Session J02: Quantum Optics

Live

We study the effects of 2-beam coupling on quantum imaging in a four-wave mixing medium. In our previous work [1] we demonstrated sub-10 Hz bright intensity-difference squeezed light from dual-seeded four-wave mixing (4WM) in Rb vapor. In this work we have observed excess noise at frequencies below the natural linewidth of Rb due to the two-beam coupling mechanism [2]. This noise, which destroys the quantum correlations between the beams, can be avoided by making sure that the input seeds do not intersect each other in the pump region. The problem is similar in the case of generating quantum-correlated images. We can eliminate this problem by imaging the seed into the 4WM region, rather than focusing it. That is, amplifying in the imaging plane rather than the Fourier plane. With sub-10 Hz squeezed light and "cross-talk" free imaging, we are closer to taking pixel-by-pixel quantum correlated images via 4WM with a CCD camera. [1] M.-C. Wu, et al., Optics Express 27, 4769 (2019). [2] M. Kauranen, et al., Phys. Rev. A 50, R929 (1994).   

Live

We will present our study on spatial quantum noise decomposition which allows us to image different quantum noise structures and manipulate the beam to optimize overall squeezing. Precision measurements are limited by intrinsic noise because of quantum uncertainty. This noise appears in two different quadratures - phase and amplitude quadratures. The noise quadratures obey the Heisenberg uncertainty principle, which sets the standard quantum limit (SQL), so we can reduce noise in one of these quadratures (at the expense of increasing noise in the other). Light with noise suppression in one of the quadratures below the SQL is called squeezed light. Squeezed light yields significant improvement of signal-to-noise ratios in many applications including precision metrology and optical communication. Squeezing, however, is not generated uniformly throughout the beam. To use these squeezed beams to our maximal advantage, we are developing methods that allow for spatial mode decomposition of the quantum beam. Advances in spatial detection and control of squeezed beams is of particular interest to optical communication technologies since it would allow quantum information transfer on individual spatial modes.   

Live

Steady-state superradiant lasers based on incoherent pumping have been shown to be promising candidates for coherent light sources of ultra-narrow linewidth. However, the incoherent pumping process can lead to many experimental difficulties due to radiative heating and other adverse effects. Here we propose a new type of superradiant laser based on a hot atomic beam. This design may be more straightforward to realize in experiments than in situ repumping, but is also rich in novel collective quantum physics. Specifically, we consider three models of the superradiant beam laser in this work. We first study a benchmark ``tight collimated model'' and show, both theoretically and numerically, that the superradiant beam laser is indeed a \textit{superradiant laser} in terms of first and second order temporal correlations and superradiant emission. We then explore a mono-velocity model to show quantum phase synchronization as the transverse velocities of atoms decrease below a phase transition point. Finally we show that with a hot atomic beam, the system will recover the benchmark superradiance below the phase transition point.   

Live

The Einstein-Podolsky-Rosen (EPR) energy-time entangled photons (biphotons) are of great interest for long-distance quantum communication and quantum network. Direct characterization of EPR energy-time entanglement requires joint correlation measurements in both time and frequency domains and remains a challenge. In this work, we produce narrowband (1.8 MHz) biphotons from spontaneous four-wave mixing in cold 85Rb atoms. The temporal correlation and uncertainties are measured by commercial single-photon counting modules. We map the biphoton joint spectrum and energy uncertainties using a narrow linewidth (72 kHz) optical cavity. We obtain the joint frequency-time uncertainty product as low as 0.063$+$/-0.0044, which not only violates the separability criterion but also satisfies the continuous variable EPR steering inequality. Our result of joint frequency-time uncertainty product is significantly smaller than the previously reported values and pushes its lower bound a step closer to zero. The work was supported by the Hong Kong Research Grants Council (Project No. 16304817), and the William Mong Institute of Nano Science and Technology (Project No. WMINST19SC05).   

Live

As a resonant photon passes through a cloud of two-level atoms, it weakly saturates the atomic transition, modifying the index of refraction of the cloud, which can be measured as a nonlinear cross-phase shift (XPS) on a second, off-resonant beam. In cases where a photon is observed at a detector on the far side of the cloud, one might conclude that it had not been absorbed and that there should therefore be no XPS. In our experiment on absorptive optical nonlinearities in cold $^{\mathrm{85}}$Rb, we use post-selection to isolate the phase shift imparted by a transmitted photon. We find that despite not having been absorbed, transmitted photons impart a significant fraction (0.77$+$/-0.17) of the nonlinear XPS of the average incident photon, raising questions about the relationship between atomic excitation, absorption, and scattering at the fully quantized level.   

Live

The reduced noise properties of squeezed light make it an ideal quantum state for quantum-enhanced metrology based on optical sensors. To extend its applicability to atomic-based sensors, squeezed light that can be tuned to and around atomic resonance is needed. While we have previously shown that it is possible to generate resonant two-mode squeezed light using four-wave mixing (FWM) in atomic Rb vapor, its tunabilty was limited by atomic absorption. To overcome this limitation, we have designed a vacuum chamber with internal electrodes that can be used to apply a large electric field to a rubidium vapor cloud. Our system can support electric fields of the order of 10 MV/m for a Rb number density of $\sim 10^{16}/{\rm m}^{-3}$, which leads to a DC stark energy level shift of 500 MHz for the D1 transition. Furthermore, for the number densities required for the FWM process ($\sim 10^{18}/{\rm m}^{-3}$) the system can support electric fields of the order of 7 MV/m. This allows us to tune the frequency of the squeezed light by as much as 250 MHz while preserving a level of 4dB of squeezing. This dressed energy level approach can also enable a tunable single-mode vacuum squeezed state source via polarized self-rotation and a tunable narrowband optical filter near atomic resonance.   

On Demand

Hanbury Brown--Twiss interference and stimulated emission, two fundamental processes in atomic physics, have been studied in a wide range of applications in science and technology. We study interference effects that occur when a weak probe is sent through a gas of two-level atoms that are prepared in a singly excited collective (Dicke or ``superatom'') state and for atoms prepared in a factorized state. We measure the time-integrated second-order correlation function of the output field as a function of the delay between the input probe field and radiation emitted by the atoms and find that, for the Dicke state, is twice as large for zero delay as it is for large delays, while for the product state, this ratio is equal to 3/2. The results agree with those of a theoretical model in which any effects related to stimulated emission are totally neglected---the coincidence counts measured in our experiment arise from Hanbury Brown--Twiss interference between the input field and the field radiated by the atoms.   

On Demand

We investigate phase-matched scattering from an array of atoms that are confined in optical tweezers in one- and two-dimensional geometries. For a linear chain, we observe phase-matched reflective scattering in a cone about the symmetry axis of the array that scales as the square of the number of atoms in the chain. For two linear chains of atoms, the phase-matched reflective scattering is enhanced or diminished as a result of Bragg scattering. Such scattering can be used for mapping collective states within an array of neutral atoms onto propagating light fields and for establishing quantum links between separated arrays.   

Experimental Mapping of Correlations of Structured Two Mode Squeezed Twin Beams

We experimentally explore spatial and temporal correlations between two-mode squeezed twin beams which carry diverse spatial mode structure and are generated in hot Rb vapor via four-wave mixing. The phase matching conditions in FWM describe coherence areas in which correlations are expected, even for diverse mode structures. However, light with complicated structure is never truly single mode in nature and the coherence areas are complicated. In order to probe correlations of varying modes between the twin beams, we utilize machine learning and Monte-Carlo methods. This knowledge could allow for further enhancement of quantum imaging and quantum communications.   

Theory of an on-chip Josephson quantum michomaser

Solid-state superconducting qubit systems are one of the most promising systems to achieve quantum computing. One of the shortcomings of this architecture is the lack of an on-chip coherent microwave source. To solve this problem, we explore the feasibility of building a Josephson micromaser powered by tunable superconducting transmon qubit(s) (which serve as an artificial three-level atom). Specifically, we explain how to engineer a system composed of two qubits (one a conventional transmon, the other a transmon with a SNAIL element) to construct an element that behaves like a 3-level atom coupled to a dissipative bath. We construct a master equation description of the maser and estimate its properties, like its coherence time, and their dependence on the pump power, pump noise, cavity widths, etc. We show that the linewidth of the micromaser approaches the Schawlow-Townes (ST) limit with feasible experimental parameters. We further notice that the nonlinear couplings between the superconducting qubit and the cavity can suppress the linewidth beyond the ST limit. Finally, we note that the possibility for highly non-linear devices in the microwave regime allows our maser to generate quantum (i.e. non-Gaussian) light.