Laser probing of macroscopic bodies - Nondestructive quantum measurement

Zhenni Li, Chang‐shui Yu, Shu-Xue Ding +1 more

Xianpeng Zhang, Chuanchang Zeng, Zhen‐Biao Yang +2 more

Nondemolition protocols use ancilla qubits to identify the fragile quantum state of a qubit without destroying its encoded information, thus playing a crucial role in nondestructive quantum measurements particularly relevant for quantum error correction. However, the multitude of ancilla preparations, information transfers, and ancilla measurements in these protocols create an intrinsic overhead for information processing. Here we consider an Andreev qubit defined in a quantum-dot Josephson junction and show that the macroscopic time-dependent oscillatory supercurrent arising from the quantum interference of the many-body eigenstates, can be used to probe the qubit itself-arbitrarily close to the nondestructive limit-under currently available experimental capabilities. This readout of arbitrary superposition states of Andreev qubits avoids ancillae altogether and significantly reduces experimental overhead as no repetitive qubit reinitialization is needed. Our prediction of an AC-like Josephson effect without an applied external voltage, which enables the nondestructive qubit readout, is a unique macroscopic manifestation of the microscopic dynamics of the Andreev quantum state. Our findings should have an unprecedented impact on advancing research and applications involving Andreev dots, thus positioning them as promising qubit contenders for quantum processing and technologies.

Markus Aspelmeyer

Vladimir P Bykov

Patrick Windpassinger, Daniel Oblak, Plamen G. Petrov +7 more

We report on the nondestructive observation of Rabi oscillations on the Cs clock transition. The internal atomic state evolution of a dipole-trapped ensemble of cold atoms is inferred from the phase shift of a probe laser beam as measured using a Mach-Zehnder interferometer. We describe a single color as well as a two-color probing scheme. Using the latter, measurements of the collective pseudospin projection of atoms in a superposition of the clock states are performed and the observed spin fluctuations are shown to be close to the standard quantum limit.

Xin Wang, Yuan Sun, Liang Liu

Optical detection and manipulation of the thermal properties is an essential subject of cold atoms in the quantum era. For laser cooled alkali atoms, we have experimentally realized deterministic temperature measurement with time cost below 1 ms and effective filtering of colder atoms with temperature less than 1 μK, with the help of nearly nondestructive detection. The quick temperature measurement is accomplished by carefully resolving the diffusion dynamics of atoms with the information provided by a single probe laser pulse in the form of bucket detection, while suppressing the amplitude and phase noises of probe laser. The separation of colder atoms is attainable as the velocity differences of atoms translate into nontrivial position differences, when the diffusion sustains for a few tens of milliseconds. In particular, these efforts are based on a labeling process that distinguishes the cold atoms under study from the others by specific internal states, while the nearly nondestructive detection is implemented via driving a cycling transition with continuous optical pulses. Moreover, such a position-dependent labeling process can be further modified to become velocity-dependent, with which we have demonstrated a Maxwell’s demon-type operation on cold atoms, as Maxwell’s demon’s intricate abilities can be understood as measuring the velocity of an individual particle and then performing feedback according to a straightforward dichotomy of the velocity value.