Rydberg atom electric field sensing for metrology, communication and hybrid quantum systems.

Rydberg atoms-based electric field sensing has developed rapidly over the past decade. A variety of theoretical proposals and experiment configurations are suggested and realized to improve the measurement metrics, such as intensity sensitivity, bandwidth, phase, and accuracy. The Stark effect and electromagnetically induced transparency (EIT) or electromagnetically induced absorption (EIA) are fundamental physics principles behind the stage. Furthermore, various techniques such as amplitude- or frequency-modulation, optical homodyne read-out, microwave superheterodyne and frequency conversion based on multi-wave mixing in atoms are utilized to push the metrics into higher levels. In this review, different technologies and the corresponding metrics they had achieved were presented, hoping to inspire more possibilities in the improvement of metrics of Rydberg atom-based electric field sensing and broadness of application scenarios.

Einstein-Podolsky-Rosen Energy-Time Entanglement of Narrow-Band Biphotons.

We report the direct characterization of energy-time entanglement of narrow-band biphotons produced from spontaneous four-wave mixing in cold atoms. The Stokes and anti-Stokes two-photon temporal correlation is measured by single-photon counters with nanosecond temporal resolution, and their joint spectrum is determined by using a narrow linewidth optical cavity. The energy-time entanglement is verified by the joint frequency-time uncertainty product of 0.063±0.0044, which does not only violate the separability criterion but also satisfies the continuous variable Einstein-Podolsky-Rosen steering inequality.

Hybrid superconductor-atom quantum interface with Raman chirped shortcut to adiabatic passage.

Realization of the highly efficient hybrid atom-photon gates is vital to the quantum interface that integrates atoms and superconducting resonators. Here we propose a scheme to realize the hybrid state transfer and controlled-PHASE gate based on Raman chirped shortcut to adiabatic passage. The scheme is fast to protect the quantum state from the decoherence effects in the hybrid interface, as well as is robust due to the geometric phase. We show that this two-qubit gate is more resilient than the Raman pulse and Raman chirped adiabatic passage against the variations in the vacuum coupling strength and two-photon detuning. Its fast and robust features make it especially suitable for long-term storage and optical readout of superconducting qubits, and moreover, entanglement swapping between two disparate components.

δ-Quench Measurement of a Pure Quantum-State Wave Function.

The measurement of a quantum state wave function not only acts as a fundamental part in quantum physics but also plays an important role in developing practical quantum technologies. Conventional quantum state tomography has been widely used to estimate quantum wave functions, which usually requires complicated measurement techniques. The recent weak-value-based quantum measurement circumvents this resource issue but relies on an extra pointer space. Here, we theoretically propose and then experimentally demonstrate a direct and efficient measurement strategy based on a δ-quench probe: by quenching its complex probability amplitude one by one (δ quench) in the given basis, we can directly obtain the quantum wave function of a pure ensemble by projecting the quenched state onto a postselection state. We confirm its power by experimentally measuring photonic complex temporal wave functions. This new method is versatile and can find applications in quantum information science and engineering.

Geometric Rydberg quantum gate with shortcuts to adiabaticity.

We propose a controlled-PHASE gate for neutral atoms in which one of the qubit state components adiabatically evolves along the multiple-atom eigenstate formed by the chirped laser pulse coupling to Rydberg states and intrinsic dipole-dipole exchange interactions and, consequently, accumulates an interaction-induced geometric phase. The geometric Rydberg gate is not limited by an adiabatic condition, which is sped up by shortcuts to adiabaticity (STA). Analyses show that an STA scheme is more robust than a non-adiabatic case against the variations of control parameters and faster than an adiabatic case, which protects from the decay of Rydberg states. Furthermore, an intermediate value of dipole-dipole interaction strength is enough for our scheme.

Experimental test of the no-go theorem for continuous ψ-epistemic models.

Quantum states are the key mathematical objects in quantum theory; however, there is still much debate concerning what a quantum state truly represents. One such century-old debate is whether a quantum state is ontic or epistemic. Recently, a no-go theorem was proposed, stating that the continuous ψ-epistemic models cannot reproduce the measurement statistic of quantum states. Here we experimentally test this theorem with high-dimensional single photon quantum states without additional assumptions except for the fair-sampling assumption. Our experimental results reproduce the prediction of quantum theory and support the no-go theorem.

Subnatural-linewidth polarization-entangled photon pairs with controllable temporal length.

We demonstrate an efficient experimental scheme for producing polarization-entangled photon pairs from spontaneous four-wave mixing (SFWM) in a laser-cooled (85)Rb atomic ensemble, with a bandwidth (as low as 0.8 MHz) much narrower than the rubidium atomic natural linewidth. By stabilizing the relative phase between the two SFWM paths in a Mach-Zehnder interferometer configuration, we are able to produce all four Bell states. These subnatural-linewidth photon pairs with polarization entanglement are ideal quantum information carriers for connecting remote atomic quantum nodes via efficient light-matter interaction in a photon-atom quantum network.