arXiv subjects

Chandrasekhar Ramanathan

Publications and source records attributed to Chandrasekhar Ramanathan.

47 records · Page 3Linked to original sources

arXiv2007

Universal Control of Nuclear Spins Via Anisotropic Hyperfine Interactions

We show that nuclear spin subsystems can be completely controlled via microwave irradiation of resolved anisotropic hyperfine interactions with a nearby electron spin. Such indirect addressing of the nuclear spins via coupling to an electron allows us to create nuclear spin gates whose operational time is significantly faster than conventional direct addressing methods. We experimentally demonstrate the feasibility of this method on a solid-state ensemble system consisting of one electron and one nuclear spin.

quant-ph↗

arXiv2007

Dynamics and Control of a Quasi-1D Spin System

We study experimentally a system comprised of linear chains of spin-1/2 nuclei that provides a test-bed for multi-body dynamics and quantum information processing. This system is a paradigm for a new class of quantum information devices that can perform particular tasks even without universal control of the whole quantum system. We investigate the extent of control achievable on the system with current experimental apparatus and methods to gain information on the system state, when full tomography is not possible and in any case highly inefficient.

quant-ph↗

arXiv2007

Simulations of Information Transport in Spin Chains

Transport of quantum information in linear spin chains has been the subject of much theoretical work. Experimental studies by nuclear spin systems in solid-state by NMR (a natural implementation of such models) is complicated since the dipolar Hamiltonian is not solely comprised of nearest-neighbor XY-Heisenberg couplings. We present here a similarity transformation between the XY-Heisenberg Hamiltonian and the grade raising Hamiltonian, an interaction which is achievable with the collective control provided by radio-frequency pulses in NMR. Not only does this second Hamiltonian allows us to simulate the information transport in a spin chain, but it also provides a means to observe its signature experimentally.

quant-ph↗

arXiv2007

Rapid Diffusion of dipolar order enhances dynamic nuclear polarization

In a dynamic nuclear polarization experiment on a 40 mM solution of 4-amino-TEMPO in a 40:60 water/glycerol mixture, we have observed that the bulk dipolar reservoir is cooled to a spin temperature of 15.5 micro-K, following microwave irradiation for 800 s. This is significantly cooler than the 35 mK spin temperature of the Zeeman reservoir. Equilibration of the two reservoirs results in a 50 % increase in the NMR signal intensity, corresponding to a Zeeman spin temperature of 23 mK. In order to achieve this polarization directly, it was necessary to irradiate the sample with microwaves for 1500 s. Cooling of the dipolar reservoir occurs during polarization transport through the magnetic field gradient around the paramagnetic impurity, and is rapidly communicated to the bulk by dipolar spin diffusion. As dipolar spin diffusion is significantly faster than Zeeman spin diffusion, the bulk dipolar reservoir cools faster than the Zeeman reservoir. This process can be exploited to rapidly polarize the nuclear spins, by repeatedly cooling the dipolar system and transferring the polarization to the Zeeman reservoir.

cond-mat.other↗

arXiv2006

Decay of highly-correlated spin states in a dipolar-coupled solid

We have measured the decay of NMR multiple quantum coherence intensities both under the internal dipolar Hamiltonian as well as when this interaction is effectively averaged to zero, in the cubic calcium fluoride (CaF2) spin system and the pseudo one-dimensional system of fluoroapatite. In calcium fluoride the decay rates depend both on the number of correlated spins in the cluster, as well as on the coherence number. For smaller clusters, the decays depend strongly on coherence number, but this dependence weakens as the size of the cluster increases. The same scaling was observed when the coherence distribution was measured in both the usual Zeeman or z basis and the x basis. The coherence decay in the one dimensional fluoroapatite system did not change significantly as a function of the multiple quantum growth time, in contrast to the calcium fluoride case. While the growth of coherence orders is severely restricted in this case, the number of correlated spins should continue to grow, albeit more slowly. All coherence intensities were observed to decay as Gaussian functions in time. In all cases the standard deviation of the observed decay appeared to scale linearly with coherence number.

cond-mat.other↗

arXiv2005

A solid-state NMR three-qubit homonuclear system for quantum information processing: control and characterization

A three-qubit 13C solid-state nuclear magnetic resonance (NMR) system for quantum information processing, based on the malonic acid molecule, is used to demonstrate high-fidelity universal quantum control via strongly-modulating radio-frequency pulses. This control is achieved in the strong-coupling regime, in which the timescales of selective qubit addressing and of two-qubit interactions are comparable.

quant-ph↗

arXiv2005

Multi-spin dynamics of the solid-state NMR Free Induction Decay

We present a new experimental investigation of the NMR free induction decay (FID) in a lattice of spin-1/2 nuclei in a strong Zeeman field. Following a pi/2 pulse, evolution under the secular dipolar Hamiltonian preserves coherence number in the Zeeman eigenbasis, but changes the number of correlated spins in the state. The observed signal is seen to decay as single-spin, single-quantum coherences evolve into multiple-spin coherences under the action of the dipolar Hamiltonian. In order to probe the multiple-spin dynamics during the FID, we measured the growth of coherence orders in a basis other than the usual Zeeman eigenbasis. This measurement provides the first direct experimental observation of the growth of coherent multiple-spin correlations during the FID. Experiments were performed with a cubic lattice of spins (19F in calcium fluoride) and a linear spin chain (19F in fluorapatite). It is seen that the geometrical arrangement of the spins plays a significant role in the development of higher order correlations. The results are discussed in light of existing theoretical models.

cond-mat.other↗

arXiv2004

Entanglement Assisted Metrology

We propose a new approach to the measurement of a single spin state, based on nuclear magnetic resonance (NMR) techniques and inspired by the coherent control over many-body systems envisaged by Quantum Information Processing (QIP). A single target spin is coupled via the natural magnetic dipolar interaction to a large ensemble of spins. Applying external radio frequency (rf) pulses, we can control the evolution of the system so that the spin ensemble reaches one of two orthogonal states whose collective properties differ depending on the state of the target spin and are easily measured. We first describe this measurement process using QIP gates; then we show how equivalent schemes can be defined in terms of the Hamiltonian of the spin system and thus implemented under conditions of real control, using well established NMR techniques. We demonstrate this method with a proof of principle experiment in ensemble liquid state NMR and simulations for small spin systems.

quant-ph↗

arXiv2004

Selective coherence transfers in homonuclear dipolar coupled spin systems

Mapping the physical dipolar Hamiltonian of a solid-state network of nuclear spins onto a system of nearest-neighbor couplings would be extremely useful for a variety of quantum information processing applications, as well as NMR structural studies. We demonstrate such a mapping for a system consisting of an ensemble of spin pairs, where the coupling between spins in the same pair is significantly stronger than the coupling between spins on different pairs. An amplitude modulated RF field is applied on resonance with the Larmor frequency of the spins, with the frequency of the modulation matched to the frequency of the dipolar coupling of interest. The spin pairs appear isolated from each other in the regime where the RF power (omega_1) is such that omega_weak << omega_1 << omega_strong. Coherence lifetimes within the two-spin system are increased from 19 us to 11.1 ms, a factor of 572.

quant-ph↗

arXiv2004

NMR Quantum Information Processing

Nuclear Magnetic Resonance (NMR) has provided a valuable experimental testbed for quantum information processing (QIP). Here, we briefly review the use of nuclear spins as qubits, and discuss the current status of NMR-QIP. Advances in the techniques available for control are described along with the various implementations of quantum algorithms and quantum simulations that have been performed using NMR. The recent application of NMR control techniques to other quantum computing systems are reviewed before concluding with a description of the efforts currently underway to transition to solid state NMR systems that hold promise for scalable architectures.

quant-ph↗

arXiv2004

Encoding multiple quantum coherences in non-commuting bases

Multiple quantum coherences are typically characterised by their coherence number and the number of spins that make up the state, though only the coherence number is normally measured. We present a simple set of measurements that extend our knowledge of the multiple quantum state by recording the coherences in both the $x$ basis and the usual $z$ basis. The coherences in the two bases are related by a similarity transformation. We characterize the growth of the multiple quantum coherences via measurements in the two bases, and show that the rate varies with the coefficient of the driving term in the Hamiltonian. Such measurements in non-commuting bases provides additional information over the 1D method about the state of the spin system. In particular the measurement of coherences in a basis other than the usual $z$ basis allows us to study the dynamics of the spin system under Hamiltonians, such as the secular dipolar Hamiltonian, that conserve $z$ basis coherence number.

quant-ph↗