arXiv subjects

I. Farrer

Publications and source records attributed to I. Farrer.

At least 73 records · Page 4Linked to original sources

arXiv2015

Energy-Tunable Quantum Dot with Minimal Fine Structure Created by Using Simultaneous Electric and Magnetic Fields

The neutral biexciton cascade of single quantum dots is a promising source of entangled photon pairs. The character of the entangled state is determined by the energy difference between the excitonic eigenstates known as fine-structure splitting (FSS). Here we reduce the magnitude of the FSS by simultaneously using two independent tuning mechanisms: in-plane magnetic field and vertical electric field. We observe that there exists a minimum possible FSS in each quantum dot which is independent of these tuning mechanisms. However, with simultaneous application of electric and magnetic fields, we show the FSS can be reduced to its minimum value as the energy of emission is tuned over several meV with a 5-T magnet.

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arXiv2015

Engineering quantum dots for electrical control of the fine structure splitting

We have studied the variation in fine-structure splitting (FSS) under application of vertical electric field in a range of quantum dots grown by different methods. In each sample we confirm that this energy splitting changes linearly over the field range we can access. We conclude that this linear tuning is a general feature of self-assembled quantum dots, observed under different growth conditions, emission wavelengths and in different material systems. Statistical measurements of characteristic parameters such as emission energy, Stark shift and FSS tuning are presented which may provide a guide for future attempts to increase the yield of quantum dots that can be tuned to a minimal value of FSS with vertical electric field.

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arXiv2015

Ultrafast electrical control of a resonantly driven single photon source

We demonstrate generation of a pulsed stream of electrically triggered single photons in resonance fluorescence, by applying high frequency electrical pulses to a single quantum dot in a p-i-n diode under resonant laser excitation. Single photon emission was verifed, with the probability of multiple photon emission reduced to 2.8%. We show that despite the presence of charge noise in the emission spectrum of the dot, resonant excitation acts as a filter to generate narrow bandwidth photons.

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arXiv2015

Measurement and control of electron wave packets from a single-electron source

We report an experimental technique to measure and manipulate the arrival-time and energy distributions of electrons emitted from a semiconductor electron pump, operated as both a single-electron source and a two-electron source. Using an energy-selective detector whose transmission we control on picosecond timescales, we can measure directly the electron arrival-time distribution and we determine the upper-bound to the distribution width to be 30 ps. We study the effects of modifying the shape of the voltage waveform that drives the electron pump, and show that our results can be explained by a tunneling model of the emission mechanism. This information was in turn used to control the emission-time difference and energy gap between a pair of electrons.

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arXiv2015

Interference with a Quantum Dot Single-Photon Source and a Laser at Telecom Wavelength

The interference of photons emitted by dissimilar sources is an essential requirement for a wide range of photonic quantum information applications. Many of these applications are in quantum communications and need to operate at standard telecommunication wavelengths to minimize the impact of photon losses and be compatible with existing infrastructure. Here we demonstrate for the first time the quantum interference of telecom-wavelength photons from an InAs/GaAs quantum dot single-photon source and a laser; an important step towards such applications. The results are in good agreement with a theoretical model, indicating a high degree of indistinguishability for the interfering photons.

quant-ph↗

arXiv2015

All-electric all-semiconductor spin field effect transistors

The spin field effect transistor envisioned by Datta and Das opens a gateway to spin information processing. Although the coherent manipulation of electron spins in semiconductors is now possible, the realization of a functional spin field effect transistor for information processing has yet to be achieved, owing to several fundamental challenges such as the low spin-injection efficiency due to resistance mismatch, spin relaxation, and the spread of spin precession angles. Alternative spin transistor designs have therefore been proposed, but these differ from the field effect transistor concept and require the use of optical or magnetic elements, which pose difficulties for the incorporation into integrated circuits. Here, we present an all-electric and all-semiconductor spin field effect transistor, in which these obstacles are overcome by employing two quantum point contacts as spin injectors and detectors. Distinct engineering architectures of spin-orbit coupling are exploited for the quantum point contacts and the central semiconductor channel to achieve complete control of the electron spins -- spin injection, manipulation, and detection -- in a purely electrical manner. Such a device is compatible with large-scale integration and hold promise for future spintronic devices for information processing.

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arXiv2015

Few-second-long correlation times in a quantum dot nuclear spin bath probed by frequency-comb NMR spectroscopy

One of the key challenges in spectroscopy is inhomogeneous broadening that masks the homogeneous spectral lineshape and the underlying coherent dynamics. A variety of techniques including four-wave mixing and spectral hole-burning are used in optical spectroscopy while in nuclear magnetic resonance (NMR) spin-echo is the most common way to counteract inhomogeneity. However, the high-power pulses used in spin-echo and other sequences often create spurious dynamics obscuring the subtle spin correlations that play a crucial role in quantum information applications. Here we develop NMR techniques that allow the correlation times of the fluctuations in a nuclear spin bath of individual quantum dots to be probed. This is achieved with the use of frequency comb excitation which allows the homogeneous NMR lineshapes to be measured avoiding high-power pulses. We find nuclear spin correlation times exceeding 1 s in self-assembled InGaAs quantum dots - four orders of magnitude longer than in strain-free III-V semiconductors. The observed freezing of the nuclear spin fluctuations opens the way for the design of quantum dot spin qubits with a well-understood, highly stable nuclear spin bath.

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arXiv2015

An entangled-LED driven quantum relay over 1 km

Quantum cryptography allows confidential information to be communicated between two parties, with secrecy guaranteed by the laws of nature alone. However, upholding guaranteed secrecy over quantum communication networks poses a further challenge, as classical receive-and-resend routing nodes can only be used conditional of trust by the communicating parties. Here, we demonstrate the operation of a quantum relay over 1 km of optical fiber, which teleports a sequence of photonic quantum bits to a receiver by utilizing entangled photons emitted by a semiconductor LED. The average relay fidelity of the link is 0.90+/-0.03, exceeding the classical bound of 0.75 for the set of states used, and sufficiently high to allow error correction. The fundamentally low multi-photon emission statistics and the integration potential of the source present an appealing platform for future quantum networks.

quant-ph↗

arXiv2014

Ultra-Low-Power Hybrid Light-Matter Solitons

New functionalities in nonlinear optics will require systems with giant optical nonlinearity as well as compatibility with photonic circuit fabrication techniques. Here we introduce a new platform based on strong light-matter coupling between waveguide photons and quantum-well excitons. On a sub-millimeter length scale we generate sub-picosecond bright temporal solitons at a pulse energy of only 0.5 pico-Joules. From this we deduce an unprecedented nonlinear refractive index 3 orders of magnitude larger than in any other ultrafast system. We study both temporal and spatio-temporal nonlinear effects and for the first time observe dark-bright spatio-temporal solitons. Theoretical modelling of soliton formation in the strongly coupled system confirms the experimental observations. These results show the promise of our system as a high speed, low power, integrated platform for physics and devices based on strong interactions between photons.

nlin.PS↗

arXiv2014

Multiplexed Charge-locking Device for Large Arrays of Quantum Devices

We present a method of forming and controlling large arrays of gate-defined quantum devices. The method uses a novel, on-chip, multiplexed charge-locking system and helps to overcome the restraints imposed by the number of wires available in cryostat measurement systems. Two device innovations are introduced. Firstly, a multiplexer design which utilises split gates to allow the multiplexer to divide three or more ways at each branch. Secondly we describe a device architecture that utilises a multiplexer-type scheme to lock charge onto gate electrodes. The design allows access to and control of gates whose total number exceeds that of the available electrical contacts and enables the formation, modulation and measurement of large arrays of quantum devices. We fabricate devices utilising these innovations on n-type GaAs/AlGaAs substrates and investigate the stability of the charge locked on to the gates. Proof-of-concept is shown by measurement of the Coulomb blockade peaks of a single quantum dot formed by a floating gate in the device. The floating gate is seen to drift by approximately one Coulomb oscillation per hour.

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arXiv2014

Hierarchy of modes in an interacting system

Studying interacting fermions in 1D at high energy, we find a hierarchy in the spectral weights of the excitations theoretically and we observe evidence for second-level excitations experimentally. Diagonalising a model of fermions (without spin), we show that levels of the hierarchy are separated by powers of $\mathcal{R}^{2}/L^{2}$, where $\mathcal{R}$ is a length-scale related to interactions and $L$ is the system length. The first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalised single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power-laws at the spectral edge. We measure momentum-resolved tunnelling of electrons (fermions with spin) from/to a wire formed within a GaAs heterostructure, which shows parabolic dispersion of the first-level mode and well-resolved spin-charge separation at low energy with appreciable interaction strength. We find structure resembling the second-level excitations, which dies away quite rapidly at high momentum.

cond-mat.str-el↗

arXiv2013

Heterogeneous teleportation with laser and quantum light sources

Quantum information technology is set to transform critical network security using quantum cryptography, and complex scientific and engineering simulations with quantum computing. Quantum computer nodes may be based on a variety of systems, such as linear optics, ions, or solid state architectures such as NV-centers in diamond, semiconductor quantum dots or spins in silicon. Interfacing any of these platforms with photonic qubits in secure quantum networks will require quantum teleportation protocols to transfer the information, and matter-light teleportation has for some of these systems been demonstrated. However, although it is conceivable that the input photon originates from a dissimilar source to that supplying the entangled resources, every demonstration so far of teleportation using linear optics use the same or identical sources for the input and entangled photons, often accompanied by a fourth heralding photon. Here we show that photons from fundamentally different sources can be used in the optical quantum teleportation protocol. Input photons are generated by a laser, and teleported using polarisation-entangled photon pairs electrically generated by an entangled-light-emitting diode (ELED). The sources have bandwidth differing by a factor 1000, different photon statistics and need not be precisely degenerate- but we still observe a teleportation fidelity of 0.77, beating the quantum limit by 10 standard deviations. This is a significant leap towards practical applications, such as extending the range of existing QKD systems using quantum relays and repeaters, which usually use weak coherent laser pulses for quantum information transport. The use of an ELED offers practical advantages of electrical control, and as we show erases the multi-photon character of the laser input field, thus eliminating errors if used in a quantum optics circuit.

quant-ph↗

arXiv2013

Voltage control of electron-nuclear spin correlation time in a single quantum dot

We demonstrate bias control of the hyperfine coupling between a single electron in an InAs quantum dot and the surrounding nuclear spins monitored through the positively charged exciton X+ emission. In applied longitudinal magnetic fields we vary simultaneously the correlation time of the hyperfine interaction and the nuclear spin relaxation time and thereby the amplitude of the achieved dynamic nuclear polarization under optical pumping conditions. In applied transverse magnetic fields a change in the applied bias allows to switch from the anomalous Hanle effect to the standard electron spin depolarization curves.

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arXiv2013

A non-invasive electron thermometer based on charge sensing of a quantum dot

We present a thermometry scheme to extract the temperature of a 2DEG by monitoring the charge occupation of a weakly tunnel-coupled 'thermometer' quantum dot using a quantum point contact detector. Electronic temperatures between 97 mK and 307 mK are measured by this method with an accuracy of up to 3 mK, and agree with those obtained by measuring transport through a quantum dot. The thermometer does not pass a current through the 2DEG, and can be incorporated as an add-on to measure the temperature simultaneously with another operating device. Further, the tuning is independent of temperature.

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arXiv2013

Ultra-shallow quantum dots in an undoped GaAs/AlGaAs 2D electron gas

We report quantum dots fabricated on very shallow 2-dimensional electron gases, only 30 nm below the surface, in undoped GaAs/AlGaAs heterostructures grown by molecular beam epitaxy. Due to the absence of dopants, an improvement of more than one order of magnitude in mobility (at 2E11 /cm^2) with respect to doped heterostructures with similar depths is observed. These undoped wafers can easily be gated with surface metallic gates patterned by e-beam lithography, as demonstrated here from single-level transport through a quantum dot showing large charging energies (up to 1.75 meV) and excited state energies (up to 0.5 meV).

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arXiv2013

Clock-controlled emission of single-electron wavepackets in a solid-state circuit

We demonstrate the transmission of single electron wavepackets from a clock-controlled source through an empty high-energy edge channel. The quantum dot source is loaded with single electrons which are then emitted with high kinetic energy ($\sim$150 meV). We find at high magnetic field that these electron can be transported over several microns without inelastic electron-electron or electron-phonon scattering. Using a time-resolved spectroscopic technique, we measure the electron energy and wavepacket size at picosecond time scales. We also show how our technique can be used to switch individual electrons into different paths.

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arXiv2012

Quantized escape and formation of edge channels at high Landau levels

We present nonlocal resistance measurements in an ultra high mobility two dimensional electron gas. Our experiments show that even at weak magnetic fields classical guiding along edges leads to a strong non local resistance on macroscopic distances. In this high Landau level regime the transport along edges is dissipative and can be controlled by the amplitude of the voltage drop along the edge. We report resonances in the nonlocal transport as a function of this voltage that are interpreted as escape and formation of edge channels.

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arXiv2012

All-electrical injection and detection of a spin polarized current using 1D conductors

All-electrical control of spin transport in nanostructures has been the central interest and chal- lenge of spin physics and spintronics. Here we demonstrate on-chip spin polarizing/filtering actions by driving the gate-defined one dimensional (1D) conductor, one of the simplest geometries for integrated quantum devices, away from the conventional Ohmic regime. Direct measurement of the spin polarization of the emitted current was performed when the momentum degeneracy was lifted, wherein both the 1D polarizer for spin injection and the analyzer for spin detection were demonstrated. The results showed that a configuration of gates and applied voltages can give rise to a tunable spin polarization, which has implications for the development of spintronic devices and future quantum information processing.

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