Nonlinear Cerenkov Radiation and its Modulation

Shining Zhu (right) and Yong Zhang (left)

[This is an invited article based on recent works of the authors. -- 2Physics.com]

Authors: Yong Zhang and Shining Zhu

Affiliation: Physics Department, Nanjing University, China

Link to Dielectric Superlattice Laboratory

In 1934, P. V. Cerenkov observed the emission of blue light from a bottle of water subjected to radioactive bombardment. This phenomenon is so-called Cerenkov radiation (CR), associated with charged particles moving at speed faster than the speed of light in the medium. The coherent radiation is observable at a conical wavefront defined by Cerenkov angle θ = arccos(v′/v), where v is the speed of the moving charged particle, and v′ is the phase velocity of the radiation wave. CR has been proved to be of great importance in subsequent experimental work in nuclear physics, and extensively used in experiments for counting and identifying relativistic particles. A recent work reported the observations of coherent, impulsive radio Cerenkov radiation from electromagnetic showers in solid ice.

Light can also create such an emission, which always involves a nonlinear optical process. So we call that nonlinear Cerenkov radiation (NCR). Unlike CR from particles, NCR results from nonlinear polarization driven by light field. The earliest work is Cerenkov second harmonic generation (SHG) in ZnS waveguide. The waveguide configuration has been widely studied since 1970. Cerenkov SHG has been considered as an important method to effectively achieve visible light because it has no strict requirements of waveguide parameters and the maximum nonlinear coefficient of the material can be used. And recently we reported Cerenkov sum-frequency generation in waveguide configuration [1]. Another NCR configuration is optical beating by extreme short pulse, which is a hot field because it can realize THz emission.

Image: Cerenkov SHG in 2D nonlinear photonic crystal waveguide

When a new physical phenomenon is discovered, people will think about two questions, how to use it and how to modify it for better use. CR is important in particle physics. NCR also has a lot of potential applications, for example, multiple photonic detection, THz waves and spectral analysis etc. However, both CR and NCR have a limiting condition, the speed of the source (charged particle for CR and nonlinear polarization for NCR) has to exceed the velocity of emission in the medium. This makes it not convenient to apply this phenomenon in some cases. Can we change that? The answer is yes, at least for NCR.

Up to now, there are three methods to finish this task, phonon-assistance, photonic crystal and nonlinear photonic crystal. All the three methods can realize Cerenkov radiation below the light threshold. And all these methods are based on the fact that the phase velocity of radiation source, linear or nonlinear polarization, could be changed by the interaction between the light and the medium. In our work, nonlinear photonic crystal waveguide is used. This is a waveguide with the periodical modulation of χ⁽²⁾, which may assist in accelerating or retarding the phase velocity v_p of nonlinear polarization, even changing its direction, thereby modulating the behavior of NCR, such as threshold value, radiation angle and direction. The increase of velocity depends on the period of nonlinear photonic crystal. Smaller the period, larger the increase. Therefore, in principle, this method can realize NCR without velocity threshold and with any second-order optical parameter process. This can realize more applications of NCR, such as photonic entanglement, quantum communication and computation networks.

References
[1] “Nonlinear Cerenkov radiation in nonlinear photonic waveguide”
Y. Zhang, Z. D. Gao, Z. Qi and S. N. Zhu, Phys. Rev. Lett. 100, 163904 (2008). Abstract Link.
[2] “Visible glow of pure liquids under g-irradiation”
P. A. Čerenkov, Dokl. Akad. Nauk SSSR 2, 451 (1934).
[3] “Optical second harmonic generation in form of coherent Cerenkov radiation form a thin-film waveguide”, P. K. Tien, R. Ulrich, and R. J. Martin, Appl. Phys. Lett. 17, 447 (1970). Abstract Link.
[4] “Cherenkov radiation at speeds below the light threshold: phonon-assisted phase matching”
T. E. Stevens, J. K. Wahlstrand, J. Kuhl, R. Merlin, Science 291, 627 (2001). Abstract Link.
[5] “Cerenkov radiation in photonic crystals”
Chiyan Luo, Mihai Ibanescu, Steven G. Johnson, J. D. Joannopoulos,
Science 299, 368 (2003). Abstract Link.
[6] “Observations of the Askaryan Effect in Ice”
P. W. Gorham, S. W. Barwick, J. J. Beatty, D. Z. Besson, W. R. Binns, C. Chen, P. Chen, J. M. Clem, A. Connolly, P. F. Dowkontt, M. A. DuVernois, R. C. Field, D. Goldstein, A. Goodhue, C. Hast, C. L. Hebert, S. Hoover, M. H. Israel, J. Kowalski, J. G. Learned, K. M. Liewer, J. T. Link, E. Lusczek, S. Matsuno, B. Mercurio, C. Miki, P. Miočinović, J. Nam, C. J. Naudet, J. Ng, R. Nichol, K. Palladino, K. Reil, A. Romero-Wolf, M. Rosen, L. Ruckman, D. Saltzberg, D. Seckel, G. S. Varner, D. Walz, and F. Wu,
Phys. Rev. Lett. 99, 171101 (2007). Abstract Link.
[7] “Quasi-phase-matched Čerenkov second-harmonic generation in a hexagonally poled LiTaO3 waveguide”, Y. Zhang, Z. Qi, W. Wang, and S. N. Zhu,
Appl. Phys. Lett. 89, 171113 (2006). Abstract Link.

The Frontier of Quantum Communication is the Space

Paolo Villoresi

Author: Paolo Villoresi

Affiliation: Department of Information Engineering,
University of Padua, Italy

[This is an invited article based on recent work of the author and his collaborators. - 2Physics.com]

As the advancements in the implementation of single light-quanta exchange in the quantum channels is constantly progressing and refining, one may think that the quantum communications (QC) will soon widespread in everyday life. The appeals of this perspective may be synthesised by choosing the quality versus the quantity or to encode information in the quantum state of a single particle instead of sending a large bunch of photons to express just one bit.

But quality has its price. Each photon that is carrying information has to be clearly sorted out from these wandering around as general background, and its quantum state has also to be kept unblemished along its propagation until the receiver. As expected, QC had its cradle in the research labs, where effective countermeasures against decoherence and background photons are relatively easy to adopt. Significant steps were done in quantum communication along optical fibres, for which already viable technologies were proposed for the distribution of cryptographic keys over legs of several tens of kilometres. In the free space counterpart, where a beam with the train of quanta is aimed toward a received with no needs of infrastructures in between, the difficulties are stronger. The intense backlights, the atmospheric turbulence, the diffusion and absorption of light are some of the issues to fight against in order to implement QC. Beside, the Earth curvature set a final limit to the leg length. The actual limit is represented by a quantum channel in which the nature of quantum entanglement has been demonstrated between two parties separated by 144 km. The experiment was done between two islands of Canary archipelagos, with the stations located quite high in the mountains. But the further extension of the free-space QC has a natural direction: going in space and communicate with the Earth.

Indeed, in our experiment we aimed to establish a link between an orbiting source of single photons and a ground telescope. Our team was set up with my coworkers at the LUXOR Labs at DEI, University of Padova in Italy and colleagues in Austria, of the group of Anton Zeilinger at the University of Vienna and Academy of Science of Austria, of the group of Cesare Barbieri at the Astronomy Department of University of Padova, Italy and of Giuseppe Bianco of Italian Space Agency in Matera. The realization of this link is the first step in the communication space-ground or space-space and based on the coding of the bits of information in the quantum state of a photon, or qubit. The experiment also demonstrated that present technology is mature enough for this purpose, and the crucial crossing from the theoretical predictions and the experimental demonstration was possible. On the other hand, the experiment required a combined effort from different expertises, from classical Optics, to satellite laser-ranging for Geodesy, to Quantum Optics, to advanced electronics. Our team synthesized these points of view and succeeded in the single photon link.

More in detail, in this experiment we have essentially simulated a quantum communication source onboard a satellite, and showed how the very dim signals could be detected. Such a quantum source has to fulfill the particular requirement, that only one single photon per pulse is emitted. In this work this is realized by sending a rapid sequence of weak laser pulses (outward pulses in the figure) towards a Japanese satellite equipped with retroflectors (Ajisai) at about 1600 km of slant distance. There is a very small probability that the photons hit the satellite and are reflected back to ground, therefore this is just as if we would have a suitable quantum source on the satellite. The main challenge was to detect the very view reflected photons amongst a huge background signal which is exactly the same situation is would be if we had the real quantum communication system. The detector is an avalance-photon-detector (APD) connected to a timing circuit. the orbital data of the satellite were used to identify the returned photons out of the background.

The next step will be to board a quantum sender on a satellite. This will allow quantum physics experiments over distances impossible on ground. In particular, it will push the limits of fundamental physics tests addressing experimentally questions as if there is a spatial limit to the entanglement, the "spooky" action? Beside, technologies as the quantum key distribution may be implemented on a global scale. And a real economic impact of the quantum communication from satellite may be expected, to be based on the cryptography, on novel paradigms as quantum teleportation, on advanced atmospheric monitoring, based on the modification of the optical signal during the downlink. There could also be impact in the global distribution of temporal information, as in the case of the so called “legal time”, and advanced methods for the clock synchronization using entangled photon pairs.

The study of the quantum satellite is ongoing, under the auspices of Italian Space Agency as well also of the European Space Agency, and we really hope that the quantum satellite will soon be on its way, that is along an orbit some hundred kilometres above us.

References
[1] "Experimental verification of the feasibility of a quantum channel between space and Earth",
P Villoresi, T Jennewein, F Tamburini, M Aspelmeyer, C Bonato, R Ursin, C Pernechele, V Luceri, G Bianco, A Zeilinger and C Barbieri,
New J. Phys., v10, 033038 (March, 2008) [IOP select paper], Abstract Link.
[2] "Ground to satellite secure key exchange using quantum cryptography",
Rarity J G, Tapster P R, Gorman P M and Knight P,
New J. Phys., v4, 82 (2002), Abstract Link.
[3] European Quantum Roadmap: http://qist.ect.it/
[4] "The Physics of Quantum Information",
D Bouwmeester, A Ekert, A Zeilinger, (Springer, 2000).

Beating the Quantum Limit in Gravitational Wave Detectors

- Manipulating quantum noise can significantly improve the sensitivity to small displacements in gravitational wave detectors -

Authors: Keisuke Goda¹, Alan Weinstein², Nergis Mavalvala¹
Affiliation:
¹LIGO Laboratory, Massachusetts Institute of Technology
²LIGO Laboratory, California Institute of Technology

Nergis Mavalvala and Keisuke Goda at MIT

[This is an invited article based on recent works of the authors. -- 2Physics.com]

Gravitational waves are ripples in the fabric of spacetime. Predicted by Albert Einstein in his general theory of relativity in 1916, gravitational waves are emitted by accelerating masses analogous to electromagnetic waves emitted by accelerating charges [1]. Although gravitational waves have not yet been detected, they have been indirectly shown to exist by Russell A. Hulse and Joseph H. Taylor Jr., who received 1993 Nobel Prize in physics. What makes direct detection difficult is that these waves are extremely faint due to their weak coupling with matter – even for violent astronomical events such as collisions of black holes and supernova explosions. Direct detection, therefore, requires instruments with unprecedented precision.

Laser-interferometric gravitational-wave detectors, such as those of the Laser Interferometer Gravitational-Wave Observatory (LIGO) [2], are designed to measure displacements of the order of 10^-18 m, or one-thousandth of the diameter of the proton, caused by passing gravitational waves that propagate from the distant universe. The LIGO detectors employ Michelson interferometry in which the difference in light travel time between the two arms of the interferometer is measured with high precision using controlled laser light.

But the story is not that simple. The quantum nature of photons prohibits us from increasing the detector sensitivity infinitely – Heisenberg’s uncertainty principle imposes a fundamental limit on the precision with which displacements caused by gravitational waves can be measured. The so-called quantum limit is set by the zero-point fluctuations of the light in the interferometer. Laser-interferometric gravitational-wave detectors such as LIGO are so sensitive that they have already confronted the quantum limit. In next generation detectors such as Advanced LIGO, optimization of classical parameters will reach the limits of conventional technology.

So, is this the end of the story? Fortunately, the answer is no. We haven’t used all the resources yet. The quantum limit can be overcome by use of non-classical or squeezed states of light [3] – states in which fluctuations are reduced below the symmetric quantum limit in one quadrature (like x) at the expense of increased fluctuations in the orthogonal quadrature (like p), while preserving the uncertainty principle (Δx Δp ≥ ћ/2). In the 2Physics article dated April 3, 2008, Roman Schnabel and Henning Vahlbruch reported their achievement of a world record in the strength of squeezing – more than 10 dB of squeezing for the first time.

In a recent paper entitled “A quantum-enhanced prototype gravitational-wave detector” [4], a team of LIGO scientists demonstrated improved sensitivity in a prototype gravitational wave detector at frequencies where the detector sensitivity was limited by photon shot noise, by injecting a squeezed state of light into the output port of the detector. The prototype detector consists of suspended quasi-free optics with a readout and control scheme similar to those used in the currently operational LIGO interferometers. In the demonstration, the team prepared a squeezed state with about 9 dB of sub-shot noise using a below-threshold optical parametric oscillator pumped by a powerful second-harmonic field [5], and injected it into the interferometer.

Figure 1 shows the 40m prototype interferometer at Caltech (left) and the squeezed light generator (right)








Figure 2 shows the noise floor of the prototype gravitational wave with a simulated gravitational wave signal at 50 kHz detector without (red) and with (blue) the injection of squeezed light. The sensitivity of the interferometer is limited by photon shot noise at frequencies above 42 kHz. The broadband shot noise floor was reduced by the injected squeezing while the strength of the simulated gravitational wave signal was intact, thereby improving the signal-to-noise ratio.

The result of the squeezing-enhancement in the prototype detector is shown in Figure 2. The comparison between the two spectra shows that the noise floor of the interferometer was reduced by the squeezed light injection at frequencies where the detector sensitivity was limited by shot noise. Figure 2 also shows the noise floor with a simulated gravitational wave signal at 50 kHz, with and without the injected squeezing. The broadband quantum noise floor was reduced by about 3 dB while the strength of the simulated gravitational wave signal was intact. This corresponds to a 44% increase in signal-to-noise ratio or detector sensitivity. In kilometer-scale gravitational wave detectors, this would correspond to a factor of 1.44³ ≈ 3.0 increase in detection rate for uniformly distributed gravitational wave sources such as coalescing neutron star binaries. This increase by a factor of 3 is significant because the occurrence of detectable gravitational waves is extremely rare – once every few years for binary neutron star coalescences at the present LIGO sensitivity.

This demonstration is an important step towards implementation of squeezing injection to improve the sensitivity of existing gravitational wave detectors worldwide. In fact, the installation of a squeezed source in a LIGO interferometer in the near future is under consideration, making gravitational wave detectors an important practical application of squeezed states of light. It is expected that within the next decade all interferometric gravitational wave detectors in the world will routinely use squeezed light.

References
[1] K.S. Thorne in “300 Years of Gravitation,” Cambridge Univ. Press, Cambridge (1987)
[2] “LIGO: The Laser Interferometer Gravitational-Wave Observatory,”
Alex Abramovici, William E. Althouse, Ronald W. P. Drever, Yekta Gürsel, Seiji Kawamura, Frederick J. Raab, David Shoemaker, Lisa Sievers, Robert E. Spero, Kip S. Thorne, Rochus E. Vogt, Rainer Weiss, Stanley E. Whitcomb, and Michael E. Zucker
Science 256, 325 (1992), Abstract Link.
[3] “Squeezed states of light,” D. F. Walls, Nature 306, 141 (1983), Abstract Link.
[4] “A quantum-enhanced prototype gravitational-wave detector,”
K. Goda, O. Miyakawa, E. E. Mikhailov, S. Saraf, R. Adhikari, K. McKenzie, R. Ward, S. Vass, A. J. Weinstein, and N. Mavalvala,
Nature Physics, advance online publication, doi:10.1038/nphys920 (2008), Abstract Link.
[5] “Generation of a stable low-frequency squeezed vacuum field with periodically poled KTiOPO₄ at 1064 nm,” K. Goda, E. E. Mikhailov, O. Miyakawa, S. Saraf, S. Vass, A. J. Weinstein, and N. Mavalvala,
Optics Letters 33, 92 (2008), Abstract Link.

Silicon Photonics for Optical Quantum Technologies

Jeremy O’Brien

[This is an invited article based on recent works of the authors. -- 2Physics.com]

Authors: Jeremy O’Brien and Alberto Politi

Affiliation: Centre for Quantum Photonics, Department of Physics and Department of Electrical & Electronic Engineering, University of Bristol

Quantum information science has shown that quantum mechanical effects can dramatically improve performance for certain tasks in communication, computation and measurement. Single particles of light – photons – are an excellent choice for quantum technologies because they are relatively noise free; information can be moved around quickly – at the speed of light; and manipulating single photons is easy. For these reasons photons have been widely used in quantum communication, quantum metrology, and quantum lithography settings, as well as quantum bits (or qubits) for quantum information processing [1].

The fact that photons see low noise during the propagation is a great advantage, but, at the same time, makes two photons interact with a negligible probability. Two photons interaction is a fundamental task for quantum information processing, and it is at the heart of the Controlled-NOT (CNOT) gate –one of the building block of a future quantum computer. A 2 photon CNOT gate was demonstrated experimentally back in 2003 [2].

A conspicuous number of different experimental realizations of this CNOT gate have since been developed in recent years by different groups, but all the realizations performed so far are based on bulk optical elements on an optical table and photon propagation in air. This approach is useful for proof of principle quantum logic operation, however, photonic quantum technologies will require scalable, miniaturized gates, with improved performances.

The Team at Bristol University has designed and measured integrated optical devices on a chip, with dimensions measured in millimetres [3]. This impressive miniaturisation was permitted thanks to the silica-on-silicon technology used in commercial devices for modern optical telecommunications, which guides light on a chip in the same way as in optical fibers.

For the first time, the feasibility of integrated quantum information was demonstrated, by achieving the key element of all quantum optics experiments, namely non-classical interference. This effect appears when two indistinguishable photons arrive at the different inputs of a beam-splitter (a half refractive mirror) at the same time. In this case, contrary to the classical analysis, the two photons always exit together from one of the two ports, and they never exit different outputs. The simplest integrated analogous of a free space bream-splitter is a directional coupler, (illustrated in Figure 1). When two waveguides are close one to each other there is a non-zero overlap between the modes of the waveguides. By choosing the waveguide separation and the length of the coupling region, it is possible to choose the amount of power that goes to one waveguide to the other (coupling ratio).


Figure 1 shows a directional coupler on a chip, the integrated analogue of a beam splitter.

Sending pairs of single photons in the two inputs of the directional coupler, it was possible to demonstrate the quantum interference effect, with a very high visibility of the quantum behaviour.

Using the same technology and various directional couplers with different coupling ratios it is possible to realise a CNOT gate, schematically represented in Figure 1. With this scheme it was possible to achieve a fidelity of the CNOT operation of more than 94%.

Figure 2 shows the schematic representation of an integrated CNOT gate. The “1/2” and “1/3” numbers indicate the coupling ratio of the different couplers that compose the CNOT gate.

The experimental characterisation of the quantum chips also proved that one of the strangest phenomena of the quantum world, namely “quantum entanglement”, was achieved on-chip. Quantum entanglement of two particles means that the state of either of the particles is not defined, but only their collective state.

This on-chip entanglement has important applications in quantum metrology. Last year Dr O’Brien and his collaborator Professor Takeuchi and co-workers at Hokkaido University reported such a quantum metrology measurement with four photons [4].

The results achieved using integrated chips show that it is possible to realize sophisticated photonic quantum circuits on a silicon chip, which will be of benefit to future quantum technologies based on photons as well as the next generation of fundamental studies in quantum optics.

References
[1] “Optical Quantum Computing”
Jeremy L. O’Brien,
Science 318, 1567 (2007), Abstract.
[2] “Demonstration of an all-optical quantum controlled-NOT gate”
J. L. O'Brien, G. J. Pryde, A. G. White, T. C. Ralph, D. Branning,
Nature 426, 264 (2003), Abstract.
[3] “Silica-on-Silicon Waveguide Quantum Circuits”
A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, J. L. O'Brien,
Science, Vol. 320. no. 5876, pp. 646 - 649 (May 2, 2008)
Published Online March 27, 2008 (10.1126/science.1155441)
Abstract, Link to Full text in the website of Bristol Centre for Quantum Photonics.
[4] "Beating the Standard Quantum Limit with Four-Entangled Photons"
T. Nagata, R. Okamoto, J. L. O'Brien, K. Sasaki, S. Takeuchi,
Science 316, 726 (2007), Abstract.