MEMS-based Vapor Cells for Quantum Computing, Sensors & Atomic Clocks

Quantum computing & sensing have made numerous recent headlines.  Quantum computing has the potential to increase computing power by orders of magnitude over what conventional semiconductor computer processors and memory chips can provide. Since quantum computing could revolutionize encryption, breaking into encrypted data, increasing memory density, and dramatically boosting artificial intelligence, governments and corporations have been pouring billions into the field.

Instead of using 0 and 1 binary bits, a quantum computer uses a qubit, which is more probabilistic than a conventional binary bit. Qubits use a quantum wave-like superposition of the bits.  Quantum computers use wave interference effects and entanglement to manipulate the qubit to perform more operations or dramatically increase the memory capacity of a computer. There are two leading technologies for quantum computing: cryogenic and vapor cell quantum devices. Cryogenic CMOS quantum computing cools the silicon circuit chip down to 3 Kelvin to reduce thermal sources of error and then uses microwave bursts to drive the silicon quantum bit down to 20 milliKelvin. Another promising quantum computer technology under development uses trapped vapors of metal atoms to form the basic qubit cell.  This quantum vapor cell is contained in a micromachined cavity of a Micro Opto Electro Mechanical System (MOEMS) chip. Rb vapor cell memory records the state of light in the atom using the photon’s relative amplitude and phase. The metal vapor offers a narrow relatively infrared spectrum of states for quantum data storage. To create this metal vapor requires the heating of the metal source to between 50 and 100°C which is easier to implement than a helium-cooled cryogenic system. The cyrogenic quantum computer is akin to the ENIAC computer which was something back in the late 1940’s even though it used a room full of vacuum tubes. Today we expect something with more processing power and memory that can travel on our wrist, in eye glasses or in our pocket. The same will be sought after with quantum computers. Mobile quantum devices certainly aren’t practical if liquid helium cooling is required. MOEMS or MEMS-based quantum chips, operating at room temperature, offer a more familiar integration path to the ubiquitous implementation of quantum computing and associated advances in mobile, perhaps personal, artificial intelligence.

The vapor cell quantum computer is a derivative of the atomic clock. Cesium (Cs) atomic clocks were first made using glass tubes with Cs vapor that would be stimulated with Rubidium (Rb) discharge lamp. As the infrared light travels through the glass tube, it is modulated by an oscillator and then hits a photodetector. When the oscillator hits the frequency between two electron spin transition levels in the Cs atom, optical absorption reduces the output at the photodetector.  A feedback loop is used to keep oscillator at the correct frequency used by the atomic clock.  In the early 2000’s DARPA funded development programs to create a much smaller MEMS-based version of the Cs atomic clock as illustrated below.

Wafer-to-wafer bonding can produce a cavity to hold the Cs vapor and possibly a buffer gas of Ar and/or nitrogen. In addition to atomic clocks and memory devices, different vapor cell designs have been used to create a variety of quantum sensors, including magnetometers for geological surveying, inertial gyroscopes, Rydberg devices for quantum communication, and entangled radar systems.

Metal vapor trapping quantum computers and sensors leverage silicon and glass micromachining processes, design techniques and chip scale packaging (CSP) methods. DRIE and wafer-to-wafer bonding in ultra-high vacuum is used to contain the metal vapor atoms in both the atomic clock and quantum computer. Anodic, eutectic and thermocompression bonding have been used in vapor cells.  Glass wafers are typically used to provide an optically transparent window into the vapor cell for both the laser and photodetector interface.. Magneto-optical or electrostatic-optical ion trapping have been employed to store and retrieve quantum data in the metal vapor MOEMS devices. In addition to Cs, Rb, Ba, Y, Ca, Be have been used in vapor cells.  Rb and Cs are the most common metal in these devices and both are difficult to incorporate in a traditional wafer fabrication sequence due to the flammable nature of these metals. Other chips are needed to form a working quantum device. A quantum chiplet stack of the MOEMS vapor cell cavity with a semiconductor laser on top and a photodiode chip under that are the bare minimum. These optical components require coatings on the inner and outer surfaces of the glass or silicon windows used by the vapor cells.

 All quantum computers are plagued by a variety of micro defects and external interferences resulting in signal noise which results in computing errors. Since vapor cells deal with the manipulation of subatomic energy levels in an atom, a variety of environmental factors like temperature and electromagnetic fields can lead to quantum decoherence and the loss of data. For vapor cells, ultra-high vacuum packaging is required to prevent random gas molecules from colliding with the atom used in the qubit.  Anodic bonding of glass to silicon has been used in many optical MEMS devices including atomic clocks and quantum sensors. Anodic bonding is well known to generate oxygen from the glass during the bond process. Residual oxygen molecules in the vapor cell cavity after anodic bonding could oxidize the metal vapor source material or generate unwanted photon wavelengths during laser stimulation of the qubit. MEMS devices like resonant gyroscopes, oscillators and filters have faced this vacuum packaging problem for decades.  Trapped gases, desorbed molecules and leaks can all lower the Q of MEMS resonators. The improvements developed over decades in vacuum packaging MEMS devices have been applied to quantum vapor cells.  Advanced wafer bonders, pre and post-bond baking, thin film getters and better sealing materials are all being used to reduce this source of quantum interference in qubits. The long-term hermeticity of the vapor cells is also an issue.  This is compounded if electrical feedthroughs are needed in the device for electrodes, RTDs or thin film heaters are to be integrated into the design and process. Helium is known to penetrate most wafer-level seals, including anodic bonded glass to silicon.  Just the 5 ppm of helium found in our atmosphere has penetrated vacuum-sealed MEMS oscillator chips at 100°C.  For optical quantum vapor cells these spurious gas molecules can produce wavelength emissions that interfere with device performance. Improvements in wafer-level design are key to shrinking vapor cells from vacuum tubes, down to single sensor or qubit chips and eventually into multi-sensor and qubit arrays, just like we did with silicon-based ICs.

The vapor cell chip must also keep the overall temperature of the vapor cell constant to avoid thermal fluctuation errors. Improved thermal & mechanical vibration isolation by placing the MEMS chip on an insulating, micromachined membrane was developed to improve the performance of inertial sensors and timing oscillators over a wide temperature operating range.   The performance of the quantum chiplet stack can be improved in the same way, thin film heating elements can be incorporated in the MOEMS chip or on the platform to both enable vapor formation and enhance qubit stability. Heating of the vapor sources and perhaps the getter inside the microcavity periodically requires good TCE matching between the electrical feedthroughs. Using through-wafer vias makes the implementation of a quantum chiplet stack simpler. A variety of thermal design and material considerations must be made for MOEMS-based vapor cell chips. Magnets or electromagnetic fields may be incorporated in the assembly for Zeeman splitting.  Finally a Faraday cage will be needed to shield the cell from external electric fields. The wafer process and packaging challenges for quantum devices are substantially higher than those that any other MEMS or MOEMS device our industry has ever tried to commercialize.

References

 D. Sparks, “Quantum vapor cell chiplets: micro optic wafer integration issues for timing, quantum sensors and memory,” GROW 2025, AI & Quantum Workshop, Sao Paulo, Brazil, July (2025).

 D. Sparks, “How MEMS technology is being used in quantum devices,” 2nd International Conference on Advanced Physics and Quantum Physics, Oct 3, (2023).

 D. Sparks, “How chip-scale packaging is advancing quantum computing,” Commercial Micro Manufacturing, Vol. 16, pp.30-35, Aug. (2023).

 E. Gibney, “Quantum gold rush: the private funding pouring into quantum start-ups,” Nature. Vol.574 7776: pp.22–24, (2019).

 X. Xue, X. et al., “CMOS-based cryogenic control of silicon quantum circuits,” Nature Vol. 593, pp. 205–210, (2021).

 J. Garcia-Ripoll, et al., “Fast and robust two-qubit gates for scalable ion trap quantum computing,”  Physical Review Letters. Vol. 91 (15): p.157901, (2003).

 J. Kitching, J. “Chip-scale atomic devices,” Applied Physics Reviews, Vol. 5 (3): pp.031302.1-37, (2018).

 D. Sparks, “MEMS integration using wafer level packaging,” Commercial Micro Manufacturing, Vol. 13, No.3, pp.16-20, (2020).

 D. Sparks, “Advances in high-reliability, hermetic MEMS CSP,” Chip-Scale Review, Vol. 20, No. 6, pp.36-39, (2016).