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We observe and study a special ground state of bosons with two spin states in an optical lattice: the spin-Mott insulator, a state that consists of repulsively bound pairs that is insulating for both spin and charge transport. Because of the pairing gap created by the interaction anisotropy, it can be prepared with low entropy and can serve as a starting point for adiabatic state preparation. We find that the stability of the spin-Mott state depends on the pairing energy, and observe two qualitatively different decay regimes, one of which exhibits protection by the gap.

We investigate the generation of entanglement (spin squeezing) in an optical-transition atomic clock through the coupling to an optical cavity in its vacuum state. We show that if each atom is prepared in a superposition of the ground state and a long-lived electronic excited state, and viewed as a spin-1/2 system, then the collective vacuum light shift entangles the atoms, resulting in a squeezed distribution of the ensemble collective spin, without any light applied. This scheme reveals that even an electromagnetic vacuum can constitute a useful resource for entanglement and quantum manipulation. By rotating the spin direction while coupling to the vacuum, the scheme can be extended to implement two-axis twisting resulting in stronger squeezing., National Science Foundation (U.S.), MIT-Harvard Center for Ultracold Atoms, United States. Defense Advanced Research Projects Agency, United States. National Aeronautics and Space Administration., United States. Office of Naval Research, United States. Office of Naval Research. Multidisciplinary University Research Initiative, United States. Army Research Office. Multidisciplinary University Research Initiative, Natural Sciences and Engineering Research Council of Canada

We demonstrate a new way to extend the coherence time of separated Bose-Einstein condensates that involves immersion into a superfluid bath. When both the system and the bath have similar scattering lengths, immersion in a superfluid bath cancels out inhomogeneous potentials either imposed by external fields or inherent in density fluctuations due to atomic shot noise. This effect, which we call superfluid shielding, allows for coherence lifetimes beyond the projection noise limit. We probe the coherence between separated condensates in different sites of an optical lattice by monitoring the contrast and decay of Bloch oscillations. Our technique demonstrates a new way that interactions can improve the performance of quantum devices., Samsung Scholarship Foundation, National Science Foundation (U.S.) (MIT-Harvard Center for Ultracold Atoms. Grant 1506369), United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative (Grants FA9550-14-1-0035 and W911NF-14-1-0003)

Protocols for attaining quantum degeneracy in atomic gases almost exclusively rely on evaporative cooling, a time-consuming final step associated with substantial atom loss. We demonstrate direct laser cooling of a gas of rubidium-87 (⁸⁷Rb) atoms to quantum degeneracy. The method is fast and induces little atom loss. The atoms are trapped in a two-dimensional optical lattice that enables cycles of compression to increase the density, followed by Raman sideband cooling to decrease the temperature. From a starting number of 2000 atoms, 1400 atoms reach quantum degeneracy in 300 milliseconds, as confirmed by a bimodal velocity distribution. The method should be broadly applicable to many bosonic and fermionic species and to systems where evaporative cooling is not possible.

We experimentally demonstrate the strictly nonclassical behavior in a many-atom system using a recently derived criterion [E. Kot et al., Phys. Rev. Lett. 108, 233601 (2012)] that explicitly does not make use of quantum mechanics. We thereby show that the magnetic-moment distribution measured by McConnell et al. [Nature (London) 519, 439 (2015)] in a system with a total mass of 2.6×10[superscript 5] atomic mass units is inconsistent with classical physics. Notably, the strictly nonclassical behavior affects an area in phase space 10[superscript 3] times larger than the Planck quantum ℏ., National Science Foundation (U.S.), United States. Defense Advanced Research Projects Agency, United States. National Aeronautics and Space Administration, United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative, United States. Army Research Office. Multidisciplinary University Research Initiative, Seventh Framework Programme (European Commission) (ERC Grant QIOS 306576)

We propose a versatile and efficient method to generate a broad class of complex entangled states of many atoms via the detection of a single photon. For an atomic ensemble contained in a strongly coupled optical cavity illuminated by weak single- or multifrequency light, the atom-light interaction entangles the frequency spectrum of a transmitted photon with the collective spin of the atomic ensemble. Simple time-resolved detection of the transmitted photon then projects the atomic ensemble into a desired pure entangled state. This method can be implemented with existing technology, yields high success probability per trial, and can generate complex entangled states such as mesoscopic superposition states of coherent spin states with high fidelity., National Science Foundation (U.S.), United States. Defense Advanced Research Projects Agency. Quantum-Assisted Sensing and Readout (QuASAR) Program, United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative, United States. Army Research Office. Multidisciplinary University Research Initiative, Natural Sciences and Engineering Research Council of Canada

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Cataloged from student-submitted PDF version of thesis., Includes bibliographical references (pages 137-142)., Engineering light at the single photon level is a long-standing goal in physics. In this thesis, we investigate optical nonlinearities at the single photon level orchestrated by an optical cavity. We demonstrate an all-optical transistor where one stored 'gate' photon controls the propagation of a 'source' light beam. The system is based on EIT and a highcooperativity cavity. Each 'gate' photon is converted to an EIT polariton and strongly suppresses the cavity transmission of subsequently applied source photons. One stored gate photon can be retrieved from the atomic ensemble after switching off more than one source photon. Without retrieval, one stored gate photon can switch up to several hundreds source photons. Next, instead of storing the photon, we let it propagate slowly and operate the system in the continuous mode. We observe an anti-cross-correlation between freespace and cavity beams. The cross-correlation function at zero time is g2 = 0:89 which means a photon in one beam extinguishes one photon in the other beam with a probability of 11%. It is a deterministic photon-photon interaction at the single photon level. Using similar skills, we propose a new method to measure zero or one photon fock states, without destroying the photons. We are improving the setup which is approaching the non-destructive measurement of photons. Potentially, this method will enable new applications in quantum information and computation. It will also enable the observation of different internal degrees of the photons, such as polarization or photon number, at the same time without losing the photons. By reducing the power to zero, we investigate the nonlinearity of the vacuum field in the optical cavity. By replacing the control beam in EIT by the vacuum field, we observe vacuum induced transparency (VIT). This effect indicates that vacuum field strongly distorts the dispersion of atoms in a way that cannot be explained by classical electromagnetism. In order to reach even str, by Wenlan Chen., Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Cataloged from student-submitted PDF version of thesis., Includes bibliographical references (pages 137-142)., Engineering light at the single photon level is a long-standing goal in physics. In this thesis, we investigate optical nonlinearities at the single photon level orchestrated by an optical cavity. We demonstrate an all-optical transistor where one stored 'gate' photon controls the propagation of a 'source' light beam. The system is based on EIT and a highcooperativity cavity. Each 'gate' photon is converted to an EIT polariton and strongly suppresses the cavity transmission of subsequently applied source photons. One stored gate photon can be retrieved from the atomic ensemble after switching off more than one source photon. Without retrieval, one stored gate photon can switch up to several hundreds source photons. Next, instead of storing the photon, we let it propagate slowly and operate the system in the continuous mode. We observe an anti-cross-correlation between freespace and cavity beams. The cross-correlation function at zero time is g2 = 0:89 which means a photon in one beam extinguishes one photon in the other beam with a probability of 11%. It is a deterministic photon-photon interaction at the single photon level. Using similar skills, we propose a new method to measure zero or one photon fock states, without destroying the photons. We are improving the setup which is approaching the non-destructive measurement of photons. Potentially, this method will enable new applications in quantum information and computation. It will also enable the observation of different internal degrees of the photons, such as polarization or photon number, at the same time without losing the photons. By reducing the power to zero, we investigate the nonlinearity of the vacuum field in the optical cavity. By replacing the control beam in EIT by the vacuum field, we observe vacuum induced transparency (VIT). This effect indicates that vacuum field strongly distorts the dispersion of atoms in a way that cannot be explained by classical electromagnetism. In order to reach even str, by Wenlan Chen., Ph. D.

The realization of an all-optical transistor, in which one “gate” photon controls a “source” light beam, is a long-standing goal in optics. By stopping a light pulse in an atomic ensemble contained inside an optical resonator, we realized a device in which one stored gate photon controls the resonator transmission of subsequently applied source photons. A weak gate pulse induces bimodal transmission distribution, corresponding to zero and one gate photons. One stored gate photon produces fivefold source attenuation and can be retrieved from the atomic ensemble after switching more than one source photon. Without retrieval, one stored gate photon can switch several hundred source photons. With improved storage and retrieval efficiency, our work may enable various new applications, including photonic quantum gates and deterministic multiphoton entanglement., National Science Foundation (U.S.), United States. Air Force Office of Scientific Research, National Science Foundation (U.S.). Graduate Research Fellowship (0645960), Austrian Science Fund (Doctoral Program CoQuS W1210)

Deterministic photon-photon interactions are a long-standing goal in optical science. Using an atomic ensemble inside a cavity, we demonstrate the mutual cross modulation of two continuous light beams at the level of individual photons. The originally uncorrelated beams derived from independent lasers become anticorrelated, as evidenced by an equal-time cross-correlation function g[superscript (2)] = 0.89(1), showing that one photon in one beam extinguishes a photon in the other beam with a probability of 11(1)%. With further technical improvements, our approach should enable the nondestructive continuous detection of traveling optical photons., National Science Foundation (U.S.), MIT-Harvard Center for Ultracold Atoms, United States. Air Force Office of Scientific Research. Quantum Memories Multidisciplinary University Research Initiative, United States. Army Research Office. QuISM Multidisciplinary University Research Initiative, National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 0645960)

Photons are excellent information carriers but normally pass through each other without consequence. Engineered interactions between photons would enable applications as varied as quantum information processing and simulation of condensed matter systems. Using an ensemble of cold atoms strongly coupled to an optical cavity, we found that the transmission of light through a medium may be controlled with few photons and even by the electromagnetic vacuum field. The vacuum induces a group delay of 25 nanoseconds on the input optical pulse, corresponding to a light velocity of 1600 meters per second, and a transparency of 40% that increases to 80% when the cavity is filled with 10 photons. This strongly nonlinear effect provides prospects for advanced quantum devices such as photon number–state filters., National Science Foundation (U.S.) (Grant PHY-0855052), National Science Foundation (U.S.) (Grant PHY-0551153), United States. Army Research Office (Defense Advanced Research Projects Agency. QuASAR Program)

The realization of an all-optical transistor, in which one “gate” photon controls a “source” light beam, is a long-standing goal in optics. By stopping a light pulse in an atomic ensemble contained inside an optical resonator, we realized a device in which one stored gate photon controls the resonator transmission of subsequently applied source photons. A weak gate pulse induces bimodal transmission distribution, corresponding to zero and one gate photons. One stored gate photon produces fivefold source attenuation and can be retrieved from the atomic ensemble after switching more than one source photon. Without retrieval, one stored gate photon can switch several hundred source photons. With improved storage and retrieval efficiency, our work may enable various new applications, including photonic quantum gates and deterministic multiphoton entanglement., National Science Foundation (U.S.), United States. Air Force Office of Scientific Research, National Science Foundation (U.S.). Graduate Research Fellowship (0645960), Austrian Science Fund (Doctoral Program CoQuS W1210)

We present a novel technique to phase-lock two lasers with controllable frequency difference. In our setup, one sideband of a current modulated Vertical-Cavity Surface-Emitting Laser (VCSEL) is phase locked to the master laser by injection seeding, while another sideband of the VCSEL is used to phase lock the slave laser. The slave laser is therefore locked in phase with the master laser, with a frequency difference tunable up to about 35 GHz. The sideband suppression rate of the slave laser is more than 30dB at 30 uW seed power. The heterodyne spectrum between master and slave has a linewidth of less than 1 Hz. A coherent population trapping resonance of rubidium is achieved using such beams., Comment: 4 pages, 4 Encapsulated PostScript figures