Measuring local temperature with a spatial resolution on the order of a few nanometers has a wide range of applications from semiconductor industry over material to life sciences [1]. When combined with precision temperature measurement it promises to give excess to small temperature changes caused e.g. by chemical reactions or biochemical processes [2]. However, nanoscale temperature measurements and precision have excluded each other so far owing to the physical processes used for temperature measurement of limited stability of nanoscale probes [3]. Here we experimentally demonstrate a novel nanoscale temperature sensing technique based on single atomic defects in diamonds. Sensor sizes range from millimeter down to a few tens of nanometers. Utilizing the sensitivity of the optically accessible electron spin level structure to temperature changes [4] we achieve a temperature noise floor of 5 mK/ √ Hz for single defects in bulk sensors. Using doped nanodiamonds as sensors yields temperature measurement with 130 mK/ √ Hz noise floor and accuracies down to 1 mK at length scales of a few ten nanometers. The high sensitivity to temperature changes together with excellent spatial resolution combined with outstanding sensor stability allows for nanoscale precision temperature determination enough to measure chemical processes of few or single molecules by their reaction heat even in heterogeneous environments like cells.Several kinds of nanoscale temperature sensing techniques have been developed in the recent past [1]. These are scanning thermal microscopes (SThM) [5], dispersed or scanned individual nanoprobes [3,6], direct methods like micro-Raman spectroscopy [7] or near-field optical temperature measurements [8]. SThMs have temperature sensitive elements at a scanning tip (e.g. thermocouple), the nanoprobes have temperature dependent properties (e.g. fluorescence spectrum) which can be accessed without direct contact.In this study utilize a single quantum system in a solid state matrix as a temperature nanoprobe, namely the negatively charged nitrogen-vacancy (NV) center in diamond, which allows probe sizes down to ∼ 5 nm [9]. High fidelity control of its ground state electronic and nuclear spins has been demonstrated for various quantum information test experiments [10-15] as well as for nanometer scale metrology purposes [16][17][18][19] e.g. measuring small magnetic and electric fields. Here we show that it also allows tracking temperature with high precision. Temperature nanoprobes can be either dispersed in the specimen to be investigated or used in scanning probe geometry (see fig. 1a).The NV center is a molecular impurity in diamond comprising a substitutional nitrogen impurity and an adjacent carbon vacancy. Optical excitation in a wavelength range from 460 nm to 580 nm yields intense fluorescence emission [20]. Excitation also leads to a high degree of ground state electron spin polarization (S = 1, the actual sensor level) into its m S = 0 (|0 ) sublevel [21]. Furthermore the fluorescence decreases upo...

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Decoherence is one of the most important obstacles that must be overcome in quantum information processing. It depends on the qubit-environment coupling strength, but also on the spectral composition of the noise generated by the environment. If the spectral density is known, fighting the effect of decoherence can be made more effective. Applying sequences of inversion pulses to the qubit system, we developed a method for dynamical decoupling noise spectroscopy. We generate effective filter functions that probe the environmental spectral density without requiring assumptions about its shape. Comparing different pulse sequences, we recover the complete spectral density function and distinguish different contributions to the overall decoherence.

Dynamical decoupling (DD) is a popular technique for protecting qubits from the environment. However, unless special care is taken, experimental errors in the control pulses used in this technique can destroy the quantum information instead of preserving it. Here, we investigate techniques for making DD sequences robust against different types of experimental errors while retaining good decoupling efficiency in a fluctuating environment. We present experimental data from solid-state nuclear spin qubits and introduce a new DD sequence that is suitable for quantum computing and quantum memory.

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