California Institute of Technology Researchers Build Foundation for Quantum Network

Dev Kapadia 23′

Figure 1
The above figure is an example of a Bloch sphere. A vector on the Bloch sphere is a unit vector called a Bloch vector denoted . The components of the Bloch vector are determined by the angles of rotation in the y-direction, denoted by θ, and the x-direction, denoted by φ. The magnitudes the functions these angles are inserted into represent the probability of the qubit being observed in one of the two states.
(Source: Wikimedia Commons)

Computers are continuing to increase in complexity and capability every year. But increasingly, the changes implemented by software developers are becoming less noticeable. Many of us simply click “accept” when notified of our most recent computer update and go about our day. Unless a significant redesign is implemented, then there is little that we actually notice about the more-powerful version of our computer. Does this mean that the age of major advancement in computer software is over?

On the contrary, many researchers argue that we are far from the death of major computer development. Currently, there is great interest in “quantum computers,” a path of research that involves integration of quantum mechanics and computer science to dramatically increase the speed of computing.

Quantum computers use what are known as “qubits” to perform the calculations currently performed by the bits of classical computers. While bits inhabit only two states (either the 0 or 1 position), qubits can inhabit a state in between 0 and 1 due to a property called superposition. Once the state of these qubits is measured, however, the qubit will jump to either the 0 state or the 1 state. In theory, the states of the qubits are represented by vectors whose directions and length describe the probabilities of the qubit being measured in the 0 or 1 state.1

It is clear that fully developed quantum computers have potential to significantly improve on the current performance of classical computing. Researchers at Google were able to develop a quantum chip that performed a calculation in 200 seconds that they estimated would take a classical supercomputer 10,000 years.2 However, researchers at the California Institute of Technology (Caltech) expect that there are problems too complex for even quantum computers to solve. For this reason, the Caltech team has focused on the creation of a “quantum network” that would allow these quantum computers to communicate and work together towards a computation. Now, they believe that atoms in structures called “optical cavities,” a beam made from sculpted crystal, could be used to build the foundation of a quantum network.3

In their model, information is stored by the atom in the form of a quantum state, such as spin. Spin is determined based on the magnetic properties of the electrons in the outermost orbit of atoms. Just as bits store information in the form of 0s and 1s, the spin of the atoms will act as qubits and store information based on the probabilities of the spin direction being in the north or south direction. The researchers then determined that they can excite the atom by shooting light at it. Once that light is absorbed, the atom releases a photon whose quantum states correlates with the quantum state of the atom at any point in time (owing to a property known as quantum entanglement). The photon can then be measured at its destination to determine the spin state of the original atom.4

Unfortunately, there are some issues associated with putting quantum computers into a network. Many elements tested for quantum network viability are too susceptible to change in environmental conditions. For instance, the magnetic and electrical forces in the environment can alter the quantum states by the time the photons reach the destination. Remember, just as bits store information in the form of their states, these particles store information in the form of their spin states. Changing the spin changes the information.3 The researchers at Caltech have developed an optical cavity and found a rare ion, Ytterbium 3+ (Yb3+), that they believe have fixed the difficulties inherent in building a quantum network. In the crystal beam of the optical cavity, light bounces back and forth until the ion inside absorbs it. The ion then emits a photon, which also bounces around inside the beam, allowing the scientists to measure the quantum states.4 They have shown that the photon stays enclosed in the beam over ninety-nine percent of the time, which allows for the receiver to make measurements effectively.3 Loss of photon detection has been accredited to leakage of protons from the cavity, the detection efficiency of the photon detector (WSi2 superconducting nanowire single proton detector), along with other inaccuracies consequential of such a study on the subatomic level.4 Furthermore, the Yb3+ are stable enough to store information for at least thirty milliseconds in their spin, which is more than enough time for photons to travel the distance of the continental United States. This means that effective, long-range communication with qubits is possible, at least across the US.3

Although the promise of a quantum network still lies far in the future, the Caltech team has demonstrated that it is possible to measure the values of quantum states over long distances before environmental factors alter them. While the development of quantum computers and networks would improve upon the speed and accuracy of existing computers, they also pose many problems. For example, they are a cybersecurity risk, as the fast computation could crack many of the algorithms that are used to protect data today.5 Nevertheless, the future of computers may very well be in the quantum realm, and it will be exciting to see what the future holds.

References:

[1] Introduction to Quantum Computing. (n.d.). Retrieved April 5, 2020, from http://docs.rigetti.com/en/stable/intro.html

[2] Savage, N. (2019, October 24). Hands-On with Google’s Quantum Computer. Retrieved April 5, 2020, from https://www.scientificamerican.com/article/hands-on-with-googles-quantum-computer/

[3] Tiny optical cavity could make quantum networks possible. (2020, March 30). Retrieved April 5, 2020, from https://www.sciencedaily.com/releases/2020/03/200330152222.htm

[4] Kindem, J.M., Ruskuc, A., Bartholomew, J.G. et al. (2020). Control and single-shot readout of an ion embedded in a nanophotonic cavity. Nature. https://doi.org/10.1038/s41586-020-2160-9

[5] Adams, P. H., & Zurich Insurance Group. (2019, July 26). Why quantum computing could make today’s cybersecurity obsolete. Retrieved April 6, 2020, from https://www.weforum.org/agenda/2019/07/why-quantum-computing-could-make-todays-cybersecurity-obsolete/

 

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