2Physics Quote:
"Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe."
-- Kathrin Altwegg and the ROSINA Team
(Read Full Article:
"Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko" )
In the early days of photography, pictures were all motionless. Those were either pictures of calm and quiet landscapes or pictures of families, in which -- not just the parents -- even the children were all very sober and sensible boys and girls. They never seemed to be running around.
But people could never live happily thereafter with whatever was achieved by then. They wanted something better.
They observed how the legs of a running baby or a horse were getting ghostly blurry when they tried taking pictures of such objects in motion. Photographing the 100m race at the Olympics posed a similar problem.
So, Now they started worrying about `Shutter Speed' or how quickly the shutter of the camera `opens and closes'. Things were getting blurry because the shutter speed could not catch up with the speed of the object in the photograph.
The minimum 'shutter speed' used these days is rough 'one-sixtieth of a second', depending on what the image is and how far away the focus is.
But, as usual, people could never live happily thereafter with whatever was achieved at any point in time. As usual, They wanted something even better.
They wanted to see every minute movement of the body of an athlete running 100 meters. They wanted to see how exactly hummingbird's wings flap up and down so quickly.
Thus began the era of photography when people wanted to see what they could not even see with their naked eyes.
Some thought they would want to get a better look at how electrons move around in atoms to understand how atoms and molecules interact with each other and how various chemical reactions happen.
Electrons, however, move very quickly within atoms. The speed is about 2200 km per second. At this speed, one can travel around the earth in just 18 seconds.
So researchers gradually developed such a unique pulse laser that is capable of generating an equivalent `shutter speed' of only 4000 trillionths of 1 second.
[Remember though we are now looking at what is happening inside atoms. The concept of `shutter speed' is no longer quite appropriate here. The act of `Looking' is no longer the same as `Looking with our eyes'. The size of an atom is about ten-millionth of a hair-width. So, `looking inside an atom' is governed by quantum mechanics. Newtonian Physics and intuition do not really apply here]
This year's Nobel Prize is a recognition of that success.
Probing Particle Exchange Symmetry Using Interference
Authors of the PRL Paper [1]: From Left to Right (top row) Adrian J. Menssen, Alex E. Jones, Benjamin J. Metcalf, Malte C. Tichy; (bottom row) Stefanie Barz, W. Steven Kolthammer, Ian A. Walmsley (Image credit for Stefanie Barz: Uli Regenscheit, University of Stuttgart; image credit for Adrian Menssen: John Cairns)
The exchange symmetry of identical particles is a foundational principle in quantum physics.
If two particles are identical, then the outcomes of any measurements of the system should be unaffected if the particles are swapped. A theory of such particles should therefore give the same predictions if all of the degrees of freedom of the two particles are exchanged. Indeed, this exchange symmetry provides a useful classification for quantum particles: Bosons are described by many-particle wavefunctions that are symmetric under such exchange, whereas fermions have anti-symmetric wavefunctions.
A simple example of invariance under exchange is illustrated in figure 1. Two particles are characterized by their position (p) and shape (s). We can label their joint state as (p, s; p', s'), as shown in Fig. 1a. If the particles are exchanged, then the new joint state is (p', s'; p, s), as in Fig. 1b. It is obvious that there is no difference between these two states. What happens, though, if we were to exchange only some of the labels? Say, the positions of the particles, but not their shapes. In that case the state would be (p, s'; p', s), as in Fig.1c. It’s clear that this partially-exchanged state is different than the original state. Now, let’s imagine the case when the two particles have the same shape, so s=s'. In that case we might say that the particles were indistinguishable, since exchanging the other degree of freedom would leave the state unchanged.
Fig 1. Full and partial exchange of the degrees of freedom of two particles. The single-particle states are denoted by position, left (l) or right (r), and shape, triangle (t) or star (s).
In fact, most particles have more complicated specifications of their states, and this allows a third option: partially distinguishable particles. For instance, imagine two photons in non-overlapping positions (left and right) with identical colour, or spectra: they are both red, say. These photons are would be indistinguishable, since exchange of their positions results in no change to the two-photon state. In contrast, let’s say the photons have non-overlapping spectra: the photon on the left is blue, while the other is red. If we now exchange their positions, we obtain a state that has no overlap with the initial configuration. Therefore, a measurement can completely distinguish whether or not the exchange occurred. The two photons are said to be distinguishable with respect to exchange of position. If two photons had partly overlapping spectra, they would be partially distinguishable. In this case, the nature of the particles can be roughly understood as an interpolation of completely indistinguishable and distinguishable cases, governed by the degree of their spectral overlap.
Remarkably, however, this perspective does not work for more than two particles. In this case the fact that we are dealing with wavefunctions characterized by complex probability amplitudes is critical. In our recent study of the properties of particle exchange among several photons, we have discovered some intriguing new features, which indicate how three particles are very different from two particles and why more particles can be decomposed into properties of three photons – or triads, as we’ll call them.
interpolation of completely indistinguishable and distinguishable cases, governed by the degree of their spectral overlap.
How do we investigate the nature of distinguishability for several particles? It turns out light provides an excellent opportunity to do so. Photons are massless bosons with no charge. They do not interact easily, and effectively cannot do so at all in free space. Thus it is only their bosonic character that links the evolution of multi-particle states. Further, photons can be engineered to have a wide variety of wavefunctions, specified by their external degrees of freedom that result in properties such as time, spectrum, space, direction and polarization. This makes it possible to make them distinguishable, indistinguishable or partly distinguishable. Moreover, they can be generated, manipulated and detected in ambient conditions, revealing quantum phenomena without complicated apparatus.
What we have done is build photon networks that allow the particles to co-propagate through a series of beam splitters. Because these networks consist entirely of linear optical elements the only means by which the photons can react is through their intrinsic “boson-ness”. When two similar photons enter the two input ports of a beam splitter, for instance, they tend to “bunch” together, leaving through the same output port. This is the well-known Hong-Ou-Mandel effect, first demonstrated in 1987.
It turns out that this effect is in fact a direct measurement of the symmetry of the wavefunction under exchange of the particles’ positions. The reason for this is straightforward to see, and is illustrated in the first movies, Video 1a and 1b. One may consider the beam splitter as a 2X2 network: there are two input beams and two output. Either photon could pass directly through the network: input 1 to output 1, say. Or the two could exchange: input 1 to output 2. These two options offer two paths for one photon to emerge in each output port. The degree to which the quantum amplitudes for these paths interfere is governed by the distinguishability of the “unexchanged” and “exchanged” states.
Video 1a by Adrian Menssen:
Video 1b by Adrian Menssen:
(Caption for videos above) Beam splitter: In the case of a beam-splitter there are two possible paths the photons can take, when considering coincident detection of one photon in each output mode of the interferometer: One where both photons remain in the waveguide they started in, or where they exchange positions. Interference from paths where two photons are exchanged leads to non-classical Hong-Ou-Mandel interference.
There is a further reason why this is important. Optical quantum information processing schemes rely on the interference of multiple photons for their enhanced performance. A key goal is to demonstrate a quantum machine that can outperform a conventional computer at a well-posed computational task, and thus show quantum “supremacy” or quantum advantage.
A promising computing problem for this challenge is called BosonSampling. It addresses the following question: given N non-interacting bosons at the input of an M-port random network (where M > N2), what is the distribution of the bosons measured at the output ports? It turns out that the probability of each possible measurement outcome is related to the permanent of the complex-valued transfer matrix that describes the relevant network section. Furthermore, estimating such matrix permanents is known to be a hard computational problem for classical computers. However, since photons are boson and multiport optical networks can be composed from simple linear optical elements, it seems that constructing a quantum machine that naturally tackles BosonSampling might be feasible. Remarkably, such a machine would accomplish a task known to be unreachable by classical algorithms.
One of the challenges in building these kinds of quantum information processing technologies is to ensure that all the photons are identical. The details of how distinguishable photons interfere is known in complete detail only for one and two photons. In our work we extended such studies to three photons. That may seem a small step, but it reveals a complex landscape, not all the features of which were known. For example, we identified a new phase – the triad phase – that is not present in two-photon interference, and which appears to be a qualitatively new element that characterizes all higher order states.
We measured the probability that three photons put into the three ports of a “tritter”, labelled, say, 1, 2, and 3, would end up at the three output ports. A tritter is a device that couples each input port symmetrically to each output port. In our experiment we used on constructed out of optical fibers, in which all three modes are connected by allowing the evanescent fields of the guided modes to overlap, thereby coupling all the modes to each other with equal strength. This enables the input photons to scatter into all the output modes.
The probability that there is a photon in each of the output ports can now happen in six ways, as shown in the second set of videos, 2a and 2b. First, none of the photons are scattered out of their input mode and end up at the same mode at the output, represented as 123 -> 123. Second, pairs of photons exchange modes, and one photon remains in its original mode, so 123 - > 213, for example. There are three such terms. Third, pairs of photons exchange twice. There are two such terms: 123 - > 213 -> 231; 123 - > 132 ->312. These pathways represent possible actual events and therefore the probability of their occurence must be determined by adding together their amplitudes coherently, taking the modulus squared of this quantity and tracing over the unmeasured degrees of freedom. The resulting number is a direct measure of the difference between the input three-photon wavefunction and its exchanged versions. The more distinguishable the photons become, the larger this distance and the disappearance of any interference pattern.
Video 2a by Adrian Menssen:
Video 2b by Adrian Menssen:
(Caption for videos above) Tritter: Here we illustrate two possible scenarios how three photons can traverse a tritter. In the first case, they remain in the waveguide they started in, while in the second scenario each photon couples into a different waveguide, leading to a full exchange of all three photons. The triad phase arises precisely from interference between paths, where a full three-photon exchange occurs. We could also imagine paths where only two photons exchange position, while the third one does not participate. Contributions from these paths lead to Hong-Ou-Mandel like “two-photon” interference. Isolating “genuine” three photon interference from two-photon interference is one of the hallmarks of our paper.
In our experiment the photons can be made distinguishable by adjusting the delays between them. But delays alone are insufficient to explore the entire state space. That’s because a there are three complex numbers specifying the three state overlaps and delays provide only two parameters. Therefore we used an additional degree of freedom – the polarization of each of the photons to provide a means to access the necessary parameters.
By setting all three polarizations to be the same, the triad phase is zero (as long as some simple conditions on the photon spectra are satisfied). In this case, the three-photon exchange term dominates, yielding a coincidence rate that is larger than when the photons are completely distinguishable. As the photons are made more distinguishable, the pair-wise exchange terms dominate, leading to a reduction in the coincidences below that of fully distinguishable photons, reminiscent of the Hong-Ou-Mandel dip. But they never completely dominate, so that there is never a complete suppression of coincidences.
The triad phase can be set to a non-zero value by adjusting the relative polarizations of the three input photons. For example, when they are polarized at 120 degrees to each other, the triad phase is π. In this case, the three-photon exchange and the two-photon exchange contributions are of the same sign, and both contribute to a dip. The photons are of course in this case rather distinguishable, so in some ways it is surprising to see such a suppression of coincidences arising from interference.
It is possible to adjust the polarizations as the photons are rendered more distinguishable by delaying them in such a way as to keep all of the two-photon coincidences constant. This is analogous to the one-photon rates in the Hong-Ou-Mandel experiment, which should be constant in order that the two-photon dip can be proven to be non-classical in origin.
We find in our experiment that the three-photon coincidence exhibits an analogous dip, which is the first time that the fully exchanged three-photon wavefunction has been isolated by interference.
Are there more complex structures for more than three photons? Of course the interference patterns are richer, and we can expect that new features appear. But it might be that there are no new classes of parameters that are needed. In other words, by measuring the triad phase of all photon triplets entering the device one can determine whether the photons are identical. If so, then pair-wise two-photon correlations would be insufficient to fully characterize a linear optical interferometer, and three-photon correlations both necessary and sufficient.
The nature of multi-particle wavefunctions, and in particular the degree to which partially distinguishable particles can still exhibit non-classical phenomena (not to say provide quantum enhancement in technological applications) is an area ripe for exploration. New features arise when one goes from one to two particles, and the richness of three particles is startling. What new ones will be found in the wavefunctions of higher particle numbers remains to be seen. Reference: [1] Adrian J. Menssen, Alex E. Jones, Benjamin J. Metcalf, Malte C. Tichy, Stefanie Barz, W. Steven Kolthammer, Ian A. Walmsley, "Distinguishability and many-particle interference". Physical Review Letters, 118, 153603 (2017). Abstract.arXiv:1609.09804.
Quantum safe cryptography has truly been thrust into the limelight recently, especially after the NSA announced “The Information Assurance Directorate (IAD) will initiate a transition to quantum resistant algorithms in the not too distant future,” in August of 2015. Previous to this announcement, there has been a multitude of work done in accomplishing the quantum key distribution (QKD) protocol [1], a symmetric key algorithm, which has been proven to be cryptanalytically secure, even against quantum computer attacks. One major concern with QKD is the distances over which secure keys can be generated. In optical fibers this is known to have an upper limit around a few hundred kilometers due to absorption in the fibers. For free space, we are limited in line of site as well as atmospheric turbulence.
Using satellites to assist in the key generation helps alleviate these distance concerns by linking multiple stations located on the ground which can be separated by much larger distances than allowed by ground links. Performing QKD with moving platforms are important to prove the viability of future satellite implementations. Demonstrations of QKD to aircraft, prior to this work, have operated in the downlink configuration [2,3], where the quantum source and transmitter are placed on the airborne platform. Most recently, the Chinese Micius satellite performed entanglement distribution to ground stations separated by at least 1200 kilometers which has shown the viability of using satellites for quantum information purposes [4]. While the downlink approach ultimately has the potential for higher key rate, it is more complex and is not as flexible as an uplink configuration, which places the quantum receiver on the airborne platform while keeping the quantum source at the ground station [5]. In September 2016, we successfully achieved a quantum uplink, generating quantum secure key, to an airplane [6].
Our experimental setup consists of a QKD source (in a trailer for temperature and humidity control) and transmitter located at a ground station at Smiths Falls—Montague Airport (Fig 1), and a QKD receiver located on a Twin Otter research aircraft from the National Research Council of Canada. Targeting and tracking were established using strong beacon lasers, an imaging camera, and tracking feedback to motors at each of the two sites. Once at the aircraft, the QKD signals were recorded for later processing to complete the QKD protocol and secure extract key.
In order to generate the photons for encoding the information, we use a weak coherent pulse source implementing polarization-encoded BB84 with decoy states [7] at a rate of 400 MHz. These signals are characterized at the source with an automated polarization compensation system to compensate for drifts due to the optical fiber portion of the transmission to the transmitting telescope.
Fig 1: Optical ground station located at Smiths Falls—Montague Airport. The airplane is shown in the background as the white dashed line in the sky.
At the receiver, the signal is coupled from the receiver telescope into a custom fine pointing unit which guides both the quantum and beacon signals with a fast-steering mirror. Inside the fine-pointing system, a dichroic mirror separates the quantum and beacon signals---the beacon is reflected towards a quad-cell photo-sensor, providing position feedback to guide the fast-steering mirror in a closed loop.
The quantum signal then passes into a custom integrated optical assembly, containing a passive-basis-choice polarization analysis module with a 50:50 beam splitter and polarizing beam splitters, resulting in four beams corresponding to the four BB84 measurement states (horizontal, vertical, diagonal, and anti-diagonal). These four modes are then coupled into multimode fibers and guided to Silicon avalanche photo diodes detectors. The detectors trigger low-voltage differential signalling pulses which are measured at a control and data processing unit based on Xiphos' Q7 processor card, which has recently flown on the GHGSat, with a custom daughter board.
The airplane flew two path types: circular arcs around the ground station, and lines past the ground station. The distances for each type of pass varied from 3 to 10 km. The circular paths were used for longer link times as well as to relax pointing requirements as the aircraft telescope would remain relatively still and the transmitter would observe a constant rate. The line passes are more representative of a satellite, showing different angular speeds for different portions of the pass.
Fig 2: The Smiths Falls—Montague Airport as seen from the airplane during a day test flight. The end of the telescope can be seen in the far left of the photo.
In total, we had successful quantum links in seven of 14 passes of the airplane over the ground station, generating asymptotic key in one pass and finite-size secure key in 5 passes, with one pass showing over 800 kbit. The loss in the various passes ranged from 34.4 to 51.1 dB. Angular speeds (at the transmitter) between 0.4 deg/s and 1.28 deg/s were achieved. A transmitter pointing at a typical low-Earth-orbit satellite with an altitude of roughly 600 kilometers would be tracking at an angular speed of approximately 0.7 deg/s whereas tracking the International Space Station would require approximately 1.2 deg/s.
Fig 3: (left to right) Christopher Pugh and Thomas Jennewein hold up a University of Waterloo/Institute for Quantum Computing sticker to be mounted on the NRC Twin Otter Research Aircraft. The QKD receiver can be seen in the open door of the airplane.
In this experiment, we have demonstrated the viability of components of a quantum receiver satellite payload by successfully performing quantum key distribution in an uplink configuration to an airplane. The major components in the receiver payload (fine pointing unit, integrated optics assembly, detector modules, control and data processing unit) have a clear path to flight for future satellite integration. Recently, the Canadian government and the Canadian Space Agency have announced the intent to build a quantum satellite and this work will be beginning shortly.
References: [1] Charles H. Bennett and Gilles Brassard, “Quantum cryptography: Public key distribution and coin tossing”. In Proceedings of the IEEE International Conference on Computers, Systems and Signal Processing, Bangalore, India, pages 175-179 (IEEE Press, New York, 1984). Article. [2] Sebastian Nauerth, Florian Moll, Markus Rau, Christian Fuchs, Joachim Horwath, Stefan Frick, Harald Weinfurter, “Air-to-ground quantum communication”. Nature Photonics, 7, 382 (2013). Abstract. [3] Jian-Yu Wang, Bin Yang, Sheng-Kai Liao, Liang Zhang, Qi Shen, Xiao-Fang Hu, Jin-Cai Wu, Shi-Ji Yang, Hao Jiang, Yan-Lin Tang, Bo Zhong, Hao Liang, Wei-Yue Liu, Yi-Hua Hu, Yong-Mei Huang, Bo Qi, Ji-Gang Ren, Ge-Sheng Pan, Juan Yin, Jian-Jun Jia, Yu-Ao Chen, Kai Chen, Cheng-Zhi Peng, Jian-Wei Pan., “Direct and full-scale experimental verifications towards ground-satellite quantum key distribution”. Nature Photonics, 7, 387 (2013). Abstract. [4] Juan Yin, Yuan Cao, Yu-Huai Li, Sheng-Kai Liao, Liang Zhang, Ji-Gang Ren, Wen-Qi Cai, Wei-Yue Liu, Bo Li, Hui Dai, Guang-Bing Li, Qi-Ming Lu, Yun-Hong Gong, Yu Xu, Shuang-Lin Li, Feng-Zhi Li, Ya-Yun Yin, Zi-Qing Jiang, Ming Li, Jian-Jun Jia, Ge Ren, Dong He, Yi-Lin Zhou, Xiao-Xiang Zhang, Na Wang, Xiang Chang, Zhen-Cai Zhu, Nai-Le Liu, Yu-Ao Chen, Chao-Yang Lu, Rong Shu, Cheng-Zhi Peng, Jian-Yu Wang, Jian-Wei Pan, “Satellite-based entanglement distribution over 1200 kilometers”. Science, 356:1140 (2017). Abstract. [5] J-P Bourgoin, E Meyer-Scott, B L Higgins, B Helou, C Erven, H Hübel, B Kumar, D Hudson, I D'Souza, R Girard, R Laflamme, T. Jennewein, “A comprehensive design and performance analysis of low Earth orbit satellite quantum communication”. New Journal of Physics, 15, 023006 (2013). Abstract. [6] Christopher J Pugh, Sarah Kaiser, Jean-Philippe Bourgoin, Jeongwan Jin, Nigar Sultana, Sascha Agne, Elena Anisimova, Vadim Makarov, Eric Choi, Brendon L Higgins, Thomas Jennewein, “Airborne demonstration of a quantum key distribution receiver payload”. Quantum Science and Technology, 2, 024009 (2017). Abstract. [7] Hoi-Kwong Lo, Xiongfeng Ma, Kai Chen, “Decoy state quantum key distribution”. Physical Review Letters, 94, 230504 (2005). Abstract.
Research in electronic devices is pushing the size of transistors below the 10 nm mark, but at this scale information processing faces difficulties in terms of heat dissipation, as well as in controlling quantum effects such as electron tunneling. An alternative to electron-based computing lies in the creation of all-optical computational devices. Although we are still far from producing functional processors, today we show how plasmonic structures are viable tools not only for the nanoscale localization of electromagnetic energy but also its transfer, contrary to what their typically high dissipation rates would have us believe.
Plasmonics, in general terms, is the field of Physics that studies the interaction of light with materials, mostly at the nanoscale, that support collective electronic oscillations [1]. These plasmonic materials are typically metallic nanoparticles (NPs) or heavily doped semiconductors that exhibit free-electron behavior. When light, as an electromagnetic wave, is incident on the NPs, the free electron gas of the NPs is excited and oscillates as a result of the polarization of the electromagnetic wave. These excited plasmonic oscillations typically have resonances in the visible spectral range and the resonance frequencies depend on the type, shape and size of the plasmon-supporting material.
Plasmonic materials afford us the capability of manipulating light and funneling its energy at the nanoscale, and has enabled the creation of novel devices, from optical metamaterials [2] to efficient nanoscale heat generators [3]. For all-optical computational devices, it is challenging to transfer and manipulate photonic energy at the nanoscale. Here, photon transfer is not feasible using traditional waveguides such as glass fibers, which require sizes comparable to the wavelength of the confined light. Plasmonic nanostructures are remarkably effective in localizing the electromagnetic energy of light, but these structures suffer from high energy losses due to the high rate of electron scattering with the background charges in the material. In our recent work [4] we show how this essential limitation can be successfully circumvented by carefully considering the interaction of different material systems with distinct plasmonic resonances.
Figure 1. TEM image of a trimer fabricated with the DNA origami technique, with a 30 nm AgNP placed between two 40 nm AuNPs. Below there is an energy diagram of the trimer. The AgNP resonance is not fully excited, acting as an efficient energy transmitter, coupling the outer AuNPs. Figure adapted from Ref. [4]
By carefully arranging two gold NPs (AuNPs) and a silver NP (AgNP) in a linear chain , our model system takes advantage of the different optical spectra of gold and silver [4]. Figure 1 shows a transmission electron microscopy (TEM) image of one of the experimentally fabricated trimers, illustrating the arrangement of our proposed hybrid nanostructure. The two 40 nm AuNPs are spatially separated by a 38 nm gap, partially occupied by a 30 nm AgNP that leaves two 4 nm interparticle gaps. To achieve this accurate NP arrangement we have used the DNA origami technique [5,6] to build a rigid bundle of 14 interconnected, parallel DNA double helices. At three points along this DNA structure, attachment sites for each particle are built, consisting of single-stranded DNA (ssDNA) extensions with orthogonal sequences for each material. Figure 1 also includes a schematic energy diagram of the trimer, which illustrates the main effect explored in the paper. The two AuNP plasmon states are only weakly coupled due to the distance separating them, but the connecting AgNP acts as a bridge for their interaction. More importantly, it mediates the transfer with a plasmonic resonance of different energy than the resonance of the AuNPs. In this way, the AgNP plasmon state is not fully occupied and it functions as a virtual state for the plasmon to travel from one AuNP to the other.
The presence of the AgNP is crucial for the coherent transfer of energy from one AuNP (transmitter) to another (receiver) with negligible energy dissipation. The AgNP bridges the large gap between the AuNPs with a virtual energy state that allows the transfer to take place. As we confirmed experimentally, the distance between the AuNPs is enough to impede their coupling in the absence of the mediating particle. In fact, the AuNPs would respond mostly as independent dipoles in the absence of the AgNP. With the three particles in place, the longitudinal plasmonic resonance of the trimer is far from the resonant frequency of the AgNP. Therefore, this mediating particle facilitates the coupling of the two AuNPs and the excitation of the global trimer plasmon mode, while avoiding the dissipation of heat that would appear in the AgNP resonant excitation. Importantly, our simulations show the hot spots that typically occur between the surfaces of metal particles in close proximity, allowing here the effective coupling of their near fields. The coupling is experimentally verified through the observation of the
scattering spectra of the trimer, evidencing the hybridization of the plasmonic modes of the NPs. Figure 2 showcases some of the scattering experimental data, alongside results from the simulated system.
Figure 2: (click on the image to view with higher resolution) Experimental (a) and simulated (b) spectra of interacting trimers, compared with a weakly interacting gold NP dimer. The scattering signal is red shifted and strongly enhanced when introducing the mediating silver NP, creating a new resonance for the whole interacting system and evidencing the hybridization of their originally non-interacting states. Figure adapted from Ref. [4]
In a second set of classical electrodynamic simulations, we observed the time resolved transfer inside the trimer after being excited by a single point dipole source, located just outside the chain. We find an extremely fast plasmon passage between the two outer particles, orders of magnitudes faster than the energy transfer observed in typical Förster-type dipole-dipole interactions [7]. By our calculations, we estimate a transfer time of about 5 fs and the oscillation of energy between the two gold particles for more than 40 fs (Figure 3). We have also described the system with a theoretical quantum model in which the three NPs are considered as three quantum oscillators that are coupled to each other via Coulomb interactions. This quantum model predicts the energy shift of the system’s bright plasmon modes that are observed experimentally, as well as the dark modes found in a full electrodynamical simulation.
Figure 3: (click on the image to view with higher resolution) Evolution in time of the excitation in the two AuNPs as obtained from simulations, showing the total electric dipole in each NP after a pulse excitation by a dipole source close to the left AuNP. The diagram on top is a representation of the trimer and the source. The middle AgNP acts as a transmitter. The data in (b), shown in a logarithmic scale, is the total amplitude of the dipolar moment in each gold NP, in which one can appreciate the back-and-forth transfer of the excitation between the AuNPs. Figure adapted from Ref. [4]
In summary, in this research we presented a novel approach toward fast and almost loss-less plasmon transfer between metal nanoparticles, which can have a great impact in applications for energy and information transfer on the nanoscale. We used a simple combination of gold and silver nanoparticles to coherently transfer energy and overcome the large heat dissipation characteristic of plasmonic systems. Our proposed quantum model provides us with a clear understanding of the physics, and our full electrodynamic simulation is in excellent agreement with the experimental measurements performed on this exciting system. This work opens new doors for designing future optical-based computing devices and for the teleportation of data over longer distances at the nanoscale.
References [1] Stefan A. Maier, "Plasmonics: fundamentals and applications" (Springer, 2007). [2] Kan Yao and Yongmin Liu, “Plasmonic metamaterials”. Nanotechnology Rev.iews 3, 177 (2014). Abstract. [3] Larousse Khosravi Khorashad, Lucas V. Besteiro, Zhiming Wang, Jason Valentine, and Alexander O. Govorov, “Localization of Excess Temperature Using Plasmonic Hot Spots in Metal Nanostructures: Combining Nano-Optical Antennas with the Fano Effect”. Journal of Physical Chemistry C, 120, 13215 (2016). Abstract. [4] Eva-Maria Roller, Lucas V. Besteiro, Claudia Pupp, Larousse Khosravi Khorashad, Alexander O. Govorov and Tim Liedl, “Hotspot-mediated non-dissipative and ultrafast plasmon passage”. Nature Physics (published online May 15, 2017). Abstract. [5] Anton Kuzyk, Robert Schreiber, Zhiyuan Fan, Günther Pardatscher, Eva-Maria Roller, Alexander Högele, Friedrich C. Simmel, Alexander O. Govorov and Tim Liedl, “DNA-based self-assembly of chiral plasmonic nanostructures with tailored optical response". Nature, 483, 311 (2012). Abstract. [6] Paul W. K. Rothemund, “Folding DNA to create nanoscale shapes and patterns”. Nature, 440, 297 (2006). Abstract. [7] Volkhard May and Oliver Kühn, “Charge and Energy Transfer Dynamics in Molecular Systems” (3rd Ed. Wiley, 2011).
Quantum network architectures consist of local nodes comprising quantum memories that are interconnected using single photons propagating in photonic channels. In such networks, the ability to transfer a quantum bit of information (qubit) from one node to another plays a central role. In a practical implementation, the photonic qubits are generated through spontaneous emission from a matter qubit – embodied for instance by a single atom or ion, a point defect or a quantum dot in a solid-state matrix. The generated photons are then collected in a fiber and sent to another matter qubit using a photonic channel.
Since photonic channels are subject to imperfect collection efficiencies and photon losses, there is a finite probability that such a state transfer based on a single photonic qubit fails. It is therefore suitable to have a heralding signal testifying that the transfer has been successful. Although some basic elements towards such heralded quantum state transfer have already been demonstrated in previous work using various physical systems [1-4], a full node-to-node heralded transfer using single photon qubits has not been achieved to date, mainly due to the dominant role of photon losses.
Using self-assembled semiconductor quantum dots (QDs), we have demonstrated heralded absorption of a neutral (source) QD generated single photonic qubit, by a single-electron charged (target) QD that is located 5 m away [5]. The photonic qubit is thereby transferred and stored in the spin degree of freedom of the single host electron. A successful process is heralded by detection of a subsequent photon that carries no information about the qubit state, which is essential to preserve the coherent quantum superposition.
The principle of our experiment is depicted in fig. 1. The source QD (QD1) is neutral and can be prepared in an arbitrary superposition of two exciton states using a two-color laser beam. This quantum state is encoded in the color of a single photon (flying qubit) generated upon radiative recombination of the QD1 exciton. This flying qubit is collected into a fiber and transferred to the target QD. Amongst the wide variety of QDs that are randomly formed during the growth of the sample, the target QD has been carefully selected to have a specific energy level scheme presenting two transitions of identical energies, such that photons emitted after absorption of the photonic qubit carry no information about the decay path. As a consequence, if the spin state is initially prepared in the superposition state |up> + |down>, upon absorption of the photonic qubit and detection of the subsequent photon, the electron spin ends in the qubit state generated by the laser excitation of the source QD.
Figure 1 : (Click on the image to view with higher resolution) Principle of the state transfer protocol based on heralded absorption of a single photon qubit.
Implementing such an experiment is particularly challenging due to finite collection efficiencies and losses in the optical elements along the chain. One of the key elements that have allowed our realization is the use of a photonic structure enabling efficient extraction of the emitted photons. More specifically, the QDs are embedded in a planar cavity and the cavity output is collected using a solid immersion lens placed on top of the sample, ensuring that about 20% of the photons from each QD are collected by the first lens. The use of sensitive superconducting single photon detector with very low dark counts as well as crossed-polarization detection to suppress the strong pump laser light and background scattering have allowed us to demonstrate heralded absorption where we detected up to about 100 successful events per second. We demonstrated that the final state of the destination quantum dot spin is correlated with the initial state of the photon (or the target QD exciton) by measuring time-resolved photon coincidences.
Our scheme can be extended to realize spin-to-spin state transfer, or to generate heralded distant entanglement between two QD spins [6]. It can also be used to connect dissimilar physical systems in the context of hybrid quantum networks [7].
References:
[1] Stephan Ritter, Christian Nölleke, Carolin Hahn, Andreas Reiserer, Andreas Neuzner, Manuel Uphoff, Martin Mücke, Eden Figueroa, Joerg Bochmann, Gerhard Rempe, “An elementary quantum network of single atoms in optical cavities”, Nature, 484, 195 (2012). Abstract. [2] Christoph Kurz, Michael Schug, Pascal Eich, Jan Huwer, Philipp Müller, Jürgen Eschner, “Experimental protocol for high-fidelity heralded photon-to-atom quantum state transfer”, Nature Communications, 5, 5527 (2014). Abstract. [3] Norbert Kalb, Andreas Reiserer, Stephan Ritter, Gerhard Rempe, “Heralded Storage of a Photonic Quantum Bit in a Single Atom”, Physical Review Letters, 114, 220501 (2015). Abstract. [4] Sen Yang, Ya Wang, D. D. Bhaktavatsala Rao, Thai Hien Tran, Ali S. Momenzadeh, M. Markham, D. J. Twitchen, Ping Wang, Wen Yang, Rainer Stöhr, Philipp Neumann, Hideo Kosaka, Jörg Wrachtrup, “High-fidelity transfer and storage of photon states in a single nuclear spin”, Nature Photonics, 10, 507 (2016). Abstract. [5] Aymeric Delteil, Zhe Sun, Stefan Fält, Atac Imamoğlu, “Realization of a Cascaded Quantum System: Heralded Absorption of a Single Photon Qubit by a Single-Electron Charged Quantum Dot”, Physical Review Letters, 118, 177401 (2017). Abstract. [6] D. Pinotsi and A. Imamoglu, “Single Photon Absorption by a Single Quantum Emitter”, Physical Review Letters, 100, 093603 (2008). Abstract. [7] H. M. Meyer, R. Stockill, M. Steiner, C. Le Gall, C. Matthiesen, E. Clarke, A. Ludwig, J. Reichel, M. Atatüre, M. Köhl, “Direct Photonic Coupling of a Semiconductor Quantum Dot and a Trapped Ion”, Physical Review Letters, 114, 123001 (2015). Abstract.
Transparent media are the foundation of almost all optical instruments, such as optical lens, etc. However, perfect transparency has never been realized in natural transparent solid materials such as glass because of the impedance mismatch with free space or air. As a consequence, there generally exist unwanted reflected waves at the surface of a glass slab, as illustrated in Fig. 1(a). It is well known that non-reflection only occurs at a particular incident angle for a specific polarization, which is known as the Brewster angle effect [1]. Our question is: is it possible to extend the Brewster angle from a particular angle to a wide range of or all angles, so that there is no reflection for any incident angle.
In addition, the virtual image formed by a glass slab placed in air is usually blurred to a certain extent [Fig. 1(a)]. Such a blur indicates the aberration of virtual images, and is caused by the mismatch of equal frequency contours (EFCs) between air (grey lines) and the glass (blue lines).
Figure 1: (a) Virtual image formation through a glass slab, with general reflection and aberration. (b) Aberration-free virtual image formation through an ultratransparent photonic crystal without any reflection due to omnidirectional impedance matching. The black arrows and blue dashed lines in represent the light rays from a point source, and the back tracing lines, respectively. The yellow dashed curves in (b) indicate equal phase surfaces of transmitted rays. The inset graphs show the corresponding EFCs. The figure is adapted from Reference [2].
The purpose of our work [2] is to explore the possibility of realizing the ultimate transparency by artificial optical structures such as photonic crystals (PhCs) [3] and metamaterials [4]. In other words, we pursue the realization of transparent media with the extreme property of omnidirectional impedance matching and the ability of forming aberration-free virtual images, which are hereby denoted as ultratransparent media.
In this work, we propose that omnidirectional impedance matching can be realized by utilizing effective medium with nonlocal parameters, i.e. permittivity and permeability that are dependent on the incident angle. Interestingly, such an effective medium can be realized by using pure dielectric PhCs. Moreover, the EFC of the ultratransparent PhC can be tuned to be a shifted ellipse (red lines) with the same height of the EFC of air (grey lines). By using ray optics, we prove that such an EFC endows the valuable ability of forming aberration-free virtual images, as presented in Fig. 1(b).
Because of the shift of EFC, the PhC is beyond the local medium framework, and effective parameters are nonlocal (i.e. spatially dispersive). Interestingly, such nonlocality leads to additional phase modulation p d, where p is the shift magnitude and d is the slab thickness [Fig. 1(b)].
Figure 2: (a) Illustration of the unit cell of the ultratransparent PhC. (b) The EFC of the PhC. (c) Transmittance through the PhC slab with N (=4, 5, 6, 15) layers of unit cells as functions of the incident angle. The figure is adapted from Reference [2].
An extreme example with almost complete transparency (T>99%) for nearly all incident angles (-89o, +89o) is shown in Fig. 2. The PhC is two-dimensional and its unit cell is shown in Fig. 2(a). For transverse electric polarization, at the working frequency, the EFC is a shifted ellipse (red dashed curve) with the same height as that in free space (grey dashed curve), as shown in Fig. 2(c). The transmittance through such a PhC slab is always near unity (>99%) for nearly all incident angles (<89o), and is almost irrespective of the layer number, N.
Figure 3: (a) Photo of the simplified PhC composed of alumina bars (white) placed inside the microwave field mapper. (b) The EFC of the PhC. (c) Transmittance through the PhC slab in simulations (solid lines) and experiments (triangular dots) and an alumina slab having the same thickness (dashed lines) as the function of incident angles. The figure is adapted from Reference [2].
To prove the theory, we performed proof-of-principle microwave experiments by utilizing a simplified PhC consisting of rectangular alumina bars in a square lattice, as shown in Fig. 3(a). Such a PhC exhibits a shifted elliptical EFC [Fig. 3(b)] and a wide-angle impedance matching effect. The measured transmission data (triangular dots) and simulation results (solid lines) both show high transmittance, which is great enhancement compared to the transmittance through an alumina slab with the same thickness (dashed lines).
Finally, we also note that such ultratransparent media can extend transformation optics (TO) [5, 6] to the general realm of nonlocal media. The traditional TO was founded in the local medium framework and require local media. Here, we demonstrate that ultratransparent media with controllable refractive indexes are also good candidates for TO applications such as invisibility cloaks. Interestingly, due to the nonlocality, the ultratransparent media also enable additional freedom in phase modulation, which is absent in the traditional TO. At optical frequencies, ultratransparent PhCs exhibit the significant advantages of omnidirectional impedance matching, low loss and micro fabrication requirement.
The concept and theory of ultratransparency gives a guideline for realizing the ultimate transparency which is broadband, omnidirectional and polarization-insensitive. Recently, we designed broadband, wide-angle and polarization-insensitive transparent media by using one-dimensional dielectric PhCs [7]. In the future, ultratransparent solid materials may be optimized to exhibit an unprecedented level of transparency and produce no reflection at all in certain ranges of frequencies.
References: [1] John D. Jackson, "Classical Electrodynamics" (3rd edition, Wiley, New York, 1975). [2] J. Luo, Y. Yang, Z. Yao, W. Lu, B. Hou, Z. H. Hang, C. T. Chan, and Y. Lai, "Ultratransparent media and transformation optics with shifted spatial dispersions", Physical Review Letters, 117, 223901 (2016). Abstract. [3] John D. Joannopoulos, Steven G. Johnson, Joshua N. Winn, Robert D. Meade, "Photonic Crystals: Molding the Flow of Light" (2nd edition, Princeton University Press, Princeton, USA, 2008). [4] Yongmin Liu, Xiang Zhang, "Metamaterials: a new frontier of science and technology", Chemical Society Reviews, 40, 2494-2507 (2011). Abstract. [5] J. B. Pendry, D. Schurig, and D. R. Smith, "Controlling electromagnetic fields", Science, 312, 1780-1782 (2006). Abstract. [6] Ulf Leonhardt, "Optical conformal mapping", Science, 312, 1777-1780 (2006). Abstract. [7] Zhongqi Yao, Jie Luo, Yun Lai, "Photonic crystals with broadband, wide-angle, and polarization-insensitive transparency", Optics Letters, 41, 5106 (2016). Abstract.
