We demonstrate fluorescence thermometry techniques with sensitivities approaching 10 mK·Hz −1/2 based on the spin-dependent photoluminescence of nitrogen vacancy (NV) centers in diamond. These techniques use dynamical decoupling protocols to convert thermally induced shifts in the NV center's spin resonance frequencies into large changes in its fluorescence. By mitigating interactions with nearby nuclear spins and facilitating selective thermal measurements, these protocols enhance the spin coherence times accessible for thermometry by 45-fold, corresponding to a 7-fold improvement in the NV center's temperature sensitivity. Moreover, we demonstrate these techniques can be applied over a broad temperature range and in both finite and near-zero magnetic field environments. This versatility suggests that the quantum coherence of single spins could be practically leveraged for sensitive thermometry in a wide variety of biological and microscale systems.spintronics | electron spin resonance | quantum control T hermometry based on thermally driven changes in fluorescence lifetimes or intensities is an essential technique in many environments that preclude electrical probes (1). Although typical fluorescence thermometers use millimeter-scale optical probes (2), the desire to noninvasively monitor intracellular thermal gradients has motivated efforts to develop analogous methods at the nanoscale (3,4). This interest has stimulated the development of nanoscale fluorescence thermometers based on quantum dots (5), rare-earth ions (6), and nanogels (7), with recent studies suggesting that intracellular temperature gradients are on the order of 1 K (8). However, the application of conventional fluorescence thermometry techniques in biological settings is limited by temperature resolutions of ∼0.2 K or worse (8-10), motivating the development of more advanced nanoscale thermometers.In recent years, solid-state electronic spins have gained considerable attention for applications in nanoscale sensing. In particular, the diamond nitrogen vacancy (NV) center (Fig. 1A) has garnered attention for its optical spin initialization and fluorescence-based spin readout (11), the ability to isolate and measure single defects (12), and the ability to manipulate its spin using microwave electron spin resonance techniques (13). NV center sensing is based on monitoring shifts in the spin resonance frequencies through the defect's fluorescence as a function of external perturbations such as magnetic fields (14-17), electric fields (18), or temperature (19,20), with the sensitivity of these techniques scaling as 1= ffiffiffiffiffiffi T C p , where T C is the relevant spin coherence time (21). These coherence times can be enhanced by two to three orders of magnitude for perturbations amenable to timeperiodic (AC) modulation through the use of dynamical decoupling techniques that periodically invert the spin state and the signal being sensed to mitigate the effects of low-frequency magnetic noise (22-24). These methods have enabled the detection...

rates. Here we present the integration of dynamical decoupling into quantum gates for a paradigmatic hybrid system, the electron-nuclear spin register. Our design harnesses the internal resonance in the coupled-spin system to resolve the conflict between gate operation and decoupling. We experimentally demonstrate these gates on a two-qubit register in diamond operating at room temperature. Quantum tomography reveals that the qubits involved in the gate operation are protected as accurately as idle qubits. We further illustrate the power of our design by executing .Decoherence is a major hurdle towards realizing scalable quantum technologies in the solid state. The inter-qubit dynamics that implement the quantum logic are unavoidably affected by uncontrolled couplings to the solid-state environment, preventing high-fidelity gate performance (Fig 1a). Dynamical decoupling 4 , a technique that employs fast qubit flips to average out the interactions with the environment, is a powerful and practical tool for mitigating decoherence [5][6][7][8][9][10][11][12]24,25 . This approach is particularly promising for the emerging class of hybrid quantum architectures [13][14][15][16][17][18][19][20][21][22][23] in which different types of qubits, such as electron and nuclear spins, superconducting resonators, and nanomechanical oscillators, perform different functions. Dynamical 2 decoupling allows for each qubit type to be decoupled at its own appropriate rate, ensuring uniform coherence protection.However, combining dynamical decoupling with quantum gate operations is generally problematic, since decoupling does not distinguish the desired inter-qubit interaction from the coupling to the decohering environment, and in general cancels both (Fig. 1b). For hybrid systems, where large difference in coherence and control timescales among the different qubit types make the encoding-based schemes 11 or synchronized application of decoupling pulses 4,12 fail, a solution has thus far remained elusive.Here we present a design that enables the integration of decoupling into gate operation for hybrid quantum architectures. We demonstrate such decoherence-protected gates on a prototype hybrid quantum system: a two-qubit register consisting of an electron and a nuclear spin (Fig. 1c). The key idea is to precisely adapt the time intervals between the electron decoupling pulses to the nuclear spin dynamics. When combined with continuous nuclear spin driving, this synchronization yields selective rotations of the nuclear spin while the electron spin is dynamically protected, as explained below. This design preserves all of the advantages of dynamical decoupling without requiring additional qubits or controllable inter-qubit couplings. It can be readily implemented to yield decoherence-protected quantum gates in a range of hybrid systems, such as various electron-nucleus spin registers [13][14][15][16][17]20 , and interface gates between the qubits and a spinchain quantum databus 22,23 .We experimentally demonstrate the scheme on ...

Two-level systems are at the core of numerous real-world technologies such as magnetic resonance imaging and atomic clocks. Coherent control of the state is achieved with an oscillating field that drives dynamics at a rate determined by its amplitude. As the strength of the field is increased, a different regime emerges where linear scaling of the manipulation rate breaks down and complex dynamics are expected. By calibrating the spin rotation with an adiabatic passage, we have measured the room-temperature "strong-driving" dynamics of a single nitrogen vacancy center in diamond. With an adiabatic passage to calibrate the spin rotation, we observed dynamics on sub-nanosecond time scales. Contrary to conventional thinking, this breakdown of the rotating wave approximation provides opportunities for time-optimal quantum control of a single spin.

We demonstrate nanometer-precision depth control of nitrogen-vacancy (NV) center creation near the surface of synthetic diamond using an in situ nitrogen delta-doping technique during plasma-enhanced chemical vapor deposition. Despite their proximity to the surface, doped NV centers with depths (d) ranging from 5 - 100 nm display long spin coherence times, T2 > 100 \mus at d = 5 nm and T2 > 600 \mus at d \geq 50 nm. The consistently long spin coherence observed in such shallow NV centers enables applications such as atomic-scale external spin sensing and hybrid quantum architectures.Comment: 14 pages, 4 figures, 11 pages of additional supplementary materia

We study the spin and orbital dynamics of single nitrogen-vacancy (NV) centers in diamond between room temperature and 700 K. We find that the ability to optically address and coherently control single spins above room temperature is limited by nonradiative processes that quench the NV center's fluorescence-based spin readout between 550 and 700 K. Combined with electronic structure calculations, our measurements indicate that the energy difference between the 3 E and 1 A 1 electronic states is approximately 0.8 eV. We also demonstrate that the inhomogeneous spin lifetime (T Ã 2 ) is temperature independent up to at least 625 K, suggesting that single NV centers could be applied as nanoscale thermometers over a broad temperature range. DOI: 10.1103/PhysRevX.2.031001 Subject Areas: Quantum Information, Semiconductor Physics, SpintronicsThe negatively charged nitrogen-vacancy (NV) center spin in diamond stands out among individually addressable qubit systems because it can be initialized, coherently controlled, and read out at room temperature [1]. The defect's robust spin coherence [2] and optical addressability via spin-dependent orbital transitions [3] have enabled applications ranging from quantum information processing [4][5][6][7] to nanoscale-magnetic and electric-field sensing [8][9][10]. While it has been shown that NV center spins can be optically polarized up to at least 500 K [11,12], little is known about what processes limit the spin's optical addressability and coherence at higher temperatures. Understanding these processes is important to hightemperature field-sensing applications and will aid the search for new defect-based spin qubits analogous to the NV center [13,14] by identifying the aspects of its orbital structure responsible for its high-temperature operation.The NV center's optical-spin polarization and opticalspin readout result from a spin-selective intersystem crossing (ISC). Although optical transitions between the spin-triplet ground ( 3 A 2 ) and excited ( 3 E) states [1.945 eV zero-phonon line (ZPL)] are typically spin conserving, the 3 E state can also relax to the 3 A 2 state via an indirect pathway that involves a nonradiative, triplet to singlet ISC and subsequent transitions through at least one additional singlet [ Fig. 1(a)]. The 3 E ISC is much stronger for the m s ¼ AE1 3 E sublevels than for the m s ¼ 0 sublevel, which facilitates spin readout through the resulting spindependent photoluminescence (PL) and initializes the spin into the m s ¼ 0 3 A 2 sublevel with high probability (P m s ¼0 $ 0:8) through repeated optical excitation [15]. Despite the singlet pathway's critical role in preparing and interrogating the spin, open questions remain regarding the number and energies of the singlets involved. Recent experiments have established that it consists of at least two singlets of 1 A 1 and 1 E symmetry separated by 1.19 eV [16][17][18], and have shown that the 1 E state lifetime is strongly temperature dependent below 300 K [17,19]. The details of the 3 E ISC remain unc...

We demonstrate a technique to nanofabricate nitrogen vacancy (NV) centers in diamond based on broad-beam nitrogen implantation through apertures in electron beam lithography resist. This method enables high-throughput nanofabrication of single NV centers on sub-100-nm length scales. Secondary ion mass spectroscopy measurements facilitate depth profiling of the implanted nitrogen to provide three-dimensional characterization of the NV center spatial distribution. Measurements of NV center coherence with on-chip coplanar waveguides suggest a pathway for incorporating this scalable nanofabrication technique in future quantum applications.

The ability to generate and verify multipartite entanglement is an important benchmark for nearterm quantum devices. We develop a scalable entanglement metric based on multiple quantum coherences, and demonstrate experimentally on a 20-qubit superconducting device. We report a state fidelity of 0.5165 ± 0.0036 for an 18-qubit GHZ state, indicating multipartite entanglement across all 18 qubits. Our entanglement metric is robust to noise and only requires measuring the population in the ground state; it can be readily applied to other quantum devices to verify multipartite entanglement. arXiv:1905.05720v1 [quant-ph]

We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. 58, 2539-2542], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments.