Physicists build first 500 GHz photon switch


The work took nearly four years to complete and it opens a fundamentally new direction in photonics – with far-reaching potential consequences for the control of photons in optical fiber channels.
Physicists build first 500 GHz photon switch

Researchers at the University of California, San Diego have built the first 500 Gigahertz (GHz) photon switch. “Our switch is more than an order of magnitude faster than any previously published result to date,” said UC San Diego electrical and computer engineering professor Stojan Radic. “That exceeds the speed of the fastest lightwave information channels in use today.”

According to an article in the journal Science, switching at such high speeds was made possible by advances in the control of a strong optical beam using only a few photons, and by the scientists’ ability to engineer the itself with accuracy down to the molecular level.

In the research paper, Radic and his colleagues in the UC San Diego Jacobs School of Engineering argue that ultrafast optical control is critical to applications that must manipulate light beyond the conventional electronic limits. In addition to very fast beam control and fast switching, the latest work opens the way to a new class of sensitive receivers (also capable of operating at very high rates), faster photon sensors, and optical processing devices.

To build the new switch, the UC San Diego team developed a new measurement technique capable of resolving sub-nanometer fluctuations in the fiber core. This was critical because local fiber dispersion varies substantially, even with small core fluctuations, and until recently, control of such small variations was not considered feasible, particularly over long device lengths.

In the experiment, a three-photon input was used to manipulate a Watt-scale beam at a speed exceeding 500 Gigahertz.

Physicists build first 500 GHz photon switch
Schematic for static and fast switching characterization: A continuous-wave (CW) laser pump combined with classically attenuated weak CW or pulsed signal, launched into the nonlinear fiber.

In their research, the engineers in the Photonic Systems Laboratory of UC San Diego’s Qualcomm Institute demonstrated that fast control becomes possible in fiber made of silica glass. “Silica fiber represents a nearly ideal physical platform because of very low optical loss, exceptional transparency and kilometer-scale interaction lengths,” noted Radic. “We showed that a silica fiber core can be controlled with sub-nanometer precision and be used for fast, few-photon control.” Until recently, control of small variations was not considered feasible – particularly over long scales. But once they were able to profile the fluctuation of the actual fiber, it became clear that the silica fiber core could be controlled with sub-nanometer precision – and be used for fast, few-photon control.

Controlling a strong optical beam is not easy. Departing from the conventional approaches that rely on highly resonant physical processes or optical cavities to control an optical beam directly, the UC San Diego team used specially designed, highly nonlinear fibers and they generated all the pulses necessary for the experiments. To design the new switch, they had to derive new theory to describe interaction between photons in fiber core controlled at molecular scale. To build the device, the team also developed a new measurement technique capable of resolving sub-nanometer fiber core fluctuations. “We were able to use the technique to synthesize the first photon gate actuated by only three photons at 500GHz,” said Radic, adding that two key contributors to synthesizing the photon gate include “the ability to predict the optimal microscopic variation, and our ability to measure such variations in physical fiber.”

Fibers may look identical to conventional measurement instruments, and even possess the same standard core variation, yet they may offer dramatically different switching performance. This is primarily due to extreme sensitivity to core fluctuations. (To be used for switching and processing, photons must interact with each other, whereas photons used to communicate information through long-distance fiber travel in a vacuum and do not interact.)

Sensitivity to fluctuations is particularly noteworthy in the context of the fiber’s core structure, which is traditionally made of glass. Its basic building block, the silicon-oxide (Si-O) molecular ring, has a 0.6-nanometer diameter, and it defines the ultimate precision with which a physical fiber core can be realized. Until recently, control of small variations was not considered feasible – particularly over long scales. But once the researchers were able to profile the fluctuation of the actual fiber, it became clear that a silica fiber core could be controlled with sub-nanometer precision, and be used for fast, few-photon control.

What made it not just feasible, but practical, was progress in measuring fiber with sub-nanometer precision. They came up with a way to measure lengths of fiber without doing damage to the fiber. “We measured kilometers of fiber samples and recorded the core variations,” said Nikola Alic, a research scientist in the Photonic Systems Laboratory. Added Radic: “The technique is so sensitive, that if a fly landed on a fiber many miles away, it would distort the core ever so slightly – and we could detect and measure it.”

After measuring the fiber, the UC San Diego researchers were able to generate what they call a “nanoscale signature library.” From there, they identified a specific core fluctuation profile that would correspond to the maximum depletion of the photon pump. Once the calculation yielded a unique core variation profile for a length of fiber, the scientists combined two distinct fiber sections from the core fluctuation library with the same variation profile.

Then came the hard part: figuring out the efficiency and speed of few-photon control. They were able to estimate the minimum number of photons in the control pulse permitted by the specific fiber. The resulting 2.5 picosecond-long pulse (one second equals one trillion picoseconds) with a peak power of 178 nW, contains less than three photons – indicating the feasibility of few-photon switching at a 500GHz rate.

“We addressed the feasibility of few-photon switching in locally controlled fiber,” said Radic, “and it is not very difficult to predict the broader implications of this approach. Specifically, the technology could be implemented for photon sensors that operate in fields that were previously not deemed possible based on the current technology roadmap.” An example is a receiver that could detect a handful of photons but very slowly – with the time delay between such pulses on the order of nanoseconds, not picoseconds (one nanosecond equals 1,000 picoseconds).

Another example: long-scale, locally-controlled four-photon mixing may trigger a multi-frequency photon avalanche, meaning that a few-photon signal could induce massive pump photon annihilation.

To take full advantage of photon switching, says Radic, a new class of fibers is needed – fibers in which the fluctuation of stochastic (randomly determined) dispersions could be minimized. These must be engineered fibers, and the UC San Diego team has already built a first prototype of a fiber engineered for this purpose.

Camera takes 3D photos in the dark


3D images of mannequinOn the left is an image created using current technology – the photo on the right was produced from the MIT team‘s new camera technology
A camera that can create 3D-images in almost pitch black conditions has been developed by researchers at Massachusetts Institute of Technology.

The team captured images of objects, using just single particles of light, known as a photons.

“Billions” of photons would be required to take a photo using the camera on a mobile phone.

The researchers say the technology could be used to help soldiers on combat operations.

Ahmed Kirmani, who wrote the paper containing the findings, said the research has been called “counter-intuitive” as normally the number of photons detected would tell you how bright an image was.

“With only one photon per pixel you would expect the image to be completely featureless,” he told the BBC.

Combat advantage

The camera technology already existed and is similar to the Lidar system used by Google for its Streetview service he explained.

Mannequin with laser
Lidar uses laser pulses and the team used the reflected photons to create their 3D image

“We borrowed the principles form this, the detectors can identify single photons but they still need hundreds of thousands to form images. But we took the system to its limit.”

Lidar uses a laser to fire pulses of light towards an object in a grid sequence. Each location on the grid corresponds to a pixel in the final image.

Normally the laser would fire a large number of times at each grid position and detect multiple reflected photons.

In contrast the system used by the MIT team moved on to the next position in the grid as soon as it had detected a single photon.

A conventional Lidar system would require about 100 times as many photons to make a similar image to the one the team captured which means the system could provide “substantial savings in energy and time”.

The team say the technology could be used in many different fields. It could help ophthalmologists when they want to create an image of a patient’s eye without having to shine a bright light in someone’s eye.

The research was part funded by the US Defense Advanced Research Projects Agency which commissions research for the Department of Defense. Mr Kirmani said the military could use the technology to allow soldiers to see in the dark, giving them an advantage in combat situations.

Slide from MIT presentation
Current 3D imaging techniques require more than single photons unlike the team’s new system

“Any technology that enhances a military’s ability to navigate, target or engage in near-total darkness would be highly prized. 3D imagery married with existing imagery and navigation technologies could significantly enhance the capabilities currently possessed,” said Reed Foster, a defence analyst at IHS.

Eventually, the researchers explain, the technology could be developed to make 3D cameras for mobile phones. The camera requires less light than the ones currently available and therefore uses less power.

Camera takes 3D photos in the dark


A camera that can create 3D-images in almost pitch black conditions has been developed by researchers at Massachusetts Institute of Technology.

3D images of mannequin

The team captured images of objects, using just single particles of light, known as a photons.

“Billions” of photons would be required to take a photo using the camera on a mobile phone.

The researchers say the technology could be used to help soldiers on combat operations.

Ahmed Kirmani, who wrote the paper containing the findings, said the research has been called “counter-intuitive” as normally the number of photons detected would tell you how bright an image was.

“With only one photon per pixel you would expect the image to be completely featureless,” he told the BBC.

Combat advantage

The camera technology already existed and is similar to the Lidar system used by Google for its Streetview service he explained.

Mannequin with laser
Lidar uses laser pulses and the team used the reflected photons to create their 3D image

“We borrowed the principles form this, the detectors can identify single photons but they still need hundreds of thousands to form images. But we took the system to its limit.”

Lidar uses a laser to fire pulses of light towards an object in a grid sequence. Each location on the grid corresponds to a pixel in the final image.

Normally the laser would fire a large number of times at each grid position and detect multiple reflected photons.

In contrast the system used by the MIT team moved on to the next position in the grid as soon as it had detected a single photon.

A conventional Lidar system would require about 100 times as many photons to make a similar image to the one the team captured which means the system could provide “substantial savings in energy and time”.

The team say the technology could be used in many different fields. It could help ophthalmologists when they want to create an image of a patient’s eye without having to shine a bright light in someone’s eye.

The research was part funded by the US Defense Advanced Research Projects Agency which commissions research for the Department of Defense. Mr Kirmani said the military could use the technology to allow soldiers to see in the dark, giving them an advantage in combat situations.

Slide from MIT presentation
Current 3D imaging techniques require more than single photons unlike the team’s new system

“Any technology that enhances a military’s ability to navigate, target or engage in near-total darkness would be highly prized. 3D imagery married with existing imagery and navigation technologies could significantly enhance the capabilities currently possessed,” said Reed Foster, a defence analyst at IHS.

Eventually, the researchers explain, the technology could be developed to make 3D cameras for mobile phones. The camera requires less light than the ones currently available and therefore uses less power.

Single photon detected but not destroyed.


First instrument built that can witness the passage of a light particle without absorbing it.

Physicists have seen a single particle of light and then let it go on its way. The feat was possible thanks to a new technique that, for the first time, detects optical photons without destroying them. The technology could eventually offer perfect detection of photons, providing a boost to quantum communication and even biological imaging.

Plenty of commercially available instruments can identify individual light particles, but these instruments absorb the photons and use the energy to produce an audible click or some other signal of detection.

Quantum physicist Stephan Ritter and his colleagues at the Max Planck Institute of Quantum Optics in Garching, Germany, wanted to follow up on a 2004 proposal of a nondestructive method for detecting photons. Instead of capturing photons, this instrument would sense their presence, taking advantage of the eccentric realm of quantum mechanics in which particles can exist in multiple states and roam in multiple places simultaneously.

Ritter and his team started with a pair of highly reflective mirrors separated by a half-millimeter-wide cavity. Then they placed a single atom of rubidium in the cavity to function as a security guard. They chose rubidium because it can take on two distinct identities, which are determined by the arrangement of its electrons. In one state, it’s a 100 percent effective sentry, preventing photons from entering the cavity. In the other, it’s a totally useless lookout, allowing photons to enter the cavity. When photons get in, they bounce back and forth about 20,000 times before exiting.

The trick was manipulating the rubidium so that it was in a so-called quantum superposition of these two states, allowing one atom to be an overachiever and a slacker at the same time. Consequently, each incoming photon took multiple paths simultaneously, both slipping into the cavity undetected and being stopped at the door and reflected away. Each time the attentive state of the rubidium turned away a photon, a measurable property of the atom called its phase changed. If the phases of the two states of the rubidium atom differed, the researchers knew that the atom had encountered a photon.

To confirm their results, the researchers placed a conventional detector outside the apparatus to capture photons after their rubidium rendezvous, the team reports November 14 in Science.

“It’s a very cool experiment,” says Alan Migdall, who leads the quantum optics group at the National Institute of Standards and Technology in Gaithersburg, Md. But he warns that identifying photons without destroying them does not mean that the outgoing photon is the same as it was prior to detection. “You’ve pulled some information out of it, so you do wind up affecting it,” he says. Ritter says he expects the photons’ properties are largely unchanged, but he acknowledges that his team needs to perform more measurements to confirm that hypothesis.

Ritter notes that no photon detector is perfect, and his team’s is no exception: It failed to detect a quarter of incoming photons, and it absorbed a third of them. But he says the power of the technique is that, for many applications of single-photon detectors, each detector wouldn’t have to be perfect. Ritter envisions a nested arrangement of improved detectors that, as long as they did not absorb photons, would almost guarantee that every photon is counted. Ultimately, that could benefit fields such as medicine and molecular biology, in which scientists require precise imaging of objects in low-light environments.

Seeing light in a new light.


Scientists create never-before-seen form of matter

Harvard and MIT scientists are challenging the conventional wisdom about light, and they didn’t need to go to a galaxy far, far away to do it.

Working with colleagues at the Harvard-MIT Center for Ultracold Atoms, a group led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of Physics Vladan Vuletic have managed to coax photons into binding together to form molecules – a state of matter that, until recently, had been purely theoretical. The work is described in a September 25 paper inNature.

The discovery, Lukin said, runs contrary to decades of accepted wisdom about the nature of light. Photons have long been described as massless particles which don’t interact with each other – shine two laser beams at each other, he said, and they simply pass through one another.

“Photonic molecules,” however, behave less like traditional lasers and more like something you might find in science fiction – the light saber.

“Most of the properties of light we know about originate from the fact that photons are massless, and that they do not interact with each other,” Lukin said. “What we have done is create a special type of medium in which photons interact with each other so strongly that they begin to act as though they have mass, and they bind together to form molecules. This type of photonic bound state has been discussed theoretically for quite a while, but until now it hadn’t been observed.

“It’s not an in-apt analogy to compare this to light sabers,” Lukin added. “When these photons interact with each other, they’re pushing against and deflect each other. The physics of what’s happening in these molecules is similar to what we see in the movies.”

To get the normally-massless photons to bind to each other, Lukin and colleagues, including Harvard post-doctoral fellow Ofer Fisterberg, former Harvard doctoral student Alexey Gorshkov and MIT graduate students Thibault Peyronel and Qiu Liang couldn’t rely on something like the Force – they instead turned to a set of more extreme conditions.

Researchers began by pumped rubidium atoms into a vacuum chamber, then used lasers to cool the cloud of atoms to just a few degrees above absolute zero. Using extremely weak laser pulses, they then fired single photons into the cloud of atoms.

As the photons enter the cloud of cold atoms, Lukin said, its energy excites atoms along its path, causing the photon to slow dramatically. As the photon moves through the cloud, that energy is handed off from atom to atom, and eventually exits the cloud with the photon.

“When the photon exits the medium, its identity is preserved,” Lukin said. “It’s the same effect we see with refraction of light in a water glass. The light enters the water, it hands off part of its energy to the medium, and inside it exists as light and matter coupled together, but when it exits, it’s still light. The process that takes place is the same it’s just a bit more extreme – the light is slowed considerably, and a lot more energy is given away than during refraction.”

When Lukin and colleagues fired two photons into the cloud, they were surprised to see them exit together, as a single molecule.

The reason they form the never-before-seen molecules?

An effect called a Rydberg blockade, Lukin said, which states that when an atom is excited, nearby atoms cannot be excited to the same degree. In practice, the effect means that as two photons enter the atomic cloud, the first excites an atom, but must move forward before the second photon can excite nearby atoms.

The result, he said, is that the two photons push and pull each other through the cloud as their energy is handed off from one atom to the next.

“It’s a photonic interaction that’s mediated by the atomic interaction,” Lukin said. “That makes these two photons behave like a molecule, and when they exit the medium they’re much more likely to do so together than as single photons.”

While the effect is unusual, it does have some practical applications as well.

“We do this for fun, and because we’re pushing the frontiers of science,” Lukin said. “But it feeds into the bigger picture of what we’re doing because photons remain the best possible means to carry quantum information. The handicap, though, has been that photons don’t interact with each other.”

To build a quantum computer, he explained, researchers need to build a system that can preserve quantum information, and process it using quantum logic operations. The challenge, however, is that quantum logic requires interactions between individual quanta so that quantum systems can be switched to perform information processing.

“What we demonstrate with this process allows us to do that,” Lukin said. “Before we make a useful, practical quantum switch or photonic logic gate we have to improve the performance, so it’s still at the proof-of-concept level, but this is an important step. The physical principles we’ve established here are important.”

The system could even be useful in classical computing, Lukin said, considering the power-dissipation challenges chip-makers now face. A number of companies – including IBM – have worked to develop systems that rely on optical routers that convert light signals into electrical signals, but those systems face their own hurdles.

Lukin also suggested that the system might one day even be used to create complex three-dimensional structures – such as crystals – wholly out of light.

“What it will be useful for we don’t know yet, but it’s a new state of matter, so we are hopeful that new applications may emerge as we continue to investigate these photonic molecules’ properties,” he said.

Scientists create never-before-seen form of matter.


Harvard and MIT scientists are challenging the conventional wisdom about light, and they didn’t need to go to a galaxy far, far away to do it.

Working with colleagues at the Harvard-MIT Center for Ultracold Atoms, a group led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of Physics Vladan Vuletic have managed to coax photons into binding together to form molecules – a state of matter that, until recently, had been purely theoretical. The work is described in a September 25 paper in Nature.

The discovery, Lukin said, runs contrary to decades of accepted wisdom about the nature of light. Photons have long been described as massless particles which don’t interact with each other – shine two laser beams at each other, he said, and they simply pass through one another.

“Photonic molecules,” however, behave less like traditional lasers and more like something you might find in science fiction – the light saber.

“Most of the properties of light we know about originate from the fact that photons are massless, and that they do not interact with each other,” Lukin said. “What we have done is create a special type of medium in which photons interact with each other so strongly that they begin to act as though they have mass, and they bind together to form molecules. This type of photonic bound state has been discussed theoretically for quite a while, but until now it hadn’t been observed.

“It’s not an in-apt analogy to compare this to light sabers,” Lukin added. “When these photons interact with each other, they’re pushing against and deflect each other. The physics of what’s happening in these molecules is similar to what we see in the movies.”

To get the normally-massless photons to bind to each other, Lukin and colleagues, including Harvard post-doctoral fellow Ofer Fisterberg, former Harvard doctoral student Alexey Gorshkov and MIT graduate students Thibault Peyronel and Qiu Liang couldn’t rely on something like the Force – they instead turned to a set of more extreme conditions.

Researchers began by pumped rubidium atoms into a vacuum chamber, then used lasers to cool the cloud of atoms to just a few degrees above absolute zero. Using extremely weak laser pulses, they then fired single photons into the cloud of atoms.

As the photons enter the cloud of cold atoms, Lukin said, its energy excites atoms along its path, causing the photon to slow dramatically. As the photon moves through the cloud, that energy is handed off from atom to atom, and eventually exits the cloud with the photon.

“When the photon exits the medium, its identity is preserved,” Lukin said. “It’s the same effect we see with refraction of light in a water glass. The light enters the water, it hands off part of its energy to the medium, and inside it exists as light and matter coupled together, but when it exits, it’s still light. The process that takes place is the same it’s just a bit more extreme – the light is slowed considerably, and a lot more energy is given away than during refraction.”

When Lukin and colleagues fired two photons into the cloud, they were surprised to see them exit together, as a single molecule.

The reason they form the never-before-seen molecules?

An effect called a Rydberg blockade, Lukin said, which states that when an atom is excited, nearby atoms cannot be excited to the same degree. In practice, the effect means that as two photons enter the atomic cloud, the first excites an atom, but must move forward before the second photon can excite nearby atoms.

The result, he said, is that the two photons push and pull each other through the cloud as their energy is handed off from one atom to the next.

“It’s a photonic interaction that’s mediated by the atomic interaction,” Lukin said. “That makes these two photons behave like a molecule, and when they exit the medium they’re much more likely to do so together than as single photons.”

While the effect is unusual, it does have some practical applications as well.

“We do this for fun, and because we’re pushing the frontiers of science,” Lukin said. “But it feeds into the bigger picture of what we’re doing because photons remain the best possible means to carry quantum information. The handicap, though, has been that photons don’t interact with each other.”

To build a quantum computer, he explained, researchers need to build a system that can preserve quantum information, and process it using quantum logic operations. The challenge, however, is that quantum logic requires interactions between individual quanta so that quantum systems can be switched to perform information processing.

“What we demonstrate with this process allows us to do that,” Lukin said. “Before we make a useful, practical quantum switch or photonic logic gate we have to improve the performance, so it’s still at the proof-of-concept level, but this is an important step. The physical principles we’ve established here are important.”

The system could even be useful in classical computing, Lukin said, considering the power-dissipation challenges chip-makers now face. A number of companies – including IBM – have worked to develop systems that rely on optical routers that convert light signals into electrical signals, but those systems face their own hurdles.

Lukin also suggested that the system might one day even be used to create complex three-dimensional structures – such as crystals – wholly out of light.

“What it will be useful for we don’t know yet, but it’s a new state of matter, so we are hopeful that new applications may emerge as we continue to investigate these photonic molecules’ properties,” he said.

 

 Source:  Nature 

Quantum cryptography conquers noise problem.


Encoded photons sent a record distance along busy optical fibres.

Quantum cryptography could keep messages ultra-secure — if the right detector can be developed.

It’s hard to stand out from the crowd — particularly if you are a single photon in a sea of millions in an optical fibre. Because of that, ultra-secure quantum-encryption systems that encode signals into a series of single photons have so far been unable to piggyback on existing telecommunications lines. But now, physicists using a technique for detecting dim light signals have transmitted a quantum key along 90 kilometres of noisy optical fibre1. The feat could see quantum cryptography finally enter the mainstream.

You cannot measure a quantum system without noticeably disrupting it. That means that two people can encode an encryption key — for bank transfers, for instance — into a series of photons and share it, safe in the knowledge that any eavesdropper will trip the system’s alarms. But such systems have not been able to transmit keys along telecommunications lines, because other data traffic swamps the encoded signal. As a result, quantum cryptography has had only niche applications, such as connecting offices to nearby back-up sites using expensive ‘dark’ fibres that carry no other signals. “This is really the bottleneck for quantum cryptography,” says physicist Nicolas Gisin, a scientific adviser at quantum-cryptography company ID Quantique in Geneva, Switzerland.

Physicists have attempted to solve the problem by sending photons through a shared fibre along a ‘quantum channel‘ at one characteristic wavelength. The trouble is that the fibre scatters light from the normal data traffic into that wavelength, polluting the quantum channel with stray photons. Andrew Shields, a physicist at the Toshiba Cambridge Research Laboratory, UK, and his colleagues have now developed a detector that picks out photons from this channel only if they strike it at a precise instant, calculated on the basis of when the encoded photons were sent. The team publishes its results in Physics Review X.

Designing a detector with such a sharp time focus was tough, explains Shields. Standard detectors use semiconducting devices that create an avalanche of electrical charge when struck by a single photon. But it usually takes more than one nanosecond (10−9 seconds) for the avalanche to grow large enough to stand out against the detector’s internal electrical hiss — much longer than the narrow window of 100 picoseconds (10−10 seconds) needed to filter a single photon from a crowd.

The team’s ‘self-differentiating’ detector activates for 100 picoseconds, every nanosecond. The weak charge triggered by a photon strike in this short interval would not normally stand out, but the detector measures the difference between the signal recorded during one operational cycle and the signal from the preceding cycle — when no matching photon was likely to be detected. This cancels out the background hum. Using this device, the team has transmitted a quantum key along a 90-kilometre fibre, which also carried noisy data at 1 billion bits per second in both directions — a rate typical of a telecommunications fibre. The team now intends to test the technique on a real telecommunications line.

Gisin’s team has independently developed a photon detector with a similar time window, which they presented at the QCrypt 2012 meeting at the Centre for Quantum Technologies in Singapore in September. However, Gisin has calculated that such a technique cannot be used to transmit quantum signals beyond the range of a large city of 100 kilometres2. Scattering accumulates over distance, so there would eventually be so many stray photons that it would be impossible to filter them out, even with a precisely timed detector.

Still, 90 kilometres is a “world record that is a big step forward in demonstrating the applicability of quantum cryptography in real-world telecommunications infrastructures”, says Vicente Martín, a physicist at the Technical University of Madrid.

Source: Nature