Not Portal 3, but close: quantum teleportation of information inside a diamond
We are all familiar with various superheroes and their unique abilities, whether we like it or not. Therefore, the question of what kind of superpower you would like to have is not so rare. Someone would like to be incredibly strong, like the Hulk, someone - fast, like Flash, and someone would not give up Batman's superpowers - money. But those who at least once were in a traffic jam from Mars to Venus would give everything for the opportunity to teleport. The concept of teleportation sounds very exciting from the point of view of science fiction, but in reality this superpower also exists, but far from people are endowed with it. Today we will meet with you a study in which scientists from Yokohama University (Japan) were able to teleport information inside a diamond. How scientists did it, how sideways quantum physics is, and what does this mean for the future of storage technologies? Answers await us in a report by scientists. Go.
Study basis
First of all, it is worth noting that the full name of the phenomenon under discussion is quantum teleportation. The principle of this process is crucial for quantum information technology. For example, to implement quantum communication, quantum repeaters are required that will transmit a qubit (quantum bit) to a remote node without revealing the state of this qubit itself. For computational procedures, teleportation will help to implement the safe transfer of input and output data through quantum communication for quantum blind computing.
The most important aspect of quantum teleportation is the transfer of quantum information to inaccessible space, which is to be expected, as well as the transfer of photon information to quantum memory without revealing or destroying stored quantum information.
In this work, scientists demonstrate a working scheme for transferring a quantum state of photon polarization to the nuclear spin of a carbon isotope bound to the NV center * of a diamond.
NV-center * is a nitrogen-substituted vacancy in diamond, i.e. a point defect in diamond when the structure of its crystal lattice is violated due to the removal of the carbon atom from the lattice site and the binding of the vacancy to the nitrogen atom.The carbon spin is first entangled with the electron spin, which is then allowed to absorb the photon in its own state, correlated with the spin-orbit interaction. Detection of an electron after relaxation in the ground state of the spin allows post-selective transfer of an arbitrary polarization of a photon to carbon memory.
The quantum-state transmission scheme allows quantum storage devices to be implemented both for scalable quantum repeaters (repeaters) of long-distance communication with quantum systems and for distributed quantum computing.
In this work, scientists were able to successfully initiate and manipulate the nuclear carbon spin through nitrogen as a nanomagnet to remove the degeneracy of the electron, maintaining a zero magnetic field on the carbon nuclear spin. The following is the process of transferring the state of polarization of photons to the quantum state of spin, that is, teleportation. All this has been successfully verified through practical experiments and observations, the results of which we will familiarize with a little later, because first the researchers want to explain to us the principle of their miracle technology.
Principle of operation
The basis of quantum teleportation is the preparation of entanglement and measurements in the Bell basis, which leads to the post-selective transfer of the quantum state ( 1a ).
Image No. 1
At the beginning, the entanglement between the electron spin and the carbon nuclear spin is prepared. Then, the polarization of photons by spin of electrons in the Bell basis is measured by absorption of photons in order to transfer the state of polarization of photons to the state of carbon spin ( 1b , 1c ).
In the practical protocol of a one-sided system of quantum repeaters with an NV center at each node, a photon is emitted from one node, leaving the electron entangled with this photon ( 1d) The success of storing photons in another node establishes entanglement between two neighboring nodes.
The negatively charged NV center in diamond consists of an admixture of nitrogen ( 14 N) and an adjacent vacancy ( V ), where the electron ( e ) is localized in the triplet state ( 1b ). The electron and nitrogen nucleus possess the spin 1 property, which makes up a three-level V-type system with two degenerate states m s , I = ± 1 (denoted as | ± 1⟩ e , N ), which make up a logical qubit, and the state m s , I = 0 (denoted by | 0⟩ e , N), which makes up the auxiliary qubit. Then, zero-field splitting (about 2.87 GHz) for the electron and nuclear quadrupole splitting (about 4.95 MHz) for nitrogen occur.
On the other hand, the carbon nuclear spin ( 13 C), weakly bonded to the electron through hyperfine interaction (0.9 MHz in this work), shows the 1/2 spin property, which constitutes a two-level system with two degenerate m I = ± 1/2 states (denoted as | ↑⟩ С , | ↓⟩ С ) in a zero magnetic field ( 1c ).
To prepare the spin entanglement between the electron and carbon nuclear spin, they are first initialized at | 0⟩ e , | ↓⟩ С. Despite the fact that it is difficult to initialize the nuclear carbon spin in a zero magnetic field, the nuclear quadrupole splitting of the nuclear nitrogen spin using a polarized electron spin makes it possible to polarize at | + 1⟩ N , which is used as a nanomagnet to apply a local magnetic field to an electron for initialization nuclear carbon spin ( 1c ).
Image №2
Next was used red light to implement CPT * (Coherent population trapping), resonantly excites electrons in the spin-orbit correlated own state | A 2 ⟩ = 1 / √2 (| +1, -1⟩ l, an e + | -1, + 1⟩ l, e (l and edenote the orbital and spin angular momenta of the electron), when the right circular polarization | + 1⟩ p first polarizes the electron at | + 1⟩ e , and then the nuclear nitrogen spin at | + 1⟩ N (violet line at 2a ).
CPT * is a phenomenon when a set of atoms coherently “gets stuck” in a dark state (an atom or molecule cannot absorb photons).Thus, the electron spin degeneracy is removed due to the hyperfine interaction with nitrogen, which contributes to the selective transition of the carbon nuclear spin from | ↓⟩ C to | ↑⟩ C (green line at 2a ). Figure 2b shows the processes of initialization of the nuclear spins of nitrogen and carbon.
Electron again initialized | 0⟩ e with the red light, resonant states | A l ⟩. Then the electron and carbon are processed using microwave radiation and a radio wave to create entanglement between them in | Ф + ⟩ e , C = 1 / √2 (| + 1, ↑⟩ e , C + | -1, ↓⟩ e , C, which is one of Bell’s four states. Figure 2c shows a quantum diagram of the whole process.
Further, the electron is allowed to absorb an incoming photon with an arbitrary polarization, which excites the electron in another spin-orbit eigenstate. The absorption of a photon projects the state of polarization of the photon and the spin state of the electron in one of the Bell states. The projection of the prepared state, consisting of an arbitrary polarization of photons and an electron-carbon entangled state, is expressed as follows:
As a result, the state of polarization of the photon with the additional unitary operation σ y is obtained .
Experimental teleportation implementation
And now you can go from words (or rather formulas) to action. First of all, the phase correlation between the input photon and the transferred carbon was measured, which shows the conservation of quantum coherence in the transfer operation.
№3 image
on the image 3a demonstrate the dependence of the photon polarization of the population of the nuclear spins of carbon, measured along the axis | +⟩ the C - | -⟩ the C . These data were obtained by measuring the amount of photons after application of radio frequency energy and microwave radiation, followed by red, resonating with the condition | E x ⟩. As scientists assumed, a strong antiphase correlation is observed, indicating the quantum nature of the transfer.
Then the researchers decided to check the validity of the quantum process during state transfer by applying six basic states of photon polarization ( 3b ), after which they estimated the state of the nuclear carbon spin after transfer based on tomography of the quantum state. Image 3b shows the Bloch vectors for carbon nuclear spin states transferred from six photon polarizations.
Fidelity * - in quantum informatics this is a measure of the proximity of two quantum states. It expresses the likelihood that one of the conditions will pass the test, which identifies it as the second.Fidelity reached an average of 78 ± 2%, which significantly exceeds the classical limit of 67% ( 3d ). The Bloch vectors made it possible to estimate the quantum transfer channel, as shown in 3d . The accuracy of the transfer process was 76%. This suggests that the transfer channel supports quantum coherence.
A decrease in the fidelity of the transfer of the quantum state is due to several factors: the imperfection of the entanglement and Bell state measurements, which is caused by incomplete initialization of spins ( 3f ); mixing of orbital excited states due to crystal deformation ( 3g ); phase rotation when measuring the Bell state; shutter errors.
The fidelity of initialization can be improved by repeating the initialization sequence, and the effect of deformation of the crystal can be offset by identifying e x and e y , which are x and y components of the deformation. Phase rotation can be avoided by initializing the nuclear nitrogen spin at | 0⟩ N before transfer.
For a more detailed familiarization with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.
Epilogue
The researchers themselves say that their technique is still in its infancy, as evidenced by the successful transfer of the polarization state of only one photon, while one pulse (200 nW, 20 ns) contains about 10 4 photons. Consequently, the probability of transferring at least two of them is 2.5%. This is not astronomically small, but for bragging and bravado it is still not enough, and scientists understand this. In the future, they intend to continue improving their brainchild. They are sure that their work will be very useful in the implementation of technologies so expected by many as quantum computing, quantum communication, and quantum data warehouses. No matter how long the process of creating all of the above technologies takes, this will definitely happen earlier than the release of Portal 3 (sorry, I could not resist).
Thank you for your attention, remain curious and have a good working week, guys! :)
Thank you for staying with us. Do you like our articles? Want to see more interesting materials? Support us by placing an order or recommending it to your friends, a 30% discount for Habr users on a unique analogue of entry-level servers that we invented for you: The whole truth about VPS (KVM) E5-2650 v4 (6 Cores) 10GB DDR4 240GB SSD 1Gbps from $ 20 or how to divide the server? (options are available with RAID1 and RAID10, up to 24 cores and up to 40GB DDR4).
Dell R730xd 2 times cheaper? Only we have 2 x Intel TetraDeca-Core Xeon 2x E5-2697v3 2.6GHz 14C 64GB DDR4 4x960GB SSD 1Gbps 100 TV from $ 199 in the Netherlands!Dell R420 - 2x E5-2430 2.2Ghz 6C 128GB DDR3 2x960GB SSD 1Gbps 100TB - from $ 99! Read about How to Build Infrastructure Bldg. class using Dell R730xd E5-2650 v4 servers costing 9,000 euros for a penny?