When a resonant photon traverses a sample of absorbing atoms, how much time do atoms spend in the excited state? Does the answer depend on whether the photon is ultimately absorbed or transmitted? In particular, if it is not absorbed, does it cause atoms to spend any time in the excited state at all? In a recent experiment probing single-photon–level optical nonlinearities [PRX Quantum 3, 010314 (2022)], we attempted to measure this time, and found a result which at first surprised us. I will describe this measurement, and more recent theoretical work which attempts to explain our observation. The theory makes further counter-intuitive predictions which we are planning to test experimentally. I will discuss the relationship of these times with other related effects and timescales.
Quantum non-demolition measurements play an important role in quantum theory and many of its applications. In theory they are the most fundamental quantum measurements, but in practice their realization can be chal- lenging due to realistic constraints. In optics for example, most measurements are destructive since photons get absorbed by the detector. While some simple single particle non-demolition measurements are routinely done in optical setups by using a second degree of freedom to encode the results at an intermediate stage, measurements of degenerate non-local observables involving multiple photons remain challenging, especially when these are done at intermediate measurement strengths. Here we present an optical setup for performing variable strength non-demolition measurements of non-local observables in a pre and postselected setting. At the heart of the setup is an apparatus that can be used to turn a strong (projective) measurement into an arbitrary strength measurement by using a quantum eraser. We present our initial calibration results for this apparatus.
The question of how long a tunneling particle spends in the forbidden region of a barrier has been a perplexing puzzle in foundational quantum mechanics for many decades. The Larmor time, one definition of tunneling time, uses an auxiliary degree of freedom of the tunneling particles to clock the time spent inside the barrier. Recently, we made our first measurement of the Larmor time for Bose-condensed 87Rb atoms tunneling through an optical barrier.1 Here, we report on follow up measurements with improved precision in the measured times and also study the time for the reflected atomic cloud. We observe significant discrepancies between our results and a simple theory based on weak measurement. We discuss our findings, hypothesize explanations for our results, and suggest future studies.
Generalized quantum measurements (also known as positive operator-valued measures or POVMs) are of great
importance in quantum information and quantum foundations, but often difficult to perform. We present an
experimental approach which can in principle be used to perform arbitrary POVMs in a linear-optical context.
One of the most interesting POVMs, the symmetric, informationally complete-POVM (or SIC-POVM), is the
most compact set of measurements that can be used to fully describe a quantum state. We use our technique
to carry out the first experimental characterization of the state of a qutrit using SIC-POVMs. Because of the
highly symmetric nature of this measurement, such a representation has the unique property that it permits all
other measurement outcomes to be predicted by a simple extension of the classical Bayesian sum rule, making
no use of complex amplitudes or Hilbert-space operators. We demonstrate this approach on several qutrit states
encoded in single photons.
Quantum process tomography is often cited as providing all the information that can be known about a given quantum process. In this paper we have shown that even if two processes appear identical under process tomography, it is possible to distinguish them using an interferometric setup. Using this setup, it is possible to gain more information about a process than just tomography provides.
We describe experiments with photon pairs to evaluate, correct for, and
avoid sources of error in optical quantum information processing.
It is well known that a simple beamsplitter can
non-deterministicially prepare or select entangled polarization
states. We use quantum process tomography (QPT) to fully
characterize this effect, including loss and decoherence. The QPT
results identify errors and indicate how well they can
be corrected. To evade decoherence in a
noisy quantum channel, we identify decoherence-free subspaces
using experimental channel characterization, without need for a
priori knowledge of the decoherence mechanism or simplifying
assumptions. Working with pairs of polarization-encoded photonic
qubits, we use tomographic and adaptive techniques to identify 2-
and 3-state decoherence-free subspaces for encoding
decoherence-free qubits and qutrits within the noisy channel.
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