Getting Physical: Bits and Quantum Error Correction
30 November 2017 |
Maryana Kartashevska | About a 4 minute read
Currently, the main impediment to building a fully functional quantum computer is something electrical engineers call ‘noise’ or in quantum terms, decoherence.
Decoherence occurs when particles that would otherwise exist in a quantum state interact with their environment, destroying quantum superposition and causing computational errors. This interaction can take many forms: it can come from the materials used to build a qubit, such as the thermal noise in the cables*, but it can also be triggered by observation.
Let’s think back to Schrodinger’s hypothetical cat (see Cat, Bits and Quantum Computing). The generally accepted theory of collapsing waves postulates that the actual state of the cat in the box is unknown and is determined only upon observation. In other words, it is the act of looking that triggers the cat’s mortality, which means that observing the information contained in a qubit would have the same effect of forcing it to assume a definitive state.
The result is errors in the calculation, which is where quantum error correction can help. But before we explore the concept further, we need to consider another unique property of quantum computing – the distinction between physical and logical qubits.
Physical qubits are the actual building blocks of a quantum computer, much like the bits in a classical computer. Logical qubits, on the other hand, are abstract and intangible – a term used to describe the information coded into the physical qubits. The more logical qubits working together in a quantum state, the bigger the computational power of the machine.
Maintaining enough logical qubits in a coherent state is one of the biggest challenges facing quantum computing. To prevent decoherence, a logical qubit is spread across multiple physical qubits. Theoretically this works because quantum entanglement allows logical qubits to communicate remotely (see Detangling Quantum Entanglement). In practice, it takes a lot of physical qubits to safeguard the integrity of just one logical qubit.
Without getting too technical, the physical qubits create something of a neighbourhood watch alliance, constantly checking in with each other to ensure the information on the logical cubit remains intact without actually observing it. This form of quantum error correction, known as surface code, is particularly popular for dealing with decoherence because of its relatively simple architecture and high fault-tolerant thresholds.
But it is not without its challenges. Scalability is one as there are still too many physical qubits per logical qubit. Coherence longevity is another as sustaining the fragile superimposed state for any meaningful length of time has so far proven difficult.
The main reason is the sheer complexity of quantum error correction. In classical computers, errors occur when a bit value flips from 0 to 1 and vice versa. In a quantum computer, errors can also occur when the plus or minus sign of the relationship between 0 and 1 changes, the so-called phase errors**. Hence any quantum error correction technique needs to be able to deal with both.
To complicate matters further, classical error correction relies on redundancy – the copying and pasting of information multiple times – which is physically impossible in quantum computers. According to the no-cloning theorem, quantum data cannot be copied because it is not possible to backup something that is superimposed, existing in multiple states at once and by its very nature undetermined. Besides, copying is tantamount to observing, which as we have already established is a big no-no.
As it stands, the progress in quantum computing is a question of engineering and so a matter of time. In the next blog, I marvel over the future where fully operational quantum computers are a reality. In the interim, please feel free to reach out and we can discuss how quantum machines will change our world.
*random fluctuations in voltage that are present in all electronic systems, regardless of the quality of the materials used
**in a quantum state particles are superimposed and can exist in both 0 and 1 states simultaneously, which can also be expressed as 0+1 or 0-1Read More From This Author
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