STORING AN OPTIMUM AMOUNT OF INFORMATION IN A PHOTON or any other quantum particle is possible even in the presence of noise, researchers have concluded (B. Schumacher et al., Physical Review A, July 1997; A.S. Holevo, upcoming in IEEE Transactions on Information Theory). A photon has many different quantum states, each of which can be made to represent a different digit. Furthermore, one can store many digits simultaneously in a single photon by putting it into a combination or "superposition" of many states. However, quantum mechanics prevents a measuring device from perfectly distinguishing between all these different states. Previously, physicists discovered that the maximum amount of information that can be read from a photon can be no greater than the amount of entropy, or disorder, it acquires when a range of quantum states is created to represent different digits. Now, the researchers show that this upper limit can be reached, even in a noisy environment, by utilizing several strategies, such as employing only those quantum states that are most distinguishable. These findings provide insights into how little energy is required to store a message. (For more details, go to Physics News Preview; Ben Schumacher, Kenyon College, 614-427-5832)