PRESS RELEASE \u2014 One of the fundamental challenges of quantum computing is obtaining information from a quantum bit, or qubit. Unlike a conventional bit, which can be read repeatedly without altering its content, direct measurement of a quantum system typically causes the state to collapse before it can be determined. In other words, reading the information modifies it.<\/p>\n
A new study with support from FAPESP<\/a> proposes circumventing this problem by utilizing a unique property of a certain type of qubit called an \u201cAndreev qubit.\u201d This qubit produces an oscillating electric supercurrent through its own quantum dynamics. This supercurrent would serve as a macroscopic indicator of the qubit\u2019s state, enabling its reconstruction in a virtually non-destructive manner.<\/p>\n The study was published<\/a> in the journal Physical Review B Letters and was led by Jos\u00e9 Carlos Egues<\/a>, a full professor at the S\u00e3o Carlos Institute of Physics at the University of S\u00e3o Paulo (IFSC-USP) in Brazil, and Yugui Yao of the School of Physics at the Beijing Institute of Technology in China. According to the authors, this method could eliminate the need for ancillas, auxiliary qubits used in current techniques for non-destructive quantum readout.<\/p>\n Before delving into these dynamics, it is important to understand the context of the phenomenon. Qubits are the fundamental units of information in quantum computers. While a classical bit can only be 0 or 1, a qubit can exist in various superpositions of these two states. This property gives quantum computing some of its most powerful characteristics but also makes the information extremely vulnerable to interaction with the environment and the measurement process itself.<\/p>\n \u201cIn the conventional formulation of quantum mechanics, a projective measurement causes the system to assume one of the states associated with the measured quantity. Thus, if a qubit has been prepared in a certain superposition of 0 and 1, its direct measurement tends to destroy that superposition and, consequently, the information it contained,\u201d says Egues.<\/p>\n To overcome this difficulty, \u201cquantum nondemolition measurements,\u201d or QND, were developed. In many of these protocols, the qubit\u2019s information is transferred to an auxiliary quantum system called an ancilla. The word \u201cancilla\u201d comes from Latin and means \u201cslave\u201d or \u201cservant.\u201d Today, figuratively speaking, the term is used to refer to something that serves another thing. In this case, measurements are taken on the ancilla to preserve the original qubit as much as possible.<\/p>\n While the procedure is effective, it requires preparing the ancillas, coupling them to the qubit, transferring the information, and performing successive measurements, which can be time-consuming. In systems with many qubits, this additional infrastructure can represent considerable experimental overhead.<\/p>\n \u201cOur strategy is different. Instead of transferring the information to an auxiliary qubit, we seek to obtain it from a signal produced by the qubit under study itself,\u201d says the researcher.<\/p>\n As previously mentioned, the qubit under study is an \u201cAndreev qubit.\u201d It is named after the Soviet physicist Alexander Andreev (1939\u20132023), who, in 1964, described a quantum process characteristic of the interface between normal and superconducting materials. These states, known as \u201cAndreev states,\u201d are produced by this process and confined to the nanoscale region separating two superconducting materials. They can be used to encode the basic states 0 and 1, as well as their superpositions.<\/p>\n \u201cIn the system under consideration, this region is a quantum dot \u2013 that is, a nanoscale structure in which electrons are spatially confined \u2013 formed in an indium arsenide [InAs] nanowire connected to superconducting materials on both sides. It\u2019s in this environment that the states characterizing the Andreev qubit are formed,\u201d Egues explains.<\/p>\n One property of this system is the circulation of a \u201csupercurrent.\u201d This is an electric current that flows without resistance. The study showed that when the qubit is in a superposition of its basic states, the supercurrent exhibits an oscillating component over time. This oscillation, produced by quantum interference between the states that comprise the superposition, carries information about the qubit\u2019s state. \u201cIt\u2019s precisely that property that we use to read the qubit. It\u2019s that supercurrent that lies at the heart of our proposal,\u201d the researcher points out.<\/p>\n A signature of the quantum state<\/p>\n Any qubit state can be geometrically represented by a point on the so-called \u201cBloch sphere.\u201d Each point on the surface of this sphere corresponds to a specific superposition of the basic states 0 and 1, and the position of a point is defined by two angles. One angle indicates the proportion in which states 0 and 1 contribute to the superposition, and the other angle indicates the phase relationship between them. Therefore, knowing these two angles is equivalent to knowing the state of the qubit.<\/p>\n \u201cOur study showed that this information is encoded in the supercurrent itself. The supercurrent has a constant component and a fluctuating component. The value of the constant component and the amplitude of the oscillation reveal the proportion of each state (0 and 1) in the superposition, and the moment when that oscillation begins reveals the phase relationship between them. Thus, by measuring how the current varies over time, it\u2019s possible to reconstruct the initial state of the qubit,\u201d Egues explains.<\/p>\n However, there is another important aspect to this phenomenon. In superconducting systems of this type, an electric voltage is usually necessary to cause the supercurrent to oscillate rather than remain constant over time. This is known as the \u201cAC Josephson effect.\u201d In the system studied, however, the current can oscillate without the application of any external voltage. \u201cOur system generates a supercurrent that varies over time like an alternating current but without voltage. It\u2019s the very evolution of the qubit\u2019s state that produces that oscillation,\u201d says Egues.<\/p>\n Thus, the current constitutes a macroscopic manifestation of the system\u2019s microscopic quantum dynamics. It is precisely this bridge between the two scales that the researchers exploit to read the qubit.<\/p>\n The Andreev qubit is incorporated into a closed superconducting circuit through which the aforementioned supercurrent circulates. Near this circuit, a SQUID (superconducting quantum interference device), an instrument extremely sensitive to weak magnetic fields, is placed.<\/p>\n As the supercurrent flows through the circuit, it produces a magnetic field that varies with it. The SQUID detects these variations and uses them to determine how the current is behaving. This behavior provides information about the qubit\u2019s state, so it is possible to reconstruct that state without directly measuring it.<\/p>\n Egues uses a simple analogy: \u201cWe can think of the qubit as one compass and the SQUID as another compass. The current in the circuit produces a magnetic field, and the SQUID \u2018senses\u2019 that field. Since the coupling between the two is very weak, the effect of the measuring instrument on the qubit tends to be very small.\u201d<\/p>\n This issue is crucial because, in quantum physics, the measuring instrument is not a passive observer. By interacting with the system, it produces feedback that can alter what one intends to measure. The influence of the measuring device on what is being measured caused great perplexity among physicists in the early days of quantum physics. However, it is inevitable. It is, so to speak, an intrinsic characteristic of material reality. The proposed device also has feedback. However, simulations carried out in the study indicated that it can be reduced to values so small that they become irrelevant.<\/p>\n \u201cOur method doesn\u2019t eliminate the fundamental problem of quantum measurement. But it can approach the limit of a non-destructive readout and, at the same time, avoid the use of auxiliary qubits \u2013 the ancillas \u2013 employed in other non-destructive quantum measurement protocols,\u201d Egues notes.<\/p>\n There is another advantage. After each measurement, there is no need to reset the qubit to its initial state. Measurements can be performed successively as the system evolves, enabling the tracking of its dynamics over time.<\/p>\n Within the limits of the margin of error<\/p>\n The researchers also sought to determine if the approach would work despite the inevitable imperfections of a real experiment. To accomplish this, they ran simulations that incorporated noise into the measurements and took into account the feedback caused by the small detection coil.<\/p>\n Depending on the number of measurements considered, the simulations yielded accuracies of 99.52%, 99.81%, and 99.85%. These results fall within the standard thresholds adopted in quantum error correction schemes.<\/p>\n The study is entirely theoretical. However, according to Egues, this does not mean that the device depends on nonexistent components or future technological advances. \u201cAll the numbers we used are realistic. In principle, this can be achieved. In fact, we used parameters from existing systems in our calculations. What doesn\u2019t yet exist is that geometry, that setup that we proposed,\u201d he argues.<\/p>\n This distinguishes the work from other theoretical proposals whose realization depends on advances in material or device fabrication. The detection of extremely small currents and magnetic fields by SQUIDs, for example, is an established technology.<\/p>\n The proposal thus combines two results. The first result is in fundamental physics: the prediction of an oscillating supercurrent similar to the AC Josephson effect but generated by the qubit\u2019s own coherent evolution without an external voltage. The second result is technological: using the characteristics of this current as a signature that allows the qubit\u2019s state to be reconstructed with minimal perturbation.<\/p>\n If the prediction is confirmed through experimentation, the method could provide a simpler architecture for reading Andreev qubits by eliminating the need for ancillas and successive resets. It could also exploit a promising feature of these systems by accessing microscopic quantum information through a macroscopic electric current produced by the qubit itself.<\/p>\n","protected":false},"excerpt":{"rendered":"Insider Brief Researchers proposed a method for reading Andreev qubits through their own oscillating supercurrent, potentially reducing measurement…\n","protected":false},"author":2,"featured_media":657690,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[24],"tags":[20128,295946,295947,85,46,370,64199,141,295948],"class_list":["post-657689","post","type-post","status-publish","format-standard","has-post-thumbnail","category-physics","tag-current","tag-electric-current","tag-fapesp","tag-il","tag-israel","tag-physics","tag-qubit","tag-science","tag-supercurrent"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts\/657689","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/comments?post=657689"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts\/657689\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/media\/657690"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/media?parent=657689"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/categories?post=657689"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/tags?post=657689"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"Introducing](\"https:\/\/www.newsbeep.com\/il\/wp-content\/uploads\/2026\/10\/1793029332_638_Option-1-1-1.png\")
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