Posted in | Quantum Computing

New Program Tells When a Quantum Computer is “Leaking” Information

An innovative computer program has been developed that tells when a quantum computer is “leaking” information to undesirable states. For the first time, this potential technology will allow users to check its dependability without having any technical knowledge.

Image credit: University of Warwick

At the University of Warwick’s Department of Physics, researchers have created a unique quantum computer program to identify the presence of “leakage,” where data being processed by a quantum computer outflow from the states of 1 and 1.

The new technique has been described in a paper recently published in the journal, Physical Review A on March 19th, 2019. It contains experimental data from its application on a publicly accessible machine demonstrating that certain computations are being affected by unwanted states.

In quantum computing, the extraordinary properties of quantum physics are harnessed to process data in an entirely different manner to traditional computers. Leveraging the behavior of quantum systems, like those existing in numerous different states simultaneously, this fundamental form of computing is developed to process information in all of those states at the same time, resulting in a major benefit when compared to traditional computing.

In traditional computing, quantum computers employ combinations of 1s and 0s in order to encode data; however, the same computers can also simultaneously manipulate quantum states that are both 1 and 0. Conversely, the hardware encoding that data may at times encode it wrongly in another state—a problem usually referred to as “leakage.” Even a slight leakage building up over several millions of hardware parts can result in miscalculations and possibly grave errors, thus invalidating any quantum benefit over traditional computers. As a part of a relatively broader set of errors, this leakage is largely preventing the scaling up of quantum computers for industrial and commercial applications.

A better understanding of the extent of the occurrence of quantum leakage will allow computer engineers to develop systems that can mitigate against it, and will also enable programmers to come up with novel error-correction methods to take account of it.

Commercial interest in quantum computing is growing so we wanted to ask how we can say for certain that these machines are doing what they are supposed to do. Quantum computers are ideally made of qubits, but as it turns out in real devices some of the time they are not qubits at all—but in fact are qutrits (three states) or ququarts (four state systems). Such a problem can corrupt every subsequent step of your computing operation. Most quantum computing hardware platforms suffer from this issue—even conventional computer drives experience magnetic leakage, for example. We need quantum computer engineers to reduce leakage as much as possible through design, but we also need to allow quantum computer users to perform simple diagnostic tests for it. If quantum computers are to enter common usage, it’s important that a user with no idea of how a quantum computer works can check that it is functioning correctly without requiring technical knowledge, or if they are accessing that computer remotely.

Dr Animesh Datta, Associate Professor, Department of Physics, University of Warwick

The scientists applied their technique utilizing the IBM Q Experience quantum devices, via IBM’s publicly accessible cloud service. A method known as dimension witnessing was used by the scientists by constantly applying the same kind of operation on the IBM Q platform and they eventually achieved a dataset of results that can only be explained by a more complex and higher dimensional quantum system and not by one quantum bit. The researchers computed that the possibility of this conclusion emerging from mere chance is below 0.05%.

Although binary digits, or 1s or 0s, are used by traditional computers to encode data in transistors, subatomic particles or superconducting circuits called transmons are used by quantum computers to encode that data as a qubit. This implies that it is in a superposition of both 1 and 0 simultaneously, enabling users to calculate on varied sequences of the same qubits at the same time. Moreover, the number of processes increases considerably when the number of qubits increases. Specific kinds of issues, like those encountered in codebreaking (which depends on factoring large integers) and in chemistry (like replicating complex molecules), are especially appropriate for investigating this property.

Transmons, including other quantum computer hardware, can exist in a large number of states—that is, 0, 1, 2, 3, 4, etc. A perfect quantum computer only utilizes binary digits, or 0s and 1s, and also uses superpositions of these, or else, errors will occur in the quantum computation.

It is quite something to be able to make this conclusion at a distance of several thousand miles, with very limited access to the IBM chip itself. Although our program only made use of the permitted ‘single qubit’ instructions, the dimension witnessing approach was able to show that unwanted states were being accessed in the transmon circuit components. I see this as a win for any user who wants to investigate the advertised properties of a quantum machine without the need to refer to hardware-specific details.

Dr George Knee, University of Warwick

Dr Knee’s study was funded by a Research Fellowship from the Royal Commission for the Exhibition of 1851.

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