We study the dynamical multistability of a solid-state single-atom laser implemented in a quantum dot spin valve. The system is formed by a resonator that interacts with a two-level system in a dot in contact with two ferromagnetic leads of antiparallel polarization. We show that a spin-polarized current provides highefficiency pumping leading to regimes of multistable lasing, in which the Fock distribution of the oscillator displays a multi-peaked distribution. The emergence of multistable lasing follows from the breakdown of the usual rotating-wave approximation for the coherent spin-resonator interaction which occurs at relatively weak couplings. The multistability manifests itself directly in the charge current flowing through the dot, switching between distinct current levels corresponding to the different states of oscillation.
Photon emission by tunneling electrons can be encouraged by locating a resonator close to the tunnel junction and applying an appropriate voltage-bias. However, studies of normal metals show that the resonator also affects how the charges flow, facilitating processes in which correlated tunneling of two charges produces one photon. We develop a theory to analyze this kind of behavior in Josephson junctions by deriving an effective Hamiltonian describing processes where two Cooper-pairs generate a single photon. We determine the conditions under which the transport is dominated by incoherent tunneling of two Cooper-pairs, whilst also uncovering a regime of coherent double Cooper-pair tunneling. We show that the system can also display an unusual form of photon-blockade and hence could serve as a single-photon source.
Hybrid quantum dot-oscillator systems have become attractive platforms to inspect quantum coherence effects at the nanoscale. Here, we investigate a Cooper-pair splitter setup consisting of two quantum dots, each linearly coupled to a local resonator. The latter can be realized either by a microwave cavity or a nanomechanical resonator. Focusing on the subgap regime, we demonstrate that cross-Andreev reflection, through which Cooper pairs are split into both dots, can efficiently cool down simultaneously both resonators into their ground state. Moreover, we show that a nonlocal heat transfer between the two resonators is activated when opportune resonance conditions are matched. The proposed scheme can act as a heat-pump device with potential applications in heat control and cooling of mesoscopic quantum resonators. Nonlocality [1, 2] and quantum correlations [3] are at the heart of many quantum technologies [4][5][6]. In hybrid quantum dot devices, Cooper pairs are a source of correlated electrons and their nonlocal splitting has experimentally [7][8][9][10][11][12][13][14][15][16][17] and theoretically [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] drawn much attention over the last few years. In particular, the nonlocal breaking of the particlehole symmetry in such Cooper-pair splitters (CPSs) gives rise to peculiar thermoelectric effects [33][34][35][36]. On the other hand, hybrid cavity quantum electrodynamics (cQED) devices are suited for correlating few-level systems over a distance [37][38][39][40][41][42]. Such cQED devices have applications in the readout of charge [43][44][45][46][47][48][49], spin [50][51][52][53][54], and valley-orbit states [55,56]. Ground-state cooling of mechanical resonators in hybrid [57] and optomechanical [58,59] systems has been demonstrated, and a cooling scheme based on local Andreev reflection has been recently proposed [60,61]. The cooling of vibrational degrees of freedom due to quantum coherences has been also investigated in nanoelectromechanical systems [62]. Combining CPSs with microwave cavities or mechanical resonators opens up new avenues to tailor energy and heat flows in nanodevices [63-65] by exploiting quantum coherence.In this Letter, we consider a CPS in a double-quantum-dot setup with each dot linearly coupled to a local resonator, constituted by either a microwave cavity [46,48,51,[66][67][68][69] or a mechanical oscillator [70-73], see Fig. 1(a). We demonstrate that this system can cool efficiently and simultaneously the oscillators down to their ground state, and in addition generate a coherent transfer of photons, and hence heat, between the two originally uncoupled cavities. This interaction arises from a strong coupling between the dots and the superconducting lead, and has a purely nonlocal origin due to cross-Andreev reflection. Subsequent, we discuss the underlying physical mechanism following the lines of Ref. 74, where a single quantum dot system in the single-atom lasing regime has been investigated.For large intradot Coulomb inter...
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