Feedback control of persistent-current oscillation based on the atomic-clock technique

We propose a scheme of stabilizing the persistent-current Rabi oscillation based on the flux qubit-resonator-atom hybrid structure. The LC resonator weakly interacts with the flux qubit and maps the persistent-current Rabi oscillation onto the intraresonator electric field. This field is further coupled to a Rydberg-Rydberg transition of the $^{87}$Rb atom. The Rabi-frequency fluctuation of the flux qubit is deduced from measuring the atomic population and stabilized via feedback controlling the external flux bias. Our numerical simulation indicates that the feedback-control method can efficiently suppress the background fluctuations in the flux qubit, especially in the low-frequency limit. This technique may be extensively applicable to different types of superconducting circuits, paving a new way to long-term-coherence superconducting quantum information processing.Comment: 4 figure

Theoretical Description of Micromaser in the Ultrastrong-Coupling Regime

We theoretically investigate an ultrastrongly-coupled micromaser based on Rydberg atoms interacting with a superconducting LC resonator, where the common rotating-wave approximation and slowly-varying-envelope approximation are no longer applicable. The effect of counter-rotating terms on the masing dynamics is studied in detail. We find that the intraresonator electric energy declines and the microwave oscillation frequency shifts significantly in the regime of ultrastrong coupling. Additionally, the micromaser phase fluctuation is suppressed, resulting in a reduced spectral linewidth.Comment: 10 pages, 3 figure

Charge-Qubit-Resonator-Interface-Based Nonlinear Circuit QED

We explore applications of nonlinear circuit QED with a charge qubit inductively coupled to a microwave LC resonator in the photonic engineering and ultrastrong-coupling multiphoton quantum optics. Simply sweeping the gate-voltage bias achieves arbitrary Fock-state pulsed maser, where the single qubit plays the role of artificial gain medium. Resonantly pumping the parametric qubit-resonator interface leads to the squeezing of resonator field, which is utilizable to the quantum-limited microwave amplification. Moreover, upwards and downwards multiphoton quantum jumps may be observed in the steady state of the driving-free system.Comment: 3 figure

Topological pumping in Aharonov-Bohm rings

Topological Thouless pumping and Aharonov-Bohm effect are both fundamental effects enabled by the topological properties of the system. Here, we study both effects together: topological pumping of interacting particles through Aharonov-Bohm rings. This system can prepare highly entangled many-particle states, transport them via topological pumping and interfere them, revealing a fractional flux quantum. The type of the generated state is revealed by non-trivial Aharonov-Bohm interference patterns that could be used for quantum sensing. The reflections induced by the interference result from transitions between topological bands. Specific bands allow transport with a band gap scaling as the square-root of the particle number. Our system paves a new way for a combined system of state preparation and topological protected transport.Comment: to be published in Communications Physic

Stabilizing Rabi Oscillation of a Charge Qubit via Atomic Clock Technique

We propose a superconducting circuit-atom hybrid, where the Rabi oscillation of single excess Cooper pair in the island is stabilized via the common atomic-clock technique. The noise in the superconducting circuit is mapped onto the voltage source which biases the Cooper-pair box via an inductor and a gate capacitor. The fast fluctuations of the gate charge are significantly suppressed by an inductor-capacitor resonator, leading to a long-relaxation-time Rabi oscillation. More importantly, the residual low-frequency fluctuations are further reduced by using the general feedback-control method, in which the voltage bias is stabilized via continuously measuring the dc-Stark-shift-induced atomic Ramsey signal. The stability and coherence time of the resulting charge-qubit Rabi oscillation are both enhanced. The principal structure of this Cooper-pair-box oscillator is studied in detail.Comment: 4 figure

The Aharonov-Bohm effect in mesoscopic Bose-Einstein condensates

Ultra-cold atoms in light-shaped potentials open up new ways to explore mesoscopic physics: Arbitrary trapping potentials can be engineered with only a change of the laser field. Here, we propose using ultracold atoms in light-shaped potentials to feasibly realize a cold atom device to study one of the fundamental problems of mesoscopic physics, the Aharonov-Bohm effect: The interaction of particles with a magnetic field when traveling in a closed loop. Surprisingly, we find that the Aharonov-Bohm effect is washed out for interacting bosons, while it is present for fermions. We show that our atomic device has possible applications as quantum simulator, Mach-Zehnder interferometer and for tests of quantum foundation.Comment: 5 pages, 5 figures to be published in Physical Review A Rapid Communication

Atomoptik und Quanteninformationsverarbeitung mit mikrostrukturierten optischen Elementen

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Relaxation of Rabi Dynamics in a Superconducting Multiple-Qubit Circuit

We investigate a superconducting circuit consisting of multiple capacitively-coupled charge qubits. The collective Rabi oscillation of qubits is numerically studied in detail by imitating environmental fluctuations according to the experimental measurement. For the quantum circuit composed of identical qubits, the energy relaxation of the system strongly depends on the interqubit coupling strength. As the qubit-qubit interaction is increased, the system's relaxation rate is enhanced firstly and then significantly reduced. In contrast, the inevitable inhomogeneity caused by the nonideal fabrication always accelerates the collective energy relaxation of the system and weakens the interqubit correlation. However, such an inhomogeneous quantum circuit is an interesting test bed for studying the effect of the system inhomogeneity in quantum many-body simulation.Comment: 15 pages, 5 figure

Readout of the atomtronic quantum interference device

A Bose-Einstein condensate confined in ring shaped lattices interrupted by a weak link and pierced by an effective magnetic flux defines the atomic counterpart of the superconducting quantum interference device: the atomtronic quantum interference device (AQUID). In this paper, we report on the detection of current states in the system through a self-heterodyne protocol. Following the original proposal of the NIST and Paris groups, the ring-condensate many-body wave function interferes with a reference condensate expanding from the center of the ring. We focus on the rf-AQUID which realizes effective qubit dynamics. Both the Bose-Hubbard and Gross-Pitaevskii dynamics are studied. For the Bose-Hubbard dynamics, we demonstrate that the self-heterodyne protocol can be applied, but higher-order correlations in the evolution of the interfering condensates are measured to readout of the current states of the system. We study how states with macroscopic quantum coherence can be told apart analyzing the noise in the time of flight of the ring condensate