Hybrid Quantum Memories

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Introduction and concept
The generation and storage of single photon states using atomic systems have been demonstrated and represent important steps in the realization of the building blocks required to operate a quantum network. Albeit beautiful proof‐of‐principle experiments have been carried out with cold atomic ensembles, technical requirements currently limit their implementation in realistic devices. For example, single photon generation and storage with cold atoms requires relatively long loading and cooling cycles, leading to a non‐optimal duty cycle. Effectively, this means that the useful count rates are low, of the order of kHz, making a probabilistic quantum repeater topology based solely on atomic systems difficult to achieve. In our hybrid proposal, we will combine a high‐count‐rate solid state photon source with room temperature atoms for photon storage able to operate at extremely high rates, close the GHz range. The possibility of such powerful combination has been recently demonstrated, as quantum dots made of GaAs in an AlGaAs matrix can now efficiently emit light at 780 nm and be tuned to the D2 transition of 87Rb.

Objectives and targeted breakthroughs
This project will develop a new architecture for quantum information and communication technologies by drawing from two fields that have evolved in relative isolation: solid state and atomic optics. The targeted breakthrough is the development of hybrid quantum systems based on quantum dot devices for the generation of non‐classical light and the implementation of these solid state devices into quantum memory experiments taking advantage of the long coherence times of atomic systems: we will bridge atomic systems (rubidium) with condensed matter systems (GaAs quantum dots) and exploit quantum coherence as a resource for quantum communication. The quantum toolbox generated will result in implementations of quantum repeaters in a truly scalable platform, with the potential for physical implementation in remote, inaccessible locations. This will make the operation of quantum communication experiments possible over arbitrarily long distances, impacting profoundly the emerging quantum information industry.

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Description of work

Fabrication and characterization – TU Delft and NIST from m01 to m24.

The work focuses on the fabrication and full characterization of the novel devices to be used in the latter tasks. The devices will feature: (i) “core” consisting of optimized single GaAs/AlGaAs quantum dots as solid‐state emitters, (ii) broadband and precise tunability of emitter properties enabled by strain‐fields, and, where feasible, also electric fields and (iii) high or ultrahigh light extraction efficiency, enabled either by a planar‐cavity or photonic wire design.

Room Temperature Quantum Interconnects – Stony Brook from m01 to m12.

Concurrently, Stony Brook will develop two rubidium‐vapor‐based quantum interconnect as a test‐bed for storage‐of‐light experiments at the single‐photon level. They will use external‐cavity diode lasers phase‐locked at 6.8 GHz in order to obtain a co‐propagating electromagnetically‐induced‐transparency (EIT) setup. Stony Brook has performed preliminary measurements (unpublished) in which they have pushed these experiments towards the single‐photon level by implement a series of filtering steps to remove the unwanted control field photons from the retrieved photon pulse. These recent achievements demonstrate the potential of the system to operate with true single photons. This task requires distinguishing one probe photon from 1011 control photons and will be achieved using three different filtering elements. The first filter stage comes from polarization elements. Additionally, two temperature‐controlled silica etalons will provide further control field suppression. We expect to have an effective, relative control suppression of 130 dB. Furthermore, the system has preliminarily been observed to operate a as quantum interconnect for pulses as short as 50 ns making it an ideal candidate for the proposed hybrid experiments.

T2.1 Frequency stabilization of a quantum dot – TU Delft from m12 to m24.

We will frequency stabilize the emission from a single quantum dot device to obtain a bright flux of indistinguishable photons. Different tuning schemes will be implemented, starting with slow but easily implemented magnetic fields, followed by Stark shift tuning and strain tuning. As the tuning techniques are improved in terms of bandwidth and tuning rates, brighter quantum sources will be available enabling more complex experiments such as the interference of photons generated by distinct devices, each coupled to Rb. The stabilization is not necessarily done with the quantum dot emission used for experiments, it could be done with another emission line, such as the bi-exciton to leave a large fraction of the device available for experiments. Generation of entangled photon pairs from a frequency locked quantum dot device will enable entanglement swapping and the storage of entangled states.

T2.2 Improving background suppression in the atomic vapor quantum interconnect-  Stony Brook from m12-m18.

The next task will be to characterize and further minimize the control-light background produced at the probe frequency as this cannot be filtered out. Background photons at the probe frequency are mainly produced by two mechanisms. The first effect is the scattering of control-field photons due to non-perfect optical pumping. The second effect is four-wave mixing associated with the control field coupling off-resonantly to the second ground state. Experiments exploring different one-photon detunings will be performed, aiming at measuring the rate of the background photons propagating through the filtering system. Concurrently, Stony Brook will also characterize the storage efficiency at each detuning, aiming at determining the best operational point. This information will be crucial as it will determine the center frequency at which the quantum dots have to be locked (see T2.1).

Once Stony Brook has found the optimal frequency regarding the signal-to-background ratio and storage efficiency, they will perform storage experiments with laser pulses attenuated to the single-photon level in order to evaluate the achievable measured signal-to-background-ratio, an adequate measure of the performance of the device at the single-photon level.  The target is to achieve a detected signal-to-background-ratio (defined as the ratio between the detected number of retrieved probe photons per pulse and the detected number of background photons per pulse) of 10. Given an estimated total probe transmission of the proposed setup of 10% (see Fig. 1) and an estimated storage efficiency of 20%, Stony Brook will obtain 0.02 measured probe photons per input pulse. This means than in order to obtain a detected signal-to-background-ratio of 10, they should decrease the background to a value of 0.002 measured background photons per pulse, three orders of magnitude less than the current state of the art.

T2.3 Storage of single-photons with high bandwidth– Stony Brook from m19 to m24.

Once the previous task has been achieved, Stony Brook will adapt the acceptance bandwidth of the memory to allow for efficient storage of short pulses. This will make it possible to efficiently interface with the single photons generated from the quantum dot. The target is to obtain an operational bandwidth of 1 GHz, for example by using off-resonant Raman interaction. The storage efficiency and the bandwidth will have to be explored systematically for a large range of single-photon detunings in order to maximize the performance of the quantum interconnect. In these studies, the control laser intensity and the control laser beam diameter will also need to be varied systematically. This increased bandwidth will also allow for very fast readout, thereby further reducing the fraction of background photons and allowing for further improvement of the signal-to-background-ratio.

T2.4 Hybrid quantum storage – Stony Brook, TU Delft, NIST from m25 to m36.

Once the GaAs quantum dots with the described characteristics have been achieved within the collaboration, a hybrid storage experiment will be set up. TU Delft/NIST will bring a cryostat containing the best quantum dot samples to the location of the vapor quantum interconnects. Within the cryostat the quantum dots will be kept at temperatures around 5K. Optical excitation will be obtained using 745 nm picosecond laser pulses. Once magnetically-tuned emission of single-photons at 87Rb wavelengths has been locked, these single photons will be stored in 87Rb vapor. The quantum-dot-emitted photons will be the probe field and hence their frequency must be tuned close to the 5S1/2F = 1 5P1/2F′ = 1 transition at a wavelength of 795 nm. The control field, interacting with the 5S1/2F = 2 5P1/2F′ = 1 transition, will be obtained from a diode laser, ensuring two-photon resonance. An electro-optical modulator will allow the time-dependent switching of the control field intensity at high rates. After storage, a combination of polarization separation and frequency filtering provided by temperature-controlled etalons will separate the single-photon from the control-field light (see T1.2). A characterization of the signal-to-background ratio and the storage efficiency and their dependency on various control parameters will be performed.

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