arXiv subjects

G. Andrew D. Briggs

Publications and source records attributed to G. Andrew D. Briggs.

At least 37 records · Page 2Linked to original sources

arXiv2017

Conditioned spin and charge dynamics of a single electron quantum dot

In this article we describe the incoherent and coherent spin and charge dynamics of a single electron quantum dot. We use a stochastic master equation to model the state of the system, as inferred by an observer with access to only the measurement signal. Measurements obtained during an interval of time contribute, by a past quantum state analysis, to our knowledge about the system at any time $t$ within that interval. Such analysis permits precise estimation of physical parameters, and we propose and test a modification of the classical Baum-Welch parameter re-estimation method to systems driven by both coherent and incoherent processes.

quant-ph↗

arXiv2017

Vibrational effects in charge transport through a molecular double quantum dot

Recent progress in the field of molecular electronics has revealed the fundamental importance of the coupling between the electronic degrees of freedom and specific vibrational modes. Considering the examples of a molecular dimer and a carbon nanotube double quantum dot, we here theoretically investigate transport through a two-site system that is strongly coupled to a single vibrational mode. Using a quantum master equation approach, we demonstrate that, depending on the relative positions of the two dots, electron-phonon interactions can lead to negative differential conductance and suppression of the current through the system. We also discuss the experimental relevance of the presented results and possible implementations of the studied system.

cond-mat.mes-hall↗

arXiv2016

Resonant optomechanics with a vibrating carbon nanotube and a radio-frequency cavity

In an optomechanical setup, the coupling between cavity and resonator can be increased by tuning them to the same frequency. We study this interaction between a carbon nanotube resonator and a radio-frequency circuit. In this resonant regime, the vacuum optomechanical coupling is enhanced by the DC voltage coupling the cavity and the mechanical resonator. Using the cavity to detect the nanotube's motion, we observe and simulate interference between mechanical and electrical oscillations. We measure the mechanical ring-down and show that further improvements to the system could enable measurement of mechanical motion at the quantum limit.

cond-mat.mes-hall↗

arXiv2016

Photon-assisted tunneling and charge dephasing in a carbon nanotube double quantum dot

We report microwave-driven photon-assisted tunneling in a suspended carbon nanotube double quantum dot. From the resonant linewidth at a temperature of 13 mK, the charge dephasing time is determined to be 280 +- 30 ps. The linewidth is independent of driving frequency, but increases with increasing temperature. The moderate temperature dependence is inconsistent with expectations from electron-phonon coupling alone, but consistent with charge noise arising in the device. The extracted level of charge noise is comparable with that expected from previous measurements of a valley-spin qubit, where it was hypothesized to be the main cause of qubit decoherence. Our results suggest a possible route towards improved valley-spin qubits.

cond-mat.mes-hall↗

arXiv2016

Detecting continuous spontaneous localisation with charged bodies in a Paul trap

Continuous spontaneous localisation (CSL) is a model that captures the effects of a class of extensions to quantum theory which are expected to result from quantum gravity, and is such that wavefunction collapse is a physical process. The rate of such a process could be very much lower than the upper bounds set by searches to date, and yet still modify greatly the interpretation of quantum mechanics and solve the quantum measurement problem. Consequently experiments are sought to explore this. We describe an experiment that has the potential to extend sensitivity to CSL by many orders of magnitude. The method is to detect heating of the motion of charged macroscopic objects confined in a Paul trap. We discuss the detection and the chief noise sources. We find that CSL with standard parameters could be observed using a vibration-isolated ion trap of size 1 cm at ultra-low pressure, with optical interferometric detection.

quant-ph↗

arXiv2015

Three-terminal graphene single-electron transistor fabricated using feedback-controlled electroburning

We report room-temperature Coulomb blockade in a single layer graphene three-terminal single-electron transistor (SET) fabricated using feedback-controlled electroburning. The small separation between the side gate electrode and the graphene quantum dot results in a gate coupling up to 3 times larger compared to the value found for the back gate electrode. This allows for an effective tuning between the conductive and Coulomb blocked state using a small side gate voltage of about 1V. The technique can potentially be used in the future to fabricate all-graphene based room temperature single-electron transistors or three terminal single molecule transistors with enhanced gate coupling.

cond-mat.mes-hall↗

arXiv2014

Electrically driven spin resonance in a bent disordered carbon nanotube

Resonant manipulation of carbon nanotube valley-spin qubits by an electric field is investigated theoretically. We develop a new analysis of electrically driven spin resonance exploiting fixed physical characteristics of the nanotube: a bend and inhomogeneous disorder. The spectrum is simulated for an electron valley-spin qubit coupled to a hole valley-spin qubit and an impurity electron spin, and features that coincide with a recent measurement are identified. We show that the same mechanism allows resonant control of the full four-dimensional spin-valley space.

cond-mat.mes-hall↗

arXiv2013

Quantum sensors based on weak-value amplification cannot overcome decoherence

Sensors that harness exclusively quantum phenomena (such as entanglement) can achieve superior performance compared to those employing only classical principles. Recently, a technique based on postselected, weakly-performed measurements has emerged as a method of overcoming technical noise in the detection and estimation of small interaction parameters, particularly in optical systems. The question of which other types of noise may be combatted remains open. We here analyze whether the effect can overcome decoherence in a typical field sensing scenario. Benchmarking a weak, postselected measurement strategy against a strong, direct strategy we conclude that no advantage is achievable, and that even a small amount of decoherence proves catastrophic to the weak-value amplification technique.

quant-ph↗

arXiv2012

Comment on `A scattering quantum circuit for measuring Bell's time inequality: a nuclear magnetic resonance demonstration using maximally mixed states'

A recent paper by Souza, Oliveira and Sarthour (SOS) reports the experimental violation of a Leggett-Garg inequality (sometimes referred to as a temporal Bell inequality). The inequality tests for quantum mechanical superposition: if the inequality is violated, the dynamics cannot be explained by a large class of classical theories under the heading of macrorealism. Experimental tests of the LG inequality are beset by the difficulty of performing the necessary so-called 'non-invasive' measurements (which for the macrorealist will extract information from a system of interest without disturbing it). SOS argue that they nevertheless achieve this difficult goal by putting the system in a maximally mixed state. The system then allegedly undergoes no perturbation during their experiment. Unfortunately the method is ultimately unconvincing to a skeptical macrorealist, and so the conclusions drawn by SOS are unjustified.

quant-ph↗

arXiv2012

Opening up the Quantum Three-Box Problem with Undetectable Measurements

One of the most striking features of quantum mechanics is the profound effect exerted by measurements alone. Sophisticated quantum control is now available in several experimental systems, exposing discrepancies between quantum and classical mechanics whenever measurement induces disturbance of the interrogated system. In practice, such discrepancies may frequently be explained as the back-action required by quantum mechanics adding quantum noise to a classical signal. Here we implement the 'three-box' quantum game of Aharonov and Vaidman in which quantum measurements add no detectable noise to a classical signal, by utilising state-of-the-art control and measurement of the nitrogen vacancy centre in diamond. Quantum and classical mechanics then make contradictory predictions for the same experimental procedure, however classical observers cannot invoke measurement-induced disturbance to explain this discrepancy. We quantify the residual disturbance of our measurements and obtain data that rule out any classical model by > 7.8 standard deviations, allowing us for the first time to exclude the property of macroscopic state-definiteness from our system. Our experiment is then equivalent to a Kochen-Spekker test of quantum non-contextuality that successfully addresses the measurement detectability loophole.

quant-ph↗

arXiv2011

Violation of a Leggett-Garg inequality with ideal non-invasive measurements

The quantum superposition principle states that an entity can exist in two different states simultaneously, counter to our 'classical' intuition. Is it possible to understand a given system's behaviour without such a concept? A test designed by Leggett and Garg can rule out this possibility. The test, originally intended for macroscopic objects, has been implemented in various systems. However to-date no experiment has employed the 'ideal negative result' measurements that are required for the most robust test. Here we introduce a general protocol for these special measurements using an ancillary system which acts as a local measuring device but which need not be perfectly prepared. We report an experimental realisation using spin-bearing phosphorus impurities in silicon. The results demonstrate the necessity of a non-classical picture for this class of microscopic system. Our procedure can be applied to systems of any size, whether individually controlled or in a spatial ensemble.

quant-ph↗

arXiv2010

Storage of multiple coherent microwave excitations in an electron spin ensemble

Electron and nuclear spins have good coherence times and an ensemble of spins is a promising candidate for a quantum memory. By employing holographic techniques via field gradients a single ensemble may be used to store many bits of information. Here we present a coherent memory using a pulsed magnetic field gradient, and demonstrate the storage and retrieval of up to 100 weak 10 GHz coherent excitations in collective states of an electron spin ensemble. We further show that such collective excitations in the electron spin can then be stored in nuclear spin states, which offer coherence times in excess of seconds.

quant-ph↗

arXiv2010

Electron paramagnetic resonance study of ErSc2NC80

We present an electron paramagnetic resonance (EPR) study of ErSc2N@C80 fullerene in which there are two Er3+ sites corresponding to two different configurations of the ErSc2N cluster inside the C80 cage. For each configuration, the EPR spectrum is characterized by a strong anisotropy of the g factors (gx,y = 2.9, gz = 13.0 and gx,y = 5.3, gz = 10.9). Illumination within the cage absorption range (<600 nm) induces a rearrangement of the ErSc2N cluster inside the cage. We follow the temporal dependence of this rearrangement phenomenologically under various conditions.

physics.chem-ph↗

arXiv2010

Entangling remote nuclear spins linked by a chromophore

Molecular nanostructures may constitute the fabric of future quantum technologies, if their degrees of freedom can be fully harnessed. Ideally one might use nuclear spins as low-decoherence qubits and optical excitations for fast controllable interactions. Here, we present a method for entangling two nuclear spins through their mutual coupling to a transient optically-excited electron spin, and investigate its feasibility through density functional theory and experiments on a test molecule. From our calculations we identify the specific molecular properties that permit high entangling power gates under simple optical and microwave pulses; synthesis of such molecules is possible with established techniques.

quant-ph↗

arXiv2008

Probing the Interior Environment of Carbon Nano-test-tubes

We report the filling of single walled carbon nanotubes with an electron spin-active fullerene species where a nitroxide radical is functionalized on the fullerene cage. High resolution transmission electron microscopy (HRTEM), optical absorption and electron spin resonance (ESR) are used to determine the rotational behavior of the encapsulated molecules and determine the polar nature of the nanotube interior.

cond-mat.mtrl-sci↗

arXiv2008

Switchable ErSc2N rotor within a C80 fullerene cage: An EPR and photoluminescence excitation study

Systems exhibiting both spin and orbital degrees of freedom, of which Er3+ is one, can offer mechanisms for manipulating and measuring spin states via optical excitations. Motivated by the possibility of observing photoluminescence and electron paramagnetic resonance from the same species located within a fullerene molecule, we initiated an EPR study of Er3+ in ErSc2N@C80. Two orientations of the ErSc2N rotor within the C80 fullerene are observed in EPR, consistent with earlier studies using photoluminescence excitation (PLE) spectroscopy. For some crystal field orientations, electron spin relaxation is driven by an Orbach process via the first excited electronic state of the 4I_15/2 multiplet. We observe a change in the relative populations of the two ErSc2N configurations upon the application of 532 nm illuminations, and are thus able to switch the majority cage symmetry. This photoisomerisation, observable by both EPR and PLE, is metastable, lasting many hours at 20 K.

cond-mat.mtrl-sci↗

arXiv2007

Modelling spin qubits in carbon peapods

We have calculated electron spin interactions in chains of Sc@C82 endohedral fullerenes in isolation and inserted into a semiconducting or metallic single-walled carbon nanotube to form a peapod. Using hybrid density functional theory (DFT), we find that the spin resides mainly on the fullerene cage, whether or not the fullerenes are in a nanotube. The spin interactions decay exponentially with fullerene separation, and the system can be described by a simple antiferromagnetic Heisenberg spin chain. A generalised Hubbard-Anderson model gives an exchange parameter J and a Coulomb parameter U in good agreement with the DFT values. Within the accuracy of the calculations, neither semiconducting nor metallic nanotubes affect the interactions between the fullerene electron spins.

cond-mat.mtrl-sci↗

arXiv2007

Environmental effects on electron spin relaxation in N@C60

We examine environmental effects of surrounding nuclear spins on the electron spin relaxation of the N@C60 molecule (which consists of a nitrogen atom at the centre of a fullerene cage). Using dilute solutions of N@C60 in regular and deuterated toluene, we observe and model the effect of translational diffusion of nuclear spins of the solvent molecules on the N@C60 electron spin relaxation times. We also study spin relaxation in frozen solutions of N@C60 in CS2, to which small quantities of a glassing agent, S2Cl2 are added. At low temperatures, spin relaxation is caused by spectral diffusion of surrounding nuclear 35Cl and 37Cl spins in the S2Cl2, but nevertheless, at 20 K, T2 times as long as 0.23 ms are observed.

quant-ph↗