The advantage to a photonic crystal cavity coupled to a quantum dot or nitrogen vacancy center is that it yields long-lived spin states, which is ideal for use in a quantum network. Quantum networks require quantum memory with three energy levels in a Λ configuration. Two energy levels are in the ground state, serving as the long lived quantum memory, and one energy level is in the excited state, which connects the ground state spin coherence to optical coherence. To achieve the three level energy system a magnetic field is applied transverse to the sample. The quantum dot is controlled with a single electron, spin measurement and initialization are demonstrated with resonant laser spectroscopy, and finally, single qubit gates are attained by rotating the spin with picosecond optical pulses.
InAs quantum dots were grown in the intrinsic layer of a n-i-p GaAs diode. A photonic crystal membrane with an optical cavity defect was etched into the GaAs diode.
The quantum dot is charged by applying a forward bias across a laser diode. The charge state of the quantum dot is then determined by photoluminescence. Resonant laser excitation of the quantum dot and cavity mode is used to measure the spin state of the electron. The V polarization is 70 times greater than for H, the linewidth of X- is about 30 times greater than the radiative limit out of the cavity. The quantum dot is detuned from the cavity by increasing the temperature from 7K to 28K.
Figure 1a. Differential Reflectivity for V polarization from 7K to 34K, detuning X- from cavity. Blue line is fit. 1b. Differential Reflectivity for the H polarization
A transverse magnetic field of 4T is applied to split the electron and X- energy levels. If the laser is resonant with one of the transitions then there is a small change in reflectivity. If the system is in the spin state it is optically driven and the spin state can be measured. To characterize initialization the quantum dot is detuned from the cavity resonance, which makes the transitions easier to resolve. Another technique to measure the spin state is to use resonance fluorescence which only leads to photon detection if the quantum dot is in the spin state. Driving transitions which are not coupled to the cavity yields a fast initialization, and driving transitions coupled to the cavity yields a spin measurement.
Figure 2. Resonance Fluorescence for H polarized laser, and V polarized detection for several biases. Magnetic field of 4T and laser power of 25nW. The inner transitions disappear due to optical pumping as the bias moves towards the middle of the charge stability region.
To measure the spin rotations a short, circularly polarized pulse is applied to the system, which will couple two spin states together. The cavity is coupled strongly to the V polarization state, thus the system needs to be detuned such that the pulse that interacts with the quantum dot is not interacting through the cavity mode. The spin is measured after two rotation pulses, which serve to rotate the spin to any angle. Spin coherence can be recovered using spin echo techniques or suppressing nuclear spin fluctuation.
The applications in which a spin qubit coupled to a photonic crystal cavity will be beneficial include cavity quantum electrodynamics, spin-controlled photonics, and quantum networks. Future work includes better control of the parameters such as polarization alignment and cavity quality factor. The group also hopes to move towards multiple quantum dot spin qubits.
Note: The development and characterization of the quantum dots and photonic crystal structure was conducted in the Cryostation. The final experiments were done with a cryostat incorporating a superconducting magnet.
This article, including all figures, was based on the following paper:
Samuel G. Carter, Timothy M. Sweeney, Mijin Kim, Chul Soo Kim, Dmitry Solenov, Sophia E. Economou, Thomas L. Reinecke, Lily Yang, Allan S. Bracker & Daniel Gammon, Nature Photonics 7329-334 (2013)