By Casimer DeCusatis
Quantum computing has received a lot of attention recently, with a wide range of possible applications ranging from the financial sector to the automotive industry and beyond. It’s only appropriate that discussions on quantum information science will take center stage at OFC 2021. While many near term quantum computers require superconducting materials or other extreme operating conditions, some architectures based on photonics may offer important advantages. In this blog, we’ll take a closer look at the optical science behind quantum computers, specifically the role of new laser technologies in creating the next generation of room temperature quantum systems.
Scientific Foundation of Quantum Computers
Just as bits are the fundamental building blocks of modern digital computers, two level systems called qubits form the foundation of quantum computers. A qubit can exist in coherent superposition of two binary states (zero and one), so it can be used to perform certain calculations much more rapidly than conventional computers. In some cases, NP-hard problems can be managed in polynomial time, a significant speed-up from the exponential execution time required by even the fastest conventional digital systems. Qubits can be realized in many forms, including the internal states of ions or atoms, which can be manipulated using high purity laser sources. Quantum logic gates can be created by manipulating ions or atoms using precisely controlled laser pulses. In turn, these gates can be assembled into circuits which act on one or more qubits, and which can be programmed much like assembly language on a conventional system. Very high purity, low noise optical sources are required, otherwise noise will corrupt the desired quantum states and cause computation errors that render the quantum gates useless.
There are significant research challenges to be overcome in order to realize practical optical qubits. One of the most significant is laser linewidth, which sets an upper limit on the coherence time for interactions between the laser source and the qubit. In order to take advantage of properties like superposition, a quantum computer must perform all its calculations before the system loses coherence (this is analogous to using the calculator function on your cell phone when the battery is running out; you only have a limited time to complete your work before the phone dies). It’s desirable to maximize coherence time in a quantum system so that we can perform more lengthy calculations. This requires laser sources with extremely narrow linewidths, on the order of perhaps 1 Hz. This is well below the range of many standard laser systems, which operate with linewidths between a few hundred kHz to several MHz. Advanced linewidth reduction systems have been developed using Ti: sapphire lasers at 729 nm center wavelength, with feedback from a high finesse external optical cavity. This makes it possible to create high fidelity entangled qubit gates using calcium ions. While device specifications vary, output power on the order of hundreds of milliwatts to a watt or more should be possible in the short term using these devices.
There are many technical challenges which remain, including scaling these systems to large enough numbers of qubits for practical calculations. Systems such as those described earlier have only been demonstrated for a few qubits at a time; hundreds or thousand of qubits are desirable for many applications. This will require greatly increasing laser output power (on the order of several watts to tens of watts or higher) while maintaining low noise and fine line widths. Precision alignment of optical components in future systems may also benefit from advances in integrated optics and modulators. These building block technologies will be addressed by invited speakers at and perhaps quantum computing will be able to leverage components being explored for applications in long distance coherent communication networks. If you’d like to brush up on high power laser sources, external modulators, and linewidth considerations, why not consider one of the OFC short courses in this area or plan to sit in on one of the tutorial sessions (https://www.ofcconference.org/en-us/home/program-speakers/tutorial-speakers/ ) ? Understanding the challenges and next steps for this technology is an important part of developing near team optical quantum computing architectures, and nobody covers the breadth and depth of these topics like OFC.
Do you have any experience working with quantum computers that are currently available in the cloud? Drop me a line on Twitter (@Dr_Casimer) with your experiences, and maybe we’ll use them in a future blog.
Quantum Information Science and Tehcnology (QIST) in the Context of Optical Communications
Short Course SC177: High-speed Semiconductor Lasers and Modulators
Posted: 26 April 2021 by
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