An electron cannot be split into smaller charged pieces. Yet under extreme conditions, electrons can behave collectively and produce quasiparticles that carry only a fraction of an electron\u2019s charge.\u00a0<\/p>\n
Now, two independent experiments have measured the same fractional charge in a particularly unusual quantum state<\/a>\u2014a result that strengthens the case for studying whether it could host exotic particles that may one day help build more robust quantum computers.<\/p>\n \u201cFor the first time in the history of this field, two different groups (Weizmann and EPFL) have measured the same values of fractional charge,\u201d Mitali Banerjee, one of the study authors and a professor at EPFL (Swiss Federal Technology Institute of Lausanne), said<\/a>.<\/p>\n Hunting for a fractional charge<\/p>\n The researchers focused on the \u03bd = 1\/2 fractional quantum Hall state, an even-denominator state that does not fit neatly into the better-known hierarchy of fractional quantum Hall states<\/a>.\u00a0<\/p>\n These states are especially interesting because some theories predict they can host non-Abelian anyons<\/a>\u2014quasiparticles whose quantum properties depend on how they are arranged and moved around one another.<\/p>\n \u201cNon-Abelian states act like a structural blueprint, where information is stored into the global geometry rather than at a single point, ensuring immunity from local structural defects or noise. By simply moving such particles around each other, we can create error-free quantum computers,\u201d Banerjee said.<\/p>\n However, measuring the charge of the quasiparticles is only one piece of that puzzle. Fractional charge alone cannot establish whether the underlying state is actually non-Abelian.<\/p>\n The researchers studied electrons confined to a 70-nanometer-wide layer of gallium arsenide. The material was cooled to extremely low temperatures and placed under a strong magnetic field, forcing the electrons into a collective quantum state.<\/p>\n The key test was performed independently on two nominally identical devices, one at EPFL and one at the Weizmann Institute, using separate experimental setups. Both devices contained a tiny bottleneck called a quantum point contact (QPC).<\/p>\n Two experiments but same result<\/p>\n The researchers operated the QPC in a weak-backscattering regime, allowing most quasiparticles to pass while randomly deflecting a small fraction. That randomness produces a faint electrical signal known as shot noise.<\/p>\n Think of rain falling randomly on a roof such that even though the average rainfall may be steady, individual drops arrive at unpredictable times. Similarly, charge carriers crossing the QPC arrive randomly. Measuring the resulting fluctuations allows researchers to determine the amount of charge carried by each quasiparticle.<\/p>\n The researchers created the QPC using a distinctive etching method and then deposited metal gates around the constriction to control how much current could pass through. Before examining the mysterious \u03bd = 1\/2 state, they checked the method against known fractional quantum Hall<\/a> states carrying charges of e and 2e\/3.<\/p>\n \u201cOur measurements were performed on two nominally identical devices in two independent experimental setups,\u201d the study authors note<\/a>.<\/p>\n When they turned to \u03bd = 1\/2, the two independent experiments produced essentially the same result. One device measured a quasiparticle charge of 0.250 \u00b1 0.013 e, while the other measured 0.249 \u00b1 0.018 e.\u00a0<\/p>\n Both measurements are consistent with e\/4, or one-quarter of an electron\u2019s charge.\u00a0<\/p>\n A clue, not proof of non-Abelian particles<\/p>\n The result is significant because the \u03bd = 1\/2 state is unusually robust. Its energy gap is about 10 times larger than that of the widely studied even-denominator state at \u03bd = 5\/2. It is also stabilized in the lowest Landau level, the lowest energy level available to electrons under a strong magnetic field.\u00a0<\/p>\n Meanwhile, two recently observed daughter fractional quantum Hall states have provided theoretical support for the possibility of non-Abelian order in the parent state.<\/p>\n \u201cThis is important because the quantum Hall state that was studied here is special as it survives till a few degrees Kelvin, and is supposed to be only a second such known state in gallium arsenide to possess special non-Abelian properties that can eventually enable a topological quantum computer,\u201d says Banerjee.<\/p>\n However, the e\/4 measurement does not prove non-Abelian order. Both Abelian and non-Abelian candidate states can produce quasiparticles with e\/4 charge, so charge measurements alone<\/a> cannot distinguish between them.\u00a0<\/p>\n The researchers therefore see the result as a benchmark for future experiments that directly probe the state\u2019s topological order and the exchange statistics of its quasiparticles.<\/p>\n Those experiments could test what happens when quasiparticles are exchanged, or braided around one another.\u00a0<\/p>\n If non-Abelian statistics are eventually confirmed, the \u03bd = 1\/2 state could become an important platform for exploring quantum information stored in collective properties rather than individual particles.<\/p>\n