Ultra-low noise and high bandwidth bipolar current driver for precise magnetic field control

Scalable Multipartite Entanglement Created by Spin Exchange in an Optical Lattice

Ultracold atoms in optical lattices form a competitive candidate for quantum computation owing to the excellent coherence properties, the highly parallel operations over spins, and the ultralow entropy achieved in qubit arrays. For this, a massive number of parallel entangled atom pairs have been realized in superlattices. However, the more formidable challenge is to scale up and detect multipartite entanglement, the basic resource for quantum computation, due to the lack of manipulations over local atomic spins in retroreflected bichromatic superlattices. In this Letter, we realize the functional building blocks in quantum-gate-based architecture by developing a cross-angle spin-dependent optical superlattice for implementing layers of quantum gates over moderately separated atoms incorporated with a quantum gas microscope for single-atom manipulation and detection. Bell states with a fidelity of 95.6(5)% and a lifetime of 2.20±0.13  s are prepared in parallel, and then connected to multipartite entangled states of one-dimensional ten-atom chains and two-dimensional plaquettes of 2×4 atoms. The multipartite entanglement is further verified with full bipartite nonseparability criteria. This offers a new platform toward scalable quantum computation and simulation.

Ultra-low noise bipolar current source for ultracold atom magnetic system

We report the development of an ultralow-noise bipolar current source based on the configuration of H-bridge current switching. The measured relative current noise fluctuation reaches 4 × 10-9 Hz-1/2, which enables an ultra-stable magnetic system for cold atom experiments. We avoid the influence of the AC leakage currents induced by the large parasitic capacitance of the H-bridge. First, the current sensor is placed as close as possible to the magnetic coils so that the systematic errors from these leakage currents are minimized. Second, the large parasitic capacitance, which parallels the magnetic coils and forms an LC oscillator, is removed from the feedback loop in our setup to maintain a large self-resonance frequency of the feedback control loop. These two improvements lead to a current source that is more precise and less noisy. Remarkably, the lowest current noise density produced by the proposed method is only 500 nA Hz-1/2 at a current of 100 A, which is about ten fold smaller than the case with leakage current. To optimize the feedback control, a numerical simulation is implemented by using Matlab Simulink, and the numerical simulation results are entirely consistent with the experimental results.

ICARUS-Q: Integrated control and readout unit for scalable quantum processors

We present a control and measurement setup for superconducting qubits based on the Xilinx 16-channel radio-frequency system-on-chip (RFSoC) device. The proposed setup consists of four parts: multiple RFSoC boards, a setup to synchronize every digital to analog converter (DAC) and analog to digital converter (ADC) channel across multiple boards, a low-noise direct current supply for tuning the qubit frequency, and cloud access for remotely performing experiments. We also designed the setup to be free of physical mixers. The RFSoC boards directly generate microwave pulses using sixteen DAC channels up to the third Nyquist zone, which are directly sampled by its eight ADC channels between the fifth and the ninth zones.

Dual feedback based bipolar current source with high stability for driving voice coil motors in wide temperature ranges

Bipolar current sources with a stability better than 0.1% in the temperature range of -30 to +70 °C are demanded for driving voice coil motors applied in a new ultra-quiet satellite platform, but almost none of the existing designs satisfy the harsh requirements. This paper presents a possible solution, which is essentially a floating-load, bipolar current source circuit with a dual feedback path. The key circuit is a composite amplifier (co-amp) composed of a high precision amplifier for error correction and a high power amplifier for load driving. The first feedback path comprises a specially designed four-wire current-sense resistor for current-to-voltage conversion and a discrete instrumentation amplifier for amplifying the converted voltage and closing the loop. The second feedback path is a proposed compensation network for loop stability. Error budgets for evaluating current stability and choosing key components of the circuit are comprehensively studied based on a derived rigorous current equation. Loop-stability problems attributable to the inductive load and the high open-loop gain of the co-amp are analyzed, and the proposed dual feedback compensation method is verified by theory, simulation, and measurement. All these contributions are demonstrated by three implemented prototypes with an output of up to ±2 A. The measured results agree well with theoretical predictions. The best and the worst stability performances of the three prototypes at +2 and -2 A are, respectively, 394 and 986 ppm in the temperature range of -30 to +70 °C, which are close to the theoretical value of 776 ppm.

A digital feedback controller for stabilizing large electric currents to the ppm level for Feshbach resonance studies

Magnetic Feshbach resonances are a key tool in the field of ultracold quantum gases, but their full exploitation requires the generation of large, stable magnetic fields up to 1000 G with fractional stabilities of better than 10^-4. Design considerations for electromagnets producing these fields, such as optical access and fast dynamical response, mean that electric currents in excess of 100 A are often needed to obtain the requisite field strengths. We describe a simple digital proportional-integral-derivative current controller constructed using a field-programmable gate array and off-the-shelf evaluation boards that allows for gain scheduling, enabling optimal control of current sources with non-linear actuators. Our controller can stabilize an electric current of 337.5 A to the level of 7.5 × 10^-7 in an averaging time of 10 min and with a control bandwidth of 2 kHz.