arXiv subjects

Samuel Häusler

Publications and source records attributed to Samuel Häusler.

7 recordsLinked to original sources

arXiv2023

Quantum logical controlled-NOT gate in a lithium niobate-on-insulator photonic quantum walk

Quantum computers comprise elementary logic gates that initialize, control and measure delicate quantum states. One of the most important gates is the controlled-NOT, which is widely used to prepare two-qubit entangled states. The controlled-NOT gate for single photon qubits is normally realized as a six-mode network of individual beamsplitters. This architecture however, utilizes only a small fraction of the circuit for the quantum operation with the majority of the footprint dedicated to routing waveguides. Quantum walks are an alternative photonics platform that use arrays of coupled waveguides with a continuous interaction region instead of discrete gates. While quantum walks have been successful for investigating condensed matter physics, applying the multi-mode interference for logical quantum operations is yet to be shown. Here, we experimentally demonstrate a two-qubit controlled-NOT gate in an array of lithium niobate-on-insulator waveguides. We engineer the tight-binding Hamiltonian of the six evanescently-coupled single-mode waveguides such that the multi-mode interference corresponds to the linear optical controlled-NOT unitary. We measure the two-qubit transfer matrix with $0.938\pm0.003$ fidelity, and we use the gate to generate entangled qubits with $0.945\pm0.002$ fidelity by preparing the control photon in a superposition state. Our results highlight a new application for quantum walks that use a compact multi-mode interaction region to realize large multi-component quantum circuits.

quant-ph↗

arXiv2021

Interaction-Assisted Reversal of Thermopower with Ultracold Atoms

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 non-interacting 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 non-interacting 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.

cond-mat.quant-gas↗

arXiv2019

Quantized conductance through a spin-selective atomic point contact

We implement a microscopic spin filter for cold fermionic atoms in a quantum point contact (QPC) and create fully spin-polarized currents while retaining conductance quantization. Key to our scheme is a near-resonant optical tweezer inducing a large effective Zeeman shift inside the QPC while its local character limits dissipation. We observe a renormalization of this shift due to interactions of a few atoms in the QPC. Our work represents the analog of an actual spintronic device and paves the way to studying the interplay between spin-splitting and interactions far from equilibrium.

cond-mat.quant-gas↗

arXiv2019

Quantized conductance through a dissipative atomic point contact

Signatures of quantum transport are expected to quickly vanish as dissipation is introduced in a system. This dissipation can take several forms, including that of particle loss, which has the consequence that the total probability current is not conserved. Here, we study the effect of such losses at a quantum point contact (QPC) for ultracold atoms. Experimentally, dissipation is provided by a near-resonant optical tweezer whose power and detuning control the loss rates for the different internal atomic states as well as their effective Zeeman shifts. We theoretically model this situation by including losses in the Landauer-Büttiker formalism over a wide range of dissipative rates. We find good agreement between our measurements and our model, both featuring robust conductance plateaus. Finally, we are able to map out the atomic density by varying the position of the near-resonant tweezer inside the QPC, realizing a dissipative scanning gate microscope for cold atoms.

cond-mat.quant-gas↗

arXiv2018

Breakdown of the Wiedemann-Franz law in a unitary Fermi gas

We report on coupled heat and particle transport measurements through a quantum point contact (QPC) connecting two reservoirs of resonantly interacting, finite temperature Fermi gases. After heating one of them, we observe a particle current flowing from cold to hot. We monitor the temperature evolution of the reservoirs and find that the system evolves after an initial response into a non-equilibrium steady state with finite temperature and chemical potential differences across the QPC. In this state any relaxation in the form of heat and particle currents vanishes. From our measurements we extract the transport coefficients of the QPC and deduce a Lorenz number violating the Wiedemann-Franz law by one order of magnitude, a characteristic persisting even for a wide contact. In contrast, the Seebeck coefficient takes a value close to that expected for a non-interacting Fermi gas and shows a smooth decrease as the atom density close to the QPC is increased beyond the superfluid transition. Our work represents a fermionic analog of the fountain effect observed with superfluid helium and poses new challenges for microscopic modeling of the finite temperature dynamics of the unitary Fermi gas.

cond-mat.quant-gas↗

arXiv2018

Band and correlated insulators of cold fermions in a mesoscopic lattice

We investigate the transport properties of neutral, fermionic atoms passing through a one-dimensional quantum wire containing a mesoscopic lattice. The lattice is realized by projecting individually controlled, thin optical barriers on top of a ballistic conductor. Building an increasingly longer lattice, one site after another, we observe and characterize the emergence of a band insulating phase, demonstrating control over quantum-coherent transport. We explore the influence of atom-atom interactions and show that the insulating state persists as contact interactions are tuned from moderately to strongly attractive. Using bosonization and classical Monte-Carlo simulations we analyze such a model of interacting fermions and find good qualitative agreement with the data. The robustness of the insulating state supports the existence of a Luther-Emery liquid in the one-dimensional wire. Our work realizes a tunable, site-controlled lattice Fermi gas strongly coupled to reservoirs, which is an ideal test bed for non-equilibrium many-body physics.

cond-mat.quant-gas↗

arXiv2017

A scanning gate microscope for cold atomic gases

We present a scanning probe microscopy technique for spatially resolving transport in cold atomic gases, in close analogy with scanning gate microscopy in semiconductor physics. The conductance of a quantum point contact connected to two atomic reservoirs is measured in the presence of a tightly focused laser beam acting as a local perturbation that can be precisely positioned in space. By scanning its position and recording the subsequent variations of conductance, we retrieve a high-resolution map of transport through a quantum point contact. We demonstrate a spatial resolution comparable to the extent of the transverse wave function of the atoms inside the channel, and a position sensitivity below 10nm. Our measurements agree well with an analytical model and ab-initio numerical simulations, allowing us to identify a regime in transport where tunneling dominates over thermal effects. Our technique opens new perspectives for the high-resolution observation and manipulation of cold atomic gases.

cond-mat.quant-gas↗