Introduction to Quantum Error Correction

Transcription

1 Introduction to Quantum Error Correction Nielsen & Chuang Quantum Information and Quantum Computation, CUP 2000, Ch. 10 Gottesman quant-ph/ Steane quant-ph/ Gottesman quant-ph/

2 Errors in QIP unitary # $ # $e! " U i( + ) %%& M non-unitary! 0 + " 1 ##$ p! 0 general: pure mixed states! " " 2 # f tr f <! = " " #! = E! E = E " " E 1 $ $ from! f = tr env " U! $! env U # f % & = $ = ' e U! e0 e0 U e! ' E! E E = e U e0 U sys+env trace preserving:! EE = 1 E! E = " " tae ρ and randomly replace by, ( ) with probability (! ) p = tr E E

3 Bit flip channel Quantum noise: channel representation! " # E! E! 1 0"! 0 1" E0 = p $, E1 1 p, 0 1 % = # $ 1 0 % & ' & ' Phase flip channel E0 = p! $ ", E1 1 p! " 0 1 % = # $ 0 # 1 % & ' & ' Bit-phase flip channel 1 0 0!i E0 = p " $ #, E1 1 py 1 p " # 0 1 % =! =! $ i 0 % & ' & ' Amplitude damping channel " 1 0 # " 0! # E0 =, E1 = $ 2 0 1! % $ % ' & ( ' 0 0 ( p! " = 1# p " + X " X + Y " Y + Z " Z 3 Depolarizing channel ( ) ( ) ( ) Geometrical interpretation: Bloch sphere in r-space (NC p. 376) I + r #! " =,! =!,!,!, =,, 2 { } r { r r r } x y z x y z

4 Repetition codes classical error, e.g. 010, corrected to majority value 000 note: learned value of bits in doing so prob. for bit error p < 1: multi-bit error prob. = 3p 2 (1-p)+p 3 =3p 2-2p 3 < p when p < quantum??! ""#!!! No cloning theorem! n bits, majority n/2+1 error prob. p n/2+1 + error prob. as n (p < 0.5) suppose! #!! and " # " " then but (! + " )#(! + " )(! + " )! + " #!! + "" =!! + "" + "! +!" by linearity cannot copy unnown quantum states

5 Encode/Error/Recovery quantum information is encoded into ρ C an error occurs! (" ) = # E " E C C recovery procedure undertaen R [! ] (" ) R E " E R = # # C l C l l regain the encoded state ρ C R [ ( )]! " = " C C

6 Encoding and Recovery R encode error diagnose error fix encoding qubits ancillas measurement error and recovery are superoperators ( ) # A A! = S " =! Unitary operations Recovery operator R restores state to the code after error from environment encode into a subspace no meaurement of state, only of error achieve by adding ancilla qubits measure ancillas syndrome of error perform unitaries conditional on syndrome to correct erroneous qubits

7 Encoding e.g., 3-qubit bit flip code 0 L >= 000> 1 L >= 111>! = " 0 + # 1 $! = " 0 + # 1 C L L! 0 0! C ( ) ( )! 0 + " 1 # 0 $! 00 + " 11! 00 + " 11 # 0 $! " 111 %! 0 + " 1 L L

8 Continuous Errors R " i! / 2 # 1 0 $ i! / 2 e 0! / 2 e i! i! / 2 # $ = % 0 e & = % & ' ( ' 0 e ( (! ) (! ) = cos / 2 I " isin / 2 Z add ancilla(s), transfer error info to ancilla (c-u) Z! 0 + " 1 # 0 $ Z! 0 + " 1 # Z ( ) ( ) L L anc L L anc (! 0 " 1 ) 0 (! 0 " 1 ) I + # $ I + # noerror L L anc L L anc ancilla superposition $! % cos I " 0 # 1 ' ( ) 2 * ( ) + & no error L L anc $! % + isin' ( Z (" 0 + # 1 )& Z ) 2 * L L anc measure ancilla 2 $! % prob. sin ' ( Z (" 0 1 ) L + # L & Zanc ) 2 * 2 $! % prob. cos ' ( I (" 0 + # 1 )& no error ) 2 * L L anc invert either one restore initial state

9 3-qubit Bit Flip Code 0 L >= 000> 1 L >= 111> α+β 1> Error X with prob. p R ψ> anc M encode error diagnose fix I II III IV M X I: (α+β 1>) α 000>+β 111> II: 8 possibilities from errors XII, IXI, IIX, XXI, XIX, IXX, XXX, III

10 state after error α 000>+β 111> (1-p) 3 α 100>+β 011> p(1-p) 2 α 010>+β 101> p(1-p) 2 α 001>+β 110> p(1-p) 2 α 110>+β 001> p 2 (1-p) α 101>+β 010> p 2 (1-p) α 011>+β 100> p 2 (1-p) α 111>+β 000> p 3 Prob. of getting state 1 or no error III: a) perform CNOT between qubits 1 & 2 with ancilla 1 b) perform CNOT between qubits 1 & 3 with ancilla 2 α 000>+β 111> 00> (1-p) 3 α 100>+β 011> 11> p(1-p) 2 α 010>+β 101> 10> p(1-p) 2 α 001>+β 110> 01> p(1-p) 2 α 110>+β 001> 01> p 2 (1-p) α 101>+β 010> 10> p 2 (1-p) α 011>+β 100> 11> p 2 (1-p) α 111>+β 000> 00> p 3 syndrome syndrome redundant for 1 and 2 (0 and 3) errors, but unequal probabilities

11 III. c) M = measure ancillas: assume only 1 (or 0) error syndrome uniquely identifies error failure rate of code = rate of 2 errors = 3p 2 (1-p)+p 3 = 3p 2-2p 3 < p for p < 0.5 IV. fix by applying unitary conditional on M syndrome: 00 do nothing 01 apply σ x to 3 rd qubit 10 apply σ x to 2 nd qubit 11 apply σ x to 1 st qubit α 000>+β 111> 00> α 100>+β 011> 11> α 010>+β 101> 10> α 001>+β 110> 01> recover encoded state α 000>+β 111>

12 Decoding e.g. from syndrome 10 after IV. have α 000>+β 111> with p(1-p) 2 extract original qubit α+β 1> with circuit: i) ii) i) α 000>+β 111> α 00>+β 1> 10> ii) α 00>+β 1> 10> α 00>+β 1> 00> = (α+β 1>) 00> get correct qubit state with prob. > 1-p prob. of failure = 3p 2-2p 3 < p for p < 0.5 success = 100% if no 2 or 3 errors error prob. reduced from p to O(p 2 )

13 3-bit Phase Code σ z (α+β 1>) = α-β 1> not classical! change basis: +> =1/ 2(+ 1>) ->=1/ 2(- 1>)! + " 1! 1 1 "! 1"! 1" # = = H % $ # 2 1 % 1 $# 1 $ # 1 $ & ' & '& ' & ' H σ z H=σ x or H= +><0 + -><1 then σ z +>= -> σ z ->= +> lie bit flip! R ψ> H H H H H H Z M M X I II

14 effectively encoded into 0 L >= +++>, 1 L >= ---> I, II α +++> + β ---> phase errors ZII, IZI, ZII act as Z on 000>, 111> e.g., ZII 000> = 000> ZII 111> = > but as X on +++>, --->

15 Both bit flip and phase errors: concatenate these two codes: 0 L >=( 000>+ 111>)( 000>+ 111>)( 000>+ 111>) 1 L >=( 000>- 111>)( 000>- 111>)( 000>- 111>) inner layer corrects bit flips 000, 111 outer layer corrects phase flips +++, ---- define Bell basis: 000>± 111> 001> ± 110> 010> ± 101> 100> ± 011> Shor PRA 52, R2493 (1995) consider decoherence of qubit 1: e a 0 +a 1 1> e 1> a 2 +a 3 1> e, a 0, a 3 = states of env first triple: 000>+ 111> (a 0 +a 1 1>) 00>+ (a 2 +a 3 1>) 11> = a 0 000>+a a 2 011>+a 3 111>

16 put in Bell basis =1/2 (a 0 +a 3 ) ( 000> + 111>) +1/2 (a 0 -a 3 ) ( 000> - 111>) +1/2 (a 1 +a 2 ) ( 100> + 011>) +1/2 (a 1 -a 2 ) ( 100> - 111>) similarly 000> - 111> goes to =1/2 (a 0 +a 3 ) ( 000> - 111>) +1/2 (a 0 -a 3 ) ( 000> + 111>) +1/2 (a 1 +a 2 ) ( 100> - 011>) +1/2 (a 1 -a 2 ) ( 100> + 111>) output 2 (syndrome 2) assume 1 error only: compare all 3 triples, see which differs majority sign indicates 0 L > or 1 L > find which qubit decohered (measure 9 ancillas which syndrome) restore qubit state with a unitary operation e.g. from 000> - 111>) 1/2 (a 0 +a 3 ) ( 000> - 111>) no error output 2 1/2 (a 0 -a 3 ) ( 000> + 111>) Z error 1/2 (a 1 +a 2 ) ( 100> + 011>) X error 1/2 (a 1 -a 2 ) ( 100> - 011>) ZX=Y error

17 have diagnosed error on 1 st qubit correct with appropriate unitary Encoder: ψ> H H H [9,1,3] code: 9 physical qubits 1 logical qubit (3-1)/2=1 arbitrary error corrected not most efficient code: [7,1,3] and [5,1,3] cannot compute easily (logical X, Z OK logical H, CNOT, T hard)