Solid state quantum computation session @CTCMP2015

Theme: Topological quantum computation

Speaker: J. K. Pachos, Univ. of Leeds, UK 9.25 – 10.25 AM

Title: A useful Superconductor

Abstract: In this talk I will present tight-binding models of three-dimensional superconductors that support a variety of topological phases. I will show that gapless Majorana surface states emerge at their boundary in agreement with the bulk-boundary correspondence. At the presence of a Zeeman field the surface states become gapped and the boundary behaves as a two-dimensional topological superconductor. Importantly, the topological character of the two dimensional boundary in induced, and protected, by the one in the three dimensional bulk. Hence, this boundary can be used for the robust realization of localized Majorana zero modes that can be employed for topological quantum computation.

Speaker: K. Sengupta, IACS Kolkata, India 10.45 – 11.45 AM

Title: Majorana Fermions

Abstract: In recent years there have been several suggested realisations of Majorana fermions in solid state systems. In this talk, I shall review the history of this development which dates back to realisation of mid gap states in 1D organic compounds. In this talk, I shall also explain the pros and cons of two major detection all schemes of these objects and briefly allude to current experimentally interesting platforms for their realisation.

Speaker: G. Baskaran, IMSc Chennai, India 11.50 – 12.50 AM

Title: P-Wave Superconductivity in an Extremely Correlated Metal in 2 Dimensions

Abstract: Quantum phases of matter that support topological excitations such as `spinon' or `Majorana fermions' are being studied vigorously recently, from the point of view of topological quantum computation. Two dimensional p + ip chiral superconductor is known to support Majorana zero mode in its vortex core. Several years ago we theoretically predicted possibility of p-wave superconductivity in the currently popular $Sr_2RuO_4$. Very recently along with Zhengcheng Gu and Hong-Chen-Jang we have predicted (arXiv:1408.6820) possibility of p + i p chiral superconductivity in infinite-U repulsive Hubbard model on a honeycomb lattice, coexisting with Nagaoka ferromagnetism. We will discuss physics behind this unexpected phase and possible experimental realization.

Theme: Quantum Biology

Speaker: K. L. Sebastian, IISc Bangalore, India 2.30 – 3.30 PM

Title: Coherences, Photosynthesis and Quantum Biology?

Abstract: Photosynthesis in plants involves the absorption of photon by a light-harvesting pigment and the subsequent transfer of energy from the absorption centre to the reaction centre. Reports by Fleming and coworkers in 2007-10 propose a quantum-mechanical, coherent, wave-like transfer of excitation among the chromophores. The studies suggest that biological systems make use of quantum coherence for efficient transport and has lead to the suggestion of the newly emerging area of quantum biology. Wave like transfer is surprising because the numerous degrees of freedom of the protein scaffold surrounding the complex would interact with it and cause decoherence, which is expected to be rapid. In our studies, we attempt to understand the mechanism of the excitation transfer in photosynthetic complexes. We suggest a new and analytical method of studying coherence in finite level systems coupled to the environment and find that it is not just the coherence but an interplay between coherence and the dissipative influence of the environment which helps in propagation of the excitation. Understanding the mechanism of this would hopefully help us avoid decoherence which is the most undesired thing in quantum computing.

[1] Bhattacharyya, P.; Sebastian, K.L., Phys..Rev.E 2013, 87, 062712.

[2] Bhattacharyya, P.; Sebastian, K.L., J. Phys. Chem. A 2013, 117, 8806.

Speaker: Jayendra Bandyopadhyay, BITS Pilani, India 3.45-4.45 PM

Title: Quantum coherence in biological processes: Case of avian magnetoreception

Abstract: Migratory birds and some other species have the ability to navigate by sensing the geomagnetic field. Recent experiments indicate that the essential process in the navigation takes place in the bird’s eye and uses chemical reaction involving molecular ions with unpaired electron spins (radical pair). Sensing is achieved via geomagnetic-dependent dynamics of the spins of the unpaired electrons. Here we utilize the results of two behavioral experiments conducted on European robins to argue that the average lifetime of the radical pair is of the order of few microseconds and therefore agrees with experimental estimations of this parameter for cryptochrome—a pigment believed to form the radical pairs. We also find a reasonable parameter regime where the sensitivity of the avian compass is enhanced by environmental noise, showing that long coherence time is not always required for better navigation.

Speaker: Swaroop Ganguly, IIT Bombay, India 4.50 – 5.50 PM

Title: Spin Dynamics and Decoherence in Radical Pair Model of Avian Magnetoreception

Abstract: Avian magnetoreception is the geomagnetic field assisted navigation ability of several bird species. Behavioural studies suggest that the radical pair (RP) model describes its underlying mechanism [1, 2]. This involves a photo-excited pair of electron spins interacting with an anisotropic hyperfine nuclear environment and the geomagnetic Zeeman field [2]. One of the challenges in the RP model is to understand the role of coherence in avian magnetoreception, and in particular, the effect of nuclear and environmental decoherence in the spin dynamics. Building on prior studies [3-6], we take a microscopic view of RP spin dynamics – specifically, the state transitions involved in RP spin state evolution and analyse the roles of nuclear and environmental decoherence in the magneto-sensitive behaviour through their effect on these state transitions. Further, our conclusions are validated by applying an information theoretic measure of coherence. This state transition point of view affords new insight into the collaborative roles of the Zeeman and hyperfine interactions in making the RP spin dynamics magneto-sensitive.

[1] K. Schulten, et al., "A biomagnetic sensory mechanism based on magnetic field modulated coherent electron spin motion." Zeitschrift für Physikalische Chemie 111.1, : 1-5, (1978).