Session Y06: Quantum Hardware Development

Design and development of low cost eight channel arbitrary waveform generator for laser beam modulation

We present the design and development of an eight channel arbitrary waveform generator tailored to the voltage-control of RF drivers for acousto-optical modulators, controllers for electro-optical modulators, and similar devices. Our device uses four 600 MHz ARM Cortex-M7 microcontrollers controlling eight 12-bit DACs via SPI. Minimum pulse duration is approximately 1 microsecond. The limitations of onboard memory and processing power in the microcontrollers are overcome by the design of separate software that runs on a connected computer, facilitates the calculation of sequential DAC outputs required to closely approximate a given expression of voltage as a function of time, and provides an easily navigated graphical interface for creating output routines containing a theoretically unlimited number of ramps. A unique feature of the device is its ability to correct for signal chain nonlinearities, due to this increased calculation capability.   

Sub-nanoradian phase sensing in Optical Parametric Oscillator cavities

In a typical sensor, the quantity to be measured modifies the phase of a probe optical beam, which is detected by interference with a reference beam. If these two interfering beams are produced in an active laser cavity, the resonance condition of the laser converts the phase shift into a frequency (phase per round-trip time). The sensor measures a beat frequency, with a larger dynamic range and less noise than the amplitude measurement of two interfering beams. So far, this property is only exploited in laser gyroscopes, where the probe and reference are two counter circulating beams. This intracavity interferometry can be extended to linear cavities by using, instead of cw beams, ultrashort pulses. Detection of an electro-optic phase with a sensitivity better than 0.4 nanoradian is demonstrated with an Optical Parametric Oscillator (OPO) synchronously pumped by a mode-locked Ti:sapphire laser of half cavity length. The phase-photon number uncertainty was lower to 0.66, close to the quantum limit of 0.5. A phase difference of 0.4 nanoradians corresponds to an elongation of 0.1 fm. This type of precision is beyond mechanical stability of bulk optics but can be exploited in an integrated circuit implementation (OPO on a chip) that is described. A theory is presented showing that, for a given phase detection, the frequency of the beat note can be amplified, without a proportional increase in noise. The enhancement, obtained through resonant negative dispersion at selected modes, does not come at the expense of noise (Petermann factor remains = 1).   

A stable, narrow-linewidth laser system with broad frequency tunability and fast switching time

To change the sensing frequency of a Rydberg atom-based sensor, adjustments to the wavelength of the Rydberg state excitation laser are necessary, with required wavelength shifts on the order of 10 nm. To efficiently change the sensing frequency, a laser with a narrow linewidth and broad tuning capability is essential. In this talk, we present a narrow linewidth laser system capable of swiftly altering a coupling laser wavelength by up to 8 nm in less than 50 μs. The laser system comprises a frequency-stabilized continuous wave laser and an electro-optic frequency comb. An individual comb line is selected using a filter, and a high-speed electro-optic modulator is employed to tune the chosen comb line to a specific frequency. Through Rydberg atom-based sensing experiments, we demonstrate frequency hopping between two Rydberg states with a rapid switching time of 400 μs, limited by the radio frequency electronics. The switching time can be further reduced to approximately 50 μs with a ping-pong scheme. If updating the radio frequency is not required during frequency hopping, the switching can be done in 200 ns. These findings showcase the potential of our laser system for advanced applications in Rydberg atom-based radio frequency sensing, like communications and radar.   

Localization of disturbances along an optical fiber

Long-distance telecommunications fibers have been shown to provide a new method for detecting acoustic disturbances passing through the ground.  Passing waves stretch and compress the fiber, leading to a phase shift of light traveling within.  In a first proof-of-principle experiment [1], seismic waves from nearby earthquakes were detected by injecting a narrow-linewidth laser into ~100 km long fibers, retroreflecting the beam at the far end, and measuring the time-varying phase shift between the outgoing and returning beams.

One shortcoming of this approach is that it is not sensitive to the location of the disturbance along the fiber.  In this work, we demonstrate an improved method that can localize a single-frequency accoustic noise source to within about 5 meters and localize taps on the fiber to within 300 meters.  For this first demonstration, two labs (A and B) are connected by a 350 meter long fiber.  An ultrastable laser in Lab A is injected into the fiber and sent to Lab B.  In Lab B, the arriving beam is split: one part is frequency shifted and retroreflected back to Lab A, while the optical phase of the other part is compared to the optical phase of a second ultrastable laser in Lab B.  The phase of the retroflected light returning to Lab A is compared to the phase of the local reference using an optical beat note detector.  By analyzing the time delay between phase fluctuations observed in Lab A and in Lab B, we are able to determine the location of the noise source.

The optical phase differences are recorded locally in each lab and only later combined for analysis.  To help synchronize the two measurement traces, an amplitude modulation following a pseudrorandom binary sequence is applied to the beam from Lab A as it enters Lab B.  This imprints a synchronized 131071-element repeating pattern at a rate of 100 kHz onto both measurements.  By computing the cross-correlation between the measured amplitude and a locally-generated copy of the pseudorandom sequence, the relative timing offset between the two traces can be determined with an uncertainty of less than 10 ns with a 50-second averaging time.

[1] Giuseppe Marra et al., Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables. Science 361, 486-490 (2018).   

A compact narrowband tunable mid-IR CW source with >30 mW for spectroscopy between 3.1 and 3.5 μm

We demonstrate spectroscopy on methane using a commercially available CW optical parametric oscillator pumped by a 780 nm tapered amplifier. This source produces >30 mW output from 3.1 to 3.5 μm with a line width <2 MHz. The device has no moving parts other than a mechanical shutter on the output during normal operation. Mode-hop free tuning is accomplished by fast temperature control of the seed DFB diode laser. Coarse tuning is accomplished by crystal temperature tuning of a periodically poled MgO doped lithium niobate crystal. Cavity length is fixed by temperature stabilization. The wavelength of the signal and pump were measured with a silicon based wavemeter and the calculated idler wavelength compared to results from direct wavelength measurement with a HgCdTe based wavemeter. The results were found to be within the error of the HgCdTe based wavemeter. Next, the output was propagated through a 20 cm gas cell with sapphire windows pressurized at 10 Torr partial pressure for both methane and acetylene. Power was reduced using a calcium fluoride wedged window. The reduced reflected power from the window was measured using a HgCdTe photodetector. Our results show good agreement with HITRAN.   

Improvement of a Rydberg-based Single-photon Source with Microwave Dressing

Enhancing Rydberg interactions for quantum applications can be achieved by exciting atoms to higher principal quantum numbers. However, there are challenges associated with the exaggerated properties of large-n Rydberg states that require tradeoffs in other performance metrics. In this work we study the use of microwave-dressed Rydberg states to enhance interactions in an ensemble-based single-photon source. We show that by coupling nS to n'P states via microwaves, it is possible to turn on resonant dipole-dipole interactions. We demonstrate that the achievable single-photon purity at a given principal quantum number can be significantly increased, allowing for improved performance at lower principal quantum numbers. We study the impact of the dressing parameters on the production efficiency, the second-order correlation function g⁽²⁾(0), and the spin-wave lifetime. Our work shows that microwave dressing can be a useful tool for Rydberg-based quantum technologies.   

Toward bi-chromatic intensity squeezing at telecom wavelength using Four-Wave Mixing in Rb Vapor

In this study, we conduct experiments focusing on the generation of a two-color continuous variable(CV) entangled photon source, with one field in telecom wavelength while the other close to atomic transition,making it a promising platform for many quantum network applications. Specifically, we generate two correlated optical fields at 1367 nm and 795nm, respectively, through four-wave mixing (FWM) in a double-ladder four-level ⁸⁵Rb vapor. We achieved an off-resonance gain of 795nm field, reaching as high as 1.8 compared to the input seed, accompanied by a 73% optical transmission. Additionally, we presented preliminary evidence of intensity squeezing at -1dBm. Furthermore, we introduced several validated complementary methods that can be integrated to enhance the efficiency of photon generation. Our experiment serves as a proof of principle, showcasing the feasibility of extending bi-chromatic photon correlation into the continuous-variable (CV) regime.