Mishkatul Bhattacharya

Lasers find ubiquitous use in physics due to their coherence, spectral purity and high intensity. However, the output of a laser does not carry crucial resources such as squeezing and entanglement, and the generation of such properties requires the use of nonlinear media. In the electromagnetic domain, the nonlinearities are weak, resulting in sources of squeezing and entanglement that are not bright. In this work, we exploit the fact that strong nonlinearities can be readily implemented in a mechanical system to enable simultaneous lasing and two-mode squeezing, producing a bright and coherent source of classically correlated phonons: a two-mode thermomechanically squeezed phonon laser. Our device is experimentally implemented by combining nonlinear damping with parametric modulation of the coupling between two center-of-mass oscillation modes of an optically levitated nanoparticle at the sum of their mechanical frequencies. Above the linear stability threshold of the system, we observe parametrically sustained oscillation and subthermal two-mode squeezing coexisting with mechanical lasing. Our device is a substantial addition to the ongoing revolution in optical tweezer physics, important to studies of nonequilibrium systems in mesoscopic classical thermodynamics, and a necessary stepping stone in the longer-term development of a quantum mechanical extension.

We theoretically propose a photonic orbital angular momentum quantum memory, currently of great interest from the perspective of quantum networks, based on a ring-trapped Bose-Einstein condensate interacting with Laguerre-Gaussian beams. The optical states are stored in the large Hilbert space of topologically protected persistent currents of the condensate. In contrast to earlier work, our proposal uses a cavity, which enhances the light-matter interaction by several orders of magnitude, resulting in high fidelity, convenient detection, and no need to repeat read-write cycles degraded by atom loss. Our scheme avoids using multiple internal atomic states as in generic electromagnetically induced transparency-based protocols and thus bypasses problems like off-resonant photoassociation by the control fields and dephasing effects that limit memory storage time. For optimized parameters, our work yields a storage time 3 orders of magnitude better than presently available and opens the way for exploiting existing advantages of cavity-based noise suppression, wavelength transduction, large bandwidth, and nondestructive readout of the memory.

The Josephson effect presents a fundamental example of macroscopic quantum coherence as well as a crucial enabler for metrology (e.g., voltage standard), sensing (e.g., superconducting quantum interference device), and quantum information processing (Josephson qubits). Recently, there has been a major renewal of interest in the effect, following its observation in Bose, Fermi, and dipolar atomic condensates, in exciton-polariton condensates, and in momentum space. We present theoretically a nondestructive, , and real time protocol for observing the ac and dc Josephson effects including integer (recently observed in cold atoms) and fractional (hitherto unobserved in cold atoms) Shapiro steps, using a ring condensate coupled to an optical cavity. Our analysis presents a single-shot metrology standard that does not require measurement of atomic number and that challenges the conventional wisdom that quantum computations cannot be observed without being destroyed. Our results have implications for the fields of atomtronics, sensing, metrology, and quantum information processing.

The Andreev-Bashkin (AB) effect, corresponding to the dissipationless dragging of one superfluid by another, was predicted in the early 1970s but has so far eluded experimental detection. In this work, we theoretically introduce a detection paradigm and show that it enables the observation of the hitherto undetected AB effect for realistic parameters. We accomplish this by using the powerful techniques of cavity optomechanics, which were crucial to the observation of gravitational waves, on a spinor ring Bose-Einstein condensate. In contrast to established AB detection methods, our scheme allows for real-time, , minimally destructive and three orders-of-magnitude-more-sensitive measurement of the AB effect. Our proposal, which considers persistent currents in weakly repulsive atomic condensates and amplifies the AB signal using a dynamic Bragg spectroscopy technique, is supported by numerical simulations of the stochastic Gross-Pitaevski equation, which agree very well with our analytic Bogoliubov–de Gennes calculations. Our work suggests a tool for sensitively and nondestructively probing the dynamics of rotationally interacting superfluids using cavities and has fundamental implications for ongoing studies of superfluid hydrodynamics, atomtronics, matter-wave interferometry, and cavity optomechanical sensing.