We propose and analyze an all-magnetic scheme to perform a Young's double slit experiment with a micron-sized superconducting sphere of mass 10 13 amu. We show that its center of mass could be prepared in a spatial quantum superposition state with an extent of the order of half a micrometer. The scheme is based on magnetically levitating the sphere above a superconducting chip and letting it skate through a static magnetic potential landscape where it interacts for short intervals with quantum circuits. In this way, a protocol for fast quantum interferometry using quantum magnetomechanics is passively implemented. Such a table-top earth-based quantum experiment would operate in a parameter regime where gravitational energy scales become relevant. In particular, we show that the faint parameter-free gravitationally-induced decoherence collapse model, proposed by Diósi and Penrose, could be unambiguously falsified.

The performance of quantum heat engines is generally based on the analysis of a single cycle. We challenge this approach by showing that the total work performed by a quantum engine need not be proportional to the number of cycles. Furthermore, optimizing the engine over multiple cycles leads to the identification of scenarios with a quantum enhancement. We demonstrate our findings with a quantum Otto engine based on a two-level system as the working substance that supplies power to an external oscillator.PACS numbers: 03.65. Ta, 05.70.Ln Advances in technology have spurred the fabrication and study of thermal machines at the nanoscale, whose performance is governed by quantum fluctuations. Prominent examples include quantum heat engines (QHEs) and pumps [1][2][3][4]. Various prototypes have been realized in the laboratory by means of cold atoms and trapped ions as a working substance [5,6]. Theoretical studies of these machines are largely motivated by foundational questions that address the interplay between thermodynamics and statistical mechanics in the quantum world [7,8]. At the same time, exciting applications are in view. Processes varying from laser emission [1] to light harvesting in both artificial and natural systems [9-11] can be described in terms of QHEs.Nonetheless, the quest for quantum signatures of the performance of thermal devices remains challenging. It is understood that a universal behavior emerges in the limit of small action [12]. Identifying scenarios exhibiting quantum supremacy, with a performance surpassing that in classical thermodynamics, stands out as an open problem. To this end, the use of quantum coherence [13], nonequilibrium reservoirs [14,15], and many-particle effects [16,17] has been proposed.The performance of quantum thermal machines is usually assessed via the characterization of a single cycle, as in classical thermodynamics. This approach assumes that the average single-cycle efficiency and power carry over to an arbitrary number of cycles, i.e., work done through n cycles is expected to be equal to n times the work done per cycle. Yet, in quantum mechanics work is determined via projective energy measurements at the beginning and end of a prescribed protocol [18,19]. As a result, assessing the performance of a quantum thermal machine can severely alter its dynamics due to the quantum measurement backaction. We argue that the QHE performance can be best assessed by measurements on an external system on which work is done (see, e.g., [20] for a related discussion). By analyzing the dynamics over many cycles, we elucidate the role of the intercycle coherence and findSchematic quantum heat engine. The quantum engine E does work w on an external system S through the coupling H SE absorbing heat Q from the baths collectively represented by B, which consists of hot (B 1 ) and cold (B 2 ) baths.scenarios with quantum-enhanced performance. In particular, we demonstrate that the average amount of work through n cycles need not be proportional to n; rather, it may have an a...

Generalized measurements of an observable performed on a quantum system during a force protocol are investigated and conditions that guarantee the validity of the Jarzynski equality and the Crooks relation are formulated. In agreement with previous studies by Campisi et al. [M. Campisi, P. Talkner, and P. Hänggi, Phys. Rev. Lett. 105, 140601 (2010); Phys. Rev. E 83, 041114 (2011)], we find that these fluctuation relations are satisfied for projective measurements; however, for generalized measurements special conditions on the operators determining the measurements need to be met. For the Jarzynski equality to hold, the measurement operators of the forward protocol must be normalized in a particular way. The Crooks relation additionally entails that the backward and forward measurement operators depend on each other. Yet, quite some freedom is left as to how the two sets of operators are interrelated. This ambiguity is removed if one considers selective measurements, which are specified by a joint probability density function of work and measurement results of the considered observable. We find that the respective forward and backward joint probabilities satisfy the Crooks relation only if the measurement operators of the forward and backward protocols are the time-reversed adjoints of each other. In this case, the work probability density function conditioned on the measurement result satisfies a modified Crooks relation. The modification appears as a protocol-dependent factor that can be expressed by the information gained by the measurements during the forward and backward protocols. Finally, detailed fluctuation theorems with an arbitrary number of intervening measurements are obtained.