But the team also provides an escape route to overcome the bias. For experiments that push the limits of precision such wrong estimates are particularly deceptive. If unaccounted for, then for example estimates for the minimum signal that the quantum sensor can detect might end up being overly optimistic, as Rojkov et al. These different speeds in information acquisition then result in a distortion of the output. The upshot is that during the delay the sensor typically acquires less information about the quantity of interest, such as a magnetic or electric field, compared to the situation in which no error had occurred. Depending on the length of this delay time, the dynamics of the quantum system, which should ideally be governed by the Hamiltonian alone, becomes contaminated by interference by the error operators. Now, Ivan Rojkov, a doctoral researcher working at ETH with Reiter and collaborating with colleagues at the Massachusetts Institute of Technology (MIT), found that the sensor output experiences a systematic bias - or, 'drift' - when there is a delay between an error and its subsequent correction. The former is described by the Hamilton operator of the sensor, the latter by error operators. The reason why this does not work is that rotation and translation are not commutative - the order in which the actions of one type or the other are executed changes the outcome.įor quantum sensors somewhat of a similar situation with non-commuting actions can arise, specifically for the 'sensing action' and the 'error action'. If the sequence would not matter, then the motorist could simply perform all steering manoeuvres at home in the garage and then confidently put the foot down on the accelerator. Then, as experience tells us, the sequence of driving ahead and making corrective movements has to be finely tuned. The story is rather different when, for practical reasons, the driver can perform correcting interventions with the steering wheel only at specific points in time. In the equivalent scenario for quantum sensing, it has been shown that by constant - or very frequent - error correction, the detrimental effects of noise can be suppressed completely, at least in principle. In the ideal case, the drift is corrected by constant counter-steering. As an analogy, imagine a car that keeps departing from the centre of the lane it travels in. In applying QEC to quantum sensing, errors are repeatedly corrected as the sensor acquires information about the target quantity. But not all is lost - the researchers describe as well procedures how to restore the correct results. Writing in Physical Review Letters, they report theoretical work in which they show that in realistic settings QEC can distort the output of quantum sensors and might even lead to unphysical results. But new theory work cautions that, unexpectedly, the approach can also give rise to inaccurate and misleading results - and shows how to rectify these shortcomings.Ĭorrected quantum sensing come with major potential side effects, as a team led by Florentin Reiter, an Ambizione fellow of the Swiss National Science Foundation working in the group of Jonathan Home in the Departement of Physics at ETH Zurich, has now found. But the benefits of error-It is well established that quantum error correction can improve the performance of quantum sensors. This approach is attracting considerable and increasing attention, as it might enable practical high-precision quantum sensors in a wider range of applications than is possible today. One way to suppress these unwanted contributions is to apply schemes collectively known as quantum error correction (QEC).
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