The heat engine, a machine that extracts useful work from thermal sources, is one of the basic theoretical constructs and fundamental applications of classical thermodynamics. The classical description of a heat engine does not include coherence in its microscopic degrees of freedom. By contrast, a quantum heat engine might possess coherence between its internal states. Although the Carnot efficiency cannot be surpassed 1-3 , and coherence can be performance degrading in certain conditions 4-9 , it was recently predicted that even when using only thermal resources, internal coherence can enable a quantum heat engine to produce more power than any classical heat engine using the same resources 10,11 . Such a power boost therefore constitutes a quantum thermodynamic signature. It has also been shown that the presence of coherence results in the thermodynamic equivalence of different quantum heat engine types 10,12 , an effect with no classical counterpart. Microscopic heat machines have been recently implemented with trapped ions 13,14 , and proposals for heat machines using superconducting circuits 15,16 and optomechanics 17,18 have been made. When operated with standard thermal baths, however, the machines implemented so far have not demonstrated any inherently quantum feature in their thermodynamic quantities. Here we implement two types of quantum heat engines by use of an ensemble of nitrogen-vacancy centres in diamond, and experimentally demonstrate both the coherence power boost and the equivalence of different heat-engine types. This constitutes the first observation of quantum thermodynamic signatures in heat machines. 2A quantum heat engine consists of a microscopic system, or an ensemble of such systems, whose internal state can be a coherent superposition of energy states. The engine cycle consists of a sequence of operations (strokes), which include the interaction of the system (or part thereof) either with a thermal bath (cold or hot), or with an external classical/semi-classical field responsible for work extraction. Interactions with the thermal baths act to change the populations of the energy states of the heat engine incoherently, in contrast to the field, which changes the populations coherently. Fig. 1 schematically presents three basic quantum heat-engine types: continuous, two-stroke and four-stroke, which differ by the ordering of the different strokes. Of these types, the four-stroke engine bears the strongest resemblance to macroscopic classical engines such as the Otto engine. It can be described (classically) by a two level system undergoing a four part cycle, illustrated in the top panel of Fig. 1a, consisting of alternating couplings to the hot and cold baths, interspersed with couplings to the work reservoir, whose effect is to change the spacing between the levels. It can be shown that this dynamics is equivalent to classical swap operations in a multilevel system 10 (multilevel embedding), as shown in the middle panel of Fig. 1a (taking U = swap). In a quantum heat engine, the oper...

It is proposed that the ground-state manifold of the neutral nitrogen-vacancy center in diamond could be used as a quantum two-level system in a solid-state-based implementation of a broadband, noise-free quantum optical memory. The proposal is based on the same-spin Λ-type three-level system created between the two E orbital ground states and the A1 orbital excited state of the center, and the cross-linear polarization selection rules obtained with the application of transverse electric field or uniaxial stress. Possible decay and decoherence mechanisms of this system are discussed, and it is shown that high-efficiency, noise-free storage of photons as short as a few tens of picoseconds for at least a few nanoseconds could be possible at low temperature.

Strong light-matter interactions are critical for quantum technologies based on light, such as memories or nonlinear interactions. Solid state materials will be particularly important for such applications, because of the relative ease of fabrication of components. Silicon vacancy centers (SiV − ) in diamond feature especially narrow inhomogeneous spectral lines, which are rare in solid materials. Here, we demonstrate resonant coherent manipulation, stimulated Raman adiabatic passage, and strong light-matter interaction via four-wave mixing of a weak signal field in an ensemble of SiV − centers.

We experimentally study a broadband implementation of the atomic frequency comb (AFC) rephasing protocol with a cryogenically cooled Pr 3+ :Y 2 SiO 5 crystal. To allow for storage of broadband pulses, we explore a novel regime where the input photonic bandwidth closely matches the inhomogeneous broadening of the material (∼ 5 GHz), thereby significantly exceeding the hyperfine ground and excited state splitting (∼ 10 MHz). Through an investigation of different AFC preparation parameters, we measure a maximum efficiency of 10% after a rephasing time of 12.5 ns. With a suboptimal AFC, we witness up to 12 rephased temporal modes.

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