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What Are Challenges to Quantum Memory?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

It may seem like a contradiction to talk about memory in a system as chaotic as quantum mechanics, yet it is possible to accomplish this. However, some of the hurdles you could imagine with quantum memory do exist and are a major problem in the field of quantum computing. Advancements have been made, however, so don’t give up hope for a quantum computer. Let’s take a look at some of the challenges and advancements that are present in this emerging field of study.

The Laser Hammer Method

The basic principle behind quantum memory is the transference of quantum qubits via photonic signals. These qubits, the quantum version of bits of information, have to be stored in a superpositioned state somehow yet retain their quantum nature, and there lies the crux of the problem. Researchers have used very cold gas to act as a reservoir but the recall time for the information stored is limited because of the energy requirements. The gas has to be energized to take in the photons in a meaningful manner otherwise it would keep the photon once trapped. A laser controls the photon in just the right way to ensure memory is secured but on the flip side requires a lengthy process to extract the information. But given a broader, more energetic spectrum for our laser and we have a much faster (and useful) process (Lee “Rough”).

Nitrogen, Silicon, and Diamonds

Picture an artificial diamond that has been laced with nitrogen impurities. I know, so common place, right? Work by NTT shows how such a set up could allow for a longer duration quantum memory. They were able to insert nitrogen into artificial diamonds which is responsive to microwaves. By changing a small group of the atoms via these waves, scientists were able to cause a quantum state change. A hurdle to this has to do with “the inhomogeneous broadening of the microwave transition in the nitrogen atoms” in which the energy state increase causes a loss of information after about a microsecond due to effects from the surrounding diamond such as charge and phonon transfers. To counter this, “spectral hole burning” was used by the team to transition to an optical range and preserve the data even longer. By inserting missing places inside the diamond, scientists were able to create isolated pockets that were able to hold onto their data longer. In a similar study, researchers using silicon instead of nitrogen were able to quiet down external forces, a cantilever was employed above the silicon qubit to provide enough of a force to counter the phonons traveling through the diamond (Aigner, Lee “Straining”).

Clouds and Lasers

One component of a quantum memory system that presents great challenges is our data processing rate. With qubits having multiple states encoded in them rather than the standard binary values, it can become challenging to not only preserve the qubit data but also retrieve it with precision, agility, and efficiency. Work by the Quantum Memories Laboratory of the University of Warsaw has shown a high capacity for this using a magneto-optical trap involving a cooled cloud of rubidium atoms at 20 microKelvins placed in a glass vacuum chamber. Nine lasers are used to trap the atoms and also read the data stored in the atoms via light scattering effects of our photons. By noting the change in the angle of emission photons during the encoding and decoding phases scientists could then measure the qubit data of all photons trapped in the cloud. The isolated nature of the setup allows for minimal external factors collapsing our quantum data, making this a promising rig (Dabrowski).

A String Method

In another attempt at isolating out quantum memory from our surroundings, scientists from the Harvard John A. Paulson School of Engineering and Applied Sciences as well as the University of Cambridge used diamonds as well. However, theirs were more like strings (which conceptually is nuts) about 1 micron in width and also used holes in the diamond’s structure to store the qubits. By making the material a string-like construct, vibrations could be tuned via voltage changes altering the length of the string to lower the surrounding material’s random effects on out electrons, ensuring our qubits are stored properly (Burrows).

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Coloring Qubits

In an advancement for multi-qubit systems, scientists took their photonic elements and gave them each a different color using an electro-optic modulator (which takes refractive properties of microwaved glass to change the frequency of incoming light). One is able to ensure that the photons are in a superpositioned state while distinguishing each one from another. And when you play around with a second modulator, you can delay the signals of the qubits so they can combine in meaningful ways into a single one, with high probabilities of success (Lee “Careful”).

Works Cited

Aigner, Florian. “New Quantum States for Better Quantum Memories.” innovations report, 23 Nov. 2016. Web. 29 Apr. 2019.

Burrows, Leah. “Tunable diamond string may hold key to quantum memory.” innovations report, 23 May 2018. Web. 01 May 2019.

Dabrowski, Michal. “Quantum memory with record-breaking capacity based on laser-cooled atoms.” innovations report, 18 Dec. 2017. Web. 01 May 2019.

Lee, Chris. “Careful phasing of a photonic qubit brings light under control.” Conte Nast., 08 Feb. 2018. Web. 03 May 2019.

---. “Rough-and-ready quantum memory may link disparate quantum systems.” Conte Nast., 09 Nov. 2018. Web. 29 Apr. 2019.

---. “Straining a diamond makes silicon-based qubit behave.” Conte Nast., 20 Sept. 2018. Web. 03 May 2019.

© 2020 Leonard Kelley

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