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2Physics Quote:
"Roughly speaking, laser interferometric GW detectors are most sensitive to celestial systems of a size comparable to the interferometers’ arm length. LISA/eLISA is sitting in a frequency band, where there are most abundant GW sources. There are several known white dwarf binaries in our galaxy directly visible to LISA/eLISA. In addition, LISA/eLISA can resolve thousands of other white dwarf binaries in our galaxy. LISA/eLISA can also observe massive black hole mergers throughout the entire universe. Extreme mass ratio inspirals (i.e. stellar mass compact object orbiting a massive black hole) and primordial GWs from the birth of the universe add great scientific values to the mission as well."
-- Yan Wang (Read his article: "Space-borne Gravitational Wave Detector LISA/eLISA")

Sunday, September 20, 2015

Experimental Violation of Leggett-Garg Type Inequalities Using Quantum Memories

(From left to right) Zong-Quan Zhou, Chuan-Feng Li, Guang-Can Guo

Authors: Zong-Quan Zhou, Chuan-Feng Li, Guang-Can Guo

Affiliation: Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, China.

Link to the Group of Quantum Network >>

Ever since the birth of quantum mechanics we faced the difficulty to reconcile the superposition behavior of quantum particles and our intuitive experience in dealing with macroscopic objects which should occupy definite states at all times and independently of the observers. Leggett and Garg subsequently formulated the question qualitatively by means of the derivation of a series of inequalities based upon the premises of macroscopic realism and noninvasive measurability [1]. In practice, the main experimental challenge comes from the implementation of truly noninvasive measurements [2]. To avoid the requirement of performing noninvasive measurements, a different type of Leggett-Garg inequalities (LGtI) has been derived from the stationary assumption [3,4], which includes the time translational invariance of the probabilities and Markovianity of the system evolution [2]. If stationarity does hold for the considered system, these inequalities provide a quantitative way to witness the persistence of coherent effects, that is, they allow for benchmarking ‘quantumness’.
Figure 1: Single photons excited the superposition states of two pieces of Nd:YVO4 crystals. The horizontally (H) and vertically (V) polarized components of a single photon are converted to collective excitation of billions of atoms in the first crystal and second crystal, respectively. The two crystals have thicknesses of 3 mm and are separated by a distance of 2 mm.

Zong-Quan Zhou, Chuan-Feng Li and Guang-Can Guo from the University of Science and Technology of China, and Susana Huelga from the Ulm University have reported in Physical Review Letters [5] that they are able to violate the LGtI in a light-matter interfaced system. By separately benchmarking the Markovian character of the evolution and the translational invariance of the conditional probabilities, the observed violation of the LGtI is attributed to the quantum coherent character of the process.

In the experiment, narrowband single photons are generated from nonlinear crystal through the spontaneous parametric down-conversion process. Then the H+V-polarized single photons are employed to excite the superposition states of the collective excitation of two solid-state quantum memories. The group developed a novel technique, the polarization-dependent atomic-frequency-comb (AFC) technique, to control the dynamical evolution of the collective atomic states. A frequency detuning of δ is introduced between the H-polarized and V-polarized AFC, which determines the state evolution speed. Finally, the atomic states are read out through the AFC echo emission and analyzed with polarization-dependent single-photon detections. The recorded state evolution is shown in Figure 2(a) which is nearly perfect unitary evolution. The calculated LGtI is shown in Figure 2(b) and shows a violation of the classical bound by 6.9 standard errors. The stationary assumption was independently verified for the experimental setup. Therefore, the experimental violation of LGtI demonstrates that the dynamics of a collective excitation which is distributed across two macroscopically separated crystals can be well described by quantum mechanics, while a classical description is excluded.
Figure 2: (a) Time evolution of the probabilities to find the system in atomic state D and A for an AFC excitation initially prepared in state D and with AFC detuning δ of 5 MHz. Here D is defined as an atomic state of “first crystal excited + second crystal excited” and A is the orthogonal state, “first crystal excited - second crystal excited”. (b). The envelope evolution of LGtI. The solid lines are the ideal quantum mechanical predictions. The blue dashed line represents the lower bound of -1, as predicted by classical incoherent theories.

This is a coherent evolution of microscopic excitation in macroscopic objects and represents an interesting exploration of the crossover between quantum and classical regimes. These results provide a general procedure to benchmark quantumness when temporal correlations can be independently assessed and confirm the persistence of quantum coherence effects in systems of increasing complexity. Further efforts to combine the multi-photon entanglement [6,7] and the atomic memory could lead to a test of LGtI with quantum superposition states of larger macroscopicity [8].

[1] A. J. Leggett, Anupam Garg, “Quantum mechanics versus macroscopic realism: Is the flux there when nobody looks?”, Physical Review Letters, 54, 857 (1985). Abstract.
[2] Clive Emary, Neill Lambert, Franco Nori, “Leggett-Garg Inequality”, Reports on Progress in Physics, 77, 016001 (2014). Abstract.
[3] S. F. Huelga, T. W. Marshall, E. Santos , “Proposed test for realist theories using Rydberg atoms coupled to a high-Q resonator”, Physical Review A, 52, R2497 (1995). Abstract.
[4] Susana F. Huelga, Trevor W. Marshall, Emilio Santos, “Temporal Bell-type inequalities for two-level Rydberg atoms coupled to a high-Q resonator”, Physical Review A, 54, 1798 (1996). Abstract.
[5] Zong-Quan Zhou, Susana, F. Huelga, Chuan-Feng Li, Guang-Can Guo, “Experimental detection of quantum coherent evolution through the violation of Leggett-Garg-type inequalities”, Physical Review Letters, 115, 113002 (2015). Abstract.
[6] A. I. Lvovsky, R. Ghobadi, A. Chandra, A. S. Prasad and C. Simon,, “Observation of micro-macro entanglement of light”, Nature Physics, 9, 541 (2013). Abstract.
[7] N. Bruno, A. Martin, P. Sekatski, N. Sangouard, R. T. Thew, N. Gisin, “Displacement of entanglement back and forth between the micro and macro domains”, Nature Physics, 9, 545 (2013). Abstract.
[8] A. J. Leggett, “Macroscopic Quantum Systems and the Quantum Theory of Measurement”, Progress of Theoretical Physics Supplements, 69, 80 (1980). Full Article.


Sunday, September 13, 2015

How Time Dilation Affects Quantum Superpositions

(From left to right) Igor Pikovski, Magdalena Zych, Fabio Costa, Časlav Brukner
(Click on the image to see their faces with better resolution)

Authors: Igor Pikovski1,2,3,4, Magdalena Zych1,2,5, Fabio Costa1,2,5, Caslav Brukner1,2

1Vienna Center for Quantum Science and Technology (VCQ), Faculty of Physics, University of Vienna, Austria,
2Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Vienna, Austria,
3ITAMP, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA,
4Department of Physics, Harvard University, Cambridge, Massachusetts, USA,
5Centre for Engineered Quantum Systems, School of Mathematics and Physics, The University of Queensland, St Lucia, Queensland, Australia.

Can gravity affect quantum systems? Quantum theory and gravity seem to apply to very different physical regimes. It is often argued that gravity is irrelevant on very small scales, where typical quantum phenomena are observed. A well-known argument compares the forces between an electron and a proton: gravity is roughly 1039 times weaker than electromagnetism. But this is not the full story. As has been shown in numerous experiments, gravity can indeed influence a quantum wave function of the smallest particles. Newtonian gravity from Earth can induce a quantum phase-shift for a particle that is in a superposition between two different heights. This was first demonstrated with Neutrons in the famous COW (Colella-Overhauser-Werner) experiment in 1975 [1].

Past 2Physics article by this group:
January 15, 2012: "Quantum Complementarity Meets Gravitational Redshift" by Magdalena Zych, Fabio Costa, Igor Pikovski, Časlav Brukner.

What about Einstein’s gravity? If the Newtonian potential can influence a quantum wave function, what can one expect from post-Newtonian effects stemming from general relativity? It turns out that novel phenomena arise, with no classical analogue. A few years ago, we studied how time dilation affects a single clock in superposition [2] (see also 2Physics article “Quantum Complementarity Meets Gravitational Redshift”). Classically, two clocks placed at different heights will experience different proper times, and thus will be time dilated with respect to each other. But in quantum theory, an additional effect arises: If a single clock is brought into superposition of two heights, its internal degrees of freedom (or clock states) get entangled with its position. This entanglement affects the superposition of heights. If the two amplitudes are brought together to interfere, the interference pattern will be affected due to quantum complementarity: As the internal clock states record time, they gain “which-way information” and thus destroy the quantum coherence. This interplay has been recently demonstrated in an experiment with a BEC, where the time delay of spin precession was simulated by inhomogeneous magnetic fields [3] (see also 2Physics article “One Clock in Two Places Simultaneously”).
Figure 1: The gravitational field causes time dilation, clocks closer to Earth run slower than clocks further away. For a quantum superposition of a single clock at two heights, the clock states and its position become entangled.

In our latest work, we showed that the effect of time dilation on quantum systems is very general and affects the quantum coherence of any composite quantum system [4]. Time dilation is universal: it affects any system regardless of its structure or composition. Clocks are affected by time dilation as much as the heart beat or the half-life of a decaying particle. Also composite quantum systems are affected, as they usually have a finite temperature: It means that there is always some internal dynamics present within the system, which can be affected by time dilation. If such a composite system is brought into superposition between different heights above Earth, as in the above example with a clock, time dilation will correlate all internal dynamics with the height of the system. And this causes decoherence of the center-of-mass, just as in the case with an actual clock. In practice, any system is affected that has some internal energy spread. If the center-of-mass of such a system is brought into superposition and a sufficient difference in proper times is accumulated along the superposed paths, quantum coherence will be lost because of the relative time delay of the internal dynamics of the system. Needless to say, time dilation can be caused by a gravitational field but also by any acceleration or by velocities. There is no “external” environment, only the presence of time dilation causes the system’s center-of-mass to decohere due to the dynamics of its own composition.
Figure 2: A complex molecule in superposition in a gravitational field. Because of time dilation, the frequencies of the individual atoms depend on the height of the molecule. This causes decoherence of quantum superpositions of the center-of-mass of the molecule.

One may expect this effect to be vanishingly small. After all, it is very hard to detect time dilation on Earth. But it turns out that already on mesoscopic scales, the effect is strong: If a gram-scale object at room temperature is put into a superposition of height difference of 1µm, then time dilation will cause decoherence after about 1ms. This is because many internal degrees of freedom contribute to the effect. Each individually is affected by time dilation only a tiny bit, but for a larger composite system, the effect can become significant. For quantum systems, it is of course very challenging to prepare such large superpositions, and other decoherence effects will also be of importance. But there is a parameter range at superfluid temperatures, where experiments with very large molecules or microspheres could in principle observe the predicted phenomena in the future. But importantly, the effect is universal and any internal dynamics will contribute to decoherence. Thus one can think of other possible experiments with internal dynamics other than thermal, such as nuclear dynamics or some fast chemical processes.

All of these will contribute to the rate of decoherence; one can be creative in designing a clever experiment. In fact, one could also use such setups to reverse the logic: observing coherence of the center-of-mass might be a key to shed light on unknown internal dynamics. Maybe the best possible way for experimental verification will turn out to take an actual clock and put it in superposition -- as we proposed in [2] and which has recently been shown to work in an analogue experiment [3]. Future experiments should be able to verify decoherence due to time dilation, which follows only from quantum theory and general relativity as we know them.

Finally, what do we actually learn from this study? In our work, we do not treat the gravitational field quantum-mechanically, thus there is no direct connection to “quantum gravity”. Yet, we study how quantum mechanical test systems behave on a background space-time, as opposed to classical test particles, and new effects arise. We discovered a new decoherence mechanism, but the most exciting aspect of the work is that the interplay between quantum theory and gravity has novel phenomena to offer. Our work shows one example, but this research direction is still widely unexplored: Many more possible effects and experiments on the interplay between these two great theories are waiting to be discovered!

[1] Roberto Colella, Albert W. Overhauser, Samuel A. Werner. “Observation of Gravitationally Induced Quantum Interference”, Physical Review Letters, 34, 1472 (1975). Abstract.
[2] Magdalena Zych, Fabio Costa, Igor Pikovski, Časlav Brukner. “Quantum interferometric visibility as a witness of general relativistic proper time”, Nature Communications, 2, 505 (2011). Abstract. 2Physics Article.
[3] Yair Margalit, Zhifan Zhou, Shimon Machluf, Daniel Rohrlich, Yonathan Japha, Ron Folman. “A self-interfering clock as a 'which path' witness”, published online in 'Science Express' (August 6, 2015). Abstract. 2Physics Article.
[4] Igor Pikovski, Magdalena Zych, Fabio Costa, Časlav Brukner, “Universal decoherence due to gravitational time dilation”, Nature Physics ,11, 668-672 (2015). Abstract.

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Sunday, September 06, 2015

High-Resolution Optical Spectroscopy Using Multimode Interference in a Compact Tapered Fibre

Noel H. Wan (left) and Dirk Englund

Authors: Noel H. Wan and Dirk Englund

Affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, USA.

Link to Quantum Photonics Laboratory >>

Optical spectroscopy is a powerful technique that has found widespread application in fundamental science and in fields ranging from biochemical sensing to optical communication. In recent years, there has been strong interest in developing compact spectrometers for portable applications. A typical spectrometer uses a dispersive element such as a prism or a grating to separate the different constituent colors or wavelengths of light, each of which is channeled into separate detectors. Accordingly, a spectrometer can better distinguish different wavelengths of light if the distance between the dispersive optic and the detectors is increased. High-resolution spectrometers tend to be bulky, and miniature spectrometers based on the traditional design have so far not overcome the trade-off between footprint and performance.

Past 2Physics article by Dirk Englund:
April 05, 2015: "Efficient Photon Collection from a Nitrogen Vacancy Center in a Circular Bullseye Grating in Diamond" by Luozhou Li, Edward Chen, Dirk Englund.

In our work, we show a spectroscopy technique that circumvents the traditional spectrometer principle, leading to a millimeter-long fiber spectrometer that is simultaneously compact and has high spectral resolutions [1]. A waveguide such as an optical fiber allows only specific spatial distributions of light, or modes, as given by Maxwell’s equations. When multiple modes are launched into an optical fiber, they constructively and destructively interfere, resulting in highly random but specific intensity patterns (Figure 1). Additionally, by tapering the optical fiber, we were able excite different modes along the fiber, thereby further increasing the spectral resolution over that of an un-tapered, straight waveguide. More importantly, the taper geometry facilitates the detection and measurement of the multimode interference signals.
Figure 1: Click on the figure to view with higher resolution. (a) Illustration of the tapered fiber multimode interference (TFMMI) spectrometer. (b) Measured interference signals of different colors or wavelengths of light.

As modes of different wavelengths do not interfere, the interference can be regarded as the fingerprints of their respective wavelengths. By first mapping every wavelength of light to its fingerprint, the device can then algorithmically solve the inverse problem of determining the composition of an input light. Using our tapered fiber spectrometer, we were able to distinguish wavelengths separated by 40 picometers and 10 picometers in the visible and near-infrared regimes, respectively.
Figure 2: Click on the figure to view with higher resolution. (a) We determined the spectral resolution of the TFMMI spectrometer by resolving two closely spaced spectra. The device clearly resolved two wavelengths at 637.96 nm and 638.00 nm but did not fully resolve input wavelengths of 637.00 nm and 637.02 nm, bounding the resolution to 40 pm at these wavelengths. (b) The TFMMI device reconstructed a continuous signal with good agreement with that obtained from a commercial spectrometer.

With an optical fiber as our multimode waveguide, we were able to perform spectroscopy on light with wavelengths ranging from 500 nanometers to 1600 nanometers but in principle, our design can be implemented in a large variety of waveguides for any operational regimes. By suitably choosing the material, for example, compact spectrometers for absorption spectroscopy in the mid-infrared can be achieved. The proposed spectrometer can be realized on-chip in volume production through photolithography or printing and may also lead to the development of designer spectrometers for fundamental science, integrated photonics and lab-on-a-chip applications.

[1] Noel H. Wan, Fan Meng, Ren-Jye Shiue, Edward H. Chen, Tim Schröder, Dirk Englund, “High-resolution optical spectroscopy using multimode interference in a compact tapered fibre”, Nature Communications, 6, 7762 (2015). Abstract.


Sunday, August 30, 2015

Universal Linear Optics

The Bristol team : (from left to right) Anthony Laing, Chris Sparrow, Jacques Carolan and Christopher Harrold.

Authors: Christopher Harrold1 and Chris Sparrow1,2

1Centre for Quantum Photonics, H. H. Wills Physics Laboratory, and Department of Electrical and Electronic Engineering, University of Bristol, UK.
2Department of Physics, Imperial College London, UK.

In our recent work [1], we demonstrate a single reprogrammable optical circuit that is sufficient to implement all possible linear optical protocols up to the size of the circuit.

Since 1897, when it was shown by the German mathematician Adolf Hurwitz, it has been known that any unitary matrix can be built up from smaller 2 X 2 matrices [2]. Then in 1994 Reck et al. showed that any arbitrary unitary can be realised via some physical process [3]. In particular, they showed that if we use a certain arrangement of reconfigurable linear optical elements, we can perform any unitary matrix.

This network can be implemented as a sequence of ≈ m2/2 two-mode primitives with two free parameters. These primitives are built out of variable reflectivity beam splitters and phase shifters. The beam splitters are comprised of a phase shifter sandwiched between two directional couplers, known as a Mach-Zehnder interferometer (MZI). By tuning the phase inside the interferometer we can arbitrarily split the light between the two outputs of the interferometer. A phase shifter placed on the output of the interferometer allows us to tune the phase of the resulting state. The phase shifters are resistive heaters patterned above a small region of waveguide. A change in temperature changes the local refractive index in the waveguide with respect to another and thus realises a phase shift. Cascading these primitive units into the triangular arrangement, together with full control over these phase shifters allows us to configure any linear optical unitary on six modes.

Our device was fabricated by Japanese company Nippon Telegraph and Telephone (NTT), the world’s leading telecommunications company. The six-mode universal system consists of 15 MZIs with 30 thermo-optic phase shifters integrated into a single photonic chip that is electrically and optically interfaced for arbitrary setting of all phase shifters. Once calibrated, we could implement any unitary we wanted and switch between experiments that typically would have taken months to build in a matter of seconds.
Figure 1: A fully reconfigurable optical chip. The device allows us to implement any quantum information protocol in linear optics on up to 6 modes.

We realised heralded quantum logic gates [4] designed to be scalable for the circuit model of quantum computation (QC) for the first time in integrated photonics. We also implemented a heralded entangling gate and used measurements in different bases to verify the successful generation of an entangled state. Entangling gates such as the one implemented here are an essential requirement for one of the most promising routes to photonic QC, measurement based quantum computing [5, 6]. Not only were we able to perform these protocols, but importantly we were able to report experimental results with high fidelity.

We then turned our attention to a completely disparate aspect of linear optics and configured our device in order to perform boson sampling. Boson sampling is a computational task with one objective: sampling from a probability distribution governed by the output statistics of photons injected into a fixed linear optical network [7]. It provides a rapid route to demonstrate the intrinsic exponential advantage a quantum computer has over a classical one. We implemented 100 instances of the boson sampling problem, injecting up to three photons into our circuit and reproduced the expected theoretical probability distributions with exceptional fidelity. We also investigated techniques used to verify the successful implementation of Boson Sampling by implementing a family of unitaries known as the quantum complex Hadamards [8]. These sorts of techniques will be crucial in the future when the circuits and number of photons used mean that classical verification will be intractable even for the most powerful classical computers [9, 10]. Crucially, the reconfigurable circuit allows us to carry out Boson sampling and the verification protocols all on the same device.

The chip’s ability to implement arbitrary circuits promises to replace a multitude of existing prototype systems as well as providing a platform for those yet to be discovered. The results presented in [1] required ~ 1000 configurations of the device and point the way towards applications across quantum technologies.

[1] Jacques Carolan, Christopher Harrold, Chris Sparrow, Enrique Martín-López, Nicholas J. Russell, Joshua W. Silverstone, Peter J. Shadbolt, Nobuyuki Matsuda, Manabu Oguma, Mikitaka Itoh, Graham D. Marshall, Mark G. Thompson, Jonathan C. F. Matthews, Toshikazu Hashimoto, Jeremy L. O’Brien, Anthony Laing, "Universal linear optics", Science, 349, 711 (2015). Abstract.
[2] A. Hurwitz, "über die Erzeugung der Invarianten durch Integration", Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse, p.71 (1897). Full Article.
[3] Michael Reck, Anton Zeilinger, Herbert J. Bernstein, Philip Bertani, "Experimental realization of any discrete unitary operator", Physical Review Letters, 73, 58 (1994). Abstract.
[4] E. Knill, R. Laflamme, G.J. Milburn, "A scheme for efficient quantum computation with linear optics", Nature, 409, 46 (2001). Abstract.

[5] Robert Raussendorf, Hans J. Briegel, "A One-Way Quantum Computer", Physical Review Letters, 86, 
5188 (2001). Abstract.

[6] Daniel E. Browne, Terry Rudolph, "Resource-Efficient Linear Optical Quantum Computation", Physical Review Letters, 95, 010501 (2005). Abstract.

[7] Scott Aaronson, Alex Arkhipov, "The computational complexity of linear optics" in Proceedings of the ACM symposium on Theory of Computing, pp.333-342 (ACM, New York, 2011), Abstract.
[8] Anthony Laing, Thomas Lawson, Enrique Martín López, Jeremy L. O’Brien, "Observation of Quantum Interference as a Function of Berry’s Phase in a Complex Hadamard Optical Network", Physical Review Letters, 108, 260505 (2012). Abstract.
[9] Jacques Carolan, Jasmin D. A. Meinecke, Peter J. Shadbolt, Nicholas J. Russell, Nur Ismail, Kerstin Wörhoff, Terry Rudolph, Mark G. Thompson, Jeremy L. O'Brien, Jonathan C. F. Matthews, Anthony Laing, "On the experimental verification of quantum complexity in linear optics", Nature Photonics, 8, 621 (2014). Abstract.
[10] Malte Christopher Tichy, Markus Tiersch, Fernando de Melo, Florian Mintert, Andreas Buchleitner, "Zero-Transmission Law for Multiport Beam Splitters", Physical Review Letters, 104, 220405 (2010). Abstract.

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Sunday, August 23, 2015

One Clock in Two Places Simultaneously

Ron Folman (photo credit: Naomi Weisman)

Author: Ron Folman

Affiliation: Ben-Gurion University of the NegevBeer Sheva, Israel.

Link to atom chip group >>


This story is about Einstein’s theory of general relativity (GR) [1] and quantum mechanics (QM) [2]. These two theories constitute the physics revolutions of the 20th century, and there are numerous attempts to unify them or, at the very least, understand how they work together. This story is also about time. Some would claim that we are still far from really understanding time [3].

GR came to life in 1915, and was successful in explaining minute anomalies in the orbits of Mercury and other planets. In 1919, Sir Arthur Eddington was able, during a total solar eclipse, to verify a prediction of the theory when he headed an expedition which confirmed the deflection of light by the Sun. Some predictions, such as gravitational waves, remain unobserved. Another of the verified predictions of GR is that time is affected by gravity; it ticks at different rates depending on how close you are to a massive object. This has been confirmed for several decades now by sending rockets with clocks to large heights and recently, as atomic clocks became much more accurate, even by simply moving a clock a few centimeters higher in a lab. This effect was nicely displayed in the recent movie 'Interstellar'.

GR was always considered strange and hard to fathom. It is rumored that in one of his lectures on GR, Sir Eddington was asked by Ludwik Silberstain: "Professor Eddington, you must be one of three persons in the world who understands general relativity." Eddington paused, unable to answer. Silberstein continued "Don't be modest, Eddington!" Finally, Eddington replied "On the contrary, I'm trying to think who the third person is." It is quite amazing to see how less than a century later GR is an important part of our day-to-day life, as it is an essential part in the GPS navigation system.

QM is thought by many to be even weirder. It took away determinism, it allowed non-locality, and it even allowed objects to be in two places (or two states, e.g., of energy or of spin) at the same time. This seemed so ridiculous that one of the fathers of QM, Erwin Schrödinger came up with a thought experiment in which a cat is alive and dead at the same time, theoretically made possible due to QM. Other contributors to the birth of QM disliked it just as much. Einstein said that “God does not play with dice” and Louis de-Broglie believed determinism will come back in a more comprehensive theory. Niels Bohr said: “For those who are not shocked when they first come across quantum theory cannot possibly have understood it”. Yet, the theory is successfully withstanding endless testing for almost a century now.

It is quite strange that these two successful theories have not been put together into one simple description of the universe. It remains to be seen what exactly the interplay between the two is: for example, some claim that the crucial process of decoherence, which is responsible for defining the border between the quantum realm and the classical world (as our day-to-day world of large objects is known), is powered by GR [4]. Some even claim that the process of “wave-function collapse” is driven by GR [5]. This process is responsible for the fact that out of many possible outcomes which exist according to QM, we see only one when we make a measurement.

The experiment :

In the experiment we conducted, published recently in Science [6], we wanted to study the interplay between the two theories. Specifically we wanted to see what would happen if we take one clock and put it in two places in which time ticks at different rates, simultaneously. We then put the clock together again and observed it.

In order to do this we used a device called an atom chip (see our website for many relevant papers). It is a powerful tool in manipulating the internal and external degrees of freedom of ultra-cold atoms. Every atom in a cloud of ultra-cold atoms (a Bose-Einstein condensate at -273 deg. Celsius) was put in two places simultaneously (known in scientific language as a spatial quantum superposition [7]), by utilizing a device called an interferometer. In parallel, every atom was turned into an atomic clock by manipulating its internal (spin) degrees of freedom.

As we did not have sensitive enough clocks to observe the minute effect of GR, we induced an artificial time difference between the two locations occupied by the single clock (these two copies of the single clock are referred to in scientific language as wave-packets). Our proof-of-principle experiment indeed shows (as predicted by [8]) that time differences have a major effect on the quantum outcome expected from an interferometer (called an “interference pattern”). Specifically, we showed that time may act as a “which path” witness as if we measured in which of the two locations the clock is. Once such a measurement is made, QM tells us that the special state of quantum superposition is destroyed and the clock can no longer be in two places at the same time (this is called “complementarity”). However, we have also showed that if the hands of the clock in one wave-packet are rotated enough so that they again overlap the hands of the clock in the other wave-packet, we again cannot differentiate between the two locations and the superposition, and consequently the interference pattern is restored.

Technically, we used very strong magnetic gradients to construct a Stern-Gerlach type of interferometer, and also used similar gradients to induce the relative time lag (using the Zeeman effect). The atoms are laser-cooled and then additionally cooled by forced evaporation. The atoms are isolated from the environment by the fact that the whole experiment is conducted in a vacuum chamber. The final outcome of the experiment is registered with a CCD camera observing the atoms.

We now have a new tool at our disposal to investigate time and the interplay between GR and QM. What we will find is an open question. Usually, new tools bring new insight. The next step should be to use more accurate clocks and to separate the clock into positions farther apart so that we are able to see the effect of GR.

The Team :

The team included Ph.D. student Yair Margalit, post-doctoral fellows Dr. Zhifan Zhou and Dr. Shimon Machluf, and researchers Dr. Yonathan Japha and Dr. Daniel Rohrlich. In the picture are the first two authors of the paper: PhD student Yair Margalit (left) and post-doctoral fellow Zhifan Zhou (right).
Additional Thoughts :

It is perhaps appropriate to end a popular article with some broad thoughts and even speculations. It stands to reason that human kind knows much less than what it doesn’t know. Namely, we are just beginning to explore and understand the structure and dynamics of our world. History and Philosophy of science teach us that our understanding is constantly changing and no theory is the “end of the road”. Will we see extensions to GR or QM in our lifetime? Will they somehow be unified?

The atoms in our lab are our teachers; they are teaching us new things every day. As we build a better dictionary from atom language to human language we will be able to understand more. They are teaching us about our physical universe and about how to understand the strange mysteries of QM. They are teaching us technology (clocks, magnetic sensors, gravitational sensors, navigation, quantum communications and quantum computing). Eventually they may even teach us things that have to do directly with who we are. For example, is there such a thing as free will? On this latter topic I invite the reader to see a very short and simplified film I made [9].

It makes me envious but also very happy to know that some of the young readers of this article will know much more than I ever will. I encourage the young reader to take up the adventure wholeheartedly.

[1] “General relativity turns 100”, Special issue, Science, 347 (March 2015). Table of contents.
[2] “Foundations of quantum mechanics”, Insight issue, Nature Physics, 10 (April 2014).  Table of contents.
[3] Lee Smolin, "Time Reborn" (Mariner Books, 2014).
[4] Igor Pikovski, Magdalena Zych, Fabio Costa, Časlav Brukner, “Universal decoherence due to gravitational time dilation”, Nature Physics, 11, 668-672 (2015). Abstract.
[5] Roger Penrose, "The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics", Chapter 6 (New York: Oxford U. Press, 1989) ; Lajos Diósi, “Gravity-related wave function collapse: mass density resolution”, Journal of Physics: Conference Series, 442, 012001 (2013). Full Article ; Angelo Bassi, Kinjalk Lochan, Seema Satin, Tejinder P. Singh, Hendrik Ulbricht, “Models of wave-function collapse, underlying theories, and experimental tests”, Review of Modern Physics, 85, 471 (2013). Abstract.
[6] Yair Margalit, Zhifan Zhou, Shimon Machluf, Daniel Rohrlich, Yonathan Japha, Ron Folman, “A self-interfering clock as a 'which path' witness”, published online in 'Science Express' (August 6, 2015). Abstract.
[7] This new terminology is meaningful as all attempts to interpret the reality of this situation with day-to-day language have led to some sort of contradiction. The strange reality of the quantum world can perhaps only be accurately described by mathematics. Using words, which originate in our day-to-day (“classical”) experience, always fails those who attempt to harness them for the description of the quantum world. Perhaps one may alternatively state that the clock is in a state where it somehow “feels” how time “ticks” in several places at once.
[8] Magdalena Zych, Fabio Costa, Igor Pikovski, Časlav Brukner, “Quantum interferometric visibility as a witness of general relativistic proper time”, Nature Communications, 2, 505 (2011). Abstract. 2Physics Article.
[9] “ ‘Are we alive?’ – a thought by Ron Folman”: YouTube Link.  

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Sunday, August 16, 2015

"Secrets of the Universe"

2Physics.com is partnering with K2 Communications, a global leader in the production and distribution of IMAX films, which is currently working on the production of a IMAX 3D documentary film titled "Secrets of the Universe". K2 Communications has been commissioned by the National Science Foundation (NSF) by way of a substantial grant to promote the pursuit of an education and/or career in STEM (Science, Technology, Engineering & Mathematics) by inspiring an appreciation for science through the amazing discoveries in the world of particle physics.

The plan is to do this through a spectacular IMAX 3D experience titled "Secrets of the Universe" that will explore some of the most elusive and fascinating phenomena in the cosmos under investigation at the Large Hadron Collider (LHC) in Geneva, Switzerland, and explain today’s advance scientific discoveries in layman's terms. With greater understanding and enthusiasm from the general public, we can expect greater support and progress for STEM.

The film will address, more specifically, the exciting possibilities for discovery during the second run currently under way at the LHC: research on dark matter, antimatter, and parallel universes that has the potential to turn science fiction into science fact. Perhaps most importantly, it will feature the science and engineering feat that is the Large Hadron Collider - arguably humankind’s greatest machine - as a monument to how technology and research build off each other to create perpetual progress.

Multiple award-winning Director Stephen Low – a veteran of more than 15 giant screen documentaries – will lead the film team that includes the highly acclaimed IMAX cinematographer Sean Phillips.

Here is the Link to the "Secrets of the Universe" campaign >>

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Sunday, August 09, 2015

Computational High-Resolution Optical Imaging of the Living Human Retina

Some authors of the paper in Nature Photonics (reference [3]). From left to right: Yuan-Zhi Liu, Fredrick A. South, and Stephen A. Boppart.

Authors: Yuan-Zhi Liu, Fredrick A. South, Stephen A. Boppart

Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA.

Link to Biophotonics Imaging Laboratory >>

Eye imaging is very important not only because the vision is the primary human sense but also because the eye is the direct viewing window of the central nervous system. Acquiring the detailed information of the individual cells, blood vessels, and nerves at the retina of the living human eye could enable earlier diagnosis and better treatment for degenerative eye and neurological diseases. Optical coherence tomography, OCT, is a medical imaging technique analogous to ultrasound imaging, but has a higher resolution because it uses light rather than sound. Based on low coherence interferometry, OCT decouples the transverse and axial resolution, and can reject the out-of-focus background to achieve the depth-resolved image. Since the first application in eye imaging, OCT has become the standard of eye care for diagnosing and tracking disease, such as glaucoma and age-related macular degeneration [1].

The presence of ocular aberrations in the eye, however, limits cellular resolution tomography of the human retina. Combined with OCT, hardware-based adaptive optics (HAO) has pushed the limits of eye imaging, enabling diffraction-limited imaging of previously unresolvable structures, such as the three-dimensional visualization of photoreceptor distributions and individual nerve fiber bundles in the living human retina [2]. However, the sophisticated hardware and controlling software of HAO adds considerable complexity and cost to the standard OCT system, limiting the number of researchers and medical professionals who could benefit from the technology.

In our recent publication [3], we developed a phase stabilization technique and fully-automated computational adaptive optics (CAO) approach in conjunction with a standard en face optical coherence tomography (OCT) setup to computationally correct strong optical aberrations even in a highly-dynamic sample such as the human eye. This is the first time clear images of individual human photoreceptors, both near and far from the fovea, have been obtained in the living retina without the need of HAO.
Figure 1: (click on the image to view with higher resolution). Cone photoreceptors in the foveal region of the retina. (a) SLO image with the location of the photoreceptor mosaic outlined. (b) En face OCT mosaic before aberration correction. (c) The same OCT mosaic after automated correction. The insets are zoomed by 2X.

In HAO, a wavefront modulation device, e.g. deformable mirror, is used to physically modulate the pupil function to correct wavefront aberrations. CAO is a post-processing technique based on the complex signal from the interferometric measurement of OCT. The digital aberration correction filter is applied to the computational pupil plane, which is calculated from the Fourier transform of the focal plane image. The frequent and unavoidable motion of eye can severely interrupt the post-processing of the interference signal. In combination with the high-speed image acquisition from an en face OCT setup, the phase stabilization algorithm corrects even the sub-wavelength motion by comparing the phase differences of the adjacent frames. Afterward, a computational technique simulating the operation of the hardware Shack-Hartmann wavefront sensor is used to correct large optical aberrations which revealed individual photoreceptor cells. A technique named guide-star-based CAO is then applied to utilize the individual photoreceptors to sense and correct smaller, but still significant, aberrations.

CAO opens a new way for high-resolution imaging in ophthalmology. In addition to lower cost and simpler system configuration than HAO, the post-processing nature of CAO also allows more flexible aberration correction. Consider spatially-variant aberrations as an example. HAO needs to sense the aberrations in each small volume and then execute each correction while imaging the subject. By using CAO, researcher/doctors can shorten the image acquisition time and apply multiple aberration corrections afterward. Released from the physical limitations of the deformable mirror, e.g. the number and the stroke range of actuator, CAO has the potential to correct higher order and larger amplitude aberrations. Because CAO manipulates the complex interference signal, in principle better image quality can be achieved by applying not only the phase but also the amplitude corrections.

[1] D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, "Optical coherence tomography", Science, 254 (5035), 1178–1181 (1991). Abstract.
[2] Yan Zhang, Barry Cense, Jungtae Rha, Ravi S. Jonnal, Weihua Gao, Robert J. Zawadzki, John S. Werner, Steve Jones, Scot Olivier, Donald T. Miller, "High-speed volumetric imaging of cone photoreceptors with adaptive optics spectral-domain optical coherence tomography", Optics Express, 14, 4380-4394 (2006). Abstract.
[3] Nathan D. Shemonski, Fredrick A. South, Yuan-Zhi Liu, Steven G. Adie, P. Scott Carney, Stephen A. Boppart, "Computational high-resolution optical imaging of the living human retina", Nature Photonics, 9, 440-443 (2015). Abstract.

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Sunday, August 02, 2015

Bending Electric Discharges with Lasers

Matteo Clerici

Authors: Matteo Clerici1,2, Yi Hu1,3, Philippe Lassonde1, Carles Milián4, Arnaud Couairon4, 
Demetrios N. Christodoulides5, Zhigang Chen3,6, Luca Razzari1, François Vidal1, François Légaré1, Daniele Faccio2, Roberto Morandotti1,7

1Institut National de la Recherche Scientifique–Énergie Matériaux Télécommunications, Varennes, Québec J3X 1S2, Canada.
2School of Engineering and Physical Sciences, Scottish Universities Physics Alliance, Heriot-Watt University, Edinburgh, UK.
3The MOE Key Laboratory of Weak Light Nonlinear Photonics, School of Physics and TEDA Applied Physics School, Nankai University, Tianjin, China.
4Centre de Physique Théorique CNRS, École Polytechnique, Palaiseau, France.
5College of Optics, Center for Research and Education in Optics and Lasers, University of Central Florida, Orlando, Florida, USA.
6Department of Physics and Astronomy, San Francisco State University, California, USA.
7Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, China.


Electric discharges are part of our everyday life. On a large scale, lightning is one of the most spectacular atmospheric phenomena, while on a much smaller scale -- cars, for instance -- rely on electric discharges for igniting the fuel. Small-scale controlled discharges find relevant industrial applications, e.g. in cutting and milling, and are also investigated as a mean to control gas flows and pollution.

Since the seventies, it has been realized that intense laser pulses may trigger and guide electric discharges [1], and ground breaking experimental results started to appear few years later (see e.g. [2–4] and references therein). From these successes, scientists have theoretically and experimentally investigated further on the laser guiding effect, also stimulated by the possibility of applying this concept to lightning control and protection [5].

While at standard air temperature and pressure conditions an electric field of nearly 34 kV/cm is required to start an electric arc between two electrodes, this threshold decreases with decreasing gas density. An intense laser focused in air ionizes the gas and locally increases the temperature. To this initial stage, a localized decrease in the density follows, which allows the discharge to start at lower than a standard electric field. This is the dominant effect responsible for the laser triggering and guiding mechanism [6,7].

Curved and healing beams:

In last few years researchers have demonstrated that the intense peak of laser pulses may travel in free space along complex trajectories, instead of the well-known straight line [8]. To enable this new degree of control the laser wavefront must be manipulated with proper optical elements. For instance, to generate curved laser beams a cubic phase modulation is required, and can be obtained either by designing a specific phase mask or by exploiting the aberrations of standard lenses. The beam generated this way is named Airy beam and the generalization of its behavior recently resulted in a vast available literature. Now, curved beams that follow an almost circular path or more complex trajectories can be obtained [9,10].

The research into laser shaping also brought to attention the ability of particular beams to self-heal, i.e. to recreate their intense part even after being partially obstructed. This happens, for instance, for Airy beams, and also for Bessel beams, another class of laser beams obtained by focusing a standard laser with a conical lens, instead of an ordinary one.

Bending the currents:

In the present work, recently published on Science Advances [11], we have combined the concept of laser guided electric discharge with the possibilities allowed by the recent results on beam shaping, and we have shown how electric sparks can be guided along a curved path avoiding an obstacle (see Fig. 1). Furthermore we have demonstrated that the self-healing property of laser beams allows the discharge to go back on track also when the obstacle cannot be avoided.

In our work we have employed a high-voltage generator able to deliver a potential difference of up to 35 kV applied to two wire electrodes suspended in air. At a distance of 5 cm no discharge occurs since the applied field is, at most, 5 times lower than the air breakdown field.
Figure 1. Sketch of the experiment. A phase mask and a lens reshape the laser beam into an Airy beam, which bend along the propagation and generate plasma on a parabolic shape. The region of air heated by the plasma has a reduced density, hence allowing the discharge to occur and guiding it along its curved path, overcoming an obstacle that is in the line of sight of the two electrodes. The inset on the top-right shows the actual measurement, acquired with a photo camera (top view). The obstacle is an opaque glass.

We then focus a shaped, intense laser pulse (intensity > 1012 W/cm2) between the electrodes, ionizing the air and therefore heating the gas. We compared different shapes: a Gaussian beam (the most common beam shape), a Bessel beam, an Airy beam, and an S-shaped beam, obtained inserting in the laser path i) a standard lens, ii) an axicon (conical lens), iii) a tilted cylindrical lens system and iv) a phase mask, respectively. This way we have seen that electric discharge occurs only when the laser pulse ignites the plasma and the discharge path indeed follows the trajectory of the shaped laser pulse. For the Gaussian beam we observe a quite erratic behavior, consequence of the poor spatial localization of the heated gas; the discharge appear more deterministic when employing a Bessel beam, due to the high spatial localization of the Bessel peak. Finally, the discharge is clearly curved when an Airy beam is employed, as shown in the inset of Fig. 1. This allows conducting the charge from one electrode to the other while avoiding an obstacle that is placed in the direct line of sight.

Current self-healing:

Bessel beams and Airy beams share the ability to regenerate after hitting on an obstacle. This is mainly due to the fact that the energy responsible for the build-up of the intense laser peak flows from a reservoir that is off-axis with respect to the intense portion of the beam.

We have shown that the beam self-healing allows guiding the electric discharge along a predefined trajectory even in case of an obstacle blocking part of the beam. This is shown in Fig. 2 for the case of a Bessel beam. The laser propagates from left to right and is focused by the conical lens visible in the left of the picture. A tiny plasma plume develops over few centimeters and regenerates after hitting the obstacle (see inset in Fig.2). When the potential difference is applied to the electrodes, a discharge occurs following the path defined by the laser plasma. It performs a jump in a non-controlled way, right after the obstacle, and then goes back on track as soon as the laser plasma reappears (see Fig. 2).
Figure 2. Self-healing and electric discharges. We show that Bessel beam induced plasma reappears after the beam is obstructed by an obstacle, due to the Bessel beam self-healing property (see inset, the laser propagates from left to right). This effect finds a straightforward application in guiding electric currents: even if the dielectric obstacle cannot be avoided, the current can be delivered on target following the predefined path.


We have shown that shaping of laser beams allows extensive control of the path over which electric currents can be guided in air. Our results have shown that a reduction of up to a factor 10 in the breakdown voltage can be obtained with the proper laser beam, and this opens the possibility to realize complex discharge path and extreme curvatures that will be investigated in the future and may find applications e.g., in the machining industry.

R.M. and F.L. gratefully acknowledge Ministère de l’Enseignement Supérieur, de la Recherche, de la Science et de la Technologie (MERST), the Natural Sciences and Engineering Research Council of Canada (NSERC) and Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT) for funding this work. M.C. acknowledges the support from the People Program (Marie Curie Actions) of the European Union’s Seventh Framework Program (FP7/2007-2013) under REA grant agreement no. 299522. The work of D.N.C. was supported by the Air Force Office of Scientific Research (AFOSR, MURI grant no. FA9550-10-1-0561), and of Z.C. by the AFOSR and National Science Foundation (NSF). D.F. acknowledges financial support from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC grant agreement no. 306559, and the Engineering and Physical Sciences Research Council (EPSRC, UK, grant EP/M006514/1). A.C. and C.M. acknowledge financial support from the French Direction Générale de l’Armement (DGA). Experiments were carried out at the Advanced Laser Light Source (ALLS) facility located at the Institut National de la Recherche Scientifique.

[1] David W. Koopman, T.D. Wilkerson, "Channeling of an ionizing electrical streamer by a laser beam", Journal of Applied Physics, 42, 1883–1886 (1971). Abstract.
[2] X.M. Zhao, J.C. Diels, "Femtosecond pulses to divert lightning", Laser Focus World, 29, 113 (1993). 
[3] M. Rodriguez, R. Sauerbrey, H. Wille, L. Wöste, T. Fujii, Y.-B. André, A. Mysyrowicz, L. Klingbeil, K. Rethmeier, W. Kalkner, J. Kasparian, E. Salmon, J. Yu, J.-P. Wolf, "Triggering and guiding megavolt discharges by use of laser-induced ionized filaments", Optics Letters, 27, 772–774 (2002). Abstract.
[4] D. Comtois, C. Y. Chien, A. Desparois, F. Génin, G. Jarry, T. W. Johnston, J.-C. Kieffer, B. La Fontaine, F. Martin, R. Mawassi, H. Pépin, F.A.M. Rizk, F. Vidal, P. Couture, H. P. Mercure, C. Potvin, A. Bondiou-Clergerie, I. Gallimberti, "Triggering and guiding leader discharges using a plasma channel created by an ultrashort laser pulse", Applied Physics Letters, 76, 819–821 (2000). Abstract.
[5] Jérôme Kasparian, Roland Ackermann, Yves-Bernard André, Grégoire Méchain, Guillaume Méjean, Bernard Prade, Philipp Rohwetter, Estelle Salmon, Kamil Stelmaszczyk, Jin Yu, André Mysyrowicz, Roland Sauerbrey, Ludger Wöste, Jean-Pierre Wolf, "Electric events synchronized with laser filaments in thunderclouds", Optics Express, 16, 5757–5763 (2008). Abstract.
[6] F. Vidal, D. Comtois, Ching-Yuan Chien, Alain Desparois, B. La Fontaine, T.W. Johnston, J. Kieffer, Hubert P. Mercure, H. Pepin,  F.A. Rizk, "Modeling the triggering of streamers in air by ultrashort laser pulses", IEEE Transaction Plasma Science 28, 418–433 (2000). Abstract.
[7] S. Tzortzakis, B. Prade, M. Franco, A. Mysyrowicz, S. Hüller, P. Mora, "Femtosecond laser-guided electric discharge in air", Physical Review E, 64, 057401 (2001). Abstract.
[8] G.A. Siviloglou, J. Broky, A. Dogariu, D.N. Christodoulides, "Observation of accelerating Airy beams", Physical Review Letters, 99, 213901 (2007). Abstract.
[9] F. Courvoisier, A. Mathis, L. Froehly, R. Giust, L. Furfaro, P.A. Lacourt, M. Jacquot, J.M. Dudley, "Sending femtosecond pulses in circles: highly nonparaxial accelerating beams", Optics Letters, 37, 1736–1738 (2012). Abstract.
[10] Ido Kaminer, Rivka Bekenstein, Jonathan Nemirovsky, Mordechai Segev, "Nondiffracting accelerating wave packets of Maxwell’s equations", Physical Review Letters, 108, 163901 (2012). Abstract.
[11] Matteo Clerici, Yi Hu, Philippe Lassonde, Carles Milián, Arnaud Couairon, Demetrios N. Christodoulides, Zhigang Chen, Luca Razzari, François Vidal, François Légaré, Daniele Faccio, Roberto Morandotti, "Laser-assisted guiding of electric discharges around objects", Science Advances, 1, e1400111 (2015). Article.