Stanford University's New Experimental Hardware Integrating Mechanical Devices into Quantum Tec

2024-01-15

interactions and behaviors in an counterintuitive manner. To this end, researchers led by Amir Safavi Naini at Stanford University have demonstrated new capabilities by coupling tiny nanomechanical oscillators with a circuit capable of storing and processing energy in the form of quantum bits or quantum information "bits". By utilizing the quantum bits of this device, researchers can manipulate the quantum states of mechanical oscillators, generating various quantum mechanical effects that could one day endow advanced computing and ultra precise sensing systems.

Safavi Naini, Associate Professor of Applied Physics at the School of Humanities and Sciences at Stanford University, said, "Through this device, we demonstrate an important next step in attempting to establish quantum computers and other useful quantum devices based on mechanical systems." Safavi Naini is the senior author of a new study published in the journal Nature on April 20, 2022. "We are essentially looking to establish 'mechanical quantum mechanics' systems," he said.

Stimulating quantum effects on computer chips

The co first authors of this study, Alex Volak and Anita Cleveland, are both doctoral students at Stanford University and are leading and working on the development of this mechanical based new quantum hardware. Using the nanosharing facilities on Stanford University campus, researchers work in cleanrooms and use specialized equipment to manufacture hardware components on two silicon computer chips with nanoscale resolution. Then, the researchers glued these two chips together, making the components on the bottom chip face the components on the upper chip in a sandwich shape.

The concept diagram of Bell state, where one unit of vibrational energy is shared between two oscillators. The system has two possible states simultaneously: the first possible quantum state (in parentheses, to the left of the plus sign) shows that the right oscillator is vibrating and the left oscillator is stationary. The second possible state shows that the vibration energy occupies the left-hand oscillator, while the right-hand oscillator is stationary. The device exists in the superposition of two possible states - meaning that each oscillator is both in motion and not in motion - until it is measured. The measurement of the system will only produce one of the two described (in parentheses) results. If the oscillator on the left is observed to be vibrating, then the oscillator on the right must be stationary, and vice versa. This indicates the entanglement relationship between the two oscillators. By conducting measurements to understand information about the motion of one oscillator, observers will also determine the state of another oscillator without the need for separate measurements.

On the bottom chip, Volak and Kleland created an aluminum superconducting circuit, forming the quantum bits of the device. Sending microwave pulses to the circuit generates photons (particles of light), encoding an information qubit in the device. Unlike traditional electrical equipment, traditional electrical equipment stores bits as voltages representing 0 or 1, and quantum bits in quantum mechanical equipment can also represent a weighted combination of 0 and 1. Thisbecause it is known as a superposition quantum mechanical phenomenon, where a quantum system exists in multiple quantum states simultaneously until the system is measured.

The top chip contains two nanomechanical resonators, formed by a suspended, bridge like crystal structure, with a length of only a few tens of nanometers or one billionth of a meter. These crystals are made of lithium niobate and are a piezoelectric material. Materials with this characteristic can convert electrical energy into motion, in the case of this device, which means that the electric field conveyed by qubit photons is converted into a quantum (or single unit) of vibrational energy called phonons.

"Just like light waves are quantified as photons, sound waves are quantified as' particles' called phonons," Cleveland said. "By combining these different forms of energy in our device, we have created a hybrid quantum technology that leverages the advantages of both.". “

The generation of these phonons allows each nanomechanical oscillator to act like a register, which is the smallest possible data storage element in a computer and is provided by quantum bits. Like quantum bits, oscillators can also be in a superposition state - they can be in both excited (representing 1) and non excited (representing 0) states simultaneously. Superconducting circuits enable researchers to prepare, read out, and modify data stored in registers, conceptually similar to the workings of traditional (non quantum) computers.

Using Entanglement

In addition to stacking, the connection between photons and resonators in this device further utilizes another important quantum mechanical phenomenon, namely entanglement. The reason why entangled states are so abnormal and notoriously difficult to create in the laboratory is because information about system states is distributed across some components. In these systems, it is possible to know everything about two particles at the same time, but have no knowledge about one particle being observed separately. Imagine two coins flipped in two different places and observed to randomly land as heads or tails with the same probability, but when the measurement results in different places are compared, they are always correlated; That is to say, if one coin lands at the tail, the other coin is guaranteed to land at the head.

Manipulating multiple quantum bits, all in a state of superposition and entanglement, is an important step in providing power for computation and sensing, and is also a highly sought after quantum based core technology. Safavi Naini said, "Without superposition and a lot of entanglement, you cannot build a quantum computer.". “

To demonstrate these quantum effects in the experiment, researchers at Stanford University generated a single quantum bit that was stored as a photon in the circuit of the bottom chip. Then let the circuit exchange energy with a mechanical oscillator on the top chip, and transfer the remaining information to the second mechanical device. By exchanging energy in this way - first with a mechanical oscillator, and then with a second oscillator - researchers use the circuit as a tool to entangle two mechanical resonators in a quantum mechanical manner.

Volak said, "The strangeness of quantum mechanics is fully demonstrated here." Not only does sound appear in discrete units, but a sound particle can also be shared between two entangled macroscopic objects, each with trillions of atoms in coordinated motion - or not in motion. “

In order to perform actual calculations, the duration of sustained entanglement or coherence will need to be greatly extended - on the order of a few seconds, rather than the few tenths of a second achieved so far. Stacking and entanglement are very subtle conditions, even susceptible to slight interference from heat or other forms of energy, and correspondingly endow the proposed quantum sensing device with exquisite sensitivity. Researchers believe that longer coherence times can be easily achieved by honing manufacturing processes and optimizing related materials.

"Over the past four years, our system performance has improved nearly 10 times annually," said Safavi Naini. "In the future, we will continue to take concrete steps towards designing quantum mechanical devices, such as computers and sensors, and bring the advantages of mechanical systems into the quantum field."

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