We developed a THz bandwidth quantum memory using a photon echo technique in InAs quantum dot ensemble with a large inhomogeneous broadning. To improve photon echo generation efficiency, we applied Adiabatic Rapid Passage (ARP) technique to quantum manipulation of quantum memory. In addition, we developed pulse pumped up conversion single photon detector (UCSPD) which enables us to achieve the femtosecond scale temporal resolution and we successfully demonstrated femtosecond time bin photon echo signals.
We have developed a quantum memory using quantum dots with a photon echo method for the storage of ultraweak femtosecond pulses. This quantum memory has a large bandwidth of 7.2 THz, which can be achieved due to the large inhomogeneous broadening of quantum dots. We successfully demonstrated femtosecond timebin pulse transfer to photon-echo-based quantum memory using quantum dots. We also succeeded in measuring the retrieved time-bin pulses as a photon echo from the quantum memory using a pulse-pumped frequency upconversion single-photon detector (UCSPD) with a temporal resolution of 429 fs. It is found that the retrieved time-bin photon echo pulse maintains the sub-picosecond time duration and the relative phase.
KEYWORDS: Magnetism, Magnetic sensors, Diamond, Magnetic resonance imaging, Continuous wave operation, Signal detection, Cameras, CCD cameras, Time metrology, Spatial resolution
Nitrogen-vacancy (NV) center in diamond is a promising candidate for a highly-sensitive magnetometer with high spatial resolution at room temperature. Conventional magnetometry typically uses scanning confocal microscopic techniques that require long measurement times to achieve a wide observation area and/or high sensitivity. Wide-field imaging techniques using CCD/CMOS camera has the advantage of enabling a wide field of view and rapid acquisition time by simultaneously detecting signals from NV centers in a large number of pixels of the camera. Continuous-wave optically detected magnetic resonance (CW-ODMR) is suitable for wide-field imaging of magnetic field using cameras with slow response time. However, only DC or low-frequency (up to kHz) AC magnetic field can be detected using CW-ODMR techniques. Recently, our group has developed a new measurement protocol using CW-ODMR to detect high-frequency (MHz range) AC magnetic fields using the zero-field splitting of the spin triplet states of NV centers. This technique is compatible with CCD-based imaging techniques. In this study, we implement our CW-ODMR protocol and measure the spatial-distribution of high-frequency AC magnetic fields with a wide-field imaging setup. Using our home-built wide-field imaging setup, a bulk diamond sample was mounted on a microwave antenna and a target AC magnetic field was applied by a copper wire placed on the sample surface. By comparing the CW-ODMR spectra with and without applied field, it is demonstrated that the AC magnetic field can be detected and estimated according to the protocol. Detection was performed both in wide-field view and windows of few pixels while averaging the signal over many pixels enabled rapid measurements.
In quantum information technology, it is necessary to develop a light-matter quantum interface that transfers and stores quantum information. As a bandwidth of quantum entangled photon pairs used for quantum information increases, a quantum interface with broad bandwidth will be required. The combination of quantum dot (QD) ensemble and photon echo (PE) method is one of promising methods for broadband quantum interface. Since the bandwidth of the quantum interface using this method is limited only by the inhomogeneous width of the QDs, it is possible to implement a quantum interface with the bandwidth of 10 THz at telecommunication wavelength. However, in the PE method, the spatial inhomogeneousity of the laser intensity and the inhomogeneousity of the resonance frequency of the QDs result in the uniform quantum control of excitons in QDs. As a result, the regeneration efficiency of the PE light is significantly deteriorated.
To solve this problem, it is effective to introduce a quantum control method using chirped pulses (Adaptive Rapid Passages; ARPs) which is robust to inhomogeneousities. In this study, we demonstrate that the regeneration efficiency of PE in inhomogeneous QDs can be improved by ARPs using femtosecond pulses. By performing numerical simulation and optical experiments, it was found that the regeneration efficiency improves as the chirp amount and the pulse area increase, and saturates at a certain condition.
The efficient transfer of a quantum state from photons to matter qubits in order to momentarily store information has become a central problem in quantum information processing. A quantum memory turns out to be an essential tool to achieve advanced technologies such as quantum networks, quantum repeaters, deterministic single photon sources or linear optics quantum computers. The realization of a quantum interface has been investigated in various forms, among which one can find solid-state atomic ensembles, color centers in crystal lattices, cold atomic gases, optical phonons in diamond and many others. Here we focus on a broadband quantum interface for high repetition rate (76 MHz) ultrafast optical pulses (250 fs) at telecommunication wavelength (1530 nm) based on the photon echo process occurring in semiconductor quantum dots. We evaluated the quantum state of photonic qubits in order to characterize the impact of the storage on the transmitted signal. Homodyne traces corresponding to projections of the Wigner function of the signal on rotated quadrature components were obtained using broadband balanced homodyne detection, i.e. mixing the ultrafast optical pulses to analyze with a high repetition rate pulsed local oscillator. The reconstruction of the Wigner function from the homodyne traces was performed using three algorithms: the inverse Radon transform, the minimax adaptive reconstruction and the maximum likelihood estimation. The three methods lead to similar results, concluding that for an input pulse in a coherent state, the reemitted photon echo is also in a coherent state.
The population and coherent dynamics of excitons in InAs quantum dots were investigated using transient pump-probe and four-wave mixing spectroscopies in the telecommunications wavelength range. The sample
was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to stack over a hundred QD layers, which resulted in a significant enhancement of nonlinear signals. By controlling the polarization directions of incident pulses, we precisely estimated the radiative and non-radiative lifetimes, the transition dipole moment, and the dephasing time while taking into account their anisotropic properties. The measured radiative lifetime and dephasing time shows large anisotropies with respect to the crystal axes, which results from the anisotropic nature of the transition dipole moment. The
anisotropy is larger than that for InAs quantum dots on a GaAs(100) substrate, which seems to reflect a lack of symmetry on an (311)B substrate. A quantitative comparison of these anisotropies demonstrates that nonradiative population relaxation and pure dephasing are quite small in our QDs.
We investigate the dephasing of excitons in InAs self-assembled
quantum dots by using a transient four-wave-mixing technique. A used sample is specially designed to compensate the strain. We observe long-lived coherence of excitons at 5 K which corresponds to the dephasing time longer than a nanosecond, where the photon energy of
the excitation pulse is 0.874~eV. We find that a pure dephasing due to exciton-phonon interactions dominates in the exciton dephasing
rather than the population decay and the exciton-exciton interaction
in the weak excitation region, by analyzing the population lifetime and the polarization-dependent dephasing time.
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