Jeffrey Mohan

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Mohan

First Name

Jeffrey

ORCID

0000-0002-4407-7371

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Publications 1 - 10 of 13

DC transport in a dissipative superconducting quantum point contact
Visuri, Anne-Maria; Mohan, Jeffrey; Uchino, Shun; et al. (2023)
Physical Review Research
We study the current-voltage characteristics of a superconducting junction with particle losses at the contacts. We adopt the Keldysh formalism to compute the steady-state current for varying transmission of the contact. In the low-transmission regime, the dissipation leads to an enhancement of the current at low bias, a nonmonotonic dependence of current on dissipation, and the emergence of new structures in the current-voltage curves. The effect of dissipation by particle loss is found to be qualitatively different from that of a finite temperature and a finite inelastic scattering rate in the reservoirs.
Erratum: Quantized conductance through a dissipative atomic point contact [Phys. Rev. A 100, 053605 (2019)]
Corman, Laura; Fabritius, Philipp; Häusler, Samuel; et al. (2021)
Physical Review A
Quantized conductance through a dissipative atomic point contact
Corman, Laura; Fabritius, Philipp; Häusler, Samuel; et al. (2019)
Physical Review A
Dark State Transport between Unitary Fermi Superfluids
Talebi, Mohsen; Will, Simon; Mohan, Jeffrey; et al. (2024)
Physical Review Letters
The formation of dark states is an important concept in quantum sciences, but its compatibility with strong interparticle interactions - for example, in a quantum degenerate gas - is hardly explored. Here, we realize a dark state in one of the spins of a two-component, resonantly interacting Fermi gas using a Λ system within the D2 transitions of Li6 at high magnetic field. The dark state is created in a micrometer-sized region within a one-dimensional channel connecting two superfluid reservoirs. The particle transport between the reservoirs is used as a probe. We observe that atoms are transported in the dark state and the superfluid-assisted fast current is preserved. If the dark state resonant condition is not met, the transport is suppressed by the spontaneous emission. We also uncover an asymmetry in the transport timescale across the two-photon resonance, which is absent in the noninteracting regime and diminished at higher temperatures. This work raises questions on the interplay of dark states with interparticle interactions and opens up perspectives for optical manipulation of fermionic pairing.
Irreversible entropy transport enhanced by fermionic superfluidity
Fabritius, Philipp; Mohan, Jeffrey; Talebi, Mohsen; et al. (2024)
Nature Physics
The nature of particle and entropy flow between two superfluids is often understood in terms of reversible flow carried by an entropy-free, macroscopic wavefunction. While this wavefunction is responsible for many intriguing properties of superfluids and superconductors, its interplay with excitations in non-equilibrium situations is less understood. Here we observe large concurrent flows of both particles and entropy through a ballistic channel connecting two strongly interacting fermionic superfluids. Both currents respond nonlinearly to chemical potential and temperature biases. We find that the entropy transported per particle is much larger than the prediction of superfluid hydrodynamics in the linear regime and largely independent of changes in the channel’s geometry. By contrast, the timescales of advective and diffusive entropy transport vary significantly with the channel geometry. In our setting, superfluidity counterintuitively increases the speed of entropy transport. Moreover, we develop a phenomenological model describing the nonlinear dynamics within the framework of generalized gradient dynamics. Our approach for measuring entropy currents may help elucidate mechanisms of heat transfer in superfluids and superconducting devices.
A low-noise and scalable FPGA-based analog signal generator for quantum gas experiments
Pahl, David; Pahl, Lukas; Mustafa, Enis; et al. (2021)
Universal particle and entropy transport in strongly interacting Fermi gases far from equilibrium
Mohan, Jeffrey (2024)
Transport experiments are among the most important methods to probe and characterize strongly interacting fermionic systems and the myriad states of matter that exist therein. The majority of existing experimental and theoretical studies in this context have focused on bulk systems in the hydrodynamic limit. In this regime, the transport properties of these systems can be described by linear response coefficients such as viscosity and conductivity which are fundamentally properties of the system in a state of thermal equilibrium. This thesis presents experiments probing the transport properties of strongly interacting gases of fermionic 6Li in a regime far from thermal equilibrium. Using the experimental toolbox of ultracold atomic gases, we shape the system into two reservoirs connected by a tunable, ballistic channel and measure currents of particles, entropy, and spin flowing through it. When the reservoirs are superfluid, the particle and entropy currents exhibit a highly nonlinear response to biases between the reservoirs in chemical potential and temperature. This observation reveals that the gas in the channel is perturbed by the biases to a state far from equilibrium. Furthermore, by inducing particle loss localized in the channel, we controllably break detailed balance. This principle underlies the transport properties of thermal systems, so its violation adds another dimension to the far-from-equilibrium nature of our system. In this setting, we observe several phenomena with no known microscopic explanation and therefore pose fundamental questions regarding transport in strongly interacting Fermi systems far from equilibrium. In a first set of experiments, we study coupled particle and entropy transport between unitary superfluid reservoirs across the 1D-2D crossover of the channel. Our central observation is a superfluid-enhanced Peltier effect: nonlinear particle currents advectively transport entropy along with them, and the average entropy transported per particle --- the Seebeck coefficient --- is robust against changes in the channel's geometry. The system also exhibits a diffusive mode of entropy transport, whose strong suppression in 1D channels results in a long-lived non-equilibrium steady state --- a signature of the breakdown of the Wiedemann-Franz law. We directly measure the entropy produced by these irreversible flows. In the absence of a microscopic or hydrodynamic theory, we develop a nonlinear phenomenological model within the formalism of generalized gradient dynamics to codify our observations and characterize the system's transport properties with a similarly compact framework as in the linear response regime. In a second set of experiments, we investigate the origin of the Seebeck coefficient's robustness by exploring a large parameter space spanned by global properties of the reservoirs and local properties of the channel. The former are the interaction strength and degeneracy that characterize the universal thermodynamics of the BCS-BEC crossover and normal-superfluid phase transition, and the latter are the transverse confinement and potential in the channel. The Seebeck coefficient depends only on the global thermodynamic properties and not on the details of the channel, suggesting that this far-from-equilibrium property has its origin in the universal equilibrium properties of the reservoirs. In contrast, the magnitudes of advective and diffusive currents vary significantly with both the channel and reservoir properties, revealing a change in the nature of the excitations responsible for transport across the BCS-BEC crossover. The phenomenological model describes the system over the entire parameter range explored here, smoothly connecting regimes of linear and nonlinear response. In a final set of experiments, we induce particle loss in the channel via photon scattering by atoms or Feshbach molecules and study its influence on transport with unitary superfluid reservoirs. Both types of dissipation have the same qualitative effects: particle currents respond more linearly and are reduced in magnitude, but remain larger than the values corresponding to a dissipation-free Fermi liquid, while entropy and spin diffusion are enhanced to levels consistent with a Fermi liquid. Effective microscopic theories based on multiple Andreev reflections quantitatively reproduce our observations of particle transport but fail for entropy and spin transport. We observe that, unlike the geometric properties of the channel, dissipation changes the advectively transported entropy per particle and appears to violate Onsager reciprocity, as evidenced by a decoupling of the Seebeck and Peltier coefficients of the phenomenological model.
Interaction-Assisted Reversal of Thermopower with Ultracold Atoms
Häusler, Samuel; Fabritius, Philipp; Mohan, Jeffrey; et al. (2021)
Physical Review X
We study thermoelectric currents of neutral, fermionic atoms flowing through a mesoscopic channel connecting a hot and a cold reservoir across the superfluid transition. The thermoelectric response results from a competition between density-driven diffusion from the cold to the hot reservoir and the channel favoring transport of energetic particles from hot to cold. We control the relative strength of both contributions to the thermoelectric response using an external optical potential in a nearly noninteracting and a strongly interacting system. Without interactions, the magnitude of the particle current can be tuned over a broad range but is restricted to flow from hot to cold in our parameter regime. Strikingly, strong interparticle interactions additionally reverse the direction of the current. We quantitatively model ab initio the noninteracting observations and qualitatively explain the interaction-assisted reversal by the reduction of entropy transport due to pairing correlations. Our work paves the way to studying the coupling of spin and heat in strongly correlated matter using spin-dependent optical techniques with cold atoms.
Superfluid Signatures in a Dissipative Quantum Point Contact
Huang, Meng-Zi; Mohan, Jeffrey; Visuri, Anne-Maria; et al. (2023)
Physical Review Letters
We measure superfluid transport of strongly interacting fermionic lithium atoms through a quantum point contact with local, spin-dependent particle loss. We observe that the characteristic non-Ohmic superfluid transport enabled by high-order multiple Andreev reflections transitions into an excess Ohmic current as the dissipation strength exceeds the superfluid gap. We develop a model with mean-field reservoirs connected via tunneling to a dissipative site. Our calculations in the Keldysh formalism reproduce the observed nonequilibrium particle current, yet do not fully explain the observed loss rate or spin current.
A low-noise and scalable FPGA-based analog signal generator for quantum gas experiments
Pahl, David; Pahl, Lukas; Mustafa, Enis; et al. (2021)
2021 IEEE International Conference on Quantum Computing and Engineering (QCE)
Achieving high fidelity for the measurement and control of quantum experiments imposes strict requirements on the precision and stability of surrounding electronics. Controlling electronics from a central device is more challenging when they are distributed in a laboratory and require analog signals where effects like ground loops and radiative cross-talk can limit their performance. Here, we present our design to address these challenges with a flexible and scalable analog signal generator. Our design is based on a field-programmable gate array (FPGA) development board, a custom PCB hosting a digital-to-analog converter (DAC) with 20 bit precision at 1 MSPS, and a custom breakout board. The FPGA development board accepts data from a master PC via TCP/IP where a user programs the waveform and sampling rate of each output channel and writes the data to on-board RAM. At runtime, the direct memory access (DMA) and Serial Peripheral Interface (SPI) modules inside the FPGA stream data to the custom DAC board via an Ethernet cable carrying the samples as differential signals along with the supply voltage. We designed the DAC board to be resistant to digital and analog noise by separating ground planes to prevent ground loops and by using high-precision and low-noise power supplies and voltage reference circuits. External trigger and clock inputs can be used to synchronize the DACs and multiple FPGAs. The time resolution and precision of our solution is optimized for experiments on quantum gases though it is flexible and can be adapted for many more applications.

Publications 1 - 10 of 13

Jeffrey Mohan

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