Boris Naydenov

Quantum register based on coupled electron spins in a room-temperature solid.

Nitrogen–vacancy centres in diamond have emerged as a promising platform for quantum information processing at room temperature. Now, coherent coupling between two electron spins separated by almost 10 nm has been demonstrated. At this distance, the spins can be addressed individually, which might enable the construction of a network of connected quantum registers.

Room-temperature entanglement between single defect spins in diamond

Entanglement is the central yet fleeting phenomenon of quantum physics. Once being considered a peculiar counter-intuitive property of quantum theory1, it has developed into the most central element of quantum technology. Consequently, there have been a number of experimental demonstrations of entanglement between photons2, atoms3, ions4 and solid-state systems such as spins or quantum dots5, 6, 7, superconducting circuits8, 9 and macroscopic diamond10. Here we experimentally demonstrate entanglement between two engineered single solid-state spin quantum bits (qubits) at ambient conditions. Photon emission of defect pairs reveals ground-state spin correlation. Entanglement (fidelity = 0.67±0.04) is proved by quantum state tomography. Moreover, the lifetime of electron spin entanglement is extended to milliseconds by entanglement swapping to nuclear spins. The experiments mark an important step towards a scalable room-temperature quantum device being of potential use in quantum information processing as well as metrology.

Chemical control of the charge state of nitrogen-vacancy centers in diamond

We investigate the effect of surface termination on the charge state of nitrogen vacancy centers, which have been ion-implanted few nanometers below the surface of diamond. We find that, when changing the surface termination from oxygen to hydrogen, previously stable NV- centers convert into NV0 and, subsequently, into an unknown non-fluorescent state. This effect is found to depend strongly on the implantation dose. Simulations of the electronic band structure confirm the dissappearance of NV- in the vicinity of the hydrogen-terminated surface. The band bending, which induces a p-type surface conductive layer leads to a depletion of electrons in the nitrogen vacancies close to the surface. Therefore, hydrogen surface termination provides a chemical way for the control of the charge state of nitrogen-vacancy centers in diamond. Furthermore, it opens the way to an electrostatic control of the charge state with the use of an external gate electrode.

Creation efficiency of nitrogen-vacancy centres in diamond

Nitrogen-vacancy (NV) colour centres in diamond are attracting growing attention due to potential applications in solid-state quantum information processing and magnetometry. Although proof-of-principle experiments have been demonstrated, further development requires the controllable production of defects with a high yield. In this paper, we experimentally show that the production efficiency of NV defects strongly depends on the ion implantation energy. This can be explained in terms of the number of vacancies produced per implanted ion and surface proximity. The dependence on ion fluence is also underlined, revealing a nonlinear regime and showing how the diamond lattice is damaged at higher fluences.

High sensitivity magnetic imaging using an array of spins in diamond.

We present a solid state magnetic field imaging technique using a two-dimensional array of spins in diamond. The magnetic sensing spin array is made of nitrogen vacancy (NV) centers created at shallow depths. Their optical response is used for measuring external magnetic fields in close proximity. Optically detected magnetic resonance is read out from a 60 x 60 microm(2) field of view in a multiplexed manner using a charge coupled device camera. We experimentally demonstrate full two-dimensional vector imaging of the magnetic field produced by a pair of current carrying microwires. The presented wide-field NV magnetometer offers, in addition to its high magnetic sensitivity and vector reconstruction, an unprecedented spatiotemporal resolution and functionality at room temperature.

Charge state manipulation of qubits in diamond

The nitrogen-vacancy (NV) centre in diamond is a promising candidate for a solid-state qubit. However, its charge state is known to be unstable, discharging from the qubit state NV− into the neutral state NV0 under various circumstances. Here we demonstrate that the charge state can be controlled by an electrolytic gate electrode. This way, single centres can be switched from an unknown non-fluorescent state into the neutral charge state NV0, and the population of an ensemble of centres can be shifted from NV0 to NV−. Numerical simulations confirm the manipulation of the charge state to be induced by the gate-controlled shift of the Fermi level at the diamond surface. This result opens the way to a dynamic control of transitions between charge states and to explore hitherto inaccessible states, such as NV+.

Multiple intrinsically identical single-photon emitters in the solid state

Emitters of indistinguishable single photons are crucial for the growing field of quantum technologies. To realize scalability and increase the complexity of quantum optics technologies, multiple independent yet identical single-photon emitters are required. However, typical solid-state single-photon sources are inherently dissimilar, necessitating the use of electrical feedback or optical cavities to improve spectral overlap between distinct emitters. Here we demonstrate bright silicon vacancy (SiV(-)) centres in low-strain bulk diamond, which show spectral overlap of up to 91% and nearly transform-limited excitation linewidths. This is the first time that distinct single-photon emitters in the solid state have shown intrinsically identical spectral properties. Our results have impact on the application of single-photon sources for quantum optics and cryptography.

Detection of a few metallo-protein molecules using color centers in nanodiamonds.

Nanometer-sized diamonds containing nitrogen-vacancy defect centers (NV) are promising nanosensors in biological environments due to their biocompatibility, bright fluorescence, and high magnetic sensitivity at ambient conditions. Here we report on the detection of ferritin molecules using magnetic noise induced by the inner paramagnetic iron as a contrast mechanism. We observe a significant reduction of both coherence and relaxation time due to the presence of ferritin on the surface of nanodiamonds. Our theoretical model is in excellent agreement with the experimental data and establishes this method as a novel sensing technology for proteins.

Nuclear magnetic resonance spectroscopy with single spin sensitivity

Nuclear magnetic resonance spectroscopy and magnetic resonance imaging at the ultimate sensitivity limit of single molecules or single nuclear spins requires fundamentally new detection strategies. The strong coupling regime, when interaction between sensor and sample spins dominates all other interactions, is one such strategy. In this regime, classically forbidden detection of completely unpolarized nuclei is allowed, going beyond statistical fluctuations in magnetization. Here we realize strong coupling between an atomic (nitrogen–vacancy) sensor and sample nuclei to perform nuclear magnetic resonance on four 29Si spins. We exploit the field gradient created by the diamond atomic sensor, in concert with compressed sensing, to realize imaging protocols, enabling individual nuclei to be located with Angstrom precision. The achieved signal-to-noise ratio under ambient conditions allows single nuclear spin sensitivity to be achieved within seconds.

Dynamical decoupling of a single-electron spin at room temperature

Here we report the increase of the coherence time T