Quantum computers may look futuristic, but inside them, tiny mistakes are quietly piling up and remembering each other.<\/p>\n
A new breakthrough by Australian and international scientists shows that errors inside quantum machines are not just fleeting glitches.<\/p>\n
They can persist, evolve, and even connect across time, creating a hidden memory that undermines today\u2019s assumptions about how quantum systems behave.<\/p>\n
This is the first time researchers have been able to reconstruct a complete, time-resolved picture of how errors unfold inside a working quantum computer.<\/p>\n
The advance could reshape how scientists design, diagnose, and ultimately fix future quantum machines.<\/p>\n
The work was led by Dr Christina Giarmatzi from Macquarie University and focused on understanding why quantum computers remain so fragile despite rapid advances in hardware.<\/p>\n
\u201cWe can think of it as quantum computers retaining memory of the errors, which can be classical or quantum depending on the way these errors are linked,\u201d says Dr Giarmatzi.<\/p>\n
Errors that remember time<\/p>\n
One of the most surprising findings is that many quantum protocols assume machines are \u201cmemoryless,\u201d meaning past errors do not influence future ones.<\/p>\n
\u201cA lot of quantum protocols assume quantum computers have no such memory (known as Markovian) but that\u2019s simply not true.\u201d<\/p>\n
Instead, the team found that errors can remain correlated across multiple moments in time, \u00a0a phenomenon known as non-Markovian noise. <\/p>\n
This kind of behavior is a major obstacle to scaling up quantum computers for real-world use.<\/p>\n
\u201cWe\u2019ve been able to reconstruct the entire evolution of a quantum process across multiple points in time \u2014 something that hasn\u2019t been done before,\u201d Dr Giarmatzi said. \u201cIt lets us see not only when noise happens, but how it carries through time.\u201d<\/p>\n
The experiments were carried out on advanced superconducting quantum processors, both in laboratories at the University of Queensland and through IBM\u2019s cloud-based quantum computers.<\/p>\n
Rewinding quantum measurements<\/p>\n
Until now, one key challenge blocked progress: measuring a quantum system mid-experiment typically collapses its state, making it impossible to reset cleanly for the next step.<\/p>\n
The researchers solved this by assuming that half the time the measurement outcome was 0 and half the time it was 1, then using software to work backward and reconstruct the system\u2019s prior state.<\/p>\n
\u201cThe hardware could do it,\u201d said co-author Dr Fabio Costa from Nordita in Stockholm. <\/p>\n
\u201cWhat we figured out was how to actually prepare the system after a mid-circuit measurement.\u201d<\/p>\n
Using this approach, the team uncovered subtle but critical time-linked noise patterns, including quantum noise caused by interactions between neighboring qubits on the same chip<\/a>.<\/p>\n Understanding these patterns could help scientists build better error-correction tools, a crucial step toward fault-tolerant quantum computing.<\/p>\n \u201cIt\u2019s rewarding when theoretical models can be brought to life on real hardware,\u201d said Tyler Jones, who worked on the project as a PhD student at the University of Queensland.<\/p>\n \u201cRobust characterisation of time correlations in quantum <\/a>systems is needed on the path to building powerful quantum machines.\u201d<\/p>\n