Interaction induced phase transition in quantum many-body detection probability

We introduce and explore the physics of quantum many-body detection probability (QMBDP). Imagine a quantum many-body system starting from a far-from-equilibrium initial state. Few detectors are put at some given positions of the system. The detectors make simultaneous stroboscopic projective measurements of some chosen local operators. A particular measurement outcome is taken as the `signal'. By QMBDP we refer to the probability that the signal is detected within a given time. We find that, due to repeated stroboscopic measurements, there can emerge a time-scale within which the signal is almost certainly detected. Depending on the spectral properties of the Hamiltonian, there can be a phase transition where this time-scale increases dramatically on tuning some Hamiltonian parameters across the transition point. Consequently, over a finite but large regime of time, depending on the initial state, tuning some Hamiltonian parameters can result in sharp transition from a phase where the signal is certainly detected (QMBDP $=1$) to a phase where the the signal may not be detected (QMBDP $<1$). As an example, we present a single-impurity non-integrable model where such a far-from-equilibrium transition is achieved by varying the many-body interaction strength

Nonclassical radiation from a nonlinear oscillator driven solely by classical 1/f1/f noise

Low-frequency classical $1/f$-noise and quantum noise from low-temperature phonon modes are ubiquitous across various experimental platforms, and are usually considered a hindrance for quantum technological applications. Here we show that the simultaneous action of classical $1/f$ noise and a low-temperature phonon bath on a nonlinear oscillator can result in the generation of nonclassical antibunched radiation without the need for any additional drive. The $1/f$ noise itself provides the source of energy for generation of photons, while the phonon bath prevents heating up to infinite temperature and takes the nonlinear oscillator to a noise-averaged non-equilibrium steady state. The photon current in this non-equilibrium steady state may be detected by a standard wide-band detector. For sufficient nonlinearity and frequency dependence of the effective noise spectrum, the detected radiation can be antibunched. This opens the possibility to turn two of the most ubiquitous intrinsic noises in experimental platforms from a hindrance to a resource. It shows that wasteful heat from unavoidable noises can be converted into useful radiation. These results are based on the Redfield equation, which provides a rigorously derived general approach to treat any type of weak noise in a quantum system, specified only via the noise spectral function, as we discuss in detail

Quantum thermodynamics with fast driving and strong coupling via the mesoscopic leads approach

Understanding the thermodynamics of driven quantum systems strongly coupled to thermal baths is a central focus of quantum thermodynamics and mesoscopic physics. A variety of different methodological approaches exist in the literature, all with their own advantages and disadvantages. The mesoscopic leads approach was recently generalized to steady-state thermal machines and has the ability to replicate Landauer-Büttiker theory in the noninteracting limit. In this approach a set of discretized lead modes, each locally damped, provide a Markovian embedding for the baths. In this work we further generalize this approach to incorporate an arbitrary time dependence in the system Hamiltonian. Following a careful discussion of the calculation of thermodynamic quantities we illustrate the power of our approach by studying several driven mesoscopic examples coupled to finite-temperature fermionic baths, replicating known results in various limits. In the case of a driven noninteracting quantum dot we show how fast driving can be used to induce heat rectification

Universal Subdiffusive Behavior at Band Edges from Transfer Matrix Exceptional Points

We discover a deep connection between parity-time symmetric optical systems and quantum transport in one-dimensional fermionic chains in a two-terminal open system setting. The spectrum of one dimensional tight-binding chain with periodic on-site potential can be obtained by casting the problem in terms of 2×2 transfer matrices. We find that these non-Hermitian matrices have a symmetry exactly analogous to the parity-time symmetry of balanced-gain-loss optical systems, and hence show analogous transitions across exceptional points. We show that the exceptional points of the transfer matrix of a unit cell correspond to the band edges of the spectrum. When connected to two zero temperature baths at two ends, this consequently leads to subdiffusive scaling of conductance with system size, with an exponent 2, if the chemical potential of the baths are equal to the band edges. We further demonstrate the existence of a dissipative quantum phase transition as the chemical potential is tuned across any band edge. Remarkably, this feature is analogous to transition across a mobility edge in quasiperiodic systems. This behavior is universal, irrespective of the details of the periodic potential and the number of bands of the underlying lattice. It, however, has no analog in absence of the baths

Searching for Lindbladians obeying local conservation laws and showing thermalization

We investigate the possibility of a Markovian quantum master equation (QME) that consistently describes a finite-dimensional system, a part of which is weakly coupled to a thermal bath. In order to preserve complete positivity and trace, such a QME must be of Lindblad form. For physical consistency, it should additionally preserve local conservation laws and be able to show thermalization. First, we show that the microscopically derived Redfield equation (RE) violates complete positivity unless in extremely special cases. We then prove that imposing complete positivity and demanding preservation of local conservation laws enforces the Lindblad operators and the lamb-shift Hamiltonian to be `local', i.e, to be supported only on the part of the system directly coupled to the bath. We then cast the problem of finding `local' Lindblad QME which can show thermalization into a semidefinite program (SDP). We call this the thermalization optimization problem (TOP). For given system parameters and temperature, the solution of the TOP conclusively shows whether the desired type of QME is possible up to a given precision. Whenever possible, it also outputs a form for such a QME. For a XXZ chain of few qubits, fixing a reasonably high precision, we find that such a QME is impossible over a considerably wide parameter regime when only the first qubit is coupled to the bath. Remarkably, we find that when the first two qubits are attached to the bath, such a QME becomes possible over much of the same paramater regime, including a wide range of temperatures