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2Physics Quote:
"The magnetic compass of birds is light dependent and suggested to be mediated by light-induced, biochemical reactions taking place in specialized photoreceptors. The photopigment molecules form magnetically sensitive radical-pair intermediates upon light excitation. The ratio of the spin states of the radical pairs (i.e., singlet vs. triplet state) is affected by Earth-strength magnetic fields, which thereby alters the response of the photopigments to light. In birds, such magneto-sensitive photoreceptors have been proposed to be arranged in an ordered array in the eye. Depending on their alignment to the magnetic field, they would show an increased or decreased sensitivity to light."
-- Rachel Muheim, Sissel Sjöberg, Atticus Pinzon-Rodriguez
(Read Full Article: "Polarized Light Modulates Light-Dependent Magnetic Compass Orientation in Birds" )

Sunday, June 26, 2016

Direct Detection of the 229Th Nuclear Clock Transition

From left to right: Peter G. Thirolf, Lars v.d. Wense, Benedict Seiferle.

Authors: Lars von der Wense1, Benedict Seiferle1, Mustapha Laatiaoui2,3, Jürgen B. Neumayr1, Hans-Jörg Maier1, Hans-Friedrich Wirth1, Christoph Mokry3,4, Jörg Runke2,4, Klaus Eberhardt3,4, Christoph E. Düllmann2,3,4, Norbert G. Trautmann4, Peter G. Thirolf1

1Ludwig-Maximilians-Universität München, 85748 Garching, Germany.
2GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany.
3Helmholtz Institut Mainz, 55099 Mainz, Germany.
4Johannes Gutenberg Universität, 55099 Mainz, Germany.

The measurement of time has always been an important tool in science and society [1]. Today’s most precise time measurements are performed with optical atomic clocks, which achieve a precision of about 10-18, corresponding to 1 second uncertainty in more than 15 billion years, a time span which is longer than the age of the universe [2]. By comparing two of such clocks, which are shifted in height by just a few centimetres, also the time dilation due to general relativistic effects becomes measurable [3].

Despite such stunning precision, these clocks could be outperformed by a different type of clock, the so called “nuclear clock” [4]. The nuclear clock makes use of a nuclear transition instead of an atomic shell transition as so far applied. The expected factor of improvement in precision of such a new type of clock has been estimated to be up to 100, in this way pushing the ability of time measurement to the next level [5]. The reason for the expected improvement is the size of the nucleus, which is orders of magnitude smaller than the size of the atom, thus leading to significantly improved resilience against external influences.

Many potential applications for a nuclear clock are currently under discussion. These include practical applications such as improved satellite-based navigational systems, data transfer, gravity detectors [6] as well as fundamental physical applications like gravitational wave detection [7] and testing for potential changes in fundamental constants [8].

Using existing technology, there is only one nuclear state known, which could serve for a nuclear clock. This is the first excited nuclear isomeric state of 229Th. Among all known (more than 175,000) nuclear excitations, this isomeric state exhibits a unique standing due to its extremely low excitation energy of only a few electronvolts [9]. The energy is that low, that it would allow for a direct laser excitation of the nuclear transition, which is the prerequisite for the development of a nuclear clock.

The existence of this isomeric state was shown in 1976, based on indirect measurements [10]. However, despite significant efforts, the direct detection of the isomeric decay could not be achieved within the past 40 years [11]. In the recently presented work [12], our group was able to solve this long-standing problem, leading to the first direct detection of the 229Th nuclear clock transition. This direct detection is important, as it paves the way for the determination of all decay parameters relevant for optical excitation of the isomeric state. It is thus a breakthrough step towards the development of a nuclear clock.
Figure 1: (click on the image to view with higher resolution) Experimental setup used for the production of a purified 229Th ion beam and the direct detection of the isomeric state. For details we refer the reader to the text and to Ref. [12].

The detection was achieved by producing a low energy, pure 229Th ion beam, with a fractional content of 229Th in the isomeric state. The isomer was produced by making use of a 2% decay branch of the alpha-decay of 233U into the isomeric state. The setup used for ion beam production is shown in Fig. 1 and will be described in the following section. The ions were collected with low kinetic energy onto the surface of a micro-channel-plate (MCP) detector, triggering the isomer’s decay and leading to its detection at the same time. The obtained signal is shown in Fig. 2. A high signal-to-background ratio could be achieved owing to the concept of spatial separation of the 233U source and the point of isomer detection. Many comparative investigations were performed in order to unambiguously show that the detected signal originates from the 229Th isomeric decay [12].

Figure 2: 229Th isomeric decay signal as observed during 2000 second integration time on the MCP detector allowing for spatially resolved signal read out.

For the production of a low-energy 229Th ion beam, a 233U source was used, which was placed inside of a buffer-gas stopping cell, filled with 40 mbar of ultra-pure helium. 229Th isotopes, as produced in the alpha-decay of 233U, are leaving this source due to their kinetic recoil energy of 84 kiloelectronvolts. These recoil isotopes were stopped in the helium buffer-gas, thereby staying charged due to the large ionization potential of helium. The low-energy 229Th ions, produced in this way, were guided through the helium background towards the exit of the stopping cell by electric fields, provided by a radio-frequency funnel system. The exit of the stopping cell consists of a Laval-nozzle system, leading to the formation of a supersonic gas jet. This gas jet injects the ions into a radio-frequency quadrupole (RFQ) ion-guide, leading to the formation of an ion beam. This ion beam is further purified with the help of a quadrupole mass-separator (QMS). In this way, a low-energy, pure 229Th ion beam was produced, possessing a fractional isomeric content of about 2%.

The next envisaged steps towards the development of a nuclear clock will be performed within the framework of the EU-funded Horizon 2020 collaboration named “NuClock” (www.nuclock.eu). Experiments will be carried out that aim for a precise determination of the isomer’s energy and half-life as being the basis for the first direct laser excitation of a nuclear transition.

[1] David Landes, "Revolution in Time: Clocks and the Making of the Modern World" (Harvard University Press, Cambridge, 2000).
[2] T.L. Nicholson, S.L. Campbell, R.B. Hutson, G.E. Marti, B.J. Bloom, R.L. McNally, W. Zhang, M.D. Barrett, M.S. Safronova, G.F. Strouse, W.L. Tew, J. Ye, "Systematic evaluation of an atomic clock at 2 X 10-18 total uncertainty", Nature Communications, 6, 6896 (2015). Abstract.
[3] Andrew D. Ludlow, Martin M. Boyd, Jun Ye, E. Peik, P. O. Schmidt, "Optical atomic clocks", Review Modern Physics, 87, 637-701 (2015). Abstract.
[4] E. Peik, Chr. Tamm, "Nuclear laser spectroscopy of the 3.5 eV transition in 229Th", Europhysics Letters, 61, 181-186 (2003). Abstract.
[5] C. J. Campbell, A. G. Radnaev, A. Kuzmich, V. A. Dzuba, V. V. Flambaum, A. Derevianko, "Single-Ion nuclear clock for metrology at the 19th decimal place", Physical Review Letters, 108, 120802 (2012). Abstract.
[6] Marianna Safronova, "Nuclear physics: Elusive transition spotted in thorium", Nature, 533, 44-45 (2016). Abstract.
[7] Shimon Kolkowitz, Igor Pikovski, Nicholas Langellier, Mikhail D. Lukin, Ronald L. Walsworth, Jun Ye, "Gravitational wave detection with optical lattice atomic clocks", arXiv:1606.01859 [physics.atom-ph] (2016).
[8] V.V. Flambaum, "Enhanced effect of temporal variation of the fine structure constant and the strong interaction in 229Th", Physical Review Letters, 97, 092502 (2006). Abstract.
[9] B.R. Beck, J.A. Becker, P. Beiersdorfer, G.V. Brown, K.J. Moody, J.B. Wilhelmy, F.S. Porter, C.A. Kilbourne, R.L. Kelley, "Energy splitting of the ground-state doublet in the nucleus 229Th", Physical Review Letters, 98, 142501 (2007). Abstract.
[10] L.A. Kroger, C.W. Reich, "Features of the low energy level scheme of 229Th as observed in the alpha decay of 233U", Nuclear Physics A, 259, 29-60 (1976). Abstract.
[11] Ekkehard Peik, Maxim Okhapkin, "Nuclear clocks based on resonant excitation of gamma-transitions", Comptes Rendus Physique, 16, 516-523 (2015). Abstract.
[12] Lars von der Wense, Benedict Seiferle, Mustapha Laatiaoui, Jürgen B. Neumayr, Hans-Jörg Maier, Hans-Friedrich Wirth, Christoph Mokry, Jörg Runke, Klaus Eberhardt, Christoph E. Düllmann, Norbert G. Trautmann, Peter G. Thirolf, "Direct detection of the 229Th nuclear clock transition", Nature, 533, 47 (2016). Abstract.

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Sunday, June 12, 2016

Demonstration of Light Induced Conical Intersections in Diatomic Molecules

Left to right: Adi Natan, Matt Ware, Phil Bucksbaum

Authors: Adi Natan1, Matthew R Ware1,4, Vaibhav S. Prabhudesai2, Uri Lev3, Barry D. Bruner3, Oded Heber3, Philip H Bucksbaum1,4

1Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, CA, USA
2Department of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Mumbai, India
3Faculty of Physics, the Weizmann Institute of Science, Rehovot, Israel
4Departments of Physics, Applied Physics, and Photon Science, Stanford University, Stanford, CA, USA

About 200 femtoseconds after you started reading this line, the first step in actually seeing it took place. In the very first step of vision, the retinal chromophores in the rhodopsin proteins in your eyes were photo-excited and then driven through a conical intersection to form a trans isomer [1]. The conical intersection is the crucial part of the machinery that allows such ultrafast energy flow. Conical intersections (CIs) are the crossing points between two or more potential energy surfaces. These tend to repel when they approach each other, but cannot stay separated everywhere in the multidimensional geometry landscape of a molecule. CIs are ubiquitous in photo-processes in polyatomic molecules and govern key phenomena such as DNA photostability [2], but they are difficult to locate, measure and control, since their positions and features depend on specific molecular system under study. It is therefore of great interest to simulate their effects.

The case for diatomic molecules is different because naturally occurring CIs cannot exist for them: Energy levels are repelled when they approach each other, and there are not enough degrees of freedom for a true crossing. However a strong laser field applied to a diatomic molecule adds one additional degree of freedom, which gives rise to a light-induced artificial CI (LICI) that is predicted to have several features in common with the poly-atomic “wild-type” CI [3-5]. More recent theoretical studies have examined various aspects of this LICI phenomenon [6-9].

The molecular dynamics in the vicinity of a LICI can be explored in the simplest laser-illuminated molecular system, the ground state of the hydrogen molecular ion, H2+ [10]. An infrared laser will induce interactions between the two lowest nuclear potential energy curves, named 1sσg and 2pσu. The interaction of strong fields with H2+ can be expressed by shifting one of the potential curves by the energy of one photon. The now ”dressed” potential curve crosses at some internuclear separation R where the two states are resonantly coupled by the laser field, the analog of the curve crossings at CIs in polyatomic molecules, as seen in Fig 1(a) . We define the LICI as this crossing point, when the polarization of the laser is perpendicular to the molecular axis. For other angles, there is no crossing, and the dynamics is “adiabatic”, that is, a nuclear wavepacket will travel only on one of the dressed potential curves.

Consider for a moment what it means to have a point of crossing where the field is perpendicular to the molecular axis of a diatomic molecule: This molecule effectively “feels” zero laser field only at this specific angle, so the potential curves assume their field free R-dependence only here. The key to the dynamics induced by the LICI is the special behavior of the molecule as it rotates through this special point, from an angle on one side where it experiences the field, to an angle on the other side where the field is once again felt.

The simplest place to start is H2+ in its ground rotational state, interacting with a strong laser field that couples resonantly the two electronic states. The spherical symmetry of the ground state field-free nuclear probability density, changes rapidly when a strong field is turned on. We can follow how this probability density evolves in the dressed state framework. In the dressed picture, the LICI is a local maximum in the two-dimensional (R,θ) potential energy landscape, while in other angles, the population dissociates adiabatically on the unbound part of the light induced potential curve. This causes the part of the population to accumulate around the LICI and then scatter from it, similar to waves scattering from a cone shape potential barrier. However, unlike scattering of a free particle around a barrier, the scattering described here is of bound nuclear wave packet, which scatters into a multitude of rovibrational states on the ground electronic surface, similar to the non-adiabatic dynamics around a natural CI. The scattering from the LICI leads to dissociation, but imposes a scattering time delay. The coherent addition of all the scattering trajectories creates an interference pattern at various angles and kinetic energy releases (Fig 1 (b), and Fig 1(c)). These quantum interferences are a universal signature for LICIs because they arise from the nature of coherent scattering interference near a point of degeneracy.
Figure 1: [Click on the image to view with higher resolution] (a) The dressed potential energy surfaces of H2+ featuring LICIs. (b) The calculated instantaneous probability of dissociation P(θ,t)diss from a given vibrational state (for example, v=9) during the interaction with the laser pulse reveals the interference. (c) Experimental (top) and calculated (bottom) angular distributions of H2+ dissociation at kinetic energy releases that correspond to specific initial vibrations states.

We have experimentally demonstrated the effects of LICIs on strong-field photodissociation of H2+ by means of quantum interferences that modify the angular distributions of the dissociating fragments [10]. The interferences depend strongly on the energy difference between the initial state and the LICI. The larger the overlap between the initial state and the LICI, the larger the effective duration of interaction and the more developed the interferences. For example, we can compare the effect among different initial states, starting from an initial state that is nearly resonant, hence has a large overlap with the LICI, such as the vibrational level v = 9 in H2+, to a state that is non-resonant such as v = 7. We observe in both calculation and experiment how these initial states indeed capture the different effective duration of the interaction with the LICI (Fig 1(c)).

LICIs are particularly attractive for future quantum control experiments due to their high degree of controllability using the polarization and frequency of the laser. The interaction is not limited to just a single LICI, and allows control of the timing of its appearance as well. Understanding the dynamics induced by LICIs will facilitate understanding and applicability to systems of higher complexity. Implementing and understanding LICIs in more complex systems will open the way to novel spectroscopy techniques in physics and chemistry.

[1] Dario Polli, Piero Altoè, Oliver Weingart, Katelyn M. Spillane, Cristian Manzoni, Daniele Brida, Gaia Tomasello, Giorgio Orlandi, Philipp Kukura, Richard A. Mathies, Marco Garavelli, Giulio Cerullo, "Conical intersection dynamics of the primary photoisomerization event in vision", Nature, 467, 440 (2010). Abstract.
[2] B. K. McFarland, J. P. Farrell, S. Miyabe, F. Tarantelli, A. Aguilar, N. Berrah, C. Bostedt, J. D. Bozek, P. H. Bucksbaum, J. C. Castagna, R. N. Coffee, J. P. Cryan, L. Fang, R. Feifel, K. J. Gaffney, J. M. Glownia, T. J. Martinez, M. Mucke, B. Murphy, A. Natan, T. Osipov, V. S. Petrović, S. Schorb, Th. Schultz, L. S. Spector, M. Swiggers, I. Tenney, S. Wang, J. L. White, W. White, M. Gühr, "Ultrafast X-ray Auger probing of photoexcited molecular dynamics", Nature Communications, 5, 4235 (2014). Abstract.
[3] Nimrod Moiseyev, Milan Šindelka, Lorentz S. Cederbaum, "Laser-induced conical intersections in molecular optical lattices", Journal of Physics B,  41, 221001 (2008). Full Article.
[4] Milan Šindelka, Nimrod Moiseyev, Lorentz S. Cederbaum, "Strong impact of light-induced conical intersections on the spectrum of diatomic molecules", Journal of Physics B, 44, 045603 (2011). Abstract.
[5] Nimrod Moiseyev, Milan Šindelka, "The effect of polarization on the light-induced conical intersection phenomenon", Journal of Physics B, 44, 111002 (2011). Abstract.
[6] Gábor J. Halász, Ágnes Vibók, Nimrod Moiseyev, Lorenz S. Cederbaum, "Nuclear-wave-packet quantum interference in the intense laser dissociation of the D2+ molecule", Physical Review A, 88, 043413 (2013). Abstract.
[7] Gábor J Halász, Ágnes Vibók, Nimrod Moiseyev, Lorenz S Cederbaum, "Light-induced conical intersections for short and long laser pulses: Floquet and rotating wave approximations versus numerical exact results", Journal of Physics B, 45, 135101 (2012). Abstract.
[8] Gábor J. Halász, Ágnes Vibók, Milan Šindelka, Lorenz S. Cederbaum, Nimrod Moiseyev, "The effect of light-induced conical intersections on the alignment of diatomic molecules", Chemical Physics, 399, 146 (2012). Abstract.
[9] G.J. Halász, A. Vibók, L.S. Cederbaum, "Direct Signature of Light-Induced Conical Intersections in Diatomics", Journal of Physical Chemistry Letters, 6, 348 (2015). Abstract.
[10] Adi Natan, Matthew R. Ware, Vaibhav S. Prabhudesai, Uri Lev, Barry D. Bruner, Oded Heber, Philip H. Bucksbaum, "Observation of Quantum Interferences via Light-Induced Conical Intersections in Diatomic Molecules", Physical Review Letters, 116, 143004 (2016). Abstract.

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Sunday, May 22, 2016

Two dimensional Superconducting Quantum Interference Filter (SQIF) arrays using 20,000 YBCO Josephson Junctions

From Left to Right: (top row) Emma Mitchell, Jeina Lazar, Keith Leslie, Chris Lewis,
(bottom row) Alex Grancea, Shane Keenan, Simon Lam, Cathy Foley.

Emma Mitchell1, Kirsty Hannam1, Jeina Lazar1, Keith Leslie1, Chris Lewis1
Alex Grancea2, Shane Keenan1, Simon Lam1, Cathy Foley1

1CSIRO Manufacturing, Lindfield, NSW, Australia,
2CSIRO Data61, Epping, NSW, Australia.

Josephson junctions form the essential magnetic sensing element at the heart of most superconducting electronics. A Josephson junction consists of two superconducting electrodes separated by a thin barrier [1]. Provided the barrier width is less than the superconducting coherence length, Cooper pairs can tunnel quantum mechanically from one electrode to the other coherently when the temperature is below the critical temperature of the two electrodes. Due to the macroscopic quantum coherence of the Cooper pairs in the superconducting state, Josephson junctions not only detect magnetic fields and RF radiation over an extremely wide frequency band, but can also emit radiation. The highly sensitive response of the Josephson junction current to magnetic fields is the key to many of its applications, including magnetometers, absolute magnetic field detectors and low noise amplifiers. More recent applications also include small RF antennas which utilize the Josephson junction’s broadband (dc- THz) detection abilities.

The dc SQUID, or Superconducting Quantum Interference Device, consists of two Josephson junctions connected together in parallel via a superconducting loop. The SQUID is an extremely sensitive flux-to voltage transducer, but despite this simplicity, an exact solution of this problem can only be given in the case of negligible inductance of the loop containing the two junctions. When the SQUID is biased with a current and an external magnetic field is applied, the voltage response oscillates periodically with applied magnetic field (Figure 1a). The period of the oscillation is inversely proportional to the loop area. SQUIDs have been connected together into arrays of increasing size and complexity to improve device sensitivity.

Figure 1: Voltage responses for a (a) dc SQUID with two step-edge junctions (b) two dimensional SQIF with 20,000 step-edge junctions.

Feynman et al. (1966) first predicted [2] an enhancement in the SQUID interference effect by having multiple (identical) junctions in parallel, analogous to a multi-slit diffraction grating. This enhancement was originally observed using superconducting point contact junctions [3] and has been further developed using series arrays of low temperature superconducting (LTS) Nb SQUIDs. More recently 1D arrays of SQUIDs with incommensurate loop areas (non-identical and variable spread) with a non-periodic voltage response were suggested [4]. The voltage response of these superconducting quantum interference filters (SQIFs) is then analogous to a non-conventional optical grating where different periodic responses from individual SQUIDs with different loop areas are summed. This results in a voltage response to a magnetic field in which a dominant anti-peak develops at zero applied field due to constructive interference of the individual SQUID responses. Weaker non-periodic oscillations occur at non-zero fields where the individual SQUID responses destructively interfere. The magnitude and width of the anti-peak for a SQIF is governed by the range and distribution of SQUID loop areas and inductances.

In our recent work [5] we demonstrate high temperature superconducting (HTS) two dimensional SQIF arrays based on 20,000 YBCO step-edge Josephson junctions connected together in series and parallel (Figure 1b). The maximum SQIF response we measured had a peak-to-peak voltage of ~ 1mV and a sensitivity of (1530 V/T) using a SQIF design with twenty sub-arrays connected in series with each sub-array consisting of 50 junctions in parallel connected to 20 such rows in series. The variation in loop areas within each subarray had a pseudo- random distribution with a mean loop area designed to have an inductance factor βL = 2LIc0 ~1 [6]. Figure 2a shows part of our array with four whole sub-arrays visible. At higher magnification the variation of individual loop areas is evident (Figure 2b) with the rows of step-edge junctions indicated by arrows.
Figure 2: (a) Part of the 20,000 YBCO step-edge junction SQIF array showing four complete sub-arrays of 1,000 junctions each (b) one sub-array at higher magnification showing rows of junctions (arrows) and variable loop areas (darker material is the YBCO).

The Josephson junctions in our samples are step-edge junctions formed when a grain boundary develops between the YBCO electrodes that grow epitaxially when a thin film is deposited over a small step approximately 400nm high with an angle of ~38o, etched into the supporting MgO substrate [7, 8]. It is well documented that HTS Josephson junctions are difficult to fabricate in large numbers across a substrate. However, step-edge junctions have the advantage of being relatively simple and inexpensive to fabricate and can be placed, at high surface density almost anywhere on a substrate. To date, we have made 2D arrays showing a SQIF response with 20,000 up to 67,000 step-edge junctions on a 1cm2 substrate.

Two dimensional SQIF arrays allow for large numbers of junctions to be placed in high density across a chip, enabling increases in the output voltage and sensitivity of the device. 2D arrays also allow for impedance matching of the array to external electronics by varying the ratio of junctions in parallel to those in series, by virtue of the junction normal resistance, Rn.

In addition, we demonstrated that the sensitivity of the SQIF depends strongly on the mean junction critical current, Ic, in the array, and the inductance (area) of the average loop in the array. In both cases keeping these parameters small such that βL < 1 is necessary for improving the SQIF sensitivity, but can be difficult to achieve with HTS junctions in which the typical spread in Ic can be 30%. The SQIF response also depends on the number of junctions; a linear increase in the SQIF sensitivity with junction number was measured for our SQIF designs.

We were also able to demonstrate RF detection at 30 MHz using our HTS SQIFs at 77 K [5]. More recently a broadband SQIF response from DC to 140 MHz was demonstrated following improvements to our SQIF sensitivity (unpublished). This follows on from reports of near field RF detection to 180 MHz using 1000 low temperature superconducting (LTS) junctions [9], where more complex and expensive cryogenic requirements limit the LTS array applications outside the laboratory.

[1] B.D. Josephson,  "Possible new effects in superconductive tunneling". Physics Letters, 1, 251 (1962). Abstract.
[2] Richard P. Feynman, Robert B. Leighton, Matthew Sands, “The Feynman lectures on Physics, Vol III” (Addison-Wesley, 1966).
[3] J.E. Zimmerman, A.H. Silver, "Macroscopic quantum interference effects through superconducting point contacts", Physical Review, 141, 367 (1966). Abstract.
[4] J. Oppenländer, Ch. Häussler, N. Schopohl, "Non-Φo periodic macroscopic quantum interference in one-dimensional parallel Josephson junction arrays with unconventional grating structures", Physical Review B, 63, 024511 (2000). Abstract.
[5] E.E. Mitchell, K.E. Hannam, J. Lazar, K.E. Leslie, C.J. Lewis, A. Grancea, S.T. Keenan, S.K.H. Lam, C.P. Foley, “2D SQIF arrays using 20,000 YBCO high RN Josephson junctions”, Superconductor Science and Technology, 29, 06LT01 (2016). Abstract.
[6] “The SQUID Handbook, Vol. I Fundamentals and technology of SQUIDs and SQUID systems", eds. John Clarke and Alex I. Braginski (Wiley, 2004).
[7] C.P. Foley, E.E. Mitchell, S.K.H. Lam, B. Sankrithyan, Y.M. Wilson, D.L. Tilbrook, S.J. Morris, "Fabrication and characterisation of YBCO single grain boundary step edge junctions", IEEE Transactions on Applied Superconductivity, 9, 4281 (1999). Abstract.
[8] E.E. Mitchell, C.P. Foley, “YBCO step-edge junctions with high IcRn”, Superconductivity Science and Technology, 23, 065007 (2010). Abstract.
[9] G.V. Prokopenko, O.A. Mukhanov, A. Leese de Escobar, B. Taylor, M.C. de Andrade, S. Berggren, P. Longhini, A. Palacios, M. Nisenoff, R. L. Fagaly, “DC and RF measurements of serial bi-SQUID arrays”, IEEE Transactions on Applied Superconductivity, 23, 1400607 (2013). Abstract.

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Sunday, May 15, 2016

Observation of Genuine One-Way Einstein-Podolsky-Rosen Steering

From Left to Right: Sabine Wollmann, Howard Wiseman, Geoff Pryde.

Sabine Wollmann1, Nathan Walk1,2, Adam J. Bennet1, Howard M. Wiseman1, Geoff J. Pryde1

1Centre for Quantum Computation and Communication Technology and Centre for Quantum Dynamics, Griffith University, Brisbane, Queensland, Australia.
2Department of Computer Science, University of Oxford, United Kingdom.

Quantum entanglement, a nonlocal phenomenon, is a key resource for foundational quantum information and communication tasks, such as teleportation, entanglement swapping and quantum key distribution. The idea of this widely investigated feature was first discussed by Albert Einstein, Boris Podolsky and Nathan Rosen (EPR) in 1935 [1]. In their famous thought experiment they consider a maximally-entangled state shared between two observers, Alice and Bob. Alice makes a measurement on her system and controls Bob’s measurement outcomes by her choice of measurement setting. They concluded from this counterintuitive effect, which Einstein later called ‘spooky action at a distance’, that quantum theory must be incomplete and an underlying hidden variable model must exist. It took another 29 years until it was proven by Bell that there exist predictions of quantum mechanics for which no possible local hidden variable model could account for [2].

Figure 1: Illustration of one-way EPR steering. Alice and Bob share a state which only allows Alice to demonstrate steering.

It was not until recently that the class of nonlocality described by EPR, being intermediate to entanglement witness tests and Bell inequality violations, was formalized by Wiseman et al. [3]. While the previous classes are a symmetric feature - in the sense that, the effects persist under exchange of the parties - this does not necessarily hold for EPR steering. The asymmetry arises because one of the parties is trusted (i.e. their measurements are assumed to be faithfully described by quantum mechanics) and the other is not. In this distinctive class of nonlocality we usually consider a shared state between the two parties Alice and Bob. The question which arises is whether sharing an asymmetric state can result in one-way EPR steering, where Alice can steer Bob, for example, but not the other way around.

This foundational question was first experimentally addressed by Haendchen et al. in 2012, who demonstrated experimentally Gaussian one-way EPR steering with two-mode squeezed states [4]. However their experimental investigation was in a limited context: Gaussian measurements on Gaussian states. As we have shown [5] there exist explicit examples of supposedly one-way steerable Gaussian states actually being two-way steerable for a broader class of measurements. So one could ask, do states exist which are one-way steerable for arbitrary measurements? And the answer is yes. Two independent groups, Nicolas Brunner’s in Geneva and Howard Wiseman’s in Brisbane, proved the existence of such states. Brunner’s approach holds for arbitrary measurements with infinite settings, so called infinite-setting positive-operator-valued measures (POVM), with the cost of using an exotic family of states to demonstrate the effect over an extremely small parameter range, which is unsuitable for experimental observation [6]. David Evans and Howard Wiseman showed one-way steerability exists for projective measurements of more practical, singlet states with symmetric noise - so called Werner states – and loss [7].
Figure 2: Creation of a one-way steerable state (see text for details). One half of a Werner state ρW is sent directly to Alice, whose measurements are described by {Ma|k}, while the other is transmitted to Bob through a loss channel, which replaces a qubit with the vacuum state and is parametrized by probability p. Bob’s measurements are described by {Mb|j}. For differing values of p the final state is unsteerable by Bob for arbitrary projective measurements or arbitrary POVMs. For the same range of p values, Alice can explicitly demonstrate steering via a finite number of Pauli measurements on both sides. She does this by steering Bob’s measurement outcomes so that their shared correlations exceed the upper bound Cn allowed in an optimal local hidden state model.

In our work, recently published in [5], we ask if we can extend the result in Ref. [6] to find a simple state which is steerable in one direction but cannot be steered in the other direction even for the case of arbitrary measurements and infinite settings. For that we consider a shared Werner state
between our observers Alice and Bob. This is a probabilistic mixture of a maximally entangled singlet state with a symmetric noise state parametrised by the mixing probability, or Werner parameter, µ. Using the theorem of Quintino et al. [5] allowed us to construct a state
where the probability p of a vacuum state represents adding asymmetric loss in Bob’s arm. This state is one-way steerable for arbitrary measurements, if we can fulfil the condition
for loss .
Figure 3: In the experimental scheme, Alice and Bob are represented by black and green boxes, respectively. Both are in control of their line and their detectors. The party that is steering is additionally in control of the source. Entangled photon pairs at 820 nm were produced via SPDC in a Sagnac interferometer. Different measurement settings are realized by rotating half- and quarter-wave plates (HWP and QWP) relative to the polarizing beam splitters. A gradient neutral density (ND) filter is mounted in front of Bob’s line to control the fraction of photon qubits passing through. Long pass (LP) filters remove 410 nm pump photons copropagating with the 820 nm photons before the latter are coupled into single-mode fibers and detected by single photon counting modules and counting electronics.

In our experiment we generate an one-way steerable state for projective measurements with a fidelity of (99.6±0.01)% with a Werner state of µ = 0.991±0.003 and insert a filter into Bob’s line to generate the loss = (87±3)%. To demonstrate that Alice remains able to steer Bob’s state, it is necessary to violate the EPR steering inequality. That means measuring a correlation function – the so called steering parameter Sn – which exceeds the classically allowed value. We observe that Alice’s steering parameter of S16 = 0.970±0.004 is 7.3 standard deviations above the classical bound at an heralding efficiency of η = (17.11±0.07)%. The loss of information in Bob’s arm makes him unable to steer the other party. We observe a steering parameter of S16 = 0.963±0.006. In this case, this S value would not have violated a steering inequality even with an infinite number of measurements.

The second one-way steerable regime which we investigate, does still allow Alice to steer Bob’s state but he remains unable to steer hers even by using POVMs. To demonstrate this case, we produce a state with a fidelity of (99.1±0.3)% with a Werner state of µ = 0.978±0.008 and applied a loss p =(99.5±0.3)%. Alice remains able to steer Bob with a steering parameter S16 = 0.951±0.005, being 6.6 standard deviations above the classical bound, at an heralding efficiency of η = (17.17±0.04)%. Bob’s steering parameter S16 = 0.951±0.006 does not violate the inequality and there is no kind of measurement he could choose, even in principle, to be able to steer Alice. We note that the shared state is not exactly a Werner state, but the extremely high fidelities imply, with low probability of error, that the state is only one-way steerable.

Thus, we observe genuine one-way EPR steering for the first time. We note that an independent demonstration was realised in Ref.[8]. While their result is restricted to two measurement settings, our experimental demonstration holds for an arbitrary number of measurements.

[1] A. Einstein, B. Podolsky, N. Rosen, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?", Physical Review, 47, 777 (1935). Abstract.
[2] John S. Bell, “On the Einstein Podolsky Rosen paradox”, Physics, 1, 195 (1964). Full Text.
[3] H. M. Wiseman, S. J. Jones, and A. C. Doherty, “Steering, entanglement, nonlocality, and the Einstein-Podolsky-Rosen paradox”, Physical Review Letters, 98, 140402 (2007). Abstract.
[4] Vitus Händchen, Tobias Eberle, Sebastian Steinlechner, Aiko Samblowski, Torsten Franz, Reinhard F. Werner, and Roman Schnabel, “Observation of one-way Einstein-Podolsky-Rosen steering”, Nature Photonics, 6, 596 (2012). Abstract.
[5] Sabine Wollmann, Nathan Walk, Adam J. Bennet, Howard M. Wiseman, and Geoff J. Pryde, "Observation of Genuine One-Way Einstein-Podolsky-Rosen Steering", Physical Review Letters, 116, 160403 (2016). Abstract.
[6] Marco Túlio Quintino, Tamás Vértesi, Daniel Cavalcanti, Remigiusz Augusiak, Maciej Demianowicz, Antonio Acín, Nicolas Brunner, "Inequivalence of entanglement, steering, and Bell nonlocality for general measurements", Physical Review A, 92, 032107 (2015). Abstract.
[7] D. A. Evans, H. M. Wiseman, "Optimal measurements for tests of Einstein-Podolsky-Rosen steering with no detection loophole using two-qubit Werner states", Physical Review A, 90, 012114 (2014). Abstract.
[8] Kai Sun, Xiang-Jun Ye, Jin-Shi Xu, Xiao-Ye Xu, Jian-Shun Tang, Yu-Chun Wu, Jing-Ling Chen, Chuan-Feng Li, and Guang-Can Guo, “Experimental quantification of asymmetric Einstein-Podolsky-Rosen steering”, Physical Review Letters, 116, 160404 (2016). Abstract. 2Physics Article.


Sunday, April 24, 2016

Demonstrating One-Way Einstein-Podolsky-Rosen Steering in Two Qubits

Some authors of the PRL paper (Reference 6) published on Thursday. From Left to Right: (top row) Kai Sun, Xiang-Jun Ye, Jin-Shi Xu, (bottom row) Jing-Ling Chen, Chuan-Feng Li, Guang-Can Guo.

Authors: Kai Sun1, Xiang-Jun Ye1, Jin-Shi Xu1, Jing-Ling Chen2, Chuan-Feng Li1, Guang-Can Guo1

1Key Laboratory of Quantum Information, CAS Centre for Excellence and Synergetic Innovation Centre in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, China.
2Chern Institute of Mathematics, Nankai University, Tianjin, China.

Asymmetric Einstein-Podolsky-Rosen (EPR) steering is an important “open question” proposed when EPR steering is reformulated in 2007 [1]. Suppose Alice and Bob share a pair of two-qubit states; it is easy to imagine that if Alice entangles with Bob, then Bob must also entangle with Alice. Such a symmetric feature holds for both entanglement and Bell nonlocality [2]. However, the situation is dramatically changed when one turns to a novel kind of quantum nonlocality, the EPR steering, which stands between entanglement and Bell nonlocality. It may happen that for some asymmetric bipartite quantum states, Alice can steer Bob but Bob cannot steer Alice. This distinguished feature would be useful for the one-way quantum tasks. The first experimental verification of one-way EPR steering was performed by using two entangled continuous variable systems in 2012 [3]. However, the experiments demonstrating one-way EPR steering [3,4] are restricted to Gaussian measurements, and for more general measurements, like projective measurements, there is no experiment realizing the asymmetric feature of EPR steering, even though the theoretical analysis has been proposed [5].

Figure 1: Illustration of one-way EPR steering. In one direction (red), EPR steering is realized and this direction is safe for quantum information. In the other direction (blue), steering task fails and this direction is not safe.

Recently, we for the first time quantified the steerability and demonstrated one-way EPR steering in the simplest entangled system (two qubits) using two-setting projective measurements [6]. The asymmetric two-qubit states in the form of ρAB = η |Ψ(θ)⟩⟨Ψ(θ)| + (1-η) |Φ(θ)⟩⟨Φ(θ)|, where 0 ≤ η≤ 1, |Ψ(θ)⟩ = cos ⁡θ |0A 0B⟩ + sin⁡θ |1A 1B⟩, |Φ(θ)⟩ = cos⁡θ |1A 0B⟩ + sin⁡θ |0A 1B⟩, are prepared in this experiment (see Figure 2(a) ). Based on the steering robustness [7], an intuitive criterion R called as “steering radius” is defined to quantify the steerability (see Figure 2 (c) ). The different values of R on two sides clearly illustrate the asymmetric feature of EPR steering. Furthermore, the one-way steering is demonstrated when R > 1 on one side and R < 1 on the other side (see Figure 2 (b)).
Figure 2:  (click on the figure to view with higher resolution)  Experimental results for asymmetric EPR steering. (a) The distribution of the experimental states. The right column shows the entangled states we prepared, and the left column is a magnification of the corresponding region in the right column. The states located in the yellow (grey) regions are predicted to realize one-way (two-way) steering theoretically in the case of two-setting measurements. The blue points and red squares represent the states realizing one-way and two-way EPR steering, respectively. The black triangles represent the states for which EPR steering task fails for both observers. (b) The values of R for the states are labeled in the left column in (a). The red squares represent the situation where Alice steers Bob's system, and the blue points represent the case where Bob steers Alice's system. (c) Geometric illustration of the strategy for local hidden states (black points) to construct the four normalized conditional states (red points) obtained from the maximally entangled state.

For the failing EPR steering process, the local hidden state model, which provides a direct and convinced contradiction between the nonlocal EPR steering and classical physics, is prepared experimentally to reconstruct the conditional states obtained in the steering process (see Figure 3).
Figure 3. (click on the figure to view with higher resolution) The experimental results of the normalized conditional states and local hidden states shown in the Bloch sphere. The theoretical and experimental results of the normalized conditional states are marked by the black and red points (hollow), respectively. The blue and green points represent the results of the four local hidden states in theory and experiment, respectively. The normalized conditional states constructed by the local hidden states are shown by the brown points. Spheres (a) and (c) are for the case in which Alice steers Bob's system, whereas (b) and (d) show the case in which Bob steers Alice's system. The parameters of the shared state in (a) and (b) are θ = 0.442 and η = 0.658; the parameters of the shared state in (c) and (d) are θ = 0.429 and η = 0.819. The spheres (a), (b) and (d) show that the local hidden state models exist, and the steering tasks fail. The sphere (c) Shows that no local hidden state model exists for the steering process with the constructed hidden states located beyond the Bloch sphere and R = 1.076.

The quantification of EPR steering provides an intuitional and fundamental way to understand the EPR steering and the asymmetric nonlocality. The demonstrated asymmetric EPR steering is significant within quantum foundations and quantum information, and shows the applications in the tasks of one-way quantum key distribution [8] and the quantum sub-channel discrimination [7], even within the frame of two-setting measurements.

[1] H. M. Wiseman, S. J. Jones, and A. C. Doherty, “Steering, entanglement, nonlocality, and the Einstein-Podolsky-Rosen paradox”, Physical Review Letters, 98, 140402 (2007). Abstract.
[2] John S. Bell, “On the Einstein Podolsky Rosen paradox”, Physics, 1, 195 (1964). Full Text.
[3] Vitus Händchen, Tobias Eberle, Sebastian Steinlechner, Aiko Samblowski, Torsten Franz, Reinhard F. Werner, and Roman Schnabel, “Observation of one-way Einstein-Podolsky-Rosen steering”, Nature Photonics, 6, 596 (2012). Abstract.
[4] Seiji Armstrong, Meng Wang, Run Yan Teh, Qihuang Gong, Qiongyi He, Jiri Janousek, Hans-Albert Bachor, Margaret D. Reid, and Ping Koy Lam, “Multipartite Einstein-Podolsky-Rosen steering and genuine tripartite entanglement with optical networks”, Nature Physics, 11, 167 (2015). Abstract.
[5] Joseph Bowles, Tamás Vértesi, Marco Túlio Quintino, and Nicolas Brunner, “One-way Einstein-Podolsky-Rosen steering”, Physical Review Letters, 112, 200402 (2014). Abstract.
[6] Kai Sun, Xiang-Jun Ye, Jin-Shi Xu, Xiao-Ye Xu, Jian-Shun Tang, Yu-Chun Wu, Jing-Ling Chen, Chuan-Feng Li, and Guang-Can Guo, “Experimental quantification of asymmetric Einstein-Podolsky-Rosen steering”, Physical Review Letters, 116, 160404 (2016). Abstract.
[7] Marco Piani, John Watrous, “Necessary and sufficient quantum information characterization of Einstein-Podolsky-Rosen steering”, Physical Review Letters, 114, 060404 (2015). Abstract.
[8] Cyril Branciard, Eric G. Cavalcanti, Stephen P. Walborn, Valerio Scarani, and Howard M. Wiseman, “One-sided device-independent quantum key distribution: security, feasibility, and the connection with steering”, Physical Review A, 85, 010301 (2012). Abstract.


Sunday, March 20, 2016

Efficient Long-Distance Heat Transport by Microwave Photons

Research team behind the original discovery (Reference [11] ) from left to right: Tuomo Tanttu, Joonas Goveenius, Mikko Möttönen, Matti Partanen, and Miika Mäkelä (Missing from the figure: Kuan Yen Tan and Russell Lake). Photo Credit: Vilja Pursiainen/Kaskas Media.

Authors: Matti Partanen and Mikko Möttönen

Affiliation: QCD Labs, Department of Applied Physics, Aalto University, Finland.

Link to the Quantum Computing and Devices (QCD) Group >>

Quantum computers are predicted to vastly speed up the computation for certain problems of great practical interest [1]. One of the most promising architectures for quantum computing is based on superconducting quantum bits [2], or qubits, which are the key ingredients in circuit quantum electrodynamics [3]. In such systems, the control of heat at the quantum level is extremely important, and remote cooling may turn out to be a viable option.

In one dimension, heat transport may be described by individual heat conduction channels -- each corresponding to a certain quantized profile of the heat carriers in the transverse direction. Importantly, the maximum heat power flowing in a single channel between bodies at given temperatures is fundamentally limited by quantum mechanics [4,5]. This quantum limit has previously been observed for phonons [6], sub-wavelength photons [7,8], and electrons [9]. Among these, the longest distance of roughly 50 μm [7,8] was recorded in the photonic channel [10]. Such short distance may be undesirable in cooling quantum devices which are sensitive to spurious dissipation.

In our recent work [11], we observe quantum-limited heat conduction by microwave photons flying in a superconducting transmission line of length 20 cm and 1 m. Thus we were able to extend the maximum distance 10,000 fold compared with the previous experiments.
Figure 1: (click on the figure to view with higher resolution) Sample structure and measurement scheme. The electron temperature of the right resistor is controlled with an external voltage while the temperatures of both resistors are measured. Microwave photons transport heat through the spiraling transmission line.

Our sample is shown in Figure 1. The heat is transferred between two normal-metal resistors functioning as black-body radiators to the transmission line [10,12]. To be able to fabricate the whole sample on a single relatively small chip, the transmission line has a double spiral structure. We have measured such spiraling transmission lines without resistors and confirmed that photons travel along the line; they do not jump through vacuum from one end to the other. Thus for heat transport, the distance should be measured along the line.

We measure the electron temperatures of both normal-metal resistors while we change the temperature of one of them [13]. The obtained temperature data agrees well with our thermal model, according to which the heat conduction is very close to the quantum limit.

In contrast to subwavelength distances employed in References [7,8], we need to match the resistance of the normal-metal parts to the characteristic impedance of the transmission line to reach the quantum limit. Furthermore, the transmission line itself has to be so weakly dissipative that almost no photons are absorbed even over distances of about a meter. However, we managed to develop nanofabrication techniques which enabled us to satisfy these conditions well. In fact, the losses in the transmission line are so weak they allow a further increment of the distance by several orders of magnitude.

We consider that long-distance heat transport through transmission lines may be a useful tool for certain future applications in the quickly developing field of quantum technology. If the coupling of a quantum device to a low-temperature transmission line can be well controlled in situ, the device may be accurately initialized without disturbing its coherence properties when the coupling is turned off [14]. Furthermore, the implementation of such in-situ-tunable environments opens an interesting avenue for the study of the detailed dynamics of open quantum systems and quantum fluctuation relations [15].

Acknowledgements: We thank M. Meschke, J. P. Pekola, D. S. Golubev, J. Kokkala, M. Kaivola and J. C. Cuevas for useful discussions, and L. Grönberg, E. Mykkänen, and A. Kemppinen for technical assistance. We acknowledge the provision of facilities and technical support by Aalto University at Micronova Nanofabrication Centre. We also acknowledge funding by the European Research Council under Starting Independent Researcher Grant No. 278117 (SINGLEOUT), the Academy of Finland through its Centres of Excellence Program (project nos 251748 and 284621) and grants (nos 138903, 135794, 265675, 272806 and 276528), the Emil Aaltonen Foundation, the Jenny and Antti Wihuri Foundation, and the Finnish Cultural Foundation.

[1] T.D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, J.L. O'Brien, “Quantum computers”, Nature, 464, 45 (2010). Abstract.
[2] J. Kelly, R. Barends, A.G. Fowler, A. Megrant, E. Jeffrey, T.C. White, D. Sank, J.Y. Mutus, B. Campbell, Yu Chen, Z. Chen, B. Chiaro, A. Dunsworth, I.-C. Hoi, C. Neill, P.J.J. O’Malley, C. Quintana, P. Roushan, A. Vainsencher, J. Wenner, A.N. Cleland, John M. Martinis, “State preservation by repetitive error detection in a superconducting quantum circuit”, Nature, 519, 66 (2015). Abstract.
[3] A. Wallraff, D.I. Schuster, A. Blais, L. Frunzio, R.-S. Huang, J. Majer, S. Kumar, S.M. Girvin, R.J. Schoelkopf, “Strong coupling of a single photon to a superconducting qubit using circuit quantum Electrodynamics”, Nature, 431, 162 (2004). Abstract.
[4] J.B. Pendry, “Quantum limits to the flow of information and entropy”, Journal of Physics A: Mathematical and General, 16, 2161 (1983). Abstract.
[5] Luis G. C. Rego, George Kirczenow, “Fractional exclusion statistics and the universal quantum of thermal conductance: A unifying approach”, Physical Review B, 59, 13080 (1999). Abstract.
[6] K. Schwab, E.A. Henriksen, J.M.Worlock, M.L. Roukes, “Measurement of the quantum of thermal conductance”, Nature, 404, 974 (2000). Abstract.
[7] Matthias Meschke, Wiebke Guichard, Jukka P. Pekola, “Single-mode heat conduction by photons”, Nature, 444, 187 (2006). Abstract.
[8] Andrey V. Timofeev, Meri Helle, Matthias Meschke, Mikko Möttönen, Jukka P. Pekola, “Electronic refrigeration at the quantum limit”, Physical Review Letters, 102, 200801 (2009). Abstract.
[9] S. Jezouin, F.D. Parmentier, A. Anthore, U. Gennser, A. Cavanna, Y. Jin, and F. Pierre, “Quantum limit of heat flow across a single electronic channel”, Science, 342, 601 (2013). Abstract.
[10] D.R. Schmidt, R.J. Schoelkopf, A.N. Cleland, “Photon-mediated thermal relaxation of electrons in nanostructures”, Physical Review Letters, 93, 045901 (2004). Abstract.
[11] Matti Partanen, Kuan Yen Tan, Joonas Govenius, Russell E. Lake, Miika K. Mäkelä, Tuomo Tanttu, Mikko Möttönen, “Quantum-limited heat conduction over macroscopic distances”, Nature Physics, Advance online publication, DOI:10.1038/nphys3642 (2016). Abstract.
[12] L.M.A. Pascal, H. Courtois, F.W.J. Hekking, “Circuit approach to photonic heat transport”, Physical Review B, 83, 125113 (2011). Abstract.
[13] Francesco Giazotto, Tero T. Heikkilä, Arttu Luukanen, Alexander M. Savin, Jukka P. Pekola, “Opportunities for mesoscopics in thermometry and refrigeration: Physics and applications”, Reviews of Modern Physics, 78, 217 (2006). Abstract.
[14] P. J. Jones, J.A.M. Huhtamäki, J. Salmilehto, K.Y. Tan, M. Möttönen, “Tunable electromagnetic environment for superconducting quantum bits”, Scientific Reports, 3, 1987 (2013). Abstract.
[15] Jukka P. Pekola, “Towards quantum thermodynamics in electronic circuits”, Nature Physics, 11, 118 (2015). Abstract.

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Sunday, March 13, 2016

Why Statistical Physics Should Be A Foundation For Materials Design

Marc Miskin

Author: Marc Miskin

James Franck Institute and Department of Physics, The University of Chicago, USA.

The fact that major epochs in human history have been named after materials like bronze, steel, silicon, and stone expresses both how important materials are for technology and how long it can take before a new material is discovered. Even today, the timespan to convert materials' discoveries into functioning technologies takes upwards of 20 years. In part this is because creating technologically useful materials requires selecting a wide range of parameters to optimize the material’s performance. While tools like statistical physics are useful for describing a material’s behavior given a set of parameters, it remains unknown how to generally invert these relationships to target desired behavior. This task is called materials design and it is a new concept at the forefront of materials research.

Recently, several methods have emerged across disciplines that draw upon optimization and simulation to create computer programs that tailor material responses to specified behaviors. However, so far the methods developed either involve black-box techniques, in which the optimizer operates without explicit knowledge of the physical laws that underpin the material’s behavior, or require carefully tuned algorithms with applicability limited to a narrow subclass of materials.

Our recent publication titled “Turning Statistical Physics Models into Materials Design Engines” [1] presents a new perspective for material design. In contrast to prior approaches, it is broad enough to be applied without modification to any system that is well described by statistical mechanics and also retains much of the key insight that is at the heart of statistical physics. In short, our formalism allows a user to transform the capacity to predict material behavior into an optimizer that tunes it.

We achieved this by examining the fundamental relationship between microstate configurations and material properties. Statistical physics poses the idea that materials are intrinsically statistical objects: the properties a bulk material has are best calculated by averaging over all the possible configurations for the material's microscopic parts. Our insight was that design programs should focus on tailoring materials at the level of micro-states themselves, rather than simply focusing on the bulk emergent properties.

For instance, suppose that the pressure and temperature affect the stiffness of a given material and the goal is to set these two parameters to make the stiffest material. The black-box approach is to view this as an optimization problem with pressure and temperature as inputs and stiffness as an output. Yet this view completely ignores the micro-states. A better perspective is to treat the control parameters as means to alter the likelihood of micro-states. To design the material, an algorithm should tweak the parameters so that the material is more likely to be in micro-states with the target behavior.

This idea is the kernel of our approach: we built a formalism out of this concept and tackle materials' design at the level of micro-state information. This gave us a program that is broad enough to address the range of materials that are well described by statistical physics, and we achieve this with a boost in efficiency, thanks to the extra information extracted from the microstate configurations.

To test our approach, we constructed test problems that a good materials optimization scheme should be able to address by itself. Material scientists need optimizers that can solve problems where the search landscape has little variation between candidate materials, juggle multiple potentially competing physical effects, operate in high dimensional search spaces, tune the processing conditions that a material is subject to, and operate when on real-world scale optimization problems. We then translated these challenges into physical test problems, and compared our approach against optimization schemes that we have used successfully for materials design in the past. Given the criterion that the best optimizer is the one that has to make the fewest guesses to arrive at the material that performs a target function, our optimizer outperformed all of our old standards.

Probably the two most fascinating solutions presented in the paper [1] are the polymer folding problem and the directed self-assembly problem. In the polymer folding problem, we asked our optimizer to tune the interaction strengths between 6 beads attached to each other along a linear chain (Figure 1). Because the interactions are attractive, when they are strong enough the chain will fold itself up into a compact shape. The goal here was to make the chain fold into a specific shape: an octahedron. Its an interesting problem because it's well known that simply making all the interactions large will not produce an octahedral geometry. Instead, the interactions needed to be developed into three separate families to generate octahedron and it took hard work from the colloidal self-assembly community to show that this worked. So it was very exciting for us when our optimizer not only produced a virtually identical motif, but managed to yield the result in the span of hours.
Figure 1: Given a polymer of 6 beads each of different color, how should the strength of each short-ranged interaction be picked so that the polymer self-folds into an octahedron geometry? Each image shows a typical polymer configuration obtained at each stage in an optimization using our new approach. The optimizer essentially starts from random, chain like geometries and after ~200 cycles transforms them into the target shape.

There is a similar story behind our directed self-assembly problem. In this case, the material is a polymer made of two types of beads. The goal is to pattern a substrate with a thin strip that has an affinity for a particular one of those two beads (Figure 2). By setting the strip width and the strength of affinity just right, it is possible to make the polymer self-assemble into stripes containing only one polymer type followed by a stripe of the other polymer type and so on. This idea holds serious promise as a next-generation manufacturing technology for semiconductors because the sizes of these stripes are on the order of nanometers. By using the polymer stripes as stencils or masks, it is possible to make next generation circuits or hard drive media with features significantly smaller than what current processing techniques allow. What we found was that not only can our optimizer produce solutions to the problem of tailoring interaction strengths for this kind of directed self-assembly, but that it does so between 5-130X faster than approaches we had tried in the past. To put this into context, solving a directed self assembly problem in the past took us roughly 1 week. Now we can solve them in just under 12 hours.
Figure 2: Given melt of polymer chains each made from half a-type (red) and half b-type (blue) beads, how should one tune the interactions between the a and b beads and a substrate so that the polymer melt self-assembles into equally spaced stripes of a and b? On the top is an image of the original polymer melt configuration for randomly seeded interaction parameters. On the bottom shows the structures that result from using our algorithm to elicit self-assembly. Note the substrate has been colored based on the affinity for each type.

Speaking broadly, materials by design is a radical shift in how we transform bulk matter into useful technology. Historically materials have been either discovered by accident or appropriated from nature to perform technological functions. What we're after is the capacity to systematically identify which materials produce a target response. The benefit of this paradigm is that increasingly complex materials could be cooked up automatically to meet specific technological demands. Our algorithm is a small part of this growing field, but the hope is that it will inspire others to consider their expertise on materials within the new context of design.

Our experience in the past has been that it can be difficult to get started on building design engines for a new material. If the problem isn't posed the right way or the optimizer isn't appropriately tailored, it may require a substantial investment of time to construct an optimization scheme that actually works. What we tried to show in this paper is that our formalism works broadly over a range of very different physical problems without any need for additional modifications. It works out of the box, so to speak, for designing any material that can be simulated using a statistical physics approach. Our hope is that this robustness will translate into a reduction in the time researchers need to spend building design algorithms, and free them up to focus on the task of making exotic materials.

Some of our recent work along these lines can be found here:
[1] Marc Z. Miskin, Gurdaman Khaira, Juan J. de Pablo, Heinrich M. Jaeger, "Turning statistical physics models into materials design engines", Proceedings of the National Academy of Sciences, 113, 34-39 (2016). Full Text.
[2] Marc Z. Miskin, Heinrich M. Jaeger, "Adapting granular materials through artificial evolution", Nature Materials, 12, 326-331 (2013). Abstract.
[3] Marc Z. Miskin, Heinrich M. Jaeger, "Evolving design rules for the inverse granular packing problem", Soft Matter, 10, 3708-3715 (2014). Abstract.
[4] Jian Qin, Gurdaman S. Khaira, Yongrui Su, Grant P. Garner, Marc Miskin, Heinrich M. Jaeger, Juan J. de Pablo, "Evolutionary pattern design for copolymer directed self-assembly", Soft Matter, 9, 11467–11472 (2013). Abstract.

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