Gradient Echo Quantum Memory in Warm Atomic Vapor

The overall goal of this procedure is the storage and recall of a pulse of light in a warm rubidium vapor using magnetic field gradients. This is accomplished by first using electro optic modulators and optical cavities to generate beams of light at the frequencies required for ramen absorption in rubidium vapor. The second step is to use al optic modulators to shape the pulses that will be stored in the memory as well as fine tune the frequency of the control beam that enables ramen absorption.

Next, the pulses of light are stored in a rubidium cell whose absorption is spatially broadened by a longitudinal magnetic field gradient. The final step is to reverse the magnetic gradient to reverse the evolution of the atomic coherence, thus recalling the stored light pulses through a photon echo process. Ultimately, ho moddy detection is used to measure the characteristics of the recalled photon echo.

The main advantage of this technique of our existing methods is that it has the highest demonstrated efficiency. The unique for a domain nature of the memory means that the frequency component of the light pulses can be stored along the length of a gas cell. The memory can then be used for spectral manipulation of a store light.

Prepare for the experiment by custom making two ring resonators. Select a hollow cylinder of bulk aluminum for the cavity spacer. This cylinder is about 25 centimeters in length.

Prepare two flat mirrors with identical reflectivity in end caps. Mount these on one end of the cavity spacer with careful machining. The mirrors do not need to be glued.

Next place an O ring in an end cap for the opposite end of the cavity spacer. Place a maximum reflectivity curved mirror on the O ring. Put a piso electric actuator on the mirror and mount the end cap on the cavity spacer compress the elements of the end cap onto the cavity spacer to allow fast actuation of the end mirror.

Now begin work on the memory apparatus. Use a long cell here, 20 centimeters with anti reflection coated windows containing isotopically enhanced rubidium 87, along with 0.5 tor of Krypton buffer gas use a cell wrapped the non-magnetic heating wire for experiments. The memory cell depicted in green in this schematic will be encased in three concentric solenoids.

There are two identical inner solenoids with a variable pitch designed to create a linearly varying magnetic field. They are mounted so the gradients of the respective fields oppose each other. Switching between the solenoids reverses the gradients in the atomic ensemble and forces rephasing of the optical pulse and recall of light from the memory.

The third outer solenoid will produce a DC magnetic field to lift the degeneracy of the XEOMIN levels. To make the inner solenoids use simulations to determine the required variable pitch spiral and print its plot. Wrap the plot around A PVC pipe to provide a guide for winding the wire.

The coils should be designed to avoid edge effects and to have mostly longitudinal fields. After wrapping and assembling the three solenoids magnetically shield them with two layers of mu metal. The experiment uses a single mode laser tuned near the Rubidium D one line at 795 nanometers.

Monitor the frequency by using a beam splitter and shining a beam through a heated cell containing a natural isotopic ratio of rubidium. Observe the scattering near resonance using a camera dune the frequency by about 1.5 gigahertz above the F equals two to F prime equals two transition to get the approximate frequency of the control beam. Next along the optical path, use a beam splitter to form a control and a probe beam.

The probe beam continues through a fiber coupled electro optic modulator and one of the ring cavities. Use the fiber coupled electro optic modulator driven by a 6.8 gigahertz microwave source to detune the probe beam from the control. Eliminate side bands by locking the ring cavity on the resonance with a positive 6.8 gigahertz sideband.

The next beam splitter directs the prob beam to an kuo optic modulator to allow fine control of its frequency and intensity. The modulator is driven with a modulated Gaussian to produce a finely pulse of light for storage in the cell. Direct the prob beam to be transmitted through a second ring cavity.

Lock the cavity to the prob beam frequency using an auxiliary locking beam injected into the reverse mode of the cavity. Recombine the probe beam and the control beam at the output mirror of the cavity where the control beam is reflected Before they enter the memory cell, adjust the recombined probe and control beams to have identical approximately circular polarization with a quarter wave plate. After the memory cell, strip the control beam from propagating light with a filter cell filled with a natural mixture of rubidium at 140 degrees Celsius.

Then use a second quarter wave plate to convert the pro pulses to near linear polarization. Prepare ho moddy detection setup for the prob beam. After the memory cell, direct a beam to a third Oko optic modulator to shift its frequency and provide a local oscillator for the detector.

Use a fast oscilloscope triggered by the control program to capture and store the signal for an experiment. Ensure the memory cell is at 80 degrees Celsius and adjust the probe beam power. Start the computer controlled script for the experiment.

A typical duty cycle is about 120 microseconds trigger the oscilloscope early in the cycle. Initially, one of the inner coils around the memory cell is on and the other is off leading to a magnetic gradient in one direction. After a pulse of the prob beam is stored, reverse the gradient to recall light from memory.

Turn off the gas cell heater during the memory storage time to avoid interference with the memory operation. Switch off the control beam while light is stored in the memory if possible. This figure shows a typical heterodyne broadened ramen line when one of the gradient magnetic coils is switched on.

The thin solid line shows data from heterodyne measurements. The oscillation is due to the beat between the probe light and the local oscillator light. The dashed curve shows the envelope of this data, the shape of the broaden ramen line.

Here, a typical average efficiency gradient echo memory signal for short storage time is shown in this plot. The red curve shows the input pulse intensity profile and the blue curve shows the output of the memory. The magnetic gradient coils were switched at 10 microseconds.

The recalled echo appears to the right of the dashed line. The non-zero intensity of the output before the switch is evidence of light leakage. This high efficiency grading echo memory can be used for a variety of experiments such as shaping pulses in time, frequency space, and potentially building a quantum repeater.

Don't forget, working with high power lasers can be extremely hazardous. Always wear laser safety goggles while performing this procedure.