TY - GEN AB - Vaidman, in a recent article adopts the method of 'quantum weak measurements in pre- and postselected ensembles' to ascertain whether or not the chained-Zeno counterfactual computation scheme proposed by Hosten et al. is counterfactual; which has been the topic of a debate on the definition of counterfactuality. We disagree with his conclusion, which brings up some interesting aspects of quantum weak measurements and some concerns about the way they are interpreted. AU - Hosten, Onur AU - Kwiat, Paul ID - 574 TI - Weak measurements and counterfactual computation ER - TY - CONF AB - A source of single photons allows secure quantum key distribution, in addition, to being a critical resource for linear optics quantum computing. We describe our progress on deterministically creating single photons from spontaneous parametric downconversion, an extension of the Pittman, Jacobs and Franson scheme [Phys. Rev A, v66, 042303 (2002)]. Their idea was to conditionally prepare single photons by measuring one member of a spontaneously emitted photon pair and storing the remaining conditionally prepared photon until a predetermined time, when it would be "deterministically" released from storage. Our approach attempts to improve upon this by recycling the pump pulse in order to decrease the possibility of multiple-pair generation, while maintaining a high probability of producing a single pair. Many of the challenges we discuss are central to other quantum information technologies, including the need for low-loss optical storage, switching and detection, and fast feed-forward control. AU - Peters, Nicholas A AU - Arnold, Keith J AU - VanDevender, Aaron P AU - Jeffrey, Evan R AU - Rangarajan, Radhika AU - Onur Hosten AU - Barreiro, Julio T AU - Altepeter, Joseph B AU - Kwiat, Paul G ID - 578 TI - Towards a quasi-deterministic single-photon source VL - 6305 ER - TY - CONF AB - Visible light photon counters (VLPCs) and solid-state photomultipliers (SSPMs) are high-efficiency single-photon detectors which have multi-photon counting capability. While both the VLPCs and the SSPMs have inferred internal quantum efficiencies above 93%, the actual measured values for both the detectors were in fact limited to less than 88%, attributed to in-coupling losses. We are currently improving this overall detection efficiency via a) custom anti-reflection coating the detectors and the in-coupling fibers, b) implementing a novel cryogenic design to reduce transmission losses and, c) using low-noise electronics to obtain a better signal-to-noise ratio. AU - Rangarajan, Radhika AU - Altepeter, Joseph B AU - Jeffrey, Evan R AU - Stoutimore, Micah J AU - Peters, Nicholas A AU - Onur Hosten AU - Kwiat, Paul G ID - 577 TI - High-efficiency single-photon detectors VL - 6372 ER - TY - JOUR AB - The logic underlying the coherent nature of quantum information processing often deviates from intuitive reasoning, leading to surprising effects. Counterfactual computation constitutes a striking example: the potential outcome of a quantum computation can be inferred, even if the computer is not run 1. Relying on similar arguments to interaction-free measurements 2 (or quantum interrogation3), counterfactual computation is accomplished by putting the computer in a superposition of 'running' and 'not running' states, and then interfering the two histories. Conditional on the as-yet-unknown outcome of the computation, it is sometimes possible to counterfactually infer information about the solution. Here we demonstrate counterfactual computation, implementing Grover's search algorithm with an all-optical approach4. It was believed that the overall probability of such counterfactual inference is intrinsically limited1,5, so that it could not perform better on average than random guesses. However, using a novel 'chained' version of the quantum Zeno effect6, we show how to boost the counterfactual inference probability to unity, thereby beating the random guessing limit. Our methods are general and apply to any physical system, as illustrated by a discussion of trapped-ion systems. Finally, we briefly show that, in certain circumstances, counterfactual computation can eliminate errors induced by decoherence. AU - Onur Hosten AU - Rakher, Matthew T AU - Barreiro, Julio T AU - Peters, Nicholas A AU - Kwiat, Paul G ID - 579 IS - 7079 JF - Nature TI - Counterfactual quantum computation through quantum interrogation VL - 439 ER - TY - CONF AB - Visible light photon counters (VLPCs) and solid-state photomultipliers (SSPMs) facilitate efficient single-photon detection. We are attempting to improve their efficiency, previously limited to < 88% by coupling losses, via anti-reflection coatings, better electronics and cryogenics. AU - Rangarajan, Radhika AU - Peters, Nicholas A AU - Onur Hosten AU - Altepeter, Joseph B AU - Jeffrey, Evan R AU - Kwiat, Paul G ID - 583 TI - Improved single-photon detection ER -