl.

He adds that while the first basic functionalities are already a reality, the great challenge is now to expand them to large, complex quantum networks.

Journal reference: Nature
Provided by: Niels Bohr Institute

YOUR INPUT IS MUCH APPRECIATED! LEAVE YOUR COMMENT BELOW.

We can see the future of Quantum Systems

Scientists at the University of Sydney have demonstrated the ability to "see" the future of quantum systems, and used that knowledge to preempt their demise, in a major achievement that could help bring the strange and powerful world of quantum technology closer to reality.

The applications of quantum-enabled technologies are compelling, and already demonstrating significant impacts - especially in the realm of sensing and metrology. And the potential to build exceptionally powerful quantum computers using quantum bits, or qubits, is driving investment from the world's largest companies.

However a significant obstacle to building reliable quantum technologies has been the randomization of quantum systems by their environments, or de-coherence, which effectively destroys the useful quantum character.

The physicists have taken a technical quantum leap in addressing this, using techniques from big data to predict how quantum systems will change and then, preventing the system's breakdown from occurring.



The research is published today in Nature Communications.
"Much the way the individual components in mobile phones will eventually fail, so too do quantum systems," said the paper's senior author Professor Michael J. Biercuk.
"But in quantum Technology the lifetime is generally measured in fractions of a second, rather than years."

Professor Biercuk, from the University of Sydney's School of Physics and a chief investigator at the Australian Research Council's Centre for Engineered Quantum Systems, said his group had demonstrated it was possible to suppress de-coherence in a preventive manner. The key was to develop a technique to predict how the system would disintegrate.

Professor Biercuk highlighted the challenges of making predictions in a quantum world: "Humans routinely employ predictive techniques in our daily experience; for instance, when we play tennis we predict where the ball will end up based on observations of the airborne ball," he said.

This works because the rules that govern how the ball will move, like gravity, are regular and known. But what if the rules changed randomly while the ball was on its way to you? In that case it's next to impossible to predict the future behavior of that ball.
"And yet this situation is exactly what we had to deal with because the disintegration of quantum systems is random. Moreover, in the quantum realm observation erases ‘quantumness’, so our team needed to be able to guess how and when the system would randomly break.

"We effectively needed to swing at the randomly moving tennis ball while blindfolded."

The team turned to machine learning for help in keeping their quantum systems - qubits realized in trapped atoms - from breaking.



What might look like random behavior actually contained enough information for a computer program to guess how the system would change in the future. It could then predict the future without direct observation, which would otherwise erase the system's useful characteristics.

The predictions were remarkably accurate, allowing the team to use their guesses preemptively to compensate for the anticipated changes.

Doing this in real time allowed the team to prevent the disintegration of the quantum character, extending the useful lifetime of the qubits.
"We know that building real quantum technologies will require major advances in our ability to control and stabilize qubits - to make them useful in applications," Professor Biercuk said.

Our techniques apply to any qubit, built in any technology, including the special superconducting circuits being used by major corporations.
"We're excited to be developing new capabilities that turn quantum systems from novelties into useful technologies. The quantum future is looking better all the time," Professor Biercuk said.

YOUR INPUT IS MUCH APPRECIATED! LEAVE YOUR COMMENT BELOW.

Quantum Computing: Information can be carry using a Photon

A Princeton University-led team has built a device that advances silicon-based quantum computers, which when built will be able to solve problems beyond the capabilities of everyday computers. The device isolates an electron so that can pass its quantum information to a photon, which can then act as a messenger to carry the information to other electrons to form the circuits of the computer.

In a step that brings silicon-based quantum computers closer to reality, researchers have built a device in which a single electron can pass its quantum information to a particle of light.

In a step that brings silicon-based quantum computers closer to reality, researchers at Princeton University have built a device in which a single electron can pass its quantum information to a particle of light. The particle of light, or photon, can then act as a messenger to carry the information to other electrons, creating connections that form the circuits of a quantum computer.

The research published in the journal Science and conducted at Princeton and HRL Laboratories in Malibu, California, represents a more than five-year effort to build a robust capability for an electron to talk to a photon, said Jason Petta, a Princeton professor of physics.



"Just like in human interactions, to have good communication a number of things need to work out -- it helps to speak the same language and so forth,"Petta said. "We are able to bring the energy of the electronic state into resonance with the light particle, so that the two can talk to each other."

The discovery will help the researchers use light to link individual electrons, which act as the bits, or smallest units of data, in a quantum computer. Quantum computers are advanced devices that, when realized, will be able to perform advanced calculations using tiny particles such as electrons, which follow quantum rules rather than the physical laws of the everyday world.

Each bit in an everyday computer can have a value of a 0 or a 1. Quantum bits -- known as qubits -- can be in a state of 0, 1, or both a 0 and a 1 simultaneously. This superposition, as it is known, enables quantum computers to tackle complex questions that today's computers cannot solve.

Simple quantum computers have already been made using trapped ions and superconductors, but technical challenges have slowed the development of silicon-based quantum devices. Silicon is a highly attractive material because it is inexpensive and is already widely used in today's smartphones and computers.

The researchers trapped both an electron and a photon in the device, then adjusted the energy of the electron in such a way that the quantum information could transfer to the photon. This coupling enables the photon to carry the information from one qubit to another located up to a centimeter away.

Quantum information is extremely fragile -- it can be lost entirely due to the slightest disturbance from the environment. Photons are more robust against disruption and can potentially carry quantum information not just from qubit to qubit in a quantum computer circuit but also between quantum chips via cables.



For these two very different types of particles to talk to each other, however, researchers had to build a device that provided the right environment. First, Peter Deelman at HRL Laboratories, a corporate research-and-development laboratory owned by the Boeing Company and General Motors, fabricated the semiconductor chip from layers of silicon and silicon-germanium. This structure trapped a single layer of electrons below the surface of the chip. Next, researchers at Princeton laid tiny wires, each just a fraction of the width of a human hair, across the top of the device. These nanometer-sized wires allowed the researchers to deliver voltages that created an energy landscape capable of trapping a single electron, confining it in a region of the silicon called a double quantum dot.

The researchers used those same wires to adjust the energy level of the trapped electron to match that of the photon, which is trapped in a superconducting cavity that is fabricated on top of the silicon wafer.

Prior to this discovery, semiconductor qubits could only be coupled to neighboring qubits. By using light to couple qubits, it may be feasible to pass information between qubits at opposite ends of a chip.

The electron's quantum information consists of nothing more than the location of the electron in one of two energy pockets in the double quantum dot. The electron can occupy one or the other pocket, or both simultaneously. By controlling the voltages applied to the device, the researchers can control which pocket the electron occupies.

"We now have the ability to actually transmit the quantum state to a photon confined in the cavity," said Xiao Mi, a graduate student in Princeton's Department of Physics and first author on the paper. "This has never been done before in a semiconductor device because the quantum state was lost before it could transfer its information."



The success of the device is due to a new circuit design that brings the wires closer to the qubit and reduces interference from other sources of electromagnetic radiation. To reduce this noise, the researchers put in filters that remove extraneous signals from the wires that lead to the device. The metal wires also shield the qubit. As a result, the qubits are 100 to 1000 times less noisy than the ones used in previous experiments.

Eventually the researchers plan to extend the device to work with an intrinsic property of the electron known as its spin. "In the long run we want systems where spin and charge are coupled together to make a spin qubit that can be electrically controlled," Petta said. "We've shown we can coherently couple an electron to light, and that is an important step toward coupling spin to light."
Story Source: Princeton University

YOUR INPUT IS MUCH APPRECIATED! LEAVE YOUR COMMENT BELOW.

Parallel Worlds

By: Alexandria Addesso

Parallel worlds and alternate universes have long been a loved theme of interest in Twilight Zone episodes and science fiction movies and literature. Like much other science fiction subject matter, parallel worlds actually do have some scientific data behind it, although many are still very skeptical.

"The idea of parallel universes in quantum mechanics has been around since 1957," said physicist at Griffith University in Brisbane, Australia, Howard Wiseman, who was also part of the team of physicists that came up with the ‘Many-Worlds Interpretation’ (MWI).

"In the well-known ‘Many-Worlds Interpretation’, each universe branches into a bunch of new universes every time a quantum measurement is made. All possibilities are therefore realized – in some universes the dinosaur-killing asteroid missed Earth. In others, Australia was colonized by the Portuguese."



Yet despite what the name of the name of the MWI seems to suggest, these other worlds and universes have absolutely no proven effect on our own. Thus leaving skeptical physicists all the more doubting, while many parallel world theories often get ridiculed or put on the back burner by many mainstream scientists, the String Theory is a way to make sense of multiple dimensions mathematically. The String Theory states that within the theoretical framework of the theory, point-like particles of particle physics are replaced by one-dimensional objects called strings and describes how these strings circulate through space and interact with each other.

Superstring theory goes a step farther and attempts to explain all of the particles and fundamental forces of nature in one theory by modelling them as vibrations of tiny supersymmetric strings. For mathematical consistency, both theories require extra dimensions of space-time. The String Theory suggests that space-time is 26-dimensional, while superstring theory is 10-dimensional.

"You almost can't avoid having some version of the multiverse in your studies if you push deeply enough in the mathematical descriptions of the physical universe," said physicist Brian Greene who authored the book The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos.



"There are many of us thinking of one version of parallel universe theory or another. If it's all a lot of nonsense, then it's a lot of wasted effort going into this far-out idea. But if this idea is correct, it is a fantastic upheaval in our understanding."

YOUR INPUT IS MUCH APPRECIATED! LEAVE YOUR COMMENT BELOW.

Albert Einstein was Wrong!

Quantum Spookiness Confirmed

An international group effort and a recently issued research paper may have just settled a century old physics debate. Quantum mechanics is, indeed, spooky. Quantum Entanglement, a part of quantum mechanics, tells us that two particles can be openly linked even across huge distances. If you measure the rotation of one particle, you will instantly know the spin of its entangled particle.

Physicists have considered this behavior as “spooky” as it doesn’t follow our daily logic at all. Common sense tells us that objects across the cosmos cannot possibly be linked, but in the quantum world, they are. Quantum mechanics also states that properties of particles are only stable when the particle is perceived.



Several physicists, counting Albert Einstein, opposed this idea as it went in contradiction of the very nature of the actual world. In the 1930s when quantum mechanics was a developing field, Einstein was a supporter of “local realism,” saying that only nearby objects could affect each other. Einstein and several other physicists established the ‘hidden variables theory’ to clarify the spooky behavior.

They claimed that our knowledge of quantum mechanics was partial and there might be unseen variables that we didn’t yet understand. In the 1960s a physicist called John Bell formulated a mathematical expression, called an inequality, to check for these so-called unseen variables. He comprehended that if these unseen variables did certainly exist, there would be a boundary to how linked the particles were. If they surpassed the set limit then the hidden variables did not exist.

Though, the experiment, called as Bell’s Inequality, did not ultimately close the door on local realism. The tests involved entangled photons, which can get lost along the way, and researcher conducting the experiments might not detect all photons produced.



In the recent experiment, directed by Professor Ronald Hanson of Delft University of Technology in the Netherlands, we have two scientists, we will name them Alice and Bob, in two workshops 1.3 kilometers apart. Each laboratory has a diamond chip, containing an electron whose rotation was entangled with a photon. The photons were then directed to a third lab in between Alice and Bob, where a sensor records the entrance time. If two photons reached at the same time they would be entangled, resulting in the electrons being entangled also.

The experiment took time of nine days. In that time, scientists noted 245 positive entanglements. While other experiments over the last few decades have also maintained Bell’s limit, this new experiment acquires from their inadequacies to overcome experimental drawbacks. Previous test used incompetent detectors, only measuring a slight number of the particles passing through them. New experiments used near-perfect sensors, but the entangled particles were close enough to possibly connect. In the recent experiment, the team used high-quality sensors and measurements collected before the electrons could conceivably exchange signals with each other, creating it the first to close both loopholes.



The outcomes of this experiment have big consequences for the world of quantum cryptography, meaning entangled photons could possibly generate safe encryption keys. Terminating the loopholes would guarantee that computer systems could sense if anyone tried to interrupt the keys, as it would halt the entanglement and activate an alarm.

Source: Umer Abrar - mirzavadoodulbaig@gmail.com

YOUR INPUT IS MUCH APPRECIATED! LEAVE YOUR COMMENT BELOW.

Understanding Quantum Computing

Quantum computing is the area of study focused on developing computer technology based on the principles of quantum theory. The quantum computer, following the laws of quantum physics, would gain enormous processing power through the ability to be in multiple states, and to perform tasks using all possible permutations simultaneously.

A Comparison of Classical and Quantum Computing
Classical computing relies, at its ultimate level, on principles expressed by Boolean algebra. Data must be processed in an exclusive binary state at any point in time or bits. While the time that each transistor or capacitor need be either in 0 or 1 before switching states is now measurable in billionths of a second, there is still a limit as to how quickly these devices can be made to switch state. As we progress to smaller and faster circuits, we begin to reach the physical limits of materials and the threshold for classical laws of physics to apply. Beyond this, the quantum world takes over.

In a quantum computer, a number of elemental particles such as electrons or photons can be used with either their charge or polarization acting as a representation of 0 and/or 1. Each of these particles is known as a quantum bit, or qubit, the nature and behavior of these particles form the basis of quantum computing.



Quantum Superposition and Entanglement
The two most relevant aspects of quantum physics are the principles of superposition and entanglement.

Superposition: Think of a qubit as an electron in a magnetic field. The electron’s spin may be either in alignment with the field, which is known as a spin-up state, or opposite to the field, which is known as a spin-down state. According to quantum law, the particle enters a superposition of states, in which it behaves as if it were in both states simultaneously. Each qubit utilized could take a superposition of both 0 and 1.

Entanglement: Particles that have interacted at some point retain a type of connection and can be entangled with each other in pairs, in a process known ascorrelation. Knowing the spin state of one entangled particle – up or down – allows one to know that the spin of its mate is in the opposite direction. Quantum entanglement allows qubits that are separated by incredible distances to interact with each other instantaneously (not limited to the speed of light). No matter how great the distance between the correlated particles, they will remain entangled as long as they are isolated.

Taken together, quantum superposition and entanglement create an enormously enhanced computing power. Where a 2-bit register in an ordinary computer can store only one of four binary configurations (00, 01, 10, or 11) at any given time, a 2-qubit register in a quantum computer can store all four numbers simultaneously, because each qubit represents two values. If more qubits are added, the increased capacity is expanded exponentially.



Difficulties with Quantum Computers
Interference – During the computation phase of a quantum calculation, the slightest disturbance in a quantum system (say a stray photon or wave of EM radiation) causes the quantum computation to collapse, a process known as de-coherence. A quantum computer must be totally isolated from all external interference during the computation phase.

Error correction – Given the nature of quantum computing, error correction is ultra-critical – even a single error in a calculation can cause the validity of the entire computation to collapse.

Output observance – Closely related to the above two, retrieving output data after a quantum calculation is complete risks corrupting the data.

The Future of Quantum Computing
The biggest and most important one is the ability to factorize a very large number into two prime numbers. That’s really important because that’s what almost all encryption of internet applications use and can be de-encrypted. A quantum computer should be able to do that relatively quickly. Calculating the positions of individual atoms in very large molecules like polymers and in viruses. The way that the particles interact with each other – if you have a quantum computer you could use it to develop drugs and understand how molecules work a bit better.



Even though there are many problems to overcome, the breakthroughs in the last 15 years, and especially in the last 3, have made some form of practical quantum computing possible. However, the potential that this technology offers is attracting tremendous interest from both the government and the private sector. It is this potential that is rapidly breaking down the barriers to this technology, but whether all barriers can be broken, and when, is very much an open question.

Source: Ahmed Banafa, from OpenMind

YOUR INPUT IS MUCH APPRECIATED! LEAVE YOUR COMMENT BELOW.