Colloquium Archives (before June 2017)

More recent colloquia are posted on the Physics & Astronomy Colloquia page.

Parker Fagrelius, UC Berkeley

Title: "The Dark Energy Spectroscopic Instrument (DESI): Creating a 3D Map of the Universe" (Video)

Abstract:  The Dark Energy Spectroscopic Instrument (DESI) will be installed on the Mayall 4-m telescope at Kitt Peak National Observatory in 2018. Using 5,000 robotic fiber positioners and ten 3-arm spectrographs, DESI will measure the spectra of up to 40 million galaxies and quasars enabling us to create a 3D map of the Universe out to redshift 3.5. This map of the Large Scale Structure (LSS) of the Universe will enable us to measure Baryon Acoustic Oscillations (BAO), a standard ruler that emerges from primordial fluctuations in the Early Universe. Using BAO and Redshift Space Distortions (RSD), the DESI project will constrain our knowledge of the time evolution of Dark Energy, which is assumed to make up more that 70% of the energy density in the current universe. This talk will review the science goals and instrument design for the DESI project, highlighting recent demonstrations of its capabilities.

Lily Childress, McGill University

Title: "Fabry-Perot Microcavities for Diamond Photonics" (Video)

Abstract:  Individual defects in crystalline materials can have electronic properties akin to those of isolated trapped atoms or ions.  Like atoms, these defect centers can have spin degrees of freedom and and optical transitions that make them an attractive platform for building quantum information technologies. Their spin states might someday be used to store and manipulate quantum information, with photons connecting individual defects into a useful computational network or secure communication system. A major challenge to this vision is the efficiency of the photonic interface, particularly for emitters such as the nitrogen-vacancy (NV) center in diamond, whose optical transitions couple strongly to strain, phonons, or electric fields. This talk will present results on the path toward creating a high-efficiency spin-photon interface for NV centers using fiber-based optical microcavities. These cavities promise a complementary approach to nanophotonic devices, with reduced diamond fabrication requirements and very high quality factors. 



Michael Johnson, Harvard University

Title: "Imaging Magnetic Fields at the Event Horizon of a Black Hole" (Video)

Abstract: Magnetic fields play a central role in the accretion, emission, and outflow near black holes but have never been directly observed in this region. Yet, because linear polarization of the bright synchrotron emission from galactic cores traces these magnetic fields, polarimetric interferometry with the Event Horizon Telescope (EHT) is capable of imaging these near-horizon fields. I will present EHT observations of the Galactic Center supermassive black hole, Sgr A*. These observations, the first to resolve the polarized emission from Sgr A* at any wavelength, revealed ordered magnetic fields with vigorous activity near the event horizon. I will also discuss the emerging capabilities of the EHT to study turbulence driven by the magnetorotational instability, the role of magnetic fields in jet launching, and signatures of magnetically-dominant regions near supermassive black holes. 

Lisa Barsotti, MIT

Title: "The Dawn of Gravitational Wave Astronomy with LIGO" (Video)

Abstract: On September 14, 2015, the two Advanced detectors of the Laser Interferometer Gravitational Wave Observatory (LIGO) detected gravitational waves from a binary black hole system, opening the era of gravitational wave astronomy. The signal, denoted GW150914, swept in frequency from 35Hz to 250Hz, and had a peak gravitational-wave strain of 10^-21. This event marked the first direct detection of gravitational waves and the first observation of a binary black hole merger. A few months later, in December 2015, a second gravitational wave event from a binary black hole system was detected.

In this talk I will discuss gravitational waves, their sources, and how they can be detected. I will then review the science results from the first Observing Run O1 that ended in January 2016, and summarize the current status of the Advanced LIGO detectors. I will conclude by describing plans and prospects for the future.

Elisabeth Newton, MIT

Title: "Spin and Magnetism in Small Stars" (Video)

Abstract:  Main sequence stars with masses below approximately 0.35 solar masses (red dwarfs, or M dwarfs) are fully-convective, and are expected to have a different type of dynamo mechanism than solar-type stars. These low-mass stars are the most common type of star in the galaxy, but a lack of observational constraints at ages beyond 1 Gyr has hampered studies of rotational evolution and magnetic activity. To address this, we have made new measurements of rotation and magnetic activity in nearby, field-age M dwarfs. I will discuss the relationships we see between age, rotation, and activity, and what they mean for rotational evolution, starspot properties, and the magnetic dynamo. Upcoming ground-based instruments and space-based surveys offer exciting prospects for continuing the study of cool stars, as well as the exoplanets that orbit them.

David Kaiser, MIT

Title: “Testing Bell’s Inequality with Astrophysical Observations” (Video)

Abstract:  Albert Einstein once dubbed quantum entanglement "spooky action at a distance," and the concept remains one of the starkest examples of how quantum theory differs from our usual intutions about space, time, and matter. Physicists have tested Bell’s inequality experimentally for over four decades, and have always found results consistent with quantum theory; today entanglement is at the heart of next-generation devices like quantum computers and quantum encryption. Yet every experimental test to date has been subject to one or more "loopholes," which could possibly account for the results even in the absence of genuine quantum entanglement. This talk describes the latest experimental tests of quantum entanglement, including a new series of experiments that uses some of the oldest light in the universe to address the last major loophole and pave the way for a genuinely loophole-free test of Bell’s inequality.