Multi-dimensional photonic states from a quantum dot

Quantum states superposed across multiple particles or degrees of freedom offer an advantage in the development of quantum technologies. Creating these states deterministically and with high efficiency is an ongoing challenge. A promising approach is the repeated excitation of multi-level quantum emitters, which have been shown to naturally generate light with quantum statistics. Here we describe how to create one class of higher dimensional quantum state, a so called W-state, which is superposed across multiple time bins. We do this by repeated Raman scattering of photons from a charged quantum dot in a pillar microcavity. We show this method can be scaled to larger dimensions with no reduction in coherence or single-photon character. We explain how to extend this work to enable the deterministic creation of arbitrary time-bin encoded qudits

Quantum photonics hybrid integration platform

Fundamental to integrated photonic quantum computing is an on-chip method for routing and modulating quantum light emission. We demonstrate a hybrid integration platform consisting of arbitrarily designed waveguide circuits and single-photon sources. InAs quantum dots (QD) embedded in GaAs are bonded to a SiON waveguide chip such that the QD emission is coupled to the waveguide mode. The waveguides are SiON core embedded in a SiO2 cladding. A tuneable Mach Zehnder interferometer (MZI) modulates the emission between two output ports and can act as a path-encoded qubit preparation device. The single-photon nature of the emission was verified using the on-chip MZI as a beamsplitter in a Hanbury Brown and Twiss measurement.E.M. and T.M. acknowledge support by the Marie Curie Actions within the Seventh Framework Programme for Research of the European Commission, under the Initial Training Network PICQUE (Grant No. 608062). F.F. acknowledges support from both the EPSRC and Toshiba Research Europe Ltd., Cambridge. J.L. acknowledges support from both the EPSRC CDT in Photonic Systems Development and Toshiba Research Europe Ltd., Cambridge. The authors acknowledge funding from the EPSRC for the MBE system used in the production of the QD samples.This is the final version of the article. It first appeared from AIP via http://dx.doi.org/10.1063/1.493502

Ramsey interference in a multilevel quantum system

We report Ramsey interference in the excitonic population of a negatively charged quantum dot measured in resonant fluorescence. Our experiments show that the decay time of the Ramsey interference is limited by the spectral width of the transition. Applying a vertical magnetic field induces Zeeman split transitions that can be addressed by changing the laser detuning to reveal two-, three-, and four-level system behavior. We show that under finite field the phase-sensitive control of two optical pulses from a single laser can be used to prepare both population and spin states simultaneously. We also demonstrate the coherent optical manipulation of a trapped spin in a quantum dot in a Faraday geometry magnetic field

Nonlinear spectra of spinons and holons in short GaAs quantum wires.

One-dimensional electronic fluids are peculiar conducting systems, where the fundamental role of interactions leads to exotic, emergent phenomena, such as spin-charge (spinon-holon) separation. The distinct low-energy properties of these 1D metals are successfully described within the theory of linear Luttinger liquids, but the challenging task of describing their high-energy nonlinear properties has long remained elusive. Recently, novel theoretical approaches accounting for nonlinearity have been developed, yet the rich phenomenology that they predict remains barely explored experimentally. Here, we probe the nonlinear spectral characteristics of short GaAs quantum wires by tunnelling spectroscopy, using an advanced device consisting of 6000 wires. We find evidence for the existence of an inverted (spinon) shadow band in the main region of the particle sector, one of the central predictions of the new nonlinear theories. A (holon) band with reduced effective mass is clearly visible in the particle sector at high energies.This work was supported by the UK EPSRC [Grant Nos. EP/J01690X/1 and EP/J016888/1].This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/NCOMMS12784

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 singleelectron source and a two-electron source. Using an energy-selective detector whose transmission we control on picosecond time scales, 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.This research was supported by the UK Department for Business, Innovation and Skills, the National Physical Laboratory’s Strategic Research Programme and the UK Engineering and Physical Sciences Research Council. V.K. has been supported by the Latvian Council of Science within research project no. 146/2012.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevB.92.12530

Microscopic metallic air-bridge arrays for connecting quantum devices

We present a single-exposure fabrication technique for a very large array of microscopic air-bridges using a tri-layer resist process with electron-beam lithography. The technique is capable of forming air-bridges with strong metal-metal or metal-substrate connections. This was demonstrated by its application in an electron tunneling device consisting of 400 identical surface gates for defining quantum wires, where the air-bridges are used as suspended connections for the surface gates. This technique enables us to create a large array of uniform one-dimensional channels that are open at both ends. In this article, we outline the details of the fabrication process, together with a study and the solution of the challenges present in the development of the technique, which includes the use of water-IPA (isopropyl alcohol) developer, calibration of the resist thickness, and numerical simulation of the development.</jats:p

Hierarchy of modes in an interacting one-dimensional system.

Studying interacting fermions in one dimension at high energy, we find a hierarchy in the spectral weights of the excitations theoretically, and we observe evidence for second-level excitations experimentally. Diagonalizing a model of fermions (without spin), we show that levels of the hierarchy are separated by powers of R^{2}/L^{2}, where 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 renormalized 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 tunneling of electrons (fermions with spin) from or 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.We acknowledge financial support from the UK EPSRC through Grant No. EP/J01690X/1 and EP/J016888/1.This is the accepted manuscript. The final version is available at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.196401

Time-of-Flight Measurements of Single-Electron Wave Packets in Quantum Hall Edge States.

We report time-of-flight measurements on electrons traveling in quantum Hall edge states. Hot-electron wave packets are emitted one per cycle into edge states formed along a depleted sample boundary. The electron arrival time is detected by driving a detector barrier with a square wave that acts as a shutter. By adding an extra path using a deflection barrier, we measure a delay in the arrival time, from which the edge-state velocity v is deduced. We find that v follows 1/B dependence, in good agreement with the E[over →]×B[over →] drift. The edge potential is estimated from the energy dependence of v using a harmonic approximation.UK Department for Business, Innovation and SkillsThis is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevLett.116.12680

Controllable Photonic Time-Bin Qubits from a Quantum Dot

Photonic time bin qubits are well suited to transmission via optical fibres and waveguide circuits. The states take the form $\frac{1}{\sqrt{2}}(\alpha \ket{0} + e^{i\phi}\beta \ket{1})$, with $\ket{0}$ and $\ket{1}$ referring to the early and late time bin respectively. By controlling the phase of a laser driving a spin-flip Raman transition in a single-hole-charged InAs quantum dot we demonstrate complete control over the phase, $\phi$. We show that this photon generation process can be performed deterministically, with only a moderate loss in coherence. Finally, we encode different qubits in different energies of the Raman scattered light, demonstrating wavelength division multiplexing at the single photon level

Assisted extraction of the energy level spacings and lever arms in direct current bias measurements of one-dimensional quantum wires, using an image recognition routine

A multiplexer technique is used to individually measure an array of 256 split gates on a single GaAs/AlGaAs heterostructure. This results in the generation of large volumes of data, which requires the development of automated data analysis routines. An algorithm is developed to find the spacing between discrete energy levels, which form due to transverse confinement from the split gate. The lever arm, which relates split gate voltage to energy, is also found from the measured data. This reduces the time spent on the analysis. Comparison with estimates obtained visually show that the algorithm returns reliable results for subband spacing of split gates measured at 1:4 K. The routine is also used to assess DC bias spectroscopy measurements at lower temperatures (50 mK). This technique is versatile and can be extended to other types of measurements. For example, it is used to extract the magnetic field at which Zeeman-split 1D subbands cross one another.This work was supported by the Engineering and Physical Sciences Research Council grant No. EP/IO14268/1.This is the accepted manuscript. The final version is available from AIP at http://scitation.aip.org/content/aip/journal/jap/117/1/10.1063/1.4905484