We experimentally demonstrate a new interferometry paradigm: a self-interfering clock. We split a clock into two spatially separated wave packets, and observe an interference pattern with a stable phase showing that the splitting was coherent, i.e., the clock was in two places simultaneously. We then make the clock wave packets "tick" at different rates to simulate a proper time lag. The entanglement between the clock's time and its path yields "which path" information, which affects the visibility of the clock's self-interference. By contrast, in standard interferometry, time cannot yield "which path" information. As a clock we use an atom prepared in a superposition of two spin states. This first proof-of-principle experiment may have far-reaching implications for the study of time and general relativity and their impact on fundamental quantum effects such as decoherence and wave packet collapse. Two-slit interferometry of quanta, such as photons and electrons, figured prominently in the Bohr-Einstein debates on the consistency of quantum theory [1, 2]. A fundamental principle emerging from those debates-intimately related to the uncertainty principle-is that "which path" information about the quanta passing through slits blocks their interference. At the climax of the debates, Einstein claimed that a clock, emitting a photon at a precise time while being weighed on a spring scale to measure the change in its massenergy, could evade the uncertainty principle. Yet Bohr showed that the clock's gravitational redshift introduced enough uncertainty in the emission time to satisfy the uncertainty principle. Inspired by the subtle role time may play in quantum mechanics, we have now sent a clock through a spatial interferometer. The proof-of-principle experiment described below presents clock interferometry as a new tool for studying the interplay of general relativity[3] and quantum mechanics [4].Quantum mechanics cannot fully describe a self-interfering clock in a gravitational field.If the paths of a clock through an interferometer have different heights, then general relativity predicts that the clock must "tick" slower along the lower path. However, time in quantum mechanics is a global parameter, which cannot differ between paths. In standard interferometry (e.g.[5]), a difference in height between two paths affects their relative phase and shifts their interference pattern; but in clock interferometry, a time differential between paths yields "which path" information, degrading the visibility of the interference pattern [6]. It follows that, while standard interferometry may probe general relativity [7][8][9] In principle, any system evolving with a well defined period can be a clock. In our experiment, we utilize a quantum two-level system. Specifically, each clock is a 87 Rb atom in a superposition of two Zeeman sublevels, the m F = 1 and m F = 2 sublevels of the F = 2 hyperfine state.The general scheme of the clock interferometer is shown in Fig. 1 atoms 90 µm below the chip surface). Initially, af...

We present a unique matter-wave interferometer whose phase scales with the cube of the time the atom spends in the interferometer. Our scheme is based on a full-loop Stern-Gerlach interferometer incorporating four magnetic field gradient pulses to create a state-dependent force. In contrast to typical atom interferometers which make use of laser light for the splitting and recombination of the wave packets, this realization uses no light and can therefore serve as a high-precision surface probe at very close distances.

Work is an essential concept in classical thermodynamics, and in the quantum regime, where the notion of a trajectory is not available, its definition is not trivial. For driven (but otherwise isolated) quantum systems, work can be defined as a random variable, associated with the change in the internal energy. The probability for the different values of work captures essential information describing the behaviour of the system, both in and out of thermal equilibrium. In fact, the work probability distribution is at the core of “fluctuation theorems” in quantum thermodynamics. Here we present the design and implementation of a quantum work meter operating on an ensemble of cold atoms, which are controlled by an atom chip. Our device not only directly measures work but also directly samples its probability distribution. We demonstrate the operation of this new tool and use it to verify the validity of the quantum Jarzynksi identity.