Bit-flips

A qubit, the fundamental element of quantum information, is often characterized as being either 0 or 1, or in a superposition of 0 and 1 “at the same time.” Intentional bit flips are likewise characterized as flipping the state of a qubit from 0 to 1, and vice versa. However, a quantum superposition is better characterized as a qubit having some probability of being 0 and some probability of being 1, which we don’t know until the qubit is measured. Before a measurement is taken, a bit flip is a swap of those two probabilities. The probability of measuring 0 is now the probability of measuring 1, and vice versa.  

A bit flip error means that this has happened unintentionally, and therefore undesirably. In order to obtain useful results from a quantum computer, and thus obtain solutions to real-world problems, bit flip error correction will be required.  

Interestingly, quantum bit flip errors are not too dissimilar conceptually from their classical counterparts, which is perhaps why we use the same terminology. The IT Support Guides article titled “What is a Bit Flip: Causes, Consequences and Prevention,” despite addressing classical bit flips, could easily be adapted to be about the measurement results from a quantum computer.

What is Bit-flip Error

A bit flip quantum error is particularly problematic because it occurs in the measurement basis. This means that bit flips directly influence the information we retrieve, via measurements, from a quantum computer. If they are in error, we retrieve errors, and our solutions are incorrect.

Future quantum computers are likely to have quantum memories, often called QRAM, the quantum analogue of classical RAM. A bit flip memory error could have repercussions for all computation that accesses this stored, incorrect quantum information.  

Origins of Bit-Flip Errors

A bit flip error can have a number of different causes, including:

• Incorrect quantum circuit design
• Flawed transpilation and/or compilation of a quantum circuit
• Decoherence, which means that the number of operations that we are trying to perform on a qubit exceeds the number of operations it can handle
• Fabrication defects in qubits that are not found in nature
• Imprecise timing in control systems
• Crosstalk from operations executed on neighboring qubits

• Disturbances caused by the measurement process
• Environmental factors, including cosmic rays and electromagnetism

A University of Wisconsin-Madison news article titled “Correlated errors in quantum computers emphasize need for design changes” discusses how errors on superconducting chips can affect faraway qubits. Given the breadth of sources, therefore, it ought not be surprising that bit flip error correction is considered essential for fault-tolerant quantum computing.  

Consequences of Bit-Flip Errors

In quantum computing, because of entanglement, a single error can affect many measurement outcomes. Some of the consequences of errors include:

• Propagation, in that an uncorrected error can cascade throughout a system
• Failure of an algorithm to return a correct solution
• Legal implications, if someone’s encoded personal information becomes incorrect
• Corruption, if the quantum information is to be stored or transmitted

The consequences don’t always have to be severe, however. After all, not all qubits have to be measured. Minor errors can occur after certain qubits are no longer needed. To be safe instead of sorry, however, it’s best to treat them all as significant.

Mitigating Bit-Flip Errors

Large-scale, fault-tolerant quantum computing (FTQC) will require a three-prong strategy:

• Quantum error suppression, which seeks to prevent errors from occurring at all
• Quantum error correction (QEC), which seeks to detect and correct errors during execution
• Quantum error mitigation, which classically adjusts the measurement results

There are bit flip codes which only correct bit flip errors, and these might be useful for specialized hardware. FTQC devices, however, will likely implement only quantum error correction codes (QECC) which correct both bit flip and phase flip errors.