Because of the ultra-low photon energies in the mid-infrared and terahertz frequencies, in these bands photodetectors are notoriously underdeveloped, and broadband single photon detectors (SPDs) are non-existent. Advanced SPDs exploit thermal effects in nanostructured superconductors, and their performance is currently limited to the more energetic near-infrared photons due to their high electronic heat capacity. Here, we demonstrate a superconducting magic-angle twisted bilayer graphene (MAG) device that is capable of detecting single photons of ultra-low energies by utilizing its record-low heat capacity and sharp superconducting transition. We theoretically quantify its calorimetric photoresponse and estimate its detection limits. This device allows the detection of ultrabroad range single photons from the visible to sub-THz with response time around 4 ns and energy resolution better than 1 THz. These attributes position MAG as an exceptional material for long-wavelength single photon sensing, which could revolutionize such disparate fields as quantum information processing and radio astronomy. Introduction:The detection of single photons is a key enabling technology in many research areas including quantum sensing, quantum key distribution, information processing and radio astronomy. Single photon detectors (SPDs) for wavelengths ranging from the visible to near infrared (nIR) have already been developed and even commercialized. State-of-the-art SPD technologies rely on heat-induced breaking of the superconducting state in nano-structured superconductors (SCs). Here, superconducting transition-edge sensors (TES) and superconducting nanowire single photon detectors (SNSPDs) have developed into the SPDs with the highest detection efficiencies, lowest dark count rates and operation wavelengths up to 10 µm (1-8). However, while no theoretical performance limits are evident so far, extending the broadband detection of single photons from the nIR to the far-infrared and the terahertz (THz), has yet to be demonstrated. TESs exploit the steepness of the temperature dependent resistance at the superconducting transition edge, which enables the generation of detectable voltages pulses upon heating electrons by absorbed light quanta (4, 5,9). Because the energy of an absorbed photon is transferred to the whole ensemble of electrons, the performance of TESs is determined by the heat capacity of the calorimetric materials used. This currently limits the SPD operation of TESs to wavelengths below 8µm(10), temperatures below 100mK, and detection times above ~10s(10-13).

Heat capacity is an invaluable quantity in condensed matter physics and yet has been completely inaccessible in two-dimensional (2D) van der Waals (vdW) materials, owing to their ultrafast thermal relaxation times and the lack of suitable nanoscale thermometers. Here, we demonstrate a novel thermal relaxation calorimetry scheme that allows the first measurements of the electronic heat capacity of graphene. It is enabled by combining a radio frequency Johnson noise thermometer, which can measure the electronic temperature with a sensitivity of ∼20 mK/Hz 1/2 , and a photomixed optical heater that modulates T e with a frequency of up to Ω = 0.2 THz. This allows record sensitive measurements of the electronic heat capacity C e < 10 −19 J/K and the fastest measurement of electronic thermal relaxation time τ e < 10 −12 s yet achieved by a calorimeter. These features advance heat capacity metrology into the realm of nanoscale and lowdimensional systems and provide an avenue for the investigation of their thermodynamic quantities.

Using photoluminescence microscopy enhanced by magnetic resonance, we visualize in real space both electron and nuclear polarization occurring in nonequilibrium fraction quantum Hall (FQH) liquids. We observe stripelike domain regions comprising FQH excited states which discretely form when the FQH liquid is excited by a source-drain current. These regions are deformable and give rise to bidirectionally polarized nuclear spins as spin-resolved electrons flow across their boundaries.

Electron spin transport and dynamics are investigated in a single, high-mobility, modulation-doped, GaAs quantum well using ultrafast two-color Kerr-rotation micro-spectroscopy, supported by qualitative kinetic theory simulations of spin diffusion and transport. Evolution of the spins is governed by the Dresselhaus bulk and Rashba structural inversion asymmetries, which manifest as an effective magnetic field that can be extracted directly from the experimental coherent spin precession. A spin precession length SOI is defined as one complete precession in the effective magnetic field. It is observed that application of (a) an out-ofplane electric field changes the spin decay time and SOI through the Rashba component of the spin-orbit coupling, (b) an in-plane magnetic field allows for extraction of the Dresselhaus and Rashba parameters, and (c) an in-plane electric field markedly modifies both the SOI and diffusion coefficient. While simulations reproduce the main features of the experiments, the latter results exceed the corresponding simulations and extend previous studies of drift-current-dependent spin-orbit interactions.

Exciton, trion and electron spin dynamics in a 20 nm wide modulation-doped GaAs single quantum well are investigated using resonant ultrafast two-color Kerr rotation spectroscopy. Excitons and trions are selectively detected by resonant probe pulses while their relative spectral weight is controlled by adjusting the gate voltage which tunes the carrier density. Tuning the carrier density markedly influences the spin decay time of the two dimensional electron gas. The spin decay time can be enhanced by a factor of 3 at an intermediate carrier concentration in the quantum well, where excitons and trions coexist in the system. In addition, we explore the capability to tune the g-factor of the electron gas via the carrier density.