Dr Andrew Ramsay

Research Interests

• NanoPhotonics
• Spintronics

Andrew Ramsay earned his MPhys in 1999 from the University of Oxford. He then moved to Cambridge to do a PhD (1999-2003) under the supervison of Albert Heberle (HCL) and John Cleaver (ME) at the Cavendish Laboratory. He then worked as a postdoc in the Low Dimensional Semiconductor Devices group led by Maurice Skolnick at the University of Sheffield. In September 2012, he rejoined the Hitachi Cambridge Laboratory. His research interests have included ultrafast spectroscopy of vertical-cavity surface-emitting lasers; characterization of quantum dot lasers; coherent optical control of exciton and spin states in InAs/GaAs quantum dots; dephasing of quantum dots; quantum dot photonic devices; plasmonics and quantum memories.

Recent Research projects

• Coherent optical control of single quantum dot heavy-hole spins : The spin of an electron trapped on a single quantum dot is a promising optically active qubit with coherence times in the microsecond regime. The principle source of dephasing for the electron is the contact hyperfine interaction between the electron and the 104 nuclear spins of the group III-V ions that make up the dot. An alternative qubit is to use the pseudo-spin of a heavy-hole, where the contact term of the hyperfine interaction is suppressed due to the p-type Bloch-function of the hole, since there is zero overlap of the hole wavefunction with the nuclear site. With colleagues at the University of Sheffield, we developed an opto-electrical technique for the fast preparation and detection of the spin of a single hole, with sub 100-ps preparation times and picoseconds scale time-resolution. More recently, (in parallel with groups at Naval Research Labs and Stanford) we demonstrated the full coherent optical control of a single hole spin. This was achieved by observing a Larmor precession of the hole about an external magnetic field, and a rotation about an effective magnetic field generated by a picosecond laser pulse. These experiments demonstrated a key capability essential for using the use of hole spin qubits.
  
  
• The role of acoustic phonons in the intensity damping of exciton Rabi oscillations in single self-assembled quantum dots : The typical radiative lifetime of an exciton (electron-hole pair) on a quantum dot is less than a nanosecond. Consequently, it is easier to measure a Rabi oscillation by varying the incident power of a picoseconds pulse, than to vary the time duration as is done in electron-spin resonance (ESR) experiments. For nearly a decade, the intensity damping observed in these Rabi rotation measurements was the subject of some debate. In collaboration with theorists Gauger, Nazir and Lovett and experimentalist Achanta we made measurements demonstrating that longitudinal acoustic phonons are the main source of the intensity damping. This work is important for understanding the fidelity of coherent optical control operations. Similar mechanisms are likely to be important in a wide range of solid-state qubit.
  
  
• Interfacing quantum dots with photonic circuit : Scale-up is a major challenge in quantum information. One strategy is to build a network of spatially isolated static qubits connected by flying qubits, for example electron spins on quantum dots connected by photons. Spatial isolation of the static qubits keeps the energy spectra simple, and enables full control of the qubit-qubit interactions. In the long-term, for scalable size and cost, such a network should be built as an on-chip waveguide circuit rather than as a forest of bench-top optics. Self-assembled quantum dots are a good choice of static qubit, since they have radiative limited lifetimes, which is essential for achieving the high-contrast two-photon interference needed for measurement based entanglement. However, these dots have strong vertical confinement resulting to an optical dipole that lies in the sample xy-plane. The electron spin up/down state is detected by emission of a circularly polarized photon x∓iy. The problem is a single waveguide can only transmit one in-plane linear polarization component, hence transfer of the spin state between two points in the plane is inhibited. In collaboration with co-workers at the Universities of Sheffield, Bristol and Tata Institute of Fundamental Research in Mumbai we propose and demonstrate a spin to guide photon interface based on placing the quantum dot at the intersection of two waveguides. In the experiments, the polarization state of the emitted photon is mapped to a which-path state carried by two waveguides. This work motivates the use of path-encoded photons, better suited to photonic circuit architecture, than polarization encoded photons, for the manipulation and entanglement generation of quantum dot spins.
  
  

Last Publications

• I. J. Luxmoore, P. Khatri, R. N. E. Malein, and A. J. Ramsay, Proceedings Of Spie - The International Society For Optical Engineering 12003, (2022), Optical control of color centers in hexagonal boron nitride

• S. Baber, R. N. E. Malein, P. Khatri, P. S. Keatley, S. Guo, F. Withers, A. J. Ramsay, and I. J. Luxmoore, Nano Lett. 22, 1, (2021), Excited State Spectroscopy of Boron Vacancy Defects in Hexagonal Boron Nitride Using Time-Resolved Optically Detected Magnetic Resonance

• J. A. Haigh, A. Nunnenkamp, and A. J. Ramsay, Phys. Rev. Lett. 127, 14, 143601 (2021), Polarization Dependent Scattering in Cavity Optomagnonics