Tag Archives: photon

For first time: high-dimensional quantum encryption performed in real world city conditions

Having congratulated China on the world’s first quantum communication network a few weeks ago (August 22, 2017 posting), this quantum encryption story seems timely. From an August 24, 2017 news item on phys.org,

For the first time, researchers have sent a quantum-secured message containing more than one bit of information per photon through the air above a city. The demonstration showed that it could one day be practical to use high-capacity, free-space quantum communication to create a highly secure link between ground-based networks and satellites, a requirement for creating a global quantum encryption network.

Quantum encryption uses photons to encode information in the form of quantum bits. In its simplest form, known as 2D encryption, each photon encodes one bit: either a one or a zero. Scientists have shown that a single photon can encode even more information—a concept known as high-dimensional quantum encryption—but until now this has never been demonstrated with free-space optical communication in real-world conditions. With eight bits necessary to encode just one letter, for example, packing more information into each photon would significantly speed up data transmission.

This looks like donuts on a stick to me,

For the first time, researchers have demonstrated sending messages in a secure manner using high dimensional quantum cryptography in realistic city conditions. Image Credit: SQO team, University of Ottawa.

An Aug. 24, 2017 Optical Society news release (also on EurekAlert), which originated the news item, describes the work done by a team in Ottawa, Canada, (Note: The ‘Congratulate China’ piece (August 22, 2017 posting) includes excerpts from an article that gave a brief survey of various national teams [including Canada] working on quantum communication networks; Links have been removed),

“Our work is the first to send messages in a secure manner using high-dimensional quantum encryption in realistic city conditions, including turbulence,” said research team lead, Ebrahim Karimi, University of Ottawa, Canada. “The secure, free-space communication scheme we demonstrated could potentially link Earth with satellites, securely connect places where it is too expensive to install fiber, or be used for encrypted communication with a moving object, such as an airplane.”

For the first time, researchers have demonstrated sending messages in a secure manner using high dimensional quantum cryptography in realistic city conditions. Image Credit: SQO team, University of Ottawa.

As detailed in Optica, The Optical Society’s journal for high impact research, the researchers demonstrated 4D quantum encryption over a free-space optical network spanning two buildings 0.3 kilometers apart at the University of Ottawa. This high-dimensional encryption scheme is referred to as 4D because each photon encodes two bits of information, which provides the four possibilities of 01, 10, 00 or 11.

In addition to sending more information per photon, high-dimensional quantum encryption can also tolerate more signal-obscuring noise before the transmission becomes unsecure. Noise can arise from turbulent air, failed electronics, detectors that don’t work properly and from attempts to intercept the data. “This higher noise threshold means that when 2D quantum encryption fails, you can try to implement 4D because it, in principle, is more secure and more noise resistant,” said Karimi.

Using light for encryption

Today, mathematical algorithms are used to encrypt text messages, banking transactions and health information. Intercepting these encrypted messages requires figuring out the exact algorithm used to encrypt a given piece of data, a feat that is difficult now but that is expected to become easier in the next decade or so as computers become more powerful.

Given the expectation that current algorithms may not work as well in the future, more attention is being given to stronger encryption techniques such as quantum key distribution, which uses properties of light particles known as quantum states to encode and send the key needed to decrypt encoded data.

Although wired and free-space quantum encryption has been deployed on some small, local networks, implementing it globally will require sending encrypted messages between ground-based stations and the satellite-based quantum communication networks that would link cities and countries. Horizontal tests through the air can be used to simulate sending signals to satellites, with about three horizontal kilometers being roughly equal to sending the signal through the Earth’s atmosphere to a satellite.

Before trying a three-kilometer test, the researchers wanted to see if it was even possible to perform 4D quantum encryption outside. This was thought to be so challenging that some other scientists in the field said that the experiment would not work. One of the primary problems faced during any free-space experiment is dealing with air turbulence, which distorts the optical signal.

Real-world testing

For the tests, the researchers brought their laboratory optical setups to two different rooftops and covered them with wooden boxes to provide some protection from the elements. After much trial and error, they successfully sent messages secured with 4D quantum encryption over their intracity link. The messages exhibited an error rate of 11 percent, below the 19 percent threshold needed to maintain a secure connection. They also compared 4D encryption with 2D, finding that, after error correction, they could transmit 1.6 times more information per photon with 4D quantum encryption, even with turbulence.

“After bringing equipment that would normally be used in a clean, isolated lab environment to a rooftop that is exposed to the elements and has no vibration isolation, it was very rewarding to see results showing that we could transmit secure data,” said Alicia Sit, an undergraduate student in Karimi’s lab.

As a next step, the researchers are planning to implement their scheme into a network that includes three links that are about 5.6 kilometers apart and that uses a technology known as adaptive optics to compensate for the turbulence. Eventually, they want to link this network to one that exists now in the city. “Our long-term goal is to implement a quantum communication network with multiple links but using more than four dimensions while trying to get around the turbulence,” said Sit.

Here’s a link to and a citation for the paper,

High-dimensional intracity quantum cryptography with structured photons by Alicia Sit, Frédéric Bouchard, Robert Fickler, Jérémie Gagnon-Bischoff, Hugo Larocque, Khabat Heshami, Dominique Elser, Christian Peuntinger, Kevin Günthner, Bettina Heim, Christoph Marquardt, Gerd Leuchs, Robert W. Boyd, and Ebrahim Karimi. Optica Vol. 4, Issue 9, pp. 1006-1010 (2017) •https://doi.org/10.1364/OPTICA.4.001006

This is an open access paper.

Light-based computation made better with silver

It’s pretty amazing to imagine a future where computers run on light but according to a May 16, 2017 news item on ScienceDaily the idea is not beyond the realms of possibility,

Tomorrow’s computers will run on light, and gold nanoparticle chains show much promise as light conductors. Now Ludwig-Maximilians-Universitaet (LMU) in Munich scientists have demonstrated how tiny spots of silver could markedly reduce energy consumption in light-based computation.

Today’s computers are faster and smaller than ever before. The latest generation of transistors will have structural features with dimensions of only 10 nanometers. If computers are to become even faster and at the same time more energy efficient at these minuscule scales, they will probably need to process information using light particles instead of electrons. This is referred to as “optical computing.”

The silver serves as a kind of intermediary between the gold particles while not dissipating energy. Capture: Liedl/Hohmann (NIM)

A March 15, 2017 LMU press release (also one EurekAlert), which originated the news item, describes a current use of light in telecommunications technology and this latest research breakthrough (the discrepancy in dates is likely due to when the paper was made available online versus in print),

Fiber-optic networks already use light to transport data over long distances at high speed and with minimum loss. The diameters of the thinnest cables, however, are in the micrometer range, as the light waves — with a wavelength of around one micrometer — must be able to oscillate unhindered. In order to process data on a micro- or even nanochip, an entirely new system is therefore required.

One possibility would be to conduct light signals via so-called plasmon oscillations. This involves a light particle (photon) exciting the electron cloud of a gold nanoparticle so that it starts oscillating. These waves then travel along a chain of nanoparticles at approximately 10% of the speed of light. This approach achieves two goals: nanometer-scale dimensions and enormous speed. What remains, however, is the energy consumption. In a chain composed purely of gold, this would be almost as high as in conventional transistors, due to the considerable heat development in the gold particles.

A tiny spot of silver

Tim Liedl, Professor of Physics at LMU and PI at the cluster of excellence Nanosystems Initiative Munich (NIM), together with colleagues from Ohio University, has now published an article in the journal Nature Physics, which describes how silver nanoparticles can significantly reduce the energy consumption. The physicists built a sort of miniature test track with a length of around 100 nanometers, composed of three nanoparticles: one gold nanoparticle at each end, with a silver nanoparticle right in the middle.

The silver serves as a kind of intermediary between the gold particles while not dissipating energy. To make the silver particle’s plasmon oscillate, more excitation energy is required than for gold. Therefore, the energy just flows “around” the silver particle. “Transport is mediated via the coupling of the electromagnetic fields around the so-called hot spots which are created between each of the two gold particles and the silver particle,” explains Tim Liedl. “This allows the energy to be transported with almost no loss, and on a femtosecond time scale.”

Textbook quantum model

The decisive precondition for the experiments was the fact that Tim Liedl and his colleagues are experts in the exquisitely exact placement of nanostructures. This is done by the DNA origami method, which allows different crystalline nanoparticles to be placed at precisely defined nanodistances from each other. Similar experiments had previously been conducted using conventional lithography techniques. However, these do not provide the required spatial precision, in particular where different types of metals are involved.

In parallel, the physicists simulated the experimental set-up on the computer – and had their results confirmed. In addition to classical electrodynamic simulations, Alexander Govorov, Professor of Physics at Ohio University, Athens, USA, was able to establish a simple quantum-mechanical model: “In this model, the classical and the quantum-mechanical pictures match very well, which makes it a potential example for the textbooks.”

Here’s a link to and c citation for the paper,

Hotspot-mediated non-dissipative and ultrafast plasmon passage by Eva-Maria Roller, Lucas V. Besteiro, Claudia Pupp, Larousse Khosravi Khorashad, Alexander O. Govorov, & Tim Liedl. Nature Physics (2017) doi:10.1038/nphys4120 Published online 15 May 2017

This paper is behind a paywall.

First hologram of a single photon (light particle)

Polish scientists have created a technique for something thought to be impossible. From a July 19, 2016 news item on Nanowerk,

Until quite recently, creating a hologram of a single photon was believed to be impossible due to fundamental laws of physics. However, scientists at the Faculty of Physics, University of Warsaw, have successfully applied concepts of classical holography to the world of quantum phenomena. A new measurement technique has enabled them to register the first ever hologram of a single light particle, thereby shedding new light on the foundations of quantum mechanics.

A July 18, 2016 University of Warsaw press release on EurekAlert, which originated the news item, describes the breakthrough in more detail,

Scientists at the Faculty of Physics, University of Warsaw, have created the first ever hologram of a single light particle. The spectacular experiment, reported in the prestigious journal Nature Photonics, was conducted by Dr. Radoslaw Chrapkiewicz and Michal Jachura under the supervision of Dr. Wojciech Wasilewski and Prof. Konrad Banaszek. Their successful registering of the hologram of a single photon heralds a new era in holography: quantum holography, which promises to offer a whole new perspective on quantum phenomena.

“We performed a relatively simple experiment to measure and view something incredibly difficult to observe: the shape of wavefronts of a single photon,” says Dr. Chrapkiewicz.

In standard photography, individual points of an image register light intensity only. In classical holography, the interference phenomenon also registers the phase of the light waves (it is the phase which carries information about the depth of the image). When a hologram is created, a well-described, undisturbed light wave (reference wave) is superimposed with another wave of the same wavelength but reflected from a three-dimensional object (the peaks and troughs of the two waves are shifted to varying degrees at different points of the image). This results in interference and the phase differences between the two waves create a complex pattern of lines. Such a hologram is then illuminated with a beam of reference light to recreate the spatial structure of wavefronts of the light reflected from the object, and as such its 3D shape.

One might think that a similar mechanism would be observed when the number of photons creating the two waves were reduced to a minimum, that is to a single reference photon and a single photon reflected by the object. And yet you’d be wrong! The phase of individual photons continues to fluctuate, which makes classical interference with other photons impossible. Since the Warsaw physicists were facing a seemingly impossible task, they attempted to tackle the issue differently: rather than using classical interference of electromagnetic waves, they tried to register quantum interference in which the wave functions of photons interact.

Wave function is a fundamental concept in quantum mechanics and the core of its most important equation: the Schrödinger equation. In the hands of a skilled physicist, the function could be compared to putty in the hands of a sculptor: when expertly shaped, it can be used to ‘mould’ a model of a quantum particle system. Physicists are always trying to learn about the wave function of a particle in a given system, since the square of its modulus represents the distribution of the probability of finding the particle in a particular state, which is highly useful.

“All this may sound rather complicated, but in practice our experiment is simple at its core: instead of looking at changing light intensity, we look at the changing probability of registering pairs of photons after the quantum interference,” explains doctoral student Jachura.

Why pairs of photons? A year ago, Chrapkiewicz and Jachura used an innovative camera built at the University of Warsaw to film the behaviour of pairs of distinguishable and non-distinguishable photons entering a beam splitter. When the photons are distinguishable, their behaviour at the beam splitter is random: one or both photons can be transmitted or reflected. Non-distinguishable photons exhibit quantum interference, which alters their behaviour: they join into pairs and are always transmitted or reflected together. This is known as two-photon interference or the Hong-Ou-Mandel effect.

“Following this experiment, we were inspired to ask whether two-photon quantum interference could be used similarly to classical interference in holography in order to use known-state photons to gain further information about unknown-state photons. Our analysis led us to a surprising conclusion: it turned out that when two photons exhibit quantum interference, the course of this interference depends on the shape of their wavefronts,” says Dr. Chrapkiewicz.

Quantum interference can be observed by registering pairs of photons. The experiment needs to be repeated several times, always with two photons with identical properties. To meet these conditions, each experiment started with a pair of photons with flat wavefronts and perpendicular polarisations; this means that the electrical field of each photon vibrated in a single plane only, and these planes were perpendicular for the two photons. The different polarisation made it possible to separate the photons in a crystal and make one of them ‘unknown’ by curving their wavefronts using a cylindrical lens. Once the photons were reflected by mirrors, they were directed towards the beam splitter (a calcite crystal). The splitter didn’t change the direction of vertically-polarised photons, but it did diverge diplace horizontally-polarised photons. In order to make each direction equally probable and to make sure the crystal acted as a beam splitter, the planes of photon polarisation were bent by 45 degrees before the photons entered the splitter. The photons were registered using the state-of-the-art camera designed for the previous experiments. By repeating the measurements several times, the researchers obtained an interference image corresponding to the hologram of the unknown photon viewed from a single point in space. The image was used to fully reconstruct the amplitude and phase of the wave function of the unknown photon.

The experiment conducted by the Warsaw physicists is a major step towards improving our understanding of the fundamental principles of quantum mechanics. Until now, there has not been a simple experimental method of gaining information about the phase of a photon’s wave function. Although quantum mechanics has many applications, and it has been verified many times with a great degree of accuracy over the last century, we are still unable to explain what wave functions actually are: are they simply a handy mathematical tool, or are they something real?

“Our experiment is one of the first allowing us to directly observe one of the fundamental parameters of photon’s wave function – its phase – bringing us a step closer to understanding what the wave function really is,” explains Jachura.

The Warsaw physicists used quantum holography to reconstruct wave function of an individual photon. Researchers hope that in the future they will be able to use a similar method to recreate wave functions of more complex quantum objects, such as certain atoms. Will quantum holography find applications beyond the lab to a similar extent as classical holography, which is routinely used in security (holograms are difficult to counterfeit), entertainment, transport (in scanners measuring the dimensions of cargo), microscopic imaging and optical data storing and processing technologies?

“It’s difficult to answer this question today. All of us – I mean physicists – must first get our heads around this new tool. It’s likely that real applications of quantum holography won’t appear for a few decades yet, but if there’s one thing we can be sure of it’s that they will be surprising,” summarises Prof. Banaszek.

Here’s a link to and a citation for the paper,

Hologram of a single photon by Radosław Chrapkiewicz, Michał Jachura, Konrad Banaszek, & Wojciech Wasilewski.  Nature Photonics (2016) doi:10.1038/nphoton.2016.129 Published online 18 July 2016

This paper is behind a paywall.

Changing the colour of single photons in a diamond quantum memory

An artist’s impression of quantum frequency conversion in a diamond quantum memory. (Credit: Dr. Khabat Heshami, National Research Council Canada)

An artist’s impression of quantum frequency conversion in a diamond quantum memory. (Credit: Dr. Khabat Heshami, National Research Council Canada)

An April 5, 2016 University of Waterloo news release (also on EurekAlert) describes the research,

Researchers from the Institute for Quantum Computing at the University of Waterloo and the National Research Council of Canada (NRC) have, for the first time, converted the colour and bandwidth of ultrafast single photons using a room-temperature quantum memory in diamond.

Shifting the colour of a photon, or changing its frequency, is necessary to optimally link components in a quantum network. For example, in optical quantum communication, the best transmission through an optical fibre is near infrared, but many of the sensors that measure them work much better for visible light, which is a higher frequency. Being able to shift the colour of the photon between the fibre and the sensor enables higher performance operation, including bigger data rates.

The research, published in Nature Communications, demonstrated small frequency shifts that are useful for a communication protocol known as wavelength division multiplexing. This is used today when a sender needs to transmit large amounts of information through a transmission so the signal is broken into smaller packets of slightly different frequencies and sent through together. The information is then organized at the other end based on those frequencies.

In the experiments conducted at NRC, the researchers demonstrated the conversion of both the frequency and bandwidth of single photons using a room-temperature diamond quantum memory.

“Originally there was this thought that you just stop the photon, store it for a little while and get it back out. The fact that we can manipulate it at the same time is exciting,” said Kent Fisher a PhD student at the Institute for Quantum Computing and with the Department of Physics and Astronomy at Waterloo. “These findings could open the door for other uses of quantum memory as well.”

The diamond quantum memory works by converting the photon into a particular vibration of the carbon atoms in the diamond, called a phonon. This conversion works for many different colours of light allowing for the manipulation of a broad spectrum of light. The energy structure of diamond allows for this to occur at room temperature with very low noise. Researchers used strong laser pulses to store and retrieve the photon. By controlling the colours of these laser pulses, researchers controlled the colour of the retrieved photon.

“The fragility of quantum systems means that you are always working against the clock,” remarked Duncan England, researcher at NRC. “The interesting step that we’ve shown here is that by using extremely short pulses of light, we are able to beat the clock and maintain quantum performance.”

The integrated platform for photon storage and spectral conversion could be used for frequency multiplexing in quantum communication, as well as build up a very large entangled state – something called a cluster state. Researchers are interested in exploiting cluster states as the resource for quantum computing driven entirely by measurements.

Here’s a link to and a citation for the paper,

Frequency and bandwidth conversion of single photons in a room-temperature diamond quantum memory by Kent A. G. Fisher, Duncan G. England, Jean-Philippe W. MacLean, Philip J. Bustard, Kevin J. Resch, & Benjamin J. Sussman. Nature Communications 7, Article number: 11200  doi:10.1038/ncomms11200 Published 05 April 2016

This paper is open access.