** Weknowmemes (****קישורים למאמרים בעברית בתוך המאמר באנגלית)**

**Foundations of quantum physics**

**Quantum time machine and quantum time travel**

Einstein’s theory of general relativity allows the existence of closed timelike curves (CTCs), paths through spacetime that, if followed, allow a time traveler to interact with his/her former self. Seth Lloyd suggests that general relativistic CTCs provide one potential mechanism for time travel, but they need not provide the only one. Quantum mechanics might allow time travel even in the absence of CTCs in the geometry of spacetime. He explores a particular version of CTCs based on combining quantum teleportation (and quantum entanglement) with “postselection”. This combination results in a quantum channel to the past. The entanglement occurs between the forward- and backward going parts of the curve. Post-selection replaces the quantum measurement, allowing time travel to take place: Postselection could ensure that only a certain type of state can be teleported. The states that qualify to be teleported are those that have been postselected to be self-consistent prior to being teleported. Only after it has been identified and approved can the state be teleported, so that, in effect, the state is traveling back in time. Under these conditions, time travel could only occur in a self-consistent, non-paradoxical way. The resulting post-selected closed timelike curves (P-CTCs) provide time-travel (Quantum time machine) that avoids grandfather paradox. Entangled states of P-CTCs, allows time travel even when no space-time CTC exists. Such quantum time travel can be thought of as a kind of quantum tunneling backwards in time, which can take place even in the absence of a classical path from future to past

**פילוסופיה של הקוונטים: מסע בזמן, טלפורטציה בזמן והחתול של שרדינגר קם לתחייה**

**Wheeler’s delayed choice thought experiment**

Wave-particle duality: A photon, may behave either as a particle or a wave. The way in which it behaves depends on the kind of experimental apparatus with which it is measured. Both aspects, particle and wave, which appear to be incompatible, are never observed simultaneously (complementarity, Copenhagen interpretation). It was suggested that quantum particles may know in advance to which experiment they will be confronted, via a hidden variable, and could decide which behavior to exhibit. This was challenged by Wheeler’s delayed choice thought experiment: In this variant of the double slit experiment (Mach-Zehnder interferometer + classically controlled beam-splitters), the observer chooses to test either the particle or wave nature of a photon after it has passed through the slits. Thus, the particle could not have known in advance via a hidden variable the kind of experiment it will be confronted. Wheeler’s experiment has been implemented experimentally, and quantum predictions were confirmed. Recently, quantum delayed choice experiments were proposed using a quantum beam-splitter in superposition of being present and absent, and thus the interferometer is in a superposition of being closed and open. This forces the photon to be in a superposition of particle and wave at the same time; then we can detect the photon before choosing if the interferometer is open or closed. This implies that we can choose if the photon behaves as a particle or as a wave after it has been already detected (post-selection). This negates consistent hidden-variable theories in which particle and wave are realistic properties. The upshot of the experiment can be cast in a (“realistic”) language of Schrödinger’s cat: “Long after the cat has supposedly been killed or not, one can choose to determine if it is dead or alive or determine if it is dead and alive,” says Seth Lloyd at the MIT. See refs. in this source

**Spookier than “spooky action at a distance”: Delayed choice quantum eraser and delayed choice entanglement swapping experiments **

According to the famous words of Albert Einstein, the effects of quantum entanglement appear as “spooky action at a distance.” Here are experiments that are spookier than quantum entanglement. Two types of delayed choice experiments: delayed choice quantum eraser experiment and delayed choice entanglement swapping. Anton Zeilinger at the Institute for Quantum Optics and Quantum Information, the University of Vienna and authors experimentally realized the latter “Gedankenexperiment” formulated by Asher Peres in 2000.

Consider Wheeler’s delayed-choice experiment: Wheeler has pointed out that the experimentalist may delay his decision as to display wave like or particle like behavior in a light beam long after the beam has been split by the appropriate optics. A delayed-choice experiment with entangled photons pave the way for new possibilities, where the choice of measurement settings on the distant photon can be made even after the other photon has been registered. This has been shown in a delayed-choice quantum eraser experiment. The which-path information of one photon was erased by a later suitable measurement on the other photon. This allowed to a posteriori decide a single-particle characteristic, namely whether the already measured photon behaved as a wave or as a particle.

However, this delayed-choice experiment focused on wave-particle duality for single particles, there is an entanglement-separability duality for two particles. Entanglement and separability correspond to two mutually exclusive types of correlations between two particles. Even the degree to which the particles were entangled can be defined after the particles have been registered.

Consider entanglement swapping. Peres proposed an experiment, where entanglement is produced a posteriori, after the entangled particles have been measured and may no longer exist. This is Delayed choice for entanglement swapping. In realist’s language: quantum entanglement can reach into the past, future actions may influence past events.

In the proposed experiment, two distant observers, conventionally called Alice and Bob, independently prepare two sets of photons entangled with each other. Alice and Bob keep one particle of each pair and send the other particle to a third observer, Eve also arranges them in pairs (one from Alice and one from Bob). Alice and Bob sort the records of their measurements into four subsets, according to Eve’s results. It then follows that, the state of the particles that Alice and Bob kept was the same as the state later found by Eve. Even after Alice and Bob have recorded the results of all their measurements, Eve still has the freedom of deciding which experiment she will perform. It is not even necessary for Alice and Bob to know which experiments Eve will do. Hence, Eve has control over Alice and Bob’s particles. Eve is free to choose either to project her two photons onto an entangled state and thus project Alice’s and Bob’s photons onto an entangled state, or to measure them individually and then project Alice’s and Bob’s photons onto a separable state. If Alice and Bob measure their photons’ spin (or polarization) states before Eve makes her choice and projects her two photons either onto an entangled state or onto a separable state, it implies that whether their two photons are entangled (showing quantum correlations) or separable (showing classical correlations) can be defined after they have been measured; Eve can choose to take her action even after Bob and Alice may have destroyed their photons. Indeed Asher Peres wrote: “quantum effects mimic not only instantaneous action-at-a-distance but also, as seen here, influence of future actions on past events, even after these events have been irrevocably recorded”.

A recent experiment implements the two important steps necessary on the way from Wheeler’s to Peres’s gedankenexperiment: One needs to first extend Wheeler’s delayed-choice experiment to the delayed-choice quantum eraser to have the possibility that a choice (for one particle) can be after the measurement (of another particle). In a second step, one has to go from the delayed-choice quantum eraser to delayed-choice entanglement swapping to be able to a posteriori decide on a two-particle characteristic and show entanglement-separability duality

**פילוסופיה של הקוונטים חלק ב’. הפרדוקס של אשר פרס: האם העתיד גורם לעבר?**

**An entanglement swapping setup**** that ****generates a secrete key ****for quantum cryptography**

The peculiar properties of quantum mechanics allow two remote parties to communicate a private secret key, which is protected from eavesdropping by the laws of physics and therefore unbreakable in theory (due to Heisenberg uncertainty principle). This is Quantum cryptography, or more precisely quantum key distribution (QKD). However, practical QKD systems could be vulnerable to side-channel attacks even if it is unbreakable in theory. Researchers from the UK have proposed a new theoretical scheme for QKD that keeps the detectors from being exposed to an untrusted third party (UTP) and, even better, uses the UTP to inadvertently generate the secrete key for the detectors. The protocol is based on an entanglement swapping setup scenario. Alice and Bob, control two private spaces, A and B, respectively. Conventionally, these spaces are assumed completely inaccessible from the outside, i.e., no illegitimate system may enter A or B. For this reason every kind of side-channel attack upon the private spaces is assumed excluded. Within its own private space, each party (Alice or Bob) has a bipartite state, which entangles two systems: A, A’ for Alice and B, B’ for Bob. Systems A, B are kept within the private spaces, while systems A’, B’ are sent to a UTP, whose task is to perform a quantum measurement and communicate the corresponding result. At this point, Alice and Bob do not share any common quantum states with which to generate a key. But the UTP is Eve!! Eve’s aim is to eavesdrop the key, or else prevent Alice and Bob from generating the key. Eve applies a quantum instrument T to the incoming systems A’, B’ from Alice and Bob. This is a quantum operation with both classical and quantum outputs. The classical output of T can be simply represented by a stochastic variable L. The quantum output of T is represented by a system E which is correlated with Alice and Bob’s private systems A, B. E is the system that Eve will use for eavesdropping. Eve can store all the output systems E (generated in many independent rounds of the protocol) into a big quantum memory. Then, she can detect the whole memory using an optimal quantum measurement (corresponding to a collective attack). Oh my god!

Eve sends a classical communication to both Alice and Bob in order to “activate” the correlations. Here, Eve has another weapon in her hands, i.e., tampering with the classical outcomes. In order to decrease the correlations between the honest parties, Alice and Bob, Eve processes the output stochastic variable L via a classical channel and then communicates the fake variable L’ to Alice and Bob. Eve is now eavesdropping and entangled with Alice and Bob. After M rounds of the protocol, Alice and Bob will share M copies of a new fake quantum entangled state dependent on the fake variable L’. In general, Alice and Bob do not know anything about this physical process. They get M copies of an unknown state plus classical fake information L’. However, by measuring a suitable number M’ of these copies, they are able to deduce the explicit form of the fake quantum state for the remaining N = M – M’ copies (here M, M’ and N are large numbers). Then, by applying local measurements, Alice on her private systems and Bob on his, they are able to extract and derive a shared secret key. Hence, in the proposed protocol Eve allows the creation of correlations between the private systems A, B that Alice and Bob can exploit to generate a secret-key. According to the authors, eventually one is able to completely protect private space settings and detectors from probing side-channel attacks.

Charles Bennett’s meme

**A quantum eraser under Einstein’s locality condition**

Anton Zeilinger and authors propose and experimentally demonstrate a quantum eraser under “Einstein’s locality condition”: The locality condition imposes that if “two systems no longer interact, no real change can take place in the second system in consequence of anything that may be done to the first system”. To experimentally realize a quantum eraser under Einstein’s locality condition, the erasure event of “which-path” information has to be relativistically space-like separated from the whole passage of the interfering system through the interferometer including its final registration. This means that in any and all reference frames no subluminal or luminal physical signal can travel from one event to the other and causally influence it.

A source in a laboratory located in La Palma, on the Canary Islands, produces path-polarization entangled photon pairs: with entanglement between two different degrees of freedom, namely the **path** of one photon denoted as the **system** photon, and the **polarization** of the other photon denoted as the **environment** photon.

The **system** photon is sent to an **interferometer**, and the **environment** photon is subject to **polarization** measurements. The **environment** photon is sent away from the **system **photon to **Tenerife** via a long 144 km optical fiber (connecting the **La Palma** laboratory and a laboratory in Tenerife).

The environment photon’s polarization carries which-path information of the system photon due to the entanglement between the two photons. According to the quantum eraser experiment, by measuring the environment photon’s polarization (horizontal or vertical), Zeilinger is able to determine the which-path information of the system photon and observe no interference, or erase the which-path information and observe interference. In the latter case, it depends on the specific outcome of the **environment **photon in **Tenerife** which one out of two different interference patterns the system photon is showing. Choices to acquire which-path information or to obtain interference of the **system** photons in **La Palma** are made so that the two systems (**system** photon and **environment** photon) are not interacting; no real change is taking place in the second system (**system** photon) in consequence of something done to the first system (**environment** photon). Hence, there are no causal influences between the system photons and the environment photons. In this arrangement in order to pass information between the environment photon in Tenerife and the system photon in La Palma, the speed of a hypothetical superluminal signal would have to be about 96 times the speed of light!

Zeilinger demonstrates and confirms that, whether the correlations between two entangled photons reveal which-path information or an interference pattern of one (**system**) photon depends on the choice of measurement on the other (**environment**) Photon; this is so even when all of the events on the two sides that can be space-like separated are space-like separated. The delayed choice quantum eraser experiment or space-like quantum eraser experiment performed here shows that it is possible to decide whether a wave or particle feature manifests itself **long after**—and even space-like separated from—the measurement.

Zeilinger and authors conclude, their results demonstrate that the viewpoint that the system photon behaves either definitely as a wave or definitely as a particle would require faster-than-light communication. Because this would be in strong tension with the special theory of relativity, they believe that such a viewpoint should be given up entirely.

Source (January 2013).

**Tripartite entanglement: three-party generalization of the 1935 Einstein-Podolsky-Rosen thought experiment (EPR)**

Scholars demonstrate Entanglement between three separated particles. Three particles – photons – are created directly from a single input photon: A pump photon (a narrowband pump laser at 404 nm) will occasionally fission inside a nonlinear crystal into a pair of daughter polarized photons at 776 nm and 842 nm. The total energy in the process is conserved. The daughter photons share strong energy and time (position-momentum) correlations that are the hallmark of entanglement. The process is repeated with the 776 nm daughter photon serving as a pump and sent through a second crystal, creating a pair of granddaughter photons simultaneously at 1530 nm and 1570 nm. Again energy is conserved, and the total energy of the three photons created must sum to the energy of the pump. This process leaves the 842 nm, 1530 nm and 1570 nm photons entangled in energy and time. Hence, the three photons exhibit genuine tripartite energy-time (position-momentum) entanglement. The entanglement between the three photons is the three-party generalization of the 1935 Einstein-Podolsky-Rosen thought experiment (EPR). The new form of three-particle entanglement may prove to be a valuable part of future communications networks that operate on the principles of quantum mechanics

Quantum communications networks? Recently entanglement has been achieved between two atomic ensembles (comprised of a large collection of identical atoms) and quantum teleportation of light to matter demonstrated. In 2005 scientists reported observations of entanglement between two atomic ensembles (quantum memories) located in distinct apparatuses separated by 3 meters. Now Chinese scientists reported they have realized the first quantum teleportation between two remote atomic-ensembles (quantum memories). What about the Quantum Internet? How do we progress toward more complex quantum networks? Does entanglement extend across the whole network? Adopt the perspective of a quantum network as a quantum many body system and to search for more physical characteristics of the network (e.g., the scaling behavior of pair correlation functions and multipartite entanglement)? Distribution of quantum information over quantum networks: interaction of light with atomic ensembles

**EPR model can exhibit a metric that is analogous to a black hole and a wormhole**

The Bohm-de Broglie (BdB) “pilot wave” hidden variable theory opened up the possibility of a new physics that lied outside the domain of quantum physics: quantum cosmology. Cosmologists applied the BdB interpretation of quantum mechanics to gravity: space-time geometry sometimes looks like (semi-classical) gravity and sometimes looks like quantum effects. In the BdB approach, it is possible to interpret the quantum effects as modifying the geometry in such a way that the scalar particles see an effective geometry.

A scholar from Brasil follows this tradition and studies the two-particle wave function of a scalar field in two dimensions under the EPR condition. He first shows that a two dimensional EPR model, in a particular quantum state and under a non-tachyonic approximating condition – EPR without assuming tachyons – can exhibit in some limited region an effective metric that is analogous to a two dimensional black hole (BH). He considers the BdB theory and concludes that, Bohm’s 1952 quantum potential generates an effective metric so that the quantum potential modifies the background geometry giving a curved space-time with the metric defining a two dimensional BH type solution. After developing a causal approach to the non-tachyonic EPR two-particle correlated system, this allows him to connect the EPR correlations with an effective wormhole geometry. For a two-dimensional static EPR model he shows that quantum effects produce an effective geometry with singularities in the metric, a key ingredient of a bridge construction or a wormhole. He therefore interprets the EPR correlations as driven by an effective wormhole, through which physical signals can propagate (no need then for tachyons to “explain” via a hidden variable theory the EPR paradox?…). The two-particle system ”sees” an effective metric with singularities, a fundamental component of a wormhole, through which the physical signals can propagate from one particle to the other.

**No-cloning theorem and teleportation **

The story of FLASH—A superluminal communicator based upon a new kind of measurement. Nick Herbert proposed entanglement + cloning; faster than light communication was never mentioned. Asher Peres was the referee who approved the publication, knowing perfectly well that it was wrong. This led to the no-cloning theorem: cloning turns out not to be possible in quantum mechanics. If you can clone quantum bits (qubits), you can use this process to communicate faster than light. In fact quantum entanglement never lets you transmit information faster than light. If quantum states can be cloned then special relativity would be violated. A quantum state (quantum information) cannot be transmitted over the telephone. Suppose that Alice has an unknown quantum state. If she could send information over the telephone that was sufficient for Bob to recreate it, then Bob could recreate two copies. However, if Bob and Alice share an entangled bit in an EPR state, Alice can indeed send a qubit in an unknown state in teleportation. In teleportation Alice has destroyed the state, so the information in it is not cloned. Information is shifted from one place to another destroying the original process. Bob must wait to receive the classical outcome of Alice’s measurement, and thus teleportation cannot be used to transmit information faster than light

Charles Bennett’s meme

**The “Everettian Revolution” – Many Worlds **

A system in a superposition of states could in principle boost quantum computers; but measurement causes the states to collapse into a single state. Prof. Frank Tipler explains how one can find a solution to this problem by adopting the “Everettian Revolution”, Hugh Everett’s many-worlds interpretation (Relative State formulation of quantum mechanics, which became the many worlds interpretation, and then parallel universes, many minds, etc…): “The quantum computer, invented by the Everettian physicist David Deutsch, is one of the first results of parallel universe thinking. The idea of the quantum computer is simple: since the analogues of ourselves in the parallel universes are interested in computing the same thing at the same time, why not share the computation between the universes? Let one of us do part of the calculation, another do another part, and so on with the final result being shared between us all”. Do we share the computation with a parallel universe via a wormhole?… Raphael Bousso and Leonard Susskind resort to cosmology. They say that in both the many-worlds interpretation of quantum mechanics and the multiverse of eternal inflation the world is viewed as an unbounded collection of parallel universes. Therefore they argue that the many-worlds of quantum mechanics and the many worlds of the multiverse are the same thing (same sides of the same coin…), and that the multiverse is necessary to give exact operational meaning to probabilistic predictions from quantum mechanics.

**פילוסופיה של הקוונטים ד’: החתול של שרדינגר במסע לעולם דה קוהרנטי ולעולמות מקבילים**