Single Spins, Single Photons

Quantum Optics in the Solid-State

We apply the concepts of quantum optics to solid-state emitters. The overriding goal is to create useful hardware for quantum information applications: a single photon source and a spin qubit. The single photon source should be a fast and bright source of indistinguishable photons on demand. The spin qubit should retain its coherence over many quantum operations. These are challenging goals. Rich and varied solid-state physics enters into the work via our design work – control of charge states, photonics and phononics environments – and via the complex dephasing mechanisms. In fact, by quantum control of the solid-state emitter, we are able to probe the underlying interactions in the solid-state – examples include the central spin problem, the electron-phonon interaction and Kondo-like interactions – with unprecedented resolution. The goals are to understand what limits these devices and to come up with creative ideas to circumvent the main problems. An outstanding goal is to develop a scalable approach to couple multiple qubits.

Quantum dots
A work-horse system is a self-assembled quantum dot in a semiconductor. In the best case (resonant excitation, high-quality material, low temperature) a single quantum dot has exceptional properties: the emission is perfectly anti-bunched, and the photons are highly indistinguishable. However, nuclear spin noise limits both electron spin coherence and photon indistinguishability and the excellent photonics properties are not necessarily preserved on nano-engineering the photon and phonon modes. Also, single photon sources are required at more useful wavelengths, for instance at 780 nm to interface an “artificial Rb atom”, the quantum dot, to real Rb atoms for a quantum memory. All these problems must be addressed.

Key recent achievements

  • Demonstration of a decoupling of a hole spin qubit from the nuclear spin noise. The experiments set a lower bound on T2* of 500 ns.
  • Observation of the coupling between two remote nuclear spins mediated by a delocalized electron spin. This is an example of an RKKY interaction. It is the mechanism predicted to determine the ultimate limit on electron spin coherence in a GaAs quantum dot. The experiment uses nuclear spins to probe electronic properties.
  • Demonstration of transform-limited optical linewidths
  • Frequency-locking of a quantum dot single photon source to a reference laser
  • Nuclear magnetic resonance on a quantum dot: a new protocol based on adiabatic passage to determine the key nuclear spin properties of a single InGaAs quantum dot (indium concentration, nuclear spin temperature, isotope-dependent polarization)
  • Charge noise and spin noise in the quantum dot spectrum: application to an ultra-clean device results in a quantitative analysis of the noise. The root-mean-square charge noise (specifically, noise in the electrostatic potential at the location of the quantum dot) is remarkably small, just 1 μV/Hz1/2.

Current projects

  • Development of the hole spin qubit in a self-assembled quantum dot
  • Quantum dots in photonic structures such as one-dimensional waveguides, micro-cavities
  • Detection of quantum dot single photons with a poor and cheap detector using some tricks from the quantum optics toolbox
  • Phononics: investigation of the coupling of a quantum dot exciton to a high-Q mechanical mode; reduction in phonon-based spin dephasing by nano-engineering the phonon modes
  • Development of a single photon source matched to the Rb wavelength using droplet quantum dots in GaAs

Key collaborators

A Tunable Micro-Cavity

Solid-State Cavity-QED

A micro-cavity enhances the light-matter interaction. In quantum photonics, a micro-cavity is a versatile tool. Radiative recombination of a solid-state emitter can be accelerated and photon extraction efficiency enhanced by exploiting the weak coupling regime of cavity-QED. The strong coupling regime allows two emitters to be coupled together even when the micro-cavity is not populated with a real photon. A micro-cavity is particularly valuable in solid-state systems: the micro-cavity can accelerate the photonic interaction, effectively weakening the effects of the solid-state-related dephasing mechanisms. Solid-state monolithic micro-cavities, micro-pillars, photonic crystal cavities for instance, offer limited tuning. The emitter position is typically fixed; there are limited in situ possibilities of tuning the emitter and cavity mode into resonance. This lack of tuning represents a problem, particularly in the present development phase where it is important to quantify the effects of the micro-cavity via the detuning dependence.

We are developing a micro-cavity which is fully tunable. It is essentially a highly miniaturized Fabry-Perot cavity: the bottom mirror is a plane mirror; the top mirror is curved to confine the light. The radius of curvature of the top mirror is typically 10 microns, the distance between the two mirrors is at most a few microns: this results in a micro-cavity mode with extent not too much increased above the diffraction limit (λ/2).

The curved mirror is fabricated in silica, either in a silica substrate or in the end facet of an optical glass fibre, by laser ablation. The laser, a CO2 laser at 10.6 μm wavelength, has the huge advantage of creating atomically-flat surfaces. In the ablation process, local melting takes place. Surface tension in the molten layer pulls out any surface irregularities. The template is then coated with a high-quality Bragg mirror. The micro-cavity itself consists of bottom and top mirrors with position control: 3-axes to determine the micro-cavity properties (lateral mode location with respect to the sample, mode frequency); 3-axes to position the mode with respect to the focus of a fixed lens.

Key features of micro-cavity

  •  High-finesse, high-Q
  • Reasonably low mode volume
  • Open access: the system is highly versatile
  • Free beam coupling: excellent mode-matching and polarization control
  • Cryogenic operation, presently up to a finesse of 10,000 without any dynamic stabilization
  • Tunability: micro-cavity mode position (x and y), micro-cavity frequency (z)

Recent achievements

  • Demonstration of strong coupling, coherent photon-exciton exchange, on a single quantum dot in a tunable micro-cavity. The cooperativity is as high as 5.5; the “bare” cooperativity relevant on short time-scales is 9. We show that spectral wandering of the emitter is a key factor limiting the performance. This needs to be addressed in the quest for ultra-high cooperativity.
  • Full characterization of tunable micro-cavity: mode frequency and spatial extent, noise performance
  • First ever launch of open access, low temperature-compatible, free space-coupled tunable micro-cavity

Current projects

  • Gated semiconductor devices in tunable micro-cavities: the quest for large Purcell factors (“strong weak coupling”), large cooperativity (“strong strong coupling”)
  • Diamond NV centres in tunable micro-cavities
  • Development of micro-cavities tailored to the needs of position sensing of laser-trapped micro-particles and nano-wire mechanics
  • Fabrication of a wider range of mirror templates on both silica substrates and optical glass fibres; creation of cryogenic, noise-proof set-up

Key collaborators

Materials Development

Materials for Solid-State Quantum Photonics

The demands of contemporary quantum photonics are unlikely to be met with the conventional group IV and III-V semiconductors. For instance, detecting single photons with semiconductor-based devices has restrictions, particularly at the telecoms wavelength. Superconducting nanowires have emerged as an attractive alternative: both the timing jitter and the quantum efficiency are better, potentially much better. NbN or amorphous superconductors have emerged as suitable superconductors for single photon detection. Another example is the difficulty in creating spatial arrays of highly-uniform, high-quality quantum emitters with III-V technology. Perhaps the class of optically-active 2D materials can successfully address this issue.

Current projects

  • Development of amorphous superconducting material for single-photon detection. This is a complex operation as part of a QSIT collaboration: thin-film sputtering and characterization are carried out in Basel; nano-fabrication at EPFL; device testing at the University of Geneva. The goal is to produce single photon detectors with high quantum efficiency and low timing jitter tailored to specific quantum applications.
  • Quantum photonics with optically-active 2D layers

Key collaborators