Sensitive nanoscale magnetic resonance imaging (MRI) of target spins using nitrogen--vacancy (NV) centers in diamond will require a quantitative understanding of dominant noise at the surface. We probe this noise by applying dynamical decoupling to shallow NVs at calibrated depths. Results support a model of NV dephasing by a surface bath of electronic spins having a correlation rate of 200 kHz, much faster than that of the bulk N spin bath. Our method of combining nitrogen delta--doping growth and nanoscale depth imaging paves a way for studying spin noise present in diverse material surfaces.The negatively charged nitrogen--vacancy (NV) center in diamond is a robust quantum sensor of magnetic fields [1--4]. Although an individual NV has the capability to detect small numbers of electronic [5--7] and nuclear spins external to diamond [8--10], its widespread application in spin imaging has been limited by the ability to form shallow NVs that retain spin coherence near the surface. Shallow spins with long coherence time, T 2 , are important because quantum phase accumulation between two electronic spin states of the NV provides signal transduction, and hence the minimum detectable magnetic dipole moment scales as δµ ∝ r 3 / T 2 , with r the NV--target spin distance [3,4]. At odds with this figure of merit is strong evidence that the diamond crystal surface adversely affects T 2 , reducing it from ~2 ms for bulk NVs [11,12] to less than 10 µs for few--nm deep NVs [6,13--16], but the origin of this decoherence is an outstanding question. We consider in this letter a model of surface spin induced decoherence, a theory which has emerged from experiments on other systems [20,21] where long coherence is a requirement, such as in superconducting circuits [17,18] and spin

Electron spins and photons are complementary quantum-mechanical objects that can be used to carry, manipulate and transform quantum information. To combine these resources, it is desirable to achieve the coherent coupling of a single spin to photons stored in a superconducting resonator. Using a circuit design based on a nanoscale spin-valve, we coherently hybridize the individual spin and charge states of a double quantum dot while preserving spin coherence. This scheme allows us to achieve spin-photon coupling up to the MHz range at the single spin level. The cooperativity is found to reach 2.3, and the spin coherence time is about 60ns. We thereby demonstrate a mesoscopic device suitable for nondestructive spin read-out and distant spin coupling.The methods of cavity quantum electrodynamics hold promise for an efficient use of the spin degree of freedom in the context of quantum computation and simulation (1). Realizing a coherent coupling between a single spin and cavity photons could enable quantum nondemolition readout of a single spin, quantum spin manipulation, and facilitate the coupling of distant spins (1,2,3,4). It could also be used in hybrid architectures in which single spins are coupled to superconducting quantum bits (5), or to simulate one-dimensional spin chains (6).The natural coupling of a spin to the magnetic part of the electromagnetic field is weak (7). In order to enhance it, one needs a large spin ensemble, typically of about 10 12 spins (8,9,10,11,12,13), but these ensembles lose the intrinsic non-linearity of a single spin 1/2.Alternatively, several theoretical proposals have been put forward to electrically couple single spins to superconducting resonators in a mesoscopic circuit (14,15,16,17), building on the exquisite accuracy with which superconducting circuits can be used to couple superconducting qubits and photons and manipulate them (18). One such approach is to engineer an artificial spin-photon interaction by using ferromagnetic reservoirs (15).Noteworthy, the spin/photon coupling is also raising experimental efforts in the optical domain (19,20,21,22,23), but the circuit approach presents the significant advantage of scalability.Recent experiments have demonstrated the coupling of double quantum dot charge states to coplanar waveguide resonators, with a coupling strength gcharge ≈ 2  10 -50 MHz (24,25,26,27,28). In Ref,(29), the spin blockade read-out technique in quantum dots (30) was combined with charge sensing with a microwave resonator (31). In contrast to this spinblockade scheme, here we use the ferromagnetic proximity effect in a coherent conductor to engineer a spin-photon coupling. Our scheme relies on the use of a non collinear spin valve geometry, which realizes an artificial spin orbit interaction (15). Specifically, we contact two non collinear ferromagnets on a carbon nanotube double quantum dot.Our device is shown in Fig. 1, A-C. Our resonator is similar to a previous experiment (27) with a coupling scheme adapted from (24). It is a Nb resonator with a qua...

International audienc

Microwave cavities have been widely used to investigate the behavior of closed few-level systems. Here, we show that they also represent a powerful probe for the dynamics of charge transfer between a discrete electronic level and fermionic continua. We have combined experiment and theory for a carbon nanotube quantum dot coupled to normal metal and superconducting contacts. In equilibrium conditions, where our device behaves as an effective quantum dot-normal metal junction, we approach a universal photon dissipation regime governed by a quantum charge relaxation effect. We observe how photon dissipation is modified when the dot admittance turns from capacitive to inductive. When the fermionic reservoirs are voltage biased, the dot can even cause photon emission due to inelastic tunneling to/from a BardeenCooper-Schrieffer peak in the density of states of the superconducting contact. We can model these numerous effects quantitatively in terms of the charge susceptibility of the quantum dot circuit. This validates an approach that could be used to study a wide class of mesoscopic QED devices.

In a standard Josephson junction the current is zero when the phase difference between the superconducting leads is zero. This condition is protected by parity and time-reversal symmetries. However, the combined presence of spinorbit coupling and magnetic field breaks these symmetries [1] and can lead to a finite supercurrent even when the phase difference is zero [2,3]. This is the so called anomalous Josephson effectthe hallmark effect of superconducting spintronics -and can be characterized by the corresponding anomalous phase shift (φ 0 ) [4,5]. We report the observation of a tunable anomalous Josephson effect in InAs/Al Josephson junctions measured via a superconducting quantum interference device (SQUID). By gate controlling the density of InAs we are able to tune the spin-orbit coupling of the Josephson junction by more than one order of magnitude. This gives us the ability to tune φ 0 , and opens several new opportunities for superconducting spintronics [6], and new possibilities for realizing and characterizing topological superconductivity [7][8][9].Superconductivity and magnetism have long been two of the main focuses of condensed matter physics. Interfacing materials with these two opposed types of electron order can serve as a platform to host many new phenomena. Recently these systems have drawn renewed theoretical and experimental attention in the context of superconducting spintronics [6] and in the search for Majorana fermions [10][11][12][13]. Novel heterostructures can provide the ingredients that are typically needed: superconducting pairing, breaking of time reversal symmetry, and strong spin-orbit coupling.A basic property of superconducting systems is that we can introduce a relation between charge current and the superconductor's phase. In the canonical example of a Josephson junction (JJ), this is the current-phase relationship (CPR), where φ is the phase difference between the two superconductors. Systems with nontrivial spin texture generally introduce a relationship between charge and spin. In the case of spin-orbit coupling this can manifest in many ways including the spin Hall effect and topological edge states [14].A hybrid system, combining spin-orbit coupling and superconductivity, results in a much richer physics where phase, charge current and spin are all interdependent. This gives rise to new phenomena such as an anomalous phase shift which is the hallmark effect of superconduct-ing spintronics [6]. In a standard JJ, the CPR always satisfies the condition I(φ = 0) = 0. This condition is protected by parity and time-reversal symmetries. However the presence of spin-orbit coupling along with the application of an in-plane magnetic field can break these symmetries [1]. This allows an anomalous phase (φ 0 ), which means that with no current flowing there can be a non-zero phase across the junction or, conversely, at zero phase a current can flow. This is also understood in the context of the spin-galvanic effect, also known as the inverse Edelstein effect. It states that in a norm...

No abstract

Semiconductor-based Josephson junctions provide a platform for studying proximity effect due to the possibility of tuning junction properties by gate voltage and large-scale fabrication of complex Josephson circuits. Recently Josephson junctions using InAs weak link with epitaxial aluminum contact have improved the product of normal resistance and critical current, IcRN , in addition to fabrication process reliability. Here we study similar devices with epitaxial contact and find large supercurrent and substantial product of IcRN in our junctions. However we find a striking difference when we compare these samples with higher mobility samples in terms of product of excess current and normal resistance, IexRN . The excess current is negligible in lower mobility devices while it is substantial and independent of gate voltage and junction length in high mobility samples. This indicates that even though both sample types have epitaxial contacts only the high-mobility one has a high transparency interface. In the high mobility short junctions, we observe values of IcRN /∆ ∼ 2.2 and IexRN /∆ ∼ 1.5 in semiconductor weak links.

The interplay of superconductivity with a non-trivial spin texture holds promises for the engineering of non-abelian Majorana quasi-particles. A wide class of systems expected to exhibit exotic correlations are based on nanoscale conductors with strong spin-orbit interaction, subject to a strong external magnetic field. The strength of the spin-orbit coupling is a crucial parameter for the topological protection of Majorana modes as it forbids other trivial excitations at low energy 1,2 . The spin-orbit interaction is in principle intrinsic to a material. As a consequence, experimental efforts have been recently focused on semiconducting nanoconductors or spin-active atomic chains contacted to a superconductor 3,4,5,6,7 . Alternatively, we show how both a spin-orbit and a Zeeman effect can be autonomously induced by using a magnetic texture coupled to any low dimensional conductor, here a carbon nanotube. Transport spectroscopy through superconducting contacts reveals oscillations of Andreev like states under a change of the magnetic texture. These oscillations are well accounted for by a scattering