The 8th Gotham-Metro Condensed Matter Meeting: Tapping NYC's Science & Engineering Talent

Journal Articles

Bluhm H, Foletti S, Neder I, et. al. Dephasing time of GaAs electron-spin qubits coupled to a nuclear bath exceeding 200µs. Nature Physics. 2011;7:109-13.

Foletti S, Bluhm H, Mahalu D, Umansky V, Yacoby A. Universal quantum control of two-electron spin quantum bits using dynamic nuclear polarization. Nature Physics. 2009;5:903-08.

Furchtgott L, Wingreen NS, Huang KC. Mechanisms for maintaining cell shape in rod-shaped gram-negative bacteria. Mol Microbiol. 2011;81(2):340-53.

Huang KC, Ehrhardt DW, Shaevitz JW. The molecular origins of chiral growth in walled cells. Curr Opin Microbiol. 2012;15(6):707-14.

Huang KC, Mukhopadhyay R, Wen B, Gitai Z, Wingreen NS. Cell shape and cell-wall organization in gram-negative bacteria. Proc Natl Acad Sci U S A. 2008;105(49):19282-7.

Shulman MD, Dial OE, Harvey SP, Bluhm H, Umansky V, Yacoby A. Demonstration of entanglement of electrostatically coupled singlet-triplet qubits. Science. 2012;336(6078):202-5.

Wang S, Furchtgott L, Huang KC, Shaevitz JW. Helical insertion of peptidoglycan produces chiral ordering of the bacterial cell wall. Proc Natl Acad Sci U S A. 2012;109(10):E595-604.

van Teeffelen S, Wang S, Furchtgott L, et. al. The bacterial actin MreB rotates, and rotation depends on cell wall assembly. Proc Natl Acad Sci U S A. 2011;108(38):15822-7.

Speaker:
Amir Yacoby, Harvard University

Highlights

• Scientists now envision a new era of computer—a quantum computer—based upon the pure quantum nature of materials.
• Crucial steps toward building a quantum computer would be creating, maintaining, and detecting coherent quantum bits.
• Reliably making and measuring entanglement between two quantum bits is a milestone in quantum computing.

Quantum information processing and metrology using few electron spins in solids

Advances in computers are central to the achievements of modern technology. Scientists now envision a new era of computer—a quantum computer—based upon the pure quantum nature of materials. There is still much to be done to realize this goal, which could provide us with revolutionary progress in information processing. Quantum computers rely on the linear superposition of states, a fundamental of quantum mechanics. Unlike conventional digital computers, quantum computers represent data as a linear superposition of basis states, and thus are able to extract more information from a single operation. It is estimated that a quantum computer would take N^1/2 operations to guess a password of N combinations, whereas a digital computer takes N operations.

One hindrance to creating a quantum computer is the difficulty of maintaining coherence among quantum bits (often realized by electron spins), the building blocks of such a computer. Since a system is always coupled to complicated environments, it is difficult to reliably manipulate and maintain coherent and entangled quantum states. Amir Yacoby from Harvard University presented new advances in manipulating and measuring quantum bits.

Although a single-electron system is the simplest quantum system, manipulation of such a system is fairly difficult. Two- or few-electron systems appear to be easier to manipulate to create desired quantum states, at the price of complicated Hilbert space, in which all possible electron spin configurations live. However, it has been shown that using a double well in the solid state material GaAs (Gallium arsenide) could enable isolation of a few quantum states (out of all possible states) in Hilbert space. First, two electrons are confined in one side of the well. Because of the Pauli principle, the ground state of the two electrons forms a singlet state. Then, one electron is moved to the other side by tilting the double well without changing the electrons' spin configuration; thus, two separated spins remain in the spin singlet state, not excited to the higher-energy triplet state. This procedure allows for a reliable determination of whether the initially created two-electron system is in a spin-singlet or a spin-triplet state.

Creating a spin-singlet state of two spatially separated electrons in a double well. Initially, two electrons are confined in one side of a double well so that their ground state is a spin singlet state (left). By applying an electric field to tilt the well, it is possible to move an electron from one side to the other without changing spin configuration (right). (Image courtesy of Amir Yacoby)

One advantage of this procedure is that it is possible to tilt the double well by simply applying an electric field, which is much easier to tune than a magnetic field. Furthermore, by coupling electron spins to nuclear spins, one can pump spins and convert spin information to charge configuration, which is much easier to measure. This allows for precise control of the spin system and maintains coherence for up to hundreds of micro seconds. This also suggests that it may be possible to create an entanglement, another key element in quantum information, between two two-spin systems. Entanglement between quantum states dates back to the famous paradox proposed by Einstein, Podolsky, and Rosen (the EPR paradox). It is a mechanism to "link" spatially separated quantum states and imprint these in a density matrix. Access to a full density matrix of entangled states verifies what type of entanglement exists between two systems.

The new approach being tried by Yacoby's group has shown that it is possible to initialize, control, and detect a two-level system with 10 nT sensitivity and 100 nm spatial resolution. Many steps remain before a quantum computer could be realized, but progress is being made.

Speaker:
KC Huang, Stanford University

Highlights

• The shape of a bacterium is primarily determined by the shape of its stiff cell wall, but the mechanism by which cell shape is formed and preserved is not well understood.
• The cell wall may be the dominant mechanical player in the growth of a bacterium, and MreB may be a key factor in orchestrating cell wall synthesis.

The physics of bacterial cell growth

KC Huang from Stanford University gave the keynote soft matter presentation, in which he explored how bacteria determine and maintain their size and shape during growth. Interestingly, although cell shape has been correlated to function for many decades, the mechanism by which cell shape is formed and preserved has eluded both biologists and physicists.

The shape of a bacterium is primarily determined by the shape of its stiff cell wall. This cell wall is a network of glycan strands (sugars) cross linked with peptides, which is meant to withstand high turgor pressure. Understanding how the cell wall is built so as to maintain the symmetry of the original structure during growth is key to understanding bacteria shape. Huang focused on cell wall construction in rod-shaped bacteria such as E. coli. To study this system, Huang modeled the cell wall as a network of springs representing peptides and glycans. Physical parameters such as spring constants, orientations, and bending moduli were estimated from existing literature. The problem was then transformed into a course-grained molecular dynamics (MD) problem with a global force field from the turgor pressure. Huang went on to describe several educated guesses that could be tested with the model to determine how material is added to the cell wall. The first idea was to add material randomly to every available site on the preexisting cell wall. This model showed that such a construction mechanism quickly leads to large deformations that strongly deviate from the original rod-like shape. Local fluctuations in the insertion of new material lead to local fluctuations in wall density.

Huang hypothesized that these fluctuations are the root of the deformations observed. The next step was therefore to normalize for wall density fluctuations such that material would be added to different sites without bias toward areas where more material already existed. Impressively, the modified model maintained cell shape extremely well. The hypothesis that follows is that there is a biological factor that performs such a normalization, thus allowing a bacterium to deal with small deformations due to density variations without disrupting overall cell shape and size.

What could such a factor be? A protein that recently gained attention in the bacterial mechanics community is MreB, which is a homolog of actin, a cytoskeletal protein in eukaryotic cells. MreB occupies left-handed helical patterns around E. coli cells and is well conserved among rod-shaped bacteria. It has also been found to contribute to cell rigidity. Huang surmised that MreB is a key player in orchestrating cell wall synthesis. He simulated cell wall construction by adding new material along helical paths around the cylindrical portion of a rod-shaped cell. This patterning allowed for faithful cell-shape maintenance during growth and revealed a left-handed twist during growth. Such twisting was confirmed by experiments, with quantitative agreement in the twist angle between computation and experiment.

Although Huang's simulations do not include the effects of the cytoplasm, the membrane, and many other biological complexities of cells, the agreement of his model with experiments on shape maintenance was very good. This suggests that the cell wall is the dominant mechanical player in the growth of a bacterium. Additionally, Huang's work supports the hypothesis that MreB is a key factor in orchestrating cell wall synthesis, and his models provide a framework for evaluating mechanisms of bacterial cell wall growth.

The model of cell wall growth that adds new material along helical paths around the cell reproduces both cell shape maintenance and twisting as observed in experiments. (Image courtesy of KC Huang)

Moderator:
Seth Pinsky, New York City Economic Development Corporation

Panelists:
Shelley A. Harrison, Coller Capital
Constantine E. Kontokosta, NYU Center for Urban Science and Progress
Rajit Manohar, Cornell University
Kathleen R. McKeown, Institute for Data Sciences and Engineering, Columbia University
Michael O'Boyle, IBM

Generation Tech: Tapping NYC's Science & Engineering Talent was the topic of the meeting's panel discussion, moderated by Seth Pinsky, president of the New York City Economic Development Corporation and featuring Shelley Harrison from Coller Capital, Constantine E. Kontokosta from NYU Center for Urban Science and Progress, Rajit Manohar from Cornell University, Kathleen McKeown from the Institute for Data Sciences and Engineering at Columbia University, and Michael O'Boyle from IBM. The panelists discussed the application of scientific models to build better cities.

Pinsky opened by introducing the need for NYC to tap into its scientific resources, especially to repair damaged infrastructure in the wake of Hurricane Sandy. This effort should harness new ideas, engineering, and technology. He also outlined Mayor Bloomberg's Applied Sciences NYC initiative, which connects different science and technology institutions and aims to make NYC a leader in innovation in order to stimulate economic growth. Each of the panelists described the developmental model of their project. McKeown's institute is working to harness the colossal amount of data generated in today's world; she mentioned how data-processing technology can impact fields like environmental health, medicine, finance, and cyber security. The institute intersects with those fields via the School of Engineering and Applied Science, the Medical School, the Business School, and four other centers across Columbia's campus. Its goal is to diversify and improve NYC's economy. Kontokosta explained that his institution, CUSP, is built on a "triple helix" model, with academic institutions at the core bolstered by private industry and government agencies. Their focus is on managing cities and data (i.e., urban informatics), with a particular investment in a global collaborative effort across physical, computer, and social science disciplines.

O'Boyle spoke about IBM's collaboration with academics to find strategies for managing the complexity of a big city, "a system of systems." He emphasized IBM's dedication to research that is applicable in practice. One example of this is an effort to understand the effects of weather on transportation, the power grid, public safety, and other city variables; the importance of such work became obvious during Hurricane Sandy. Manohar explained the "partnership model" Cornell is developing with Technion–Israel Institute of Technology, which aims to develop a unique applied-science campus in NYC offering an integrated program with training in both deeply technical fields such as electrical and computer engineering as well as entrepreneurship and commercialization. This training will also be grounded on transient hubs that are sensitive to economic needs. This means that the degrees offered and training provided will change based on economic and market development. Harrison explained his perspective of NYC as a "living lab"—a space in which it is possible to use physical and technical sciences to learn the problems of the city and model potential solutions. He emphasized Coller Capital's efforts to facilitate conversation between large corporations and small start-ups in order to foster success in large, complex cities like NYC.

Pinsky asked the panelists to comment on opportunities for young researchers in NYC and on what makes these opportunities unique in this decade. McKeown mentioned that Columbia offers semester-long internships as well as two new training programs in data sciences. Kontokosta pointed out NYU's various degrees for science-inclined students who want to apply their skills in an urban setting. O'Boyle explained IBM's interest in fostering opportunities for young scientists, especially those who are open to applying their skills in innovative settings. Manohar posited that today's students are motivated by the real-world impact of their work. He emphasized the need to attract and keep those students in science.