Title: Integrated optical control and enhanced coherence of ion qubits via multi-wavelength photonics
Authors: R.J. Niffenegger, J. Stuart, C. Sorace-Agaskar, D. Kharas, S. Bramhavar, C.D. Bruzewicz, W. Loh, R. McConnell, D. Reens, G.N. West, J.M. Sage, J. Chiaverini
First author’s institution: Lincoln Laboratory, Massachusetts Institute of Technology
Status: Published in Nature Communications, https://arxiv.org/abs/2001.05052
With the implementation of the first quantum logic gate at the National Institute of Standards and Technology in 1995, a single trapped beryllium ion became the first physically realized quantum bit . Unlike man-made superconducting qubits, which tech giants Google and IBM are utilizing in their quest to build quantum computers, trapped ions are natural qubits — as a fundamental building block of nature, each ion is identical to every other ion of the same species. Additionally, they’ve been shown to have the longest coherence times of any qubit , and their control techniques had largely already been developed through decades of atomic clock engineering . However, in spite of all its attractiveness, trapped-ion technology has been missing a clear scalability pathway to the large-scale quantum computers needed to perform long-promised algorithms such as Shor’s algorithm, which requires millions of physical qubits. But that pathway is missing no longer.
Currently, trapped-ion quantum computers consist of a chain of positively charged atoms suspended in an alternating electric field, the quantum states of which are controlled via laser light of various colors guided by free-space optics . This current method restricts trapped-ion quantum technology in two ways: chains are limited to only a few dozen ions and the fidelity of quantum logic operations are limited by the susceptibility to vibrations and drift of the multitude of free-space optics, which are needed to shape and steer the light. However, this paper demonstrates the first realization of optical waveguides that are integrated into a surface-electrode ion-trap chip that allow for full control of ion qubits without the need for free-space optics. This essentially introduces a unit cell approach into trapped-ion quantum computer design, which will allow for scalability beyond a few dozen qubits to millions. Rather than a single trapping zone as is done in modern prototypes, with integrated optics it’s possible for qubits to now be shuttled around to different regions on a chip for loading, logic operations, memory storage, and state preparation and readout with complete optical access regardless of location. Additionally, the elimination of free-space optics makes all trapped ion quantum technology (computers, clocks, sensors, and communication network nodes) cheaper, more compact, and far less susceptible to environmental noise.
In their experiment, the team at Lincoln Labs utilizes a strontium-88 optical qubit in which 5s 2S1/2 is the ground qubit state and 4d 2D5/2 is the excited state. The transition between these states is an electric quadrupole transition in which the excited state is metastable with a T1 coherence time of ~1s. It should be noted that other forms of trapped ion qubits (the hyperfine states of the ground state of 171Yb+, for example) have T1 coherence times on the order of years.
In addition to the qubit transition at 674nm, control of the ion also requires light of specific wavelengths for ionization (461nm and 405nm), Doppler cooling and state detection (422nm), and for Doppler cooling and sideband cooling repumping (1092nm and 1033nm, respectively). All six of these wavelengths are fed via optical fiber into the cryogenic, ultra-high vacuum environment where the trapping chip resides. Here, the fibers are aligned to polarization-maintaining, single-mode waveguides under the chip surface, which then feed the light to each individual trapping region.
To direct the light out of the waveguide to the trapped ion, diffractive gratings were etched into the waveguide with a periodic pattern. The grating causes a periodic variation in the refractive index, which results in the diffraction of a portion of the light coupled to the waveguide out of the plane of the chip. The efficiency of this diffraction in this experiment was approximately 10%. Additionally, the teeth of the grating were made with curvature in order to focus and increase the intensity of the light at the approximate position of the ion.
In addition to the proof-of-concept demonstration, the researchers also quantified the reduction of susceptibility to vibrations with the new integrated photonics system compared to free-space optics. With free-space optics, qubit decoherence can arise from optical phase, amplitude, and pointing instabilities as a result of the ion and optics vibrating out of phase. However, with integrated optics, the vibrations of the ion and the optics are in a common mode. The researchers measured the qubit decoherence in the two systems by observing the decay contrast of Ramsey interference fringes . They found that for free-space optics the decay time of the Ramsey fringes drops dramatically as a function of ion acceleration, while it remains unchanged for the case of integrated optics. This demonstrates the nearly total immunity of the integrated optics system to even extreme environmental vibrations, which will allow for a large reduction in systematic error in trapped ion quantum computers and atomic clocks.
There are, however, limitations in this first prototype that will need to be overcome with further engineering. For example, the beams diffracted out of the chip intersect at 65 um above the surface, 10 um above the RF null where the ion sits when optimally trapped. Although the vertical position of the ion cannot be adjusted, the ion can be shifted horizontally to a point where it can interact with up to three of the laser beams at once; therefore, ion-control operations with up to three beams were demonstrated in this paper. In spite of this current limitation, the researchers were able to demonstrate a pi-pulse (an X gate or bit-flip gate on a physical qubit) time of 6.5 um and an average qubit detection fidelity of 99.6% for a 3 ms detection time.
This paper did not just present the first demonstration of full control of trapped ion optical qubits with photonics integrated into a surface electrode trap. It also revealed the pathway to scaling up trapped ion quantum computers beyond the noisy, intermediate scale to a version with millions of physical qubits capable of implementing quantum error correcting codes, executing resource-demanding quantum algorithms, and demonstrating full quantum advantage over classical computing systems. Since the publishing of this paper, first on the arXiv in January 2020 then in Nature in October, the two leading trapped ion quantum computing companies, Honeywell Quantum Solutions and IonQ, have added integrated optics into their 5-year plans for scaling up their quantum systems [6,7]. It’s clear that, while more engineering is required to perfect the technology, integrated photonics are an essential element in the way forward for trapped ion quantum computers.
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