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E 3

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Lab room: B167 Birge
Lab phone: (510) 643-8152

Photo of atom chip and cavities

E3: Cavity QED on an Atom Chip

By integrating trapped ensembles of ultracold atoms and high-finesse cavities with an atom chip we are able to study and control the classical and quantum interactions between photons and the internal/external degrees of freedom of the atom ensemble.

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For more details, see Research Highlights.

Schematic of an atom chip on a silicon substrate incorporating a Fabry Perot cavity. A hole is micromachined in the substrate to allow light in the cavity mode to pass through

The Apparatus

Using copper wires embedded in the atom chip we magnetically trap and load an ultracold ensemble of Rb 87 into the cavity.

Cross-Section of chip and cavity. Cavity mirrors are separated by 250 um

We then transfer the ensemble into an optical trap, a very far detuned longitudinal mode of the cavity.

Probe light is coupled to another longitudinal mode and its transmission is recorded exiting the cavity.

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Spin oscillators in a cavity

Spin optodynamics

Coming soon to an optical resonator near you!

Cavity-aided magnetic resonance imaging

The cavity shift is sensitive to the spin state of the atoms. By applying a large magnetic-field gradient, we can spatially address spins in different lattice sites using radio-frequency (RF) radiation.

By chirping the applied RF, we flip the spins in each lattice site via rapid adiabatic passage. Each flip causes a step in the cavity shift. By taking the derivative of the cavity shift vs. time, we obtain a spatial image of the atom density along the cavity axis.

(A) Cavity shift as a function of time as RF is swept across the magnetic resonances of atoms in neighboring lattice sites. (B) Atom density in the lattice, as extracted from the cavity shift profile. (C) Rapid adiabatic passage with no magnetic-field gradient, showing a 14 kHz Rabi frequency. This is much less than the 50 kHz resonance splitting between adjacent wells, giving us 120 nm spatial imaging resolution.

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Tunable optomechanics

Top Schematic of atom-probe coupling for different well locations. Atoms (grey points) sit at the bottom of the FORT potential (black dashed line). Since the FORT wavelength is incommensurate with that of the probe, different wells lead to different levels of coupling (green dots on blue curve). Bottom Experimentally obtained contrast plot highlighting the atom-probe coupling dependence on the location of the atom cloud.

Our experimental setup, based on a micromachined atom chip, allows us to freely position the atoms relative to the probe standing wave. This enables both linear and quadratic optomechanical coupling. The strength of this coupling, along with the mechanical resonator frequency, can be tuned by varying the intracavity probe field intensity and its detuning from atomic resonance.

Optomechanical resonators generally consist of solid-state devices. Our atom-based resonator does not suffer from many of the drawbacks found in typical solid-state systems, such as significant environmental couplings, thermal occupation of the mechanical resonator mode, and optomechanical parameters fixed during device fabrication.

The ensemble behaves similarly to a dispersive piece of glass, changing the effective length of the cavity.

Top Schematic of a piece of glass inside a cavity. Bottom In our experiments, the trapped ensemble provides the phase shift

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Optomechanics in a superlattice

With two optical dipole traps of incommensurate wavelengths, we create an array of loading sites with differing mechanical resonance frequencies, allowing us to separate spectrally the responses of atomic motion from neighboring sites.

This mechanical resonance separation technique has allowed us to apply and measure forces with unprecedented sensitivity and to couple mechanical oscillators together via the cavity probe field. For the latter experiment, we can observe the state of each oscillator evolve with or without coupling. Because the photonic coupling field can be measured as it exits the cavity, this kind of coupling necessarily introduces measurement backaction. In order to measure both the coupling-induced energy transfer and the coincident backaction, we acquire a statistical ensemble of many realizations of the coupling measurement.

Atoms trapped in optical superlattice modulate the probe at distinct frequencies.

Statistical ensemble of measured states for oscillator 1 (top) and 2 (bottom), without (left) and with (right) coupling and the resulting backction.

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Ponderomotive squeezing

The collective motion of the atoms' center of mass can act to suppress quantum noise fluctuations of the probe light. This suppression is called ponderomotive squeezing.

When the main source of noise on our probe light is photon shot noise, the optomechanical system is dominantly driven by these quantum fluctuations in radiation pressure. In such a system, by detuning our probe light from cavity resonance we close the gain loop and allow the optomechanical response to interfere constructively and destructively with input optical fluctuations, leading to spectral windows of amplification and of sub-shot-noise squeezing.

Power spectral densities of optomechanical response on a full scale (A) and magnified about the region of shot noise (B), in the phase-modulation (PM; red) and amplitude-modulation (AM; blue) quadratures (solid, data; dashed, theory).

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