Gopalakrishnan Balasubramanian

Bio: Gopalakrishnan Balasubramanian is an academic researcher from Max Planck Society. The author has contributed to research in topics: Diamond & Nitrogen-vacancy center. The author has an hindex of 20, co-authored 38 publications receiving 6995 citations. Previous affiliations of Gopalakrishnan Balasubramanian include University of Göttingen & University of Stuttgart.

Nanoscale imaging magnetometry with diamond spins under ambient conditions

Abstract: Magnetic resonance imaging and optical microscopy are key technologies in the life sciences. For microbiological studies, especially of the inner workings of single cells, optical microscopy is normally used because it easily achieves resolution close to the optical wavelength. But in conventional microscopy, diffraction limits the resolution to about half the wavelength. Recently, it was shown that this limit can be partly overcome by nonlinear imaging techniques, but there is still a barrier to reaching the molecular scale. In contrast, in magnetic resonance imaging the spatial resolution is not determined by diffraction; rather, it is limited by magnetic field sensitivity, and so can in principle go well below the optical wavelength. The sensitivity of magnetic resonance imaging has recently been improved enough to image single cells, and magnetic resonance force microscopy has succeeded in detecting single electrons and small nuclear spin ensembles. However, this technique currently requires cryogenic temperatures, which limit most potential biological applications. Alternatively, single-electron spin states can be detected optically, even at room temperature in some systems. Here we show how magneto-optical spin detection can be used to determine the location of a spin associated with a single nitrogen-vacancy centre in diamond with nanometre resolution under ambient conditions. By placing these nitrogen-vacancy spins in functionalized diamond nanocrystals, biologically specific magnetofluorescent spin markers can be produced. Significantly, we show that this nanometre-scale resolution can be achieved without any probes located closer than typical cell dimensions. Furthermore, we demonstrate the use of a single diamond spin as a scanning probe magnetometer to map nanoscale magnetic field variations. The potential impact of single-spin imaging at room temperature is far-reaching. It could lead to the capability to probe biologically relevant spins in living cells.

Ultralong spin coherence time in isotopically engineered diamond

Abstract: As quantum mechanics ventures into the world of applications and engineering, materials science faces the necessity to design matter to quantum grade purity. For such materials, quantum effects define their physical behaviour and open completely new (quantum) perspectives for applications. Carbon-based materials are particularly good examples, highlighted by the fascinating quantum properties of, for example, nanotubes or graphene. Here, we demonstrate the synthesis and application of ultrapure isotopically controlled single-crystal chemical vapour deposition (CVD) diamond with a remarkably low concentration of paramagnetic impurities. The content of nuclear spins associated with the (13)C isotope was depleted to 0.3% and the concentration of other paramagnetic defects was measured to be <10(13) cm(-3). Being placed in such a spin-free lattice, single electron spins show the longest room-temperature spin dephasing times ever observed in solid-state systems (T2=1.8 ms). This benchmark will potentially allow observation of coherent coupling between spins separated by a few tens of nanometres, making it a versatile material for room-temperature quantum information processing devices. We also show that single electron spins in the same isotopically engineered CVD diamond can be used to detect external magnetic fields with a sensitivity reaching 4 nT Hz(-1/2) and subnanometre spatial resolution.

Electric-field sensing using single diamond spins

Abstract: Point defects in diamond known as nitrogen-vacancy centres have been shown to be sensitive to minute magnetic fields, even at room temperature. A demonstration that the spin associated with these defect centres is also sensitive to electric fields holds out the prospect of a sensor that can resolve, under ambient conditions, single spins and single elementary charges at the nanoscale.

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

Abstract: 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.

Dynamic Polarization of Single Nuclear Spins by Optical Pumping of Nitrogen-Vacancy Color Centers in Diamond at Room Temperature

Abstract: We report a versatile method to polarize single nuclear spins in diamond, based on optical pumping of a single nitrogen-vacancy (NV) defect and mediated by a level anticrossing in its excited state. A nuclear-spin polarization higher than 98% is achieved at room temperature for the 15N nuclear spin associated with the NV center, corresponding to microK effective nuclear-spin temperature. We then show simultaneous initialization of two nuclear spins in the vicinity of a NV defect. Such robust control of nuclear-spin states is a key ingredient for further scaling up of nuclear-spin based quantum registers in diamond.

The properties and applications of nanodiamonds

Abstract: Nanodiamonds have excellent mechanical and optical properties, high surface areas and tunable surface structures. They are also non-toxic, which makes them well suited to biomedical applications. Here we review the synthesis, structure, properties, surface chemistry and phase transformations of individual nanodiamonds and clusters of nanodiamonds. In particular we discuss the rational control of the mechanical, chemical, electronic and optical properties of nanodiamonds through surface doping, interior doping and the introduction of functional groups. These little gems have a wide range of potential applications in tribology, drug delivery, bioimaging and tissue engineering, and also as protein mimics and a filler material for nanocomposites.

Quantum Computing

Abstract: Quantum mechanics---the theory describing the fundamental workings of nature---is famously counterintuitive: it predicts that a particle can be in two places at the same time, and that two remote particles can be inextricably and instantaneously linked These predictions have been the topic of intense metaphysical debate ever since the theory's inception early last century However, supreme predictive power combined with direct experimental observation of some of these unusual phenomena leave little doubt as to its fundamental correctness In fact, without quantum mechanics we could not explain the workings of a laser, nor indeed how a fridge magnet operates Over the last several decades quantum information science has emerged to seek answers to the question: can we gain some advantage by storing, transmitting and processing information encoded in systems that exhibit these unique quantum properties? Today it is understood that the answer is yes Many research groups around the world are working towards one of the most ambitious goals humankind has ever embarked upon: a quantum computer that promises to exponentially improve computational power for particular tasks A number of physical systems, spanning much of modern physics, are being developed for this task---ranging from single particles of light to superconducting circuits---and it is not yet clear which, if any, will ultimately prove successful Here we describe the latest developments for each of the leading approaches and explain what the major challenges are for the future

Quantum sensing

Abstract: "Quantum sensing" describes the use of a quantum system, quantum properties or quantum phenomena to perform a measurement of a physical quantity Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors, or atomic clocks More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits The field is expected to provide new opportunities - especially with regard to high sensitivity and precision - in applied physics and other areas of science In this review, we provide an introduction to the basic principles, methods and concepts of quantum sensing from the viewpoint of the interested experimentalist

Nanoscale magnetic sensing with an individual electronic spin in diamond

Abstract: Detection of weak magnetic fields with nanoscale spatial resolution is an outstanding problem in the biological and physical sciences. For example, at a distance of 10 nm, the spin of a single electron produces a magnetic field of about 1 muT, and the corresponding field from a single proton is a few nanoteslas. A sensor able to detect such magnetic fields with nanometre spatial resolution would enable powerful applications, ranging from the detection of magnetic resonance signals from individual electron or nuclear spins in complex biological molecules to readout of classical or quantum bits of information encoded in an electron or nuclear spin memory. Here we experimentally demonstrate an approach to such nanoscale magnetic sensing, using coherent manipulation of an individual electronic spin qubit associated with a nitrogen-vacancy impurity in diamond at room temperature. Using an ultra-pure diamond sample, we achieve detection of 3 nT magnetic fields at kilohertz frequencies after 100 s of averaging. In addition, we demonstrate a sensitivity of 0.5 muT Hz(-1/2) for a diamond nanocrystal with a diameter of 30 nm.

Ultralong spin coherence time in isotopically engineered diamond

Abstract: As quantum mechanics ventures into the world of applications and engineering, materials science faces the necessity to design matter to quantum grade purity. For such materials, quantum effects define their physical behaviour and open completely new (quantum) perspectives for applications. Carbon-based materials are particularly good examples, highlighted by the fascinating quantum properties of, for example, nanotubes or graphene. Here, we demonstrate the synthesis and application of ultrapure isotopically controlled single-crystal chemical vapour deposition (CVD) diamond with a remarkably low concentration of paramagnetic impurities. The content of nuclear spins associated with the (13)C isotope was depleted to 0.3% and the concentration of other paramagnetic defects was measured to be <10(13) cm(-3). Being placed in such a spin-free lattice, single electron spins show the longest room-temperature spin dephasing times ever observed in solid-state systems (T2=1.8 ms). This benchmark will potentially allow observation of coherent coupling between spins separated by a few tens of nanometres, making it a versatile material for room-temperature quantum information processing devices. We also show that single electron spins in the same isotopically engineered CVD diamond can be used to detect external magnetic fields with a sensitivity reaching 4 nT Hz(-1/2) and subnanometre spatial resolution.