Fermilab’s NEXUS Experiment Sheds Light on Qubit Errors, Boosting Quantum Computing Potential

The NEXUS dilution refrigerator experiment at Fermilab investigates the impact of ionizing radiation and cosmic ray muons on qubit decoherence, a vital issue in quantum computing. Conducted 100 meters underground to minimize cosmic ray muons, the experiment uses a chip in four different radiation configurations to collect data. The aim is to understand how ionizing radiation affects qubits and to test the efficiency of a jump detection code. The experiment also explores the impact of correlated errors and quasiparticles on qubits. The findings could advance quantum computing applications, particle detection, and our understanding of quantum mechanics.

What is the Purpose of the NEXUS Dilution Refrigerator Experiment at Fermilab?

The NEXUS dilution refrigerator at Fermilab is being used to repeat an experiment initially conducted by Wilen et al. The experiment is being conducted 100 meters underground in the MINOS cavern to significantly reduce the incident rate of cosmic ray muons. The primary focus of the experiment is to investigate the effect of ionizing radiation and cosmic ray muons on qubit decoherence. The refrigerator is being used to collect data from a chip in four different radiation configurations.

The experiment aims to gain a better understanding of how the rate of ionizing radiation affects the qubits. To achieve this, a jump detection code is used to find jumps of various sizes in the data collected. The jumps are charge offsets of the qubit, thought to be due to quasiparticle poisoning. The efficiency of the jump detection code is tested using simulated data with a known number of injected jumps of known sizes.

The experiment also involves changing two main parameters, the smoothing factor and the threshold at which a charge offset is considered a jump. A χ2 analysis is then performed to find the best combination of these two parameters.

How Does Quantum Computing and Qubit Errors Relate to the Experiment?

In quantum computing applications, correlated errors are the end of the line. These errors cannot be rectified using error correction after the fact. However, when using quantum chips for particle detection, the opposite is the case since correlated errors can be used to identify energy deposits.

An error on a qubit, a quantum chip as opposed to a classical chip and bit, is any loss of information. Wilen et al stipulates that ionizing radiation and cosmic ray muons that are incident on the chip is one cause of these errors. In this work, the researchers look to further quantize the effect of ionizing radiation and cosmic ray muons.

The errors being investigated are correlated charge jump errors. There are two parts of this type of error: charge jump errors and correlated errors. In a classical computer bit, which has two possible states 0 and 1, an error occurs when the bit flips incorrectly. In a qubit, there are two axes meaning more types of errors. The researchers are interested in two of these: decoherence and dephasing.

What are Correlated Errors and How Do They Impact the Experiment?

Correlated errors are errors that are related spatially and temporally. They are related by both time and space. It means that multiple errors are not independent of each other and therefore cannot be resolved with error correction. These correlated errors do make quantum computing difficult but they are helpful when using qubits as particle detectors.

McEwen et al explores the impact cosmic ray muons have on qubit chips, showing how errors can propagate out after a muon passes through the chip. Specifically, McEwen et al shows that this propagation of errors can be seen in time and space, that the error rates ripple outward. It is because of this ripple that we can use qubits in particle detection.

These ripples happen when quasiparticles are incident on the qubit. An example of a quasiparticle is a phonon, which is the quantization of vibrations much like a photon is the quantization of light. The difference between a phonon and a photon is that the phonon does not exist on its own, it is the excitation and thus vibration of particles.

How Does the Experiment Investigate the Impact of Quasiparticles on the Qubit?

To investigate the impact of quasiparticles on the qubit and isolate the chip from background radiation, the experiment is done in the NEXUS clean room in the MINOS cavern at Fermilab, more than 100 meters underground. In the dilution fridge is a chip with four transmon qubits with a different distance between each of the qubits. This chip is the same one used in Wilen et al.

The researchers collect T1 measurements using Ramsey Tomography from the qubit chip while the dilution fridge is in four different radiation configurations consisting of a lead shield being open or closed and the presence or lack of a radioactive source. The configurations and the radiation rate are listed in Table I in descending order.

To test the jump detection code, simulated data is used. To do so, a no jump template is created using real data from the shield closed, no source configuration. The number of points in each sweep is then upsampled from to 1265 points. The mean is then taken.

What is the Significance of the NEXUS Dilution Refrigerator Experiment at Fermilab?

The NEXUS dilution refrigerator experiment at Fermilab is significant as it provides valuable insights into the effect of ionizing radiation and cosmic ray muons on qubit decoherence. The findings from this experiment could potentially lead to advancements in quantum computing applications, particularly in the area of error correction.

The experiment also contributes to the understanding of correlated errors and their impact on quantum computing. By investigating the propagation of errors in time and space, the researchers are able to use qubits in particle detection, which could have significant implications for the field of particle physics.

Furthermore, the experiment’s investigation into the impact of quasiparticles on the qubit could lead to a better understanding of quantum mechanics and the behavior of particles at the quantum level. This could potentially lead to advancements in the development of quantum technologies and applications.