When we enjoy our favorite song, the seemingly continuous stream of music is actually transmitted in small packets of quantum particles known as phonons.
According to the principles of quantum mechanics, these phonons are considered indivisible and cannot be divided. However, scientists from the University of Chicago’s Pritzker School of Molecular Engineering (PME) have embarked on an investigation into the consequences of attempting to split a phonon.
In groundbreaking experiments, led by Professor Andrew Cleland, the research team employed an acoustic beamsplitter device to “split” phonons and observe their quantum properties. By demonstrating that the beamsplitter could induce a unique quantum superposition state in one phonon and create interference between two phonons, the team achieved significant progress in the development of a novel type of quantum computer.
The findings, published in the journal Science, represent a significant advancement based on the Pritzker Molecular Engineering team’s years of pioneering work on phonons.
“Splitting” a phonon into a superposition
During the experiments, the researchers utilized phonons with frequencies much higher than what can be detected by the human ear, approximately a million times higher. Prior to this study, Prof. Cleland and his team had already made significant advancements in generating and detecting single phonons, even achieving the entanglement of two phonons.
To demonstrate the quantum properties of these phonons, Cleland’s team, which included his graduate student Hong Qiao, developed a beamsplitter specifically designed to split sound waves in half. This beamsplitter functioned by transmitting half of the sound beam while reflecting the other half back to its source. While beamsplitters for light already existed and had been utilized to showcase the quantum properties of photons, the team successfully created a similar device for sound. The entire system, comprising two qubits for phonon generation and detection, operated under extremely low temperatures and employed individual surface acoustic wave phonons, which travel along the surface of a material, specifically lithium niobate in this case.
Interestingly, according to quantum physics, a single phonon is considered indivisible. Therefore, when the researchers directed a single phonon towards the beamsplitter, instead of splitting, it entered a quantum superposition state. In this state, the phonon existed in a simultaneous combination of being both reflected and transmitted. The act of observing or measuring the phonon caused this quantum superposition to collapse into one of the two possible outcomes.
To maintain the superposition state, the team devised a method involving the capture of the phonon in two qubits. A qubit serves as the fundamental unit of information in quantum computing. Although only one qubit physically captured the phonon, the researchers could not determine which qubit until after the measurement. In other words, the quantum superposition of the phonon was transferred to the two qubits. By measuring the superposition state of these two qubits, the team obtained conclusive evidence demonstrating that the beamsplitter created a quantum entangled state, which is considered the “gold standard” proof, as noted by Prof. Cleland.
Showing phonons behave like photons
In the second phase of the experiments, the researchers aimed to demonstrate an essential quantum phenomenon that was originally observed with photons in the 1980s and is now known as the Hong-Ou-Mandel effect. This effect occurs when two identical photons are simultaneously directed from opposite directions into a beamsplitter. The superposed outputs of the beamsplitter result in an interference pattern where the two photons always travel together, exiting the beamsplitter in one of the two possible directions.
Significantly, the same effect was observed when the team conducted the experiment with phonons. The superposed output indicated that only one of the two detector qubits captured the phonons, moving in one direction while the other direction remained phonon-free. Despite the fact that the qubits had the capability to capture only a single phonon at a time, not two, the qubit positioned in the opposite direction did not detect any phonons, providing evidence that both phonons were traveling in the same direction. This phenomenon is referred to as two-phonon interference.
Achieving quantum entanglement with phonons represents a much more significant leap compared to photons. The phonons used in these experiments, although indivisible, required the coordination of quadrillions of atoms to exhibit quantum mechanical behavior. This raises intriguing questions about the boundary between the quantum and classical realms, as quantum mechanics governs the behavior of particles at the tiniest scales. The experiment delves deeper into this transition, exploring where classical physics emerges as the quantum realm fades away.
“It’s kind of amazing,” remarked Cleland. “All those atoms must exhibit coherent behavior collectively to uphold the predictions of quantum mechanics. The peculiar aspects of quantum mechanics are not constrained by size.”
Creating a new linear mechanical quantum computer
The potential of quantum computers lies in the peculiar characteristics of the quantum realm. Scientists aim to leverage the phenomena of superposition and entanglement to tackle previously unsolvable problems. One avenue for achieving this is through the use of photons in a “linear optical quantum computer.”
In a similar vein, a linear mechanical quantum computer, utilizing phonons instead of photons, has the potential to perform novel types of computations. Prof. Cleland emphasizes that the success of the two-phonon interference experiment solidifies the equivalence of phonons and photons. This outcome provides assurance that the necessary technology is available to construct a linear mechanical quantum computer.
Unlike photon-based linear optical quantum computing, the approach pursued at the University of Chicago integrates phonons directly with qubits. This integration opens up possibilities for hybrid quantum computers that combine the strengths of linear quantum computers and qubit-based quantum computers, with phonons playing a crucial role.
The next phase of the research involves the development of a logic gate, a fundamental component of computing, utilizing phonons. Prof. Cleland and his team are actively engaged in this area of investigation.
The paper includes additional authors such as É. Dumur, G. Andersson, H. Yan, M.-H. Chou, J. Grebel, C. R. Conner, Y. J. Joshi, J. M. Miller, R. G. Povey, and X. Wu.
Source: University of Chicago