Entanglement distillation

Entanglement distillation (also called "entanglement purification") is the transformation of "N" copies of an arbitrary entangled state formula_1 into some number of approximately pure Bell pairs, using only local operations and classical communication.
Quantum entanglement distillation can in this way overcome the degenerative influence of noisy quantum channels by transforming previously shared less entangled pairs into a smaller number of maximally entangled pairs.
History.
The limits for entanglement dilution and distillation are due to C. H. Bennett, H. Bernstein, S. Popescu, and B. Schumacher, who presented the first distillation protocols for pure states in 1996; entanglement distillation protocols for mixed states were introduced by Bennett, Brassard, Popescu, Schumacher, Smolin and Wootters the same year. Bennett, DiVincenzo, Smolin and Wootters established the connection to quantum error-correction in a ground-breaking paper published in August 1996, also in the journal of Physical Review, which has stimulated a lot of subsequent research.
Quantifying entanglement.
A two qubit system can be written as a superposition of possible computational basis qubit states: formula_2, each with an associated complex coefficient formula_3:
formula_4
As in the case of a single qubit, the probability of measuring a particular computational basis state formula_5 is the square of the modulus of its amplitude, or associated coefficient, formula_6, subject to the normalization condition formula_7. The normalization condition guarantees that the sum of the probabilities add up to 1, meaning that upon measurement, one of the states will be observed. 
The Bell state is a particularly important example of a two qubit state: formula_8
Bell states possess the property that measurement outcomes on the two qubits are correlated. As can be seen from the expression above, the two possible measurement outcomes are zero and one, both with probability of 50%. As a result, a measurement of the second qubit always gives the same result as the measurement of the first qubit.
Bell states can be used to quantify entanglement. Let "m" be the number of high-fidelity copies of a Bell state that can be produced using LOCC. Given a large number of Bell states the amount of entanglement present in a pure state formula_9 can then be defined as the ratio of formula_10, called the distillable entanglement of a particular state formula_11, which gives a quantified measure of the amount of entanglement present in a given system. The process of entanglement distillation aims to saturate this limiting ratio. The number of copies of a pure state that may be converted into a maximally entangled state is equal to the von Neumann entropy formula_12 of the state, which is an extension of the concept of classical entropy for quantum systems. Mathematically, for a given density matrix formula_13, the von Neumann entropy formula_12 is formula_15. Entanglement can then be quantified as the entropy of entanglement, which is the von Neumann entropy of either formula_16 or formula_17 as:
formula_18
Which ranges from 0 for a product state to formula_19 for a maximally entangled state (if the formula_20 is replaced by formula_21 then maximally entangled has a value of 1).
Motivation.
Suppose that two parties, Alice and Bob, would like to communicate classical information over a noisy quantum channel. Either classical or quantum information can be transmitted over a quantum channel by encoding the information in a quantum state. With this knowledge, Alice encodes the classical information that she intends to send to Bob in a (quantum) product state, as a tensor product of reduced density matrices formula_22 where each formula_13 is diagonal and can only be used as a one time input for a particular channel formula_24.
The fidelity of the noisy quantum channel is a measure of how closely the output of a quantum channel resembles the input, and is therefore a measure of how well a quantum channel preserves information. If a pure state formula_25 is sent into a quantum channel emerges as the state represented by density matrix formula_13, the fidelity of transmission is defined as formula_27.
The problem that Alice and Bob now face is that quantum communication over large distances depends upon successful distribution of highly entangled quantum states, and due to unavoidable noise in quantum communication channels, the quality of entangled states generally decreases exponentially with channel length as a function of the fidelity of the channel. Entanglement distillation addresses this problem of maintaining a high degree of entanglement between distributed quantum states by transforming N copies of an arbitrary entangled state formula_1 into approximately formula_29 Bell pairs, using only local operations and classical communication. The objective is to share strongly correlated qubits between distant parties (Alice and Bob) in order to allow reliable quantum teleportation or quantum cryptography.
Entanglement concentration.
Pure states.
Given n particles in the singlet state shared between Alice and Bob, local actions and classical communication will suffice to prepare m arbitrarily good copies of formula_30 with a yield
Let an entangled state formula_9 have a Schmidt decomposition:
formula_32
where coefficients p(x) form a probability distribution, and thus are positive valued and sum to unity. The tensor product of this state is then,
formula_33
Now, omitting all terms formula_34 which are not part of any sequence which is likely to occur with high probability, known as the typical set: formula_35 the new state is
formula_36
And renormalizing,
formula_37
Then the fidelity
Suppose that Alice and Bob are in possession of m copies of formula_9. Alice can perform a measurement onto the typical set formula_39 subset of formula_40, converting the state formula_41 with high fidelity. The theorem of typical sequences then shows us that formula_42 is the probability that the given sequence is part of the typical set, and may be made arbitrarily close to 1 for sufficiently large m, and therefore the Schmidt coefficients of the renormalized Bell state formula_43 will be at most a factor formula_44 larger. Alice and Bob can now obtain a smaller set of n Bell states by performing LOCC on the state formula_43 with which they can overcome the noise of a quantum channel to communicate successfully.
Mixed states.
Many techniques have been developed for doing entanglement distillation for mixed states, giving a lower bounds on the value of the distillable entanglement formula_46 for specific classes of states formula_13.
One common method involves Alice not using the noisy channel to transmit source states directly but instead preparing a large number of Bell states, sending half of each Bell pair to Bob. The result from transmission through the noisy channel is to create the mixed entangled state formula_13, so that Alice and Bob end up sharing formula_49 copies of formula_13. Alice and Bob then perform entanglement distillation, producing formula_51 almost perfectly entangled states from the mixed entangled states formula_13 by performing local unitary operations and measurements on the shared entangled pairs, coordinating their actions through classical messages, and sacrificing some of the entangled pairs to increase the purity of the remaining ones. Alice can now prepare an formula_51 qubit state and teleport it to Bob using the formula_51 Bell pairs which they share with high fidelity. What Alice and Bob have then effectively accomplished is having simulated a noiseless quantum channel using a noisy one, with the aid of local actions and classical communication.
Let formula_55 be a general mixed state of two spin-1/2 particles which could have resulted from the transmission of an initially pure singlet state
formula_56
through a noisy channel between Alice and Bob, which will be used to distill some pure entanglement. The fidelity of 
formula_57
is a convenient expression of its purity relative to a perfect singlet. Suppose that M is already a pure state of two particles formula_58 for some formula_30. The entanglement for formula_30, as already established, is the von Neumann entropy formula_61 where
formula_62
and likewise for formula_17, represent the reduced density matrices for either particle. The following protocol is then used:
Repeating the outlined protocol above will distill Werner states whose purity may be chosen to be arbitrarily high formula_78 from a collection "M" of input mixed states of purity formula_79 but with a yield tending to zero in the limit formula_80. By performing another bilateral XOR operation, this time on a variable number formula_81 of source pairs, as opposed to 1, into each target pair prior to measuring it, the yield can be made to approach a positive limit as formula_80. This method can then be combined with others to obtain an even higher yield.
Procrustean method.
The Procrustean method of entanglement concentration can be used for as little as one partly entangled pair, being more efficient than the Schmidt projection method for entangling less than 5 pairs, and requires Alice and Bob to know the bias (formula_83) of the n pairs in advance. The method derives its name from Procrustes because it produces a perfectly entangled state by chopping off the extra probability associated with the larger term in the partial entanglement of the pure states:
formula_84
Assuming a collection of particles for which formula_83 is known as being either less than or greater than formula_86 the Procrustean method may be carried out by keeping all particles which, when passed through a polarization-dependent absorber, or a polarization-dependent-reflector, which absorb or reflect a fraction formula_87 of the more likely outcome, are not absorbed or deflected. Therefore, if Alice possesses particles for which formula_88, she can separate out particles which are more likely to be measured in the up/down basis, and left with particles in maximally mixed state of spin up and spin down. This treatment corresponds to a POVM (positive-operator-valued measurement). To obtain a perfectly entangled state of two particles, Alice informs Bob of the result of her generalized measurement while Bob doesn't measure his particle at all but instead discards his if Alice discards hers.
Stabilizer protocol.
The purpose of an formula_89 entanglement distillation protocol is to distill formula_90 pure ebits from formula_91 noisy ebits where formula_92. The yield of such a protocol is formula_93. Two parties can then use the noiseless ebits for quantum communication protocols.
The two parties establish a set of shared noisy ebits in the following way. The sender Alice first prepares formula_91 Bell states formula_95 locally. She sends the second qubit of each pair over a noisy quantum channel to a receiver Bob. Let formula_96 be the state formula_97 rearranged so that all of Alice's qubits are on the left and all of Bob's qubits are on the right. The noisy quantum channel applies a Pauli error in the error set formula_98 to the set of formula_91 qubits sent over the channel. The sender and receiver then share a set of formula_91 noisy ebits of the form formula_101 where the identity formula_102 acts on Alice's qubits and formula_103 is some Pauli operator in formula_104 acting on Bob's qubits.
A one-way stabilizer entanglement distillation protocol uses a stabilizer code for the distillation procedure. Suppose the stabilizer formula_105 for an formula_89 quantum error-correcting code has generators formula_107. The distillation procedure begins with Alice measuring the formula_108 generators in formula_105. Let formula_110 be the set of the formula_111 projectors that project onto the formula_111 orthogonal subspaces corresponding to the generators in formula_105. The measurement projects formula_96 randomly onto one of the formula_115 subspaces. Each formula_116 commutes with the noisy operator formula_103 on Bob's side so that
formula_118
The following important "Bell-state matrix identity" holds for an arbitrary matrix formula_119:
formula_120
Then the above expression is equal to the following:
formula_121
Therefore, each of Alice's projectors formula_116 projects Bob's qubits onto a subspace formula_123 corresponding to Alice's projected subspace formula_116. Alice restores her qubits to the simultaneous +1-eigenspace of the generators in formula_105. She sends her measurement results to Bob. Bob measures the generators in formula_105. Bob combines his measurements with Alice's to determine a syndrome for the error. He performs a recovery operation on his qubits to reverse the error. He restores his qubits formula_105. Alice and Bob both perform the decoding unitary corresponding to stabilizer formula_105 to convert their formula_90 logical ebits to formula_90 physical ebits.
Entanglement-assisted stabilizer code.
Luo and Devetak provided a straightforward extension of the above protocol (Luo and Devetak 2007). Their method converts an entanglement-assisted stabilizer code into an entanglement-assisted entanglement distillation protocol.
Luo and Devetak form an entanglement distillation protocol that has entanglement assistance from a few noiseless ebits. The crucial assumption for an entanglement-assisted entanglement distillation protocol is that Alice and Bob possess formula_131 noiseless ebits in addition to their formula_91 noisy ebits. The total state of the noisy and noiseless ebits is
formula_133
where formula_134 is the formula_135 identity matrix acting on Alice's qubits and the noisy Pauli operator formula_136 affects Bob's first formula_91 qubits only. Thus the last formula_131 ebits are noiseless, and Alice and Bob have to correct for errors on the first formula_91 ebits only.
The protocol proceeds exactly as outlined in the previous section. The only difference is that Alice and Bob measure the generators in an entanglement-assisted stabilizer code. Each generator spans over formula_140 qubits where the last formula_131 qubits are noiseless.
We comment on the yield of this entanglement-assisted entanglement distillation protocol. An entanglement-assisted code has formula_108 generators that each have formula_140 Pauli entries. These parameters imply that the entanglement distillation protocol produces formula_144 ebits. But the protocol consumes formula_131 initial noiseless ebits as a catalyst for distillation. Therefore, the yield of this protocol is formula_93.
Entanglement dilution.
The reverse process of entanglement distillation is entanglement dilution, where large copies of the Bell state are converted into less entangled states using LOCC with high fidelity. The aim of the entanglement dilution process, then, is to saturate the inverse ratio of n to m, defined as the distillable entanglement.
Applications.
Besides its important application in quantum communication, entanglement purification also plays a crucial role in error correction for quantum computation, because it can significantly increase the quality of logic operations between different qubits. The role of entanglement distillation is discussed briefly for the following applications.
Quantum error correction.
Entanglement distillation protocols for mixed states can be used as a type of error-correction for quantum communications channels between two parties Alice and Bob, enabling Alice to reliably send mD(p) qubits of information to Bob, where D(p) is the distillable entanglement of p, the state that results when one half of a Bell pair is sent through the noisy channel formula_24 connecting Alice and Bob.
In some cases, entanglement distillation may work when conventional quantum error-correction techniques fail. Entanglement distillation protocols are known which can produce a non-zero rate of transmission D(p) for channels which do not allow the transmission of quantum information due to the property that entanglement distillation protocols allow classical communication between parties as opposed to conventional error-correction which prohibits it.
Quantum cryptography.
The concept of correlated measurement outcomes and entanglement is central to quantum key exchange, and therefore the ability to successfully perform entanglement distillation to obtain maximally entangled states is essential for quantum cryptography.
If an entangled pair of particles is shared between two parties, anyone intercepting either particle will alter the overall system, allowing their presence (and the amount of information they have gained) to be determined so long as the particles are in a maximally entangled state. Also, in order to share a secret key string, Alice and Bob must perform the techniques of privacy amplification and information reconciliation to distill a shared secret key string. Information reconciliation is error-correction over a public channel which reconciles errors between the correlated random classical bit strings shared by Alice and Bob while limiting the knowledge that a possible eavesdropper Eve can have about the shared keys. After information reconciliation is used to reconcile possible errors between the shared keys that Alice and Bob possess and limit the possible information Eve could have gained, the technique of privacy amplification is used to distill a smaller subset of bits maximizing Eve's uncertainty about the key.
Quantum teleportation.
In quantum teleportation, a sender wishes to transmit an arbitrary quantum state of a particle to a possibly distant receiver. Quantum teleportation is able to achieve faithful transmission of quantum information by substituting classical communication and prior entanglement for a direct quantum channel. Using teleportation, an arbitrary unknown qubit can be faithfully transmitted via a pair of maximally-entangled qubits shared between sender and receiver, and a 2-bit classical message from the sender to the receiver. Quantum teleportation requires a noiseless quantum channel for sharing perfectly entangled particles, and therefore entanglement distillation satisfies this requirement by providing the noiseless quantum channel and maximally entangled qubits.