Florian Rempp

Multipartite entanglement among single spins in diamond.

Robust entanglement at room temperature is a necessary requirement for practical applications in quantum technology. We demonstrate the creation of bipartite- and tripartite-entangled quantum states in a small quantum register consisting of individual 13C nuclei in a diamond lattice. Individual nuclear spins are controlled via their hyperfine coupling to a single electron at a nitrogen-vacancy defect center. Quantum correlations are of high quality and persist on a millisecond time scale even at room temperature, which is adequate for sophisticated quantum operations.

Electric-field sensing using single diamond spins.

The ability to sensitively detect charges under amb ient conditions would be a fascinating new tool benefitting a wide range of researchers ac ross disciplines. However, most current techniques are limited to low-temperature methods l ike single-electron transistors (SET)[1,2], single–electron electrostatic force microscopy[3] a nd scanning tunnelling microscopy [4]. Here we open up a new quantum metrology technique demons trating precision electric field measurement using a single nitrogen-vacancy defect entre(NV) spin in diamond. An AC electric field sensitivity reaching ~ 140V/cm/ √Hz has been achieved. This corresponds to the electric field produced by a single elementary char ge located at a distance of ~ 150 nm from our spin sensor with averaging for one second. By caref ul analysis of the electronic structure of the defect centre, we show how an applied magnetic fiel d influences the electric field sensing properties. By this we demonstrate that diamond defct centre spins can be switched between electric and magnetic field sensing modes and ident ify suitable parameter ranges for both detector schemes. By combining magnetic and electri c field sensitivity, nanoscale detection and ambient operation our study opens up new frontiers in imaging and sensing applications ranging from material science to bioimaging.

Single-Shot Readout of a Single Nuclear Spin

Probed But Not Perturbed The processing and manipulation of quantum information holds great promise in terms of outperforming classical computers and secure communication. However, quantum information is delicate, and even reading the information is a destructive and probabilistic process requiring a number of measurements to home in on the information stored as a quantum state. For the nitrogen vacancy in diamond, Neumann et al. (p. 542, published online 1 July) show that these limitations can be eliminated. A measurement protocol was designed and implemented where the spin state of the nuclear spin of the vacancy could be mapped onto and read out from the surrounding electronic spins in a single-shot measurement nondestructively. The quantum state of a single nitrogen vacancy in diamond can be read out nondestructively in a single-shot measurement. Projective measurement of single electron and nuclear spins has evolved from a gedanken experiment to a problem relevant for applications in atomic-scale technologies like quantum computing. Although several approaches allow for detection of a spin of single atoms and molecules, multiple repetitions of the experiment that are usually required for achieving a detectable signal obscure the intrinsic quantum nature of the spin’s behavior. We demonstrated single-shot, projective measurement of a single nuclear spin in diamond using a quantum nondemolition measurement scheme, which allows real-time observation of an individual nuclear spin’s state in a room-temperature solid. Such an ideal measurement is crucial for realization of, for example, quantum error correction protocols in a quantum register.

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.

Coherence of single spins coupled to a nuclear spin bath of varying density

The dynamics of single electron and nuclear spins in a diamond lattice with different 13 C nuclear spin concentration is investigated. It is shown that coherent control of up to three individual nuclei in a dense nuclear spin cluster is feasible. The free-induction decays of nuclear spin Bell states and single nuclear coherences among 13 C nuclear spins are compared and analyzed. Reduction in a free-induction-decay time T2 and a coherence time T2 upon increase in nuclear spin concentration has been found. For pure diamond, T 2 as long as 30 s and T2 of up to 0.65 ms for the electron spin has been observed. The 13 C concentration dependence of T 2 is explained by Fermi contact and dipolar interactions with nuclei in the lattice. It has been found that T2 decreases approximately as 1 / n, where n is 13 C concentration, which corresponds to the reported theoretical line of T2 for an electron spin interacting with a nuclear spin bath.

Local effective dynamics of quantum systems: A generalized approach to work and heat

By computing the local energy expectation values with respect to some local measurement basis we show that for any quantum system there are two fundamentally different contributions: changes in energy that do not alter the local von Neumann entropy and changes that do. We identify the former as work and the latter as heat. Since our derivation makes no assumptions on the system Hamiltonian or its state, the result is valid even for states arbitrarily far from equilibrium. Examples are discussed ranging from the classical limit to purely quantum-mechanical scenarios, i.e. where the Hamiltonian and the density operator do not commute.

Cyclic cooling algorithm

We introduce a scheme to perform the cooling algorithm, first presented by Boykin et al. in 2002, for an arbitrary number of times on the same set of qbits. We achieve this goal by adding an additional SWAP gate and a bath contact to the algorithm. This way one qbit may repeatedly be cooled without adding additional qbits to the system. By using a product Liouville space to model the bath contact we calculate the density matrix of the system after a given number of applications of the algorithm.

Towards T1-limited magnetic resonance imaging using Rabi beats

Two proof-of-principle experiments toward T1-limited magnetic resonance imaging with NV centers in diamond are demonstrated. First, a large number of Rabi oscillations is measured and it is demonstrated that the hyperfine interaction due to the NV’s 14N can be extracted from the beating oscillations. Second, the Rabi beats under V-type microwave excitation of the three hyperfine manifolds is studied experimentally and described theoretically.

Response to Comment on “Multipartite Entanglement Among Single Spins in Diamond”

Our study reported entanglement among single spins in diamond. Lovett and Benjamin argue that three of six described entangled states were not achieved. Here, we explain our choice of entangled states and discuss their importance for quantum information processing. We also show that the eigenstates discussed by Lovett and Benjamin, although formally entangled and routinely generated in our experiments, cannot be used to detect nonlocal correlations.