We present measurements of the electron temperature using gate-defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1 mK, the device electron temperature saturates, remaining close to 10 mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature.

We report a thermally activated metastability in a GaAs double quantum dot exhibiting real-time charge switching in diamond shaped regions of the charge stability diagram. Accidental charge traps and sensor backaction are excluded as the origin of the switching. We present an extension of the canonical double dot theory based on an intrinsic, thermal electron exchange process through the reservoirs, giving excellent agreement with the experiment. The electron spin is randomized by the exchange process, thus facilitating fast, gate-controlled spin initialization. At the same time, this process sets an intrinsic upper limit to the spin relaxation time. DOI: 10.1103/PhysRevLett.115.106804 PACS numbers: 73.21.La, 73.23.Hk, 73.63.Kv Spins in quantum dots [1] are promising candidates for the realization of qubits-the elementary units of a quantum computer. Great progress was made in recent years towards implementing quantum information processing schemes with electron spins in GaAs quantum dots [2][3][4][5][6][7][8], which hold the potential for scaling to a large number of qubits [9][10][11]. Stable qubits with long coherence times are of crucial importance to execute numerous coherent quantum gates. Spin echo and dynamical decoupling techniques were successfully employed to isolate the electronic system from the slowly fluctuating nuclear spins of the GaAs host material [3,[12][13][14][15], enhancing the coherence time T 2 from below 1 μs to much longer times exceeding 0.2 ms. A fundamental limit T 2 ≤ 2T 1 is set by the spin relaxation time T 1 . In a magnetic field, spins relax through spin-phonon coupling mediated by the spin-orbit interaction [2,[16][17][18]. Since here the spin-orbit coupling is weak, very long T 1 times result, exceeding 1 s at 1 T [19], leaving ample room for further improvements of the spin qubit coherence.In this Letter, we report the experimental observation of a thermal electron exchange process via the reservoirs of a quantum dot, setting an intrinsic upper bound to T 1 , which can be orders of magnitude lower than the fundamental spin-phonon limit [16]. The resulting metastable charge states-appearing in the double dot (DD) in the absence of interdot tunneling-make the exchange process detectable with a charge sensor. Within a diamond shaped region, the DD switches its charge state back and forth over time from an electron on the left dot to an electron on the right dot without direct interdot tunneling. After excluding unintentional charge traps and sensor backaction, we present an extension of the orthodox DD transport theory accounting very well for the observations. The exchange process can be used for fast qubit initialization [7]. Finally, we outline ways to extend T 1 up to the spin-phonon limit.The sample is fabricated from a GaAs heterostructure with a 2D electron gas 110 nm below the surface (density 2.6 × 10 11 cm −2 and mobility 4 × 10 5 cm 2 =V s). The device layout, see Fig. 1(a), is adopted from Ref. [20]. Each dot adjacent to the DD (center) acts as a charge sensor [...

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