Quantum computers must be able to function in the presence of decoherence. The simplest strategy for decoherence reduction is dynamical decoupling (DD), which requires no encoding overhead and works by converting quantum gates into decoupling pulses. Here, using the IBM and Rigetti platforms, we demonstrate that the DD method is suitable for implementation in today's relatively noisy and small-scale cloud-based quantum computers. Using DD, we achieve substantial fidelity gains relative to unprotected, free evolution of individual superconducting transmon qubits. To a lesser degree, DD is also capable of protecting entangled two-qubit states. We show that dephasing and spontaneous emission errors are dominant in these systems, and that different DD sequences are capable of mitigating both effects. Unlike previous work demonstrating the use of quantum error correcting codes on the same platforms, we make no use of post-selection and hence report unconditional fidelity improvements against natural decoherence. arXiv:1807.08768v2 [quant-ph]
High frequency electron spin resonance (ESR) spectroscopy is an invaluable tool for identification and characterization of spin systems. Nanoscale ESR using the nitrogen-vacancy (NV) center has been demonstrated down to the level of a single spin. However, NVdetected ESR has exclusively been studied at low magnetic fields, where spectral overlap prevents clear identification of spectral features. Within this work, we demonstrate NVdetected ESR measurements of single-substitutional nitrogen impurities in diamond at a NV Larmor frequency of 115 GHz and the corresponding magnetic field of 4.2 Tesla. The NV-ESR measurements utilize a double electron-electron resonance sequence and are performed using both ensemble and single NV spin systems. In the single NV experiment, chirp pulses are used to improve the population transfer and for NV-ESR measurements.This work provides the basis for NV-based ESR measurements of external spins at high magnetic fields.The nitrogen-vacancy (NV) center has unique properties that make it an excellent candidate for high sensitivity magnetic sensing. 1-3 The NV center is a two atom defect in the diamond lattice, with the capacity for optical spin-state initialization and readout, long coherence times, and high sensitivity to external magnetic fields. 4-6 NV-detected electron spin resonance (ESR) offers the capability to detect a single or a small number of electron spins 7-12 and to investigate biological molecules at the single molecule level. 13,14 Such an ESR technique with single spin sensitivity potentially eliminates ensembles averaging in heterogeneous and complex systems, and has great promise to directly probe fundamental interactions and biochemical function. In ESR, the measurement of the g-factor is extremely useful for the identification of spin species. However, a featureless "g = 2" signal is often observed, causing spectral overlap with target ESR signals, which may prevent spin identification. [14][15][16][17] Similar to nuclear magnetic resonance spectroscopy, pulsed ESR spectroscopy at higher frequencies (HF) and magnetic fields becomes more powerful for finer spectral resolution, enabling clear spectral separation of systems with similar g values. 18,19 This is advantageous in the investigation of complex and heterogeneous spin systems. 20,21 A high frequency of Larmor precession is also less sensitive to motional narrowing, enabling the ESR investigation of structures for molecules in motion. 22,23 In addition, a high Larmor frequency provides greater spin polarization:improving sensitivity 18,19 and providing control of spin dynamics. 24,25 On the other hand, pulsed HF ESR often has the disadvantage of long pulse times due to low HF microwave power. Moreover, the low microwave power limits the excitation bandwidth, and consequently the sensitivity of pulse ESR measurements. NV-detected ESR (indicated as NV-ESR) at a high frequency will enable ESR with drastically improved sensitivity and spectral resolution. However, only a few investigations of NV centers have been...
The nitrogen-vacancy (NV) center has enabled widespread study of nanoscale nuclear magnetic resonance (NMR) spectroscopy at low magnetic fields. NMR spectroscopy at high magnetic fields significantly improves the technique’s spectral resolution, enabling clear identification of closely related chemical species. However, NV-detected NMR is typically performed using AC sensing through electron spin echo envelope modulation, a hyperfine spectroscopic technique that is not feasible at high magnetic fields. Within this paper, we have explored an NV-detected NMR technique for applications of high field NMR. We have demonstrated optically detected magnetic resonance with the NV Larmor frequency of 230 GHz at 8.3 T, corresponding to a proton NMR frequency of 350 MHz. We also demonstrated the first measurement of electron–electron double resonance detected NMR using the NV center and successfully detected 13C nuclear bath spins. The described technique is limited by the longitudinal relaxation time (T1), not the transverse relaxation time (T2). Future applications of the method to perform nanoscale NMR of external spins at 8.3 T and even higher magnetic fields are also discussed.
Spectral analysis of electron spin resonance (ESR) is a powerful technique for various investigations including characterization of spin systems, measurements of spin concentration, and probing spin dynamics. The nitrogen-vacancy (NV) center in diamond is a promising magnetic sensor enabling improvement of ESR sensitivity to the level of a single spin. Therefore, understanding the nature of NV-detected ESR (NV-ESR) spectrum is critical for applications to nanoscale ESR. Within this work we investigate the linewidth of NV-ESR from single substitutional nitrogen centers (called P1 centers). NV-ESR is detected by a double electron-electron resonance (DEER) technique. By studying the dependence of the DEER excitation bandwidth on NV-ESR linewidth, we find that the spectral resolution is improved significantly and eventually limited by inhomogeneous broadening of the detected P1 ESR. Moreover, we show that the NV-ESR linewidth can be as narrow as 0.3 MHz.
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