Back-Reaction of Clocks and Limitations on Observability in Closed Systems

Autonomous quantum machines and the finite sized Quasi-Ideal clock

Processes such as quantum computation, or the evolution of quantum cellular automata are typically described by a unitary operation implemented by an external observer. In particular, an interaction is generally turned on for a precise amount of time, using a classical clock. A fully quantum mechanical description of such a device would include a quantum description of the clock whose state is generally disturbed because of the back-reaction on it. Such a description is needed if we wish to consider finite sized autonomous quantum machines requiring no external control. The extent of the back-…

Entanglement of quantum clocks through gravity

In general relativity, the picture of spacetime assigns an ideal clock to each worldline. Being ideal, gravitational effects due to these clocks are ignored and the flow of time according to one clock is not affected by the presence of clocks along nearby worldlines. However, if time is defined operationally, as a pointer position of a physical clock that obeys the principles of general relativity and quantum mechanics, such a picture is at most a convenient fiction. Specifically, we show that the general relativistic mass-energy equivalence implies gravitational interaction between the clocks…

Improving Short-Term Stability in Optical Lattice Clocks by Quantum Nondemolition Measurements

We propose a multimeasurement estimation protocol for Quantum Nondemolition (QND) measurements in a Rabi clock interferometer. The method is well suited for current state-of-the-art optical lattice clocks with QND measurement capabilities. The protocol exploits the correlations between multiple non-destructive measurements of the initially prepared coherent spin state. A suitable Gaussian estimator for the clock laser detuning is presented, and an analytic expression for the sensitivity of the protocol is derived. We use this analytic expression to optimise the protocol using available experim…

Page-Wootters形式论中的时间膨胀与质心归一化

Time Dilation and center of Mass Normalization in the Page-Wootters Formalism

该研究在Page-Wootters量子力学框架下，揭示复合量子钟的相对论效应，发现内部能量对质心的反作用会引发时间非局域的薛定谔方程，还得到干涉可见度的二次方能量修正项，可通过差分测量分离。

关于“作为多信使天文学奇异场望远镜的量子传感器网络”的评论

Comment on ''Quantum sensor networks as exotic field telescopes for multi-messenger astronomy''

本文指出Dailey等人的工作忽略了普通物质对二次相互作用标量波的反作用，考虑该效应会导致标量波在地球表面被强烈屏蔽并延缓其在星际介质中的传播，从而使时钟网络探测失效并阻碍多信使天文学。

Wigner's friend and the quasi-ideal clock

In 1962, Eugene P. Wigner introduced a thought experiment that highlighted the incompatibility in quantum theory between unitary evolution and wave function reduction in a measurement. This work resulted in a class of thought experiments often called Wigner's Friend Scenarios, which have been providing insights over many frameworks and interpretations of quantum theory. Recently, a no-go theorem obtained by Daniela Frauchiger and Renato Renner brought attention back to the Wigner's Friend and its potential of putting theories to test. Many answers to this result pointed out how timing in the t…

Direct comparison of two spin squeezed optical clocks below the quantum projection noise limit

Building scalable quantum systems that demonstrate genuine performance enhancement based on entanglement is a major scientific goal for fields including computing, networking, simulations, and metrology. The tremendous challenge arises from the fragility of entanglement in increasingly larger sized quantum systems. Optical atomic clocks utilizing a large number of atoms have pushed the frontier of measurement science, building on precise engineering of quantum states and control of atomic interactions. However, today's state-of-the-art optical atomic clocks are limited by the quantum projectio…

Measurement of many-body chaos using a quantum clock

There has been recent progress in understanding chaotic features in many-body quantum systems. Motivated by the scrambling of information in black holes, it has been suggested that the time dependence of out-of-time-ordered (OTO) correlation functions such as $\langle O_2(t) O_1(0) O_2(t) O_1(0) \rangle $ is a faithful measure of quantum chaos. Experimentally, these correlators are challenging to access since they apparently require access to both forward and backward time evolution with the system Hamiltonian. Here, we propose a protocol to measure such OTO correlators using an ancilla which …

Proposal for a solid-state magnetoresistive Larmor quantum clock

We propose a solid-state implementation of the Larmor clock that exploits tunnel magnetoresistance to distill information on how long itinerant spins take to traverse a barrier embedded in it. Keeping in mind that the tunnelling time innately involves pristine pre-selection and post-selection, our proposal takes into account the detrimental aspects of multiple reflections by incorporating multiple contacts, multiple current measurements and suitably defined magnetoresistance signals. Our analysis provides a direct mapping between the magnetoresistance signals and the tunneling times and aligns…

静止量子钟中的共振延迟：提升阈值掩模

Resonant delay in a stationary quantum clock: Lifting the threshold mask

本文通过引入阈值减除的静止佩雷斯钟观测量，从散射相位能量导数中分离出低能阈值项与共振延迟，解决了静止量子钟在连续谱边缘的阈值奇异性问题。