We measure isotope shifts for five Yb þ isotopes with zero nuclear spin on two narrow optical quadrupole transitions 2 S 1=2 → 2 D 3=2 , 2 S 1=2 → 2 D 5=2 with an accuracy of ∼300 Hz. The corresponding King plot shows a 3 × 10 −7 deviation from linearity at the 3σ uncertainty level. Such a nonlinearity can indicate physics beyond the Standard Model (SM) in the form of a new bosonic force carrier, or arise from higherorder nuclear effects within the SM. We identify the quadratic field shift as a possible nuclear contributor to the nonlinearity at the observed scale, and show how the nonlinearity pattern can be used in future, more accurate measurements to separate a new-boson signal from nuclear effects.

Nitrogen-vacancy (NV) quantum defects in diamond are sensitive detectors of magnetic fields. Due to their atomic size and optical readout capability, they have been used for magnetic resonance spectroscopy of nanoscale samples on diamond surfaces. Here we present a protocol for fabricating NV-diamond chips and for constructing and operating a simple, low-cost "quantum diamond spectrometer" for performing nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy in nanoscale volumes. The instrument is based on a commercially-available diamond chip, with an ion-implanted NVensemble at a depth of ~ 10 nm below the diamond surface. The spectrometer operates at low magnetic fields (~ 300 G) and requires standard optical and microwave components for NV spin preparation, manipulation and readout. We demonstrate the utility of this device for nanoscale proton and fluorine NMR spectroscopy, as well as for the detection of transition metals via ESR noise spectroscopy. We estimate that the full protocol requires 2-3 months to implement, depending on the availability of equipment, diamond substrates, and user experience. Introduction:Magnetic resonance spectroscopy of electrons and nuclei comprises a family of ubiquitous and essential analytical tools in modern chemical and biological research 1 . Electron spin resonance (ESR), also known as electron paramagnetic resonance (EPR), spectroscopy is a useful means for probing molecules possessing unpaired electrons, such as transition metal complexes and radicals 2 . (Bio)molecules that lack an unpaired electronic spin can be probed via ESR-active spin labels. Nuclear magnetic resonance (NMR), on the other hand, is a more widely utilized technique, as NMR-active nuclei (e.g., 1 H, 13 C, 14 N, and 31 P) are commonly encountered in organic and biological chemistry. The narrow spectral lines of NMR afford unprecedented information about molecular structure and dynamics. NMR is less sensitive than ESR, however, owing to the lower gyromagnetic ratios of nuclei compared to that of the electron. In fact, both NMR and ESR are relatively insensitive when compared to the state of the art in other analytical techniques like mass spectrometry or fluorescence microscopy. The low sensitivity of magnetic resonance is particularly challenging for life science applications, where biomolecules of interest commonly occur in small absolute quantities or concentrations. Thus, there is great interest in new techniques to increase the sensitivity of magnetic resonance spectroscopy 3-5 . One promising approach employs a magnetic sensor based on fluorescent quantum defects in diamond, such as nitrogen-vacancy (NV) color centers, enabling interrogation of sample volumes down to the nanoscale 6,7 , including single proteins 8,9 , single protons 10 and 2D materials 11 . In this protocol, we describe the procedure for generating NV-diamond sensor chips and the construction of a "quantum diamond spectrometer" for NMR and ESR of nanoscale samples placed on the diamond chip. Physical back...

Entanglement is one of the most fundamental properties of quantum mechanics, and is the key resource for quantum information processing (QIP). Bipartite entangled states of identical particles have been generated and studied in several experiments, and post-selected or heralded entangled states involving pairs of photons, single photons and single atoms, or different nuclei in the solid state, have also been produced. Here we use a deterministic quantum logic gate to generate a 'hybrid' entangled state of two trapped-ion qubits held in different isotopes of calcium, perform full tomography of the state produced, and make a test of Bell's inequality with non-identical atoms. We use a laser-driven two-qubit gate, whose mechanism is insensitive to the qubits' energy splittings, to produce a maximally entangled state of one (40)Ca(+) qubit and one (43)Ca(+) qubit, held 3.5 micrometres apart in the same ion trap, with 99.8 ± 0.6 per cent fidelity. We test the CHSH (Clauser-Horne-Shimony-Holt) version of Bell's inequality for this novel entangled state and find that it is violated by 15 standard deviations; in this test, we close the detection loophole but not the locality loophole. Mixed-species quantum logic is a powerful technique for the construction of a quantum computer based on trapped ions, as it allows protection of memory qubits while other qubits undergo logic operations or are used as photonic interfaces to other processing units. The entangling gate mechanism used here can also be applied to qubits stored in different atomic elements; this would allow both memory and logic gate errors caused by photon scattering to be reduced below the levels required for fault-tolerant quantum error correction, which is an essential prerequisite for general-purpose quantum computing.

Optical precision spectroscopy of isotope shifts can be used to test for new forces beyond the Standard Model, and to determine basic properties of atomic nuclei. We measure isotope shifts on the highly forbidden 2 S 1/2 → 2 F 7/2 octupole transition of trapped 168,170,172,174,176 Yb ions. When combined with previous measurements in Yb + and very recent measurements in Yb, the data reveal a King plot nonlinearity of up to 240σ. The trends exhibited by experimental data are explained by nuclear density functional theory calculations with the Fayans functional. We also find, with 4.3σ confidence, that there is a second distinct source of nonlinearity, and discuss its possible origin.

We propose a surface ion trap design incorporating microwave control electrodes for near-field single-qubit control. The electrodes are arranged so as to provide arbitrary frequency, amplitude and polarization control of the microwave field in one trap zone, while a similar set of electrodes is used to null the residual microwave field in a neighbouring zone. The geometry is chosen to reduce the residual field to the 0.5% level without nulling fields; with nulling, the crosstalk may be kept close to the 0.01% level for realistic microwave amplitude and phase drift. Using standard photolithography and electroplating techniques, we have fabricated a proofof-principle electrode array with two trapping zones. We discuss requirements for the microwave drive system and prospects for scalability to a large two-dimensional trap array.

Individual addressing of qubits is essential for scalable quantum computation. Spatial addressing allows unlimited numbers of qubits to share the same frequency, whilst enabling arbitrary parallel operations. We demonstrate addressing of long-lived 43 Ca + "atomic clock" qubits held in separate zones (960 µm apart) of a microfabricated surface trap with integrated microwave electrodes. Such zones could form part of a "quantum CCD" architecture for a large-scale quantum information processor. By coherently cancelling the microwave field in one zone we measure a ratio of Rabi frequencies between addressed and non-addressed qubits of up to 1400, from which we calculate a spin-flip probability on the qubit transition of the non-addressed ion of 1.3 × 10 −6 . Off-resonant excitation then becomes the dominant error process, at around 5 × 10 −3 . It can be prevented either by working at higher magnetic field, or by polarization control of the microwave field. We implement polarization control with error 2 × 10 −5 , which would suffice to suppress off-resonant excitation to the ∼ 10 −9 level if combined with spatial addressing. Such polarization control could also enable fast microwave operations.

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